- Email: [email protected]
An improved design for the dust extraction system of orbital sanders used on wood
Ann. occup. Hyg., Vol British
39, No. 2, pp. 155-165. 1995 Elsevier Science Ltd Occupational Hygiene Society Printed in Great Britam
0003-4878(94)00121-9
AN IMPROVED DESIGN SYSTEM OF ORBITAL
FOR THE DUST EXTRACTION SANDERS USED ON WOOD
A. Thorpe and R. C. Brown Occupational
Medicine and Hygiene Laboratory, Health and Safety Executive, Broad Lane, Sheffield S3 7HQ, U.K. (Received injnalform
20 September 1994)
Abstract-The dust extraction systems of available orbital sanders do not perform well when the sanders are used on edges or pieces that do not contact the entire sander base. In an effort to improve the situation a number of experimental bases have been fitted to a sander, with holes in a variety of patterns. Many patterns give good performance, and particularly good dust extraction can be obtained through 10 carefully placed holes, only two more than the number in a standard base. Holes with a diameter of 5 mm give a better performance than 10 mm dia. holes. The effect of varying the extraction flow rate has also been investigated and it is found that the efficiency of dust collection varies approximately linearly with flow rate until a maximum of about 90% is reached, after which little improvement results.
INTRODUCTION
In previous papers measurements have been reported of the effectiveness of dust control systemsfitted to hand-sanding devices (Regnier et al., 1988;Hampl et al., 1992; Thorpe and Brown, 1994), and of factors influencing the rate of production of dust (Thorpe and Brown, 1995). In summary, dust control was found to be good in most instances where the sander was usedon a flat piece of wood, but when testswere carried out on a slowly rotating dowel, simulating an edge, the capture efficiency of an orbital sander with an integral dust extraction system could be as low as 5%. This could be increased to approximately 50% when the integral extraction system was replaced by one giving much higher extraction flow rates, such as a vacuum cleaner. However, in spite of the improvement the concentration of airborne dust was still unacceptable (Thorpe and Brown, 1994). Furthermore, it has become clear, during visits to several woodworking factories, that workers prefer the integral filter bag method of dust extraction since the trailing hose of external extractors makes them awkward to manoeuvre. The aim of the present work was, therefore, to develop an improved method of dust extraction for orbital sanders used with integral dust extraction. DUST
CAPTURE
EFFICIENCY
OF ORBITAL
SANDERS
USED
TO SAND
EDGES
OR DOWELS
Dust extraction, in all of the orbital sanders tested in the exercisedescribed above, takes place through holes in the sander base and the attached abrasive paper. In this article attention will be focused on a typical device referred to in a previous paper c; Crown copyright
1994 155
156
A. Thorpe and R. C. Brown
\ Position of holes in rubber base /
Fig. 1. Base of orbital sander showing extraction holes and air channels.
(Thorpe and Brown, 1994),as sander C. This sander has a weight of 2.8 kg and a power of 280 W, and by the use of an electronic control its speedcan be varied between 8000 and 20000 strokes min-‘. The hole pattern in the baseof this sander, which is typical of many orbital sanders of its size, is shown in Fig. 1. Air is drawn through the holes, which are 10 mm in diameter, by an internal fan powered by the sander’smotor. The air and any entrained dust particles then pass through an exhaust port on the rear of the sander either to an integral filter bag or to an external dust extractor. The holes are unevenly spacedand much of the area of the baseis not close to any of them, but when flat wood is sandedthe dust would be subjected to inward airflow at the edge of the sander base whatever the position of the holes on the base might be. The low capture efficiency for orbital sanders when sanding dowel is undoubtedly due to the relatively large distance between the extraction holes in the baseofthe sander and the region of dust generation. A rule of thumb for local exhaust ventilation (LEV) is that at a distance of one diameter from the source of extraction (a hole in the sander basein this instance) the extraction velocity drops to one-tenth of the value it takes at the entrance to the hole. If the dowel is sanded by the central part of the sander basethe distance between the point of contact of the dowel and the nearest extraction hole would be several hole diameters. This suggeststhat the capture efficiency would be improved if the distance between
Dust extraction with orbital wood sanders
157
the extraction holes were decreased, for instance by increasing their number. The diameter of the holes may also affect the capture efficiency. Some inroad into this study has already been made by staff at a woodworking factory where the allergenic wood, Western Red Cedar, is used (R. Snowdon, private communication). They modified the basesof their pneumatic orbital sandersby adding an extra row of dust extraction holes positioned down the centre of the sanding base. Workers using the sander found that wood dust emissions were visibly lower than those from a standard sander, when edgesand corners of components or thin sections were being sanded. We have attempted to carry out work of quantitive type aimed at finding an optimal dust extraction pattern.
MODIFICATION
OF THE SANDER
BASE
Several approaches to altering the hole pattern of the sander base are possible, depending on whether the requirement is simplicity in the processof modification or a more fundamental assessmentof alternative patterns. The easiestmethod of introducing extra holes is simply to drill them in the existing sander base.The baseof the sander used in tests has four holes along the length of each long edge, as shown in Fig. 1, which line up with channels in the metal base of the sander itself. The rubber basefastensup flush to the sander so that any dust that passes through the holes travels along the channels and into the four inlets through which the air is drawn by the internal fan. The drawback of adding holes in this way is that their position is limited to the parts of the base lying over the extraction channels. Alternatively, channels can be milled in the rubber base itself, connecting any additional holes to the existing holes and giving a route to the fan inlet apertures. This method allows additional extraction holes to be positioned centrally along the length of the sander base, and it is an improvement over the previous method, but it is still limited in the number and position of holes that can be added. By far the most flexible method of altering hole pattern and size is to use a completely redesigned sander base, and this was the method chosen for our studies. Removal of the existing base and its replacement by a new one would be a complicated feat of engineering, but the effect of hole pattern can be investigated by attaching a new baseplate to the existing one, after the removal ofits rubber cover. This makes the sander slightly unwieldy, which would be a problem in practical use,but not in our simulation. Holes could be drilled almost anywhere in the new base. The number of hole patterns that could be investigated is extremely large, but this can be limited by restricting the choice to patterns that have the symmetry of the base itself, which has two axes of symmetry passing through the centre, one parallel to each side. A pattern that did not have this symmetry could be optimal only if the sander were used in an asymmetrical fashion; and there is no reason to expect that this is the case. The size of hole can also be varied from the 10 mm dia. hole used in existing equipment. Since the extraction systems to be investigated are likely to have more rather than fewer holes it is reasonable to consider smaller rather than larger holes, and so a diameter of 5 mm was chosen as the alternative. The abrasive area of the sander is reduced by the presenceof holes, and holes larger than 10 mm in diameter might affect
158
A. Thorpe and R. C. Brown
IO I
0
0
0
0 0
24
0
00 19
r
000
0
0
000
0
0
000
0
0
000
0
0
000
0
0
000
0
0 0
000
0
0
000
0
0
000
0
0
0
0 Pattern
0
0
0
Pattern
1
I
2
Fig. 2. Modified sander basesof the two basic patterns, with all holes uncovered (the basesshown have 5 mm dia. holes; bases with an identical pattern of 10 mm dia. holes were also used).
the quality of finish on the sanded wood. Holes much smaller than 5 mm in diameter might need to be so numerous as to be impractical and would probably clog too easily. For the investigation, four milled aluminium baseswere made, with 5 and 10 mm dia. holes in two different patterns, as illustrated in Fig. 2. The baseswere covered with 5 mm thick neoprene rubber with holes punched in the appropriate positions. Each basehad a large number of holes, and could give rise to many different hole patterns if sets of holes were selectively sealed. EXPERIMENTAL
METHOD
The performance of the orbital sander with modified sander baseswas investigated using automated apparatus situated inside a large recirculating dust tunnel as described in previous work (Thorpe and Brown, 1994).The sander was set to full speed (setting number 6) and was fitted with 50 grade sandpaper (coarse grit); and the apparatus moved it at a speedof 0.3 m s- ’ with a total downward force of 50 N, in a reciprocating motion across the specimen,with a stroke length of 0.8 m. The specimen was, in most instances, a 5 cm dia. rotating dowel of beech, since this gave a good simulation of an edge whilst preventing the development of flats. Final tests were carried out on flat wood to ensure that the previously high dust capture efficiency on
Dust extraction
with orbital wood sanders
159
such specimenswas not adversely affected by modifications that improved the capture efficiency when dowel was sanded. The tests were carried out in a uniform air stream with a velocity of 0.25 m s- ’ and, where sampling was carried out, the sampling instruments were placed downstream, as in previous work. In most assessments,however, the efficiency was calculated from a comparison between the massof wood removed from the test specimenand the massof dust collected in the filter bag. In previous work, during which an orbital sander was applied to a dowel, the latter had been parallel to and equidistant from the long edges of the sander base. This resulted in the dowel being as far as possible from the original holes, but in a real sanding situation the sander would be used in various orientations with the dowel making contact with any part of the sander base.This variable was accommodated, to some extent, by carrying out tests with the sander baseat a variety of positions and at two orientations: with the long edgeof the baseparallel to the dowel, and with the long edge perpendicular to it. In each casethe test was carried out at a number of positions. The choices made for the lines of contact between the dowel and the base depended both on the number of holes and on the hole pattern in any particular base.In order to minimize the number of tests, wherever possible the orientation and position which gave the largest distance between hole and dowel contact point was tried first, and if this did not give an acceptable capture efficiency no further tests were carried out using this hole pattern. When the best hole pattern was realized the sander was tested with this for a full 60 min whilst the gravimetric total dust concentration downstream was measuredwith an isokinetic membrane filter sampler. A sampling rate of 10 1.min- ’ was necessaryto ensure that a ‘weighable’ sample was obtained.
HOLE
PATTERNS
USED
IN THE
TESTS
The patterns used in the tests are illustrated in Fig. 3, and in Table 1. Patterns B, F and G, are derived by blocking holes in array 1, and patterns A, C, D, E, H and J are obtained from array 2. All following referencesto basetypes are given in capital letters, and all referencesto sander position and orientation are shown by numbers. In many instances both 5 and 10 mm dia. holes were usedin the samepattern. Table 1 gives both the number of holes in each pattern and the area of the holes in the sandpaper as a percentage of its total area, since this might affect the quality of the finish of the wood.
MEASUREMENT
OF AIR VELOCITY
THROUGH
THE
EXTRACTION
HOLES
The resistance of the paths taken by air entering the dust extraction system of the sander may vary according to which hole is entered, and so there will not necessarilybe uniform airflow among the holes. Severeimbalance will have an adverse effect on the performance even if the pattern of extraction holes is sound. The air velocities through the extraction holes, shown in Table 1, were calculated from the measured total
160
A. Thorpe and R. C. Brown
extraction flow rate and the area of extraction holes in the sander base and are, therefore, mean values for each pattern. The variation in air velocity among the holes in a given pattern was quantified by mounting the sander with the basefacing upwards and measuring the velocity through each hole using a Dantec model 54N50 hot wire anemometer with an omnidirectional probe. The probe was placed 3 mm above the sander base, becauseif it were closer it could be damaged by the rapid vibration of the sander. The air velocity was measured with the sander running at full speed,and with a filter bag attached. Two sander bases were investigated in this way: the standard sander base with eight IO-mm dia. extraction holes, and base C with 18 5-mm dia. extraction holes. With the standard base the velocity was highest through the middle four holes and significantly lower 1
0
0
0
0
0
0
0
0
0
0
0
0
0 *
0 h
-3
i-
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
h
n”
h
*
-3
h
h
h
h
-2
0
0
0
0
0
0
0
0
-4
-2
0
0
t
0
0
’
0
0
0
0
0 0
0
0 -2
0
0
0 0
0
0
0
A
*
r\
0
0
0
0
0
0
0
0
0 m
h 0
-2
0
0 0
0
0 c
D
Fig. 3. (A-D).
-1
Dust extraction with orbital wood sanders 1
2
1
161
2
-
3
0
t t
-4
-
c.
h
-
0
0
c
-3
0
8
-4
-
-3
0
0
0
0
0
0
0
0
-1 i
-L F
E 1
0
2
0
0 -6
0 -3
-1
-5 e
A
h
n
0
0
H
0
-1
0
0
0
0
0 G
-1
0
0 J
Fig. 3. (E-J). Fig. 3. Hole patterns used in the tests, achieved by selective blocking of holes in patterns 1 or 2, Fig. 2, and showing positions of contact with the dowel.
through the outer four holes, the mean velocity and standard deviation through all the holes being 2.22f0.41 m s-r air. The velocity varied much less with hole position for base C, being 2.12f 0.14 m s-l. All of the velocity measurements were made with the sander operating under zero load, and the air flow rates when the sander is used in practice are likely to be lower than these values. The position of the probe during these measurementswas less than one hole diameter from the sander surface, and the measured mean air velocity lay between the mean entry velocity and one-tenth of this value, as expected.
162
A. Thorpe and R. C. Brown
Table 1. Variation in sander capture efficiency with size, and pattern of extraction holes and extraction flow rate
Hole pattern
Dowel position
Hole diameter (mm)
A A A B B B B C C C C D D D D E E E E E E E E F F F F F G G G G G G G H H H H J
1 2 3 1 2 3 4 1 2 1 2 1 2 1 2 1 2 3 4 1 2 3 4 4 1 2 3 4 1 2 3 4 5 6 6 1 2 3 3 1
5 5 5 5 5 5 5 10 10 5 5 10 10 5 5 10 10 10 10 5 5 5 5 10 5 5 5 5 5 5 10 5 5 5 10 5 5 5 10 5
PERFORMANCE
Number of holes
% of base taken by holes
32 32 32 25 26 26 26 18 18 18 18 15 15 15 15 13 13 13 13 13 13 13 13 14 14 14 14 14 10 10 10 10 10 10 10 10 10 10 10 I
4.1 4.1 4.1 3.3 3.3 3.3 3.3 9.1 9.1 2.3 2.3 1.6 1.6 1.9 1.9 6.6 6.6 6.6 6.6 1.6 1.6 1.6 1.6 7.1 1.8 1.8 1.8 1.8 1.3 1.3 5.1 1.3 1.3 1.3 5.1 1.3 1.3 1.3 5.1 0.9
AS A FUNCTION
Flow rate (1. min-‘)
Air velocity through holes (m SK’)
Capture efficiency (%)
166.9 196.5 197.8 174.9 181.4 196.5 187.8 165.5 212.2 197.4 192.8 187.8 185.2 161.4 170.9 167.9 194.1 216.8 219.1 177.5 223.7 176.2 173.5 196.5 189.1 180.1 158.7 161.4 166.9 200.2 178.8 162.8 172.2 177.5 196.5 161.4 180.1 186.5 243.3 111.5
4.43 5.22 5.26 5.72 5.93 6.43 6.14 1.95 2.50 9.32 9.11 2.66 2.62 9.15 9.69 2.74 3.17 3.54 3.58 11.61 14.63 11.52 11.35 2.98 11.48 10.94 9.64 9.80 14.19 17.02 3.80 13.84 14.64 15.09 4.17 13.72 15.31 15.86 5.16 21.56
71.8 68.6 61.3 67.0 70.7 54.2 61.0 60.6 53.2 68.6 52.8 58.3 52.5 69.9 44.5 62.7 57.0 67.3 61.0 80.3 60.7 70.0 68.9 49.3 70.6 65.7 67.3 65.3 76.4 60.8 55.2 67.9 82.7 63.4 46.8 75.1 61.0 36.3 46.5 39.5
OF HOLE
PATTERN
The results of the tests are detailed in Table 1, and it is clear that many of the patterns give good dust capture efficiency in someor all orientations. In summary, base A showed good capture efficiency in all sander orientations and positions but is probably unacceptable becauseof the large number, 26, of extraction holes. This is also the casefor baseB. BasesC and D would probably be acceptablefrom the point of view of the number of holes, especially if they were 5 mm in diameter. However, the capture efficiencies,although promising, did show a marked dependenceon sander orientation
Dust extraction
with orbital wood sanders
163
and position. Base E was very promising, especially with the smaller holes, where efficiency was high at all orientations and positions. BasesF and G had a relatively poor capture efficiency with large holes but good with small. BasesH and J were poor. Since baseG has the fewestholes, this should be least likely to affect the finish of the wood adversely. The fractional area taken up by the 5 mm holes is also less than onethird of that with a standard sander base. Bases with fewer holes than this did not perform as well and can, therefore, be ruled out as possible alternatives. As a check, base G was tested again with flat wood and gave a capture efficiency of 88.3%, indicating that the hole arrangement does not adversely affect its performance in this situation. All of the patterns gave consistently better capture efficiencieswith 5 mm dia. holes than with 10 mm dia. holes. The reason for this is not obvious. The air velocity at the entrance of a 5 mm dia. hole will be four times as great as that at the entrance to a 10 mm dia. hole, provided that the volume flow rate is constant. At distances significantly larger than a hole diameter the difference will be small. However, the larger area of the 10 mm holes should result in more particles being under the direct influence of the airstream. Inside the sander these small holes would be expected to cause a slight reduction in the transit time of the dust to the collecting bag. TESTS
OF OPTIMUM
HOLE
ARRANGEMENT
AT A NUMBER
OF FLOW
RATES
Previous work (Thorpe and Brown, 1994) has shown that the dust extraction efficiency of sanders is higher when they are used with external extraction systems, which gave a high flow rate, than with integral extraction systems,for which the flow rate is much lower. It is useful to put this observation on a quantitive basis, and to this end baseG, the optimal pattern, was tested with dowel at position 2, for a range of flow rates, provided by a vacuum cleaner with flow control. The results, illustrated in Figs 4 and 5, show that the effectivenessof the dust control system increases as the extraction air flow rate is increased. Figure 4 shows results obtained by comparing the weight increase of the dust collection bag with the weight loss of the specimen of wood being sanded, and Fig. 5 shows results obtained from measurements of airborne dust concentration. There is more scatter in the latter results, and the two methods of estimation suggestthat approximately 90% efficiency is obtained at a flow rate of 60&900 1.min- ‘, but that little benefit is obtained by the use of higher flow rates than 900 1.min-‘. When an integral extraction system is used the flow rate will drop as the filter bag becomesloaded, owing to increased resistanceto air flow, and this also results in a drop in capture efficiency. This indicates that the bag should be replaced regularly. In contrast, when flat wood was sanded (Thorpe and Brown, 1994), the loading of the integral filter bag was found to have very little effect on the capture efficiency until the bag was practically full, when the flow rate dropped almost to zero and the capture efficiency fell very quickly. CONCLUSIONS
The addition of extra holes in the standard sander baseof an orbital sander results in an increased efficiency of dust capture when the sander is used on dowel or edges.
A. Thorpe and R. C. Brown
164
0
200
400
800 Flow rate (Vmin)
800
1000
1200
Fig. 4. Dust capture efficiency of sander with base G at position 2 as a function of extraction flow rate, estimated from weights of wood removed and dust collected: 0, experiment; -, by-eye fit.
f
1.5-
z 5 ‘i: e E $
1.0-
E zi 2 0.5 -
0
200
400
800 Flow rate (Vmin)
800
1000
1200
Fig. 5. Airborne dust concentration as a function of extraction flow rate for sander with baseG, at position 2: 0, experiment; -, by-eye fit.
The degree of improvement and its variation with the orientation and position of the dowel depend critically on the hole size and hole pattern. A pattern of holes has been found which gives a much increased capture efficiency for all orientations and positions of sander baserelative to the axis of the dowel, and the sander fitted with the improved basecontinues to work well on flat wood. For this and other patterns, bases with 5 mm extraction holes consistently gave better capture efficienciesthan baseswith the samepattern of 10 mm holes. Even with. the optimized sander base the capture efficiency depends critically on flow rate. If the flow rate
Dust extraction with orbital wood sanders
165
decreasesowing to clogging of the filter bag or if the speed setting on the sander is reduced, the capture efficiency drops dramatically. Likewise, an increase in flow rate results in a much improved capture efficiency.
Acknowledgements-We would like to thank the manufacturer who lent us the sander used in the experiments, and the staff of Amdega Ltd, Darlington, for the loan of a sander with additional extraction holes. We are also grateful to our coieagues at HSE, in particular Mr K. A. Dobson, Mr R. Boland, Mrs J. P. Rhodes, Mr R. Johns and Mrs P. M. Waldron, of the Field Operations Division, and Mr P. A. Roberts of the Research and Laboratory Services Division.
REFERENCES Hampl, V., Topmiller, J. L., Watkins, D. S. and Murdock, D. J. (1992)Control of wood dust from rotational hand-held sanders. Appl. occup. Environ. Hyg. 7,263-270. Regnier, R., Braconnier, R. and Aubertin, G. (1988) Efficacite des dispositits, de captage integres aux machines poratives. INRS 33 9-354. HSE Translation 13698 (1990). Thorpe, A. and Brown, R. C. (1994) Measurements of the effectivenessof dust extraction systems of hand sanders used on wood. Ann. occup. Hyg. 38,279-302. Thorpe, A. and Brown, R. C. (1995) Factors influencing the production of dust during the hand sanding of wood. Am. ind. Hyg. Ass. J. (in press).
39, No. 2, pp. 155-165. 1995 Elsevier Science Ltd Occupational Hygiene Society Printed in Great Britam
0003-4878(94)00121-9
AN IMPROVED DESIGN SYSTEM OF ORBITAL
FOR THE DUST EXTRACTION SANDERS USED ON WOOD
A. Thorpe and R. C. Brown Occupational
Medicine and Hygiene Laboratory, Health and Safety Executive, Broad Lane, Sheffield S3 7HQ, U.K. (Received injnalform
20 September 1994)
Abstract-The dust extraction systems of available orbital sanders do not perform well when the sanders are used on edges or pieces that do not contact the entire sander base. In an effort to improve the situation a number of experimental bases have been fitted to a sander, with holes in a variety of patterns. Many patterns give good performance, and particularly good dust extraction can be obtained through 10 carefully placed holes, only two more than the number in a standard base. Holes with a diameter of 5 mm give a better performance than 10 mm dia. holes. The effect of varying the extraction flow rate has also been investigated and it is found that the efficiency of dust collection varies approximately linearly with flow rate until a maximum of about 90% is reached, after which little improvement results.
INTRODUCTION
In previous papers measurements have been reported of the effectiveness of dust control systemsfitted to hand-sanding devices (Regnier et al., 1988;Hampl et al., 1992; Thorpe and Brown, 1994), and of factors influencing the rate of production of dust (Thorpe and Brown, 1995). In summary, dust control was found to be good in most instances where the sander was usedon a flat piece of wood, but when testswere carried out on a slowly rotating dowel, simulating an edge, the capture efficiency of an orbital sander with an integral dust extraction system could be as low as 5%. This could be increased to approximately 50% when the integral extraction system was replaced by one giving much higher extraction flow rates, such as a vacuum cleaner. However, in spite of the improvement the concentration of airborne dust was still unacceptable (Thorpe and Brown, 1994). Furthermore, it has become clear, during visits to several woodworking factories, that workers prefer the integral filter bag method of dust extraction since the trailing hose of external extractors makes them awkward to manoeuvre. The aim of the present work was, therefore, to develop an improved method of dust extraction for orbital sanders used with integral dust extraction. DUST
CAPTURE
EFFICIENCY
OF ORBITAL
SANDERS
USED
TO SAND
EDGES
OR DOWELS
Dust extraction, in all of the orbital sanders tested in the exercisedescribed above, takes place through holes in the sander base and the attached abrasive paper. In this article attention will be focused on a typical device referred to in a previous paper c; Crown copyright
1994 155
156
A. Thorpe and R. C. Brown
\ Position of holes in rubber base /
Fig. 1. Base of orbital sander showing extraction holes and air channels.
(Thorpe and Brown, 1994),as sander C. This sander has a weight of 2.8 kg and a power of 280 W, and by the use of an electronic control its speedcan be varied between 8000 and 20000 strokes min-‘. The hole pattern in the baseof this sander, which is typical of many orbital sanders of its size, is shown in Fig. 1. Air is drawn through the holes, which are 10 mm in diameter, by an internal fan powered by the sander’smotor. The air and any entrained dust particles then pass through an exhaust port on the rear of the sander either to an integral filter bag or to an external dust extractor. The holes are unevenly spacedand much of the area of the baseis not close to any of them, but when flat wood is sandedthe dust would be subjected to inward airflow at the edge of the sander base whatever the position of the holes on the base might be. The low capture efficiency for orbital sanders when sanding dowel is undoubtedly due to the relatively large distance between the extraction holes in the baseofthe sander and the region of dust generation. A rule of thumb for local exhaust ventilation (LEV) is that at a distance of one diameter from the source of extraction (a hole in the sander basein this instance) the extraction velocity drops to one-tenth of the value it takes at the entrance to the hole. If the dowel is sanded by the central part of the sander basethe distance between the point of contact of the dowel and the nearest extraction hole would be several hole diameters. This suggeststhat the capture efficiency would be improved if the distance between
Dust extraction with orbital wood sanders
157
the extraction holes were decreased, for instance by increasing their number. The diameter of the holes may also affect the capture efficiency. Some inroad into this study has already been made by staff at a woodworking factory where the allergenic wood, Western Red Cedar, is used (R. Snowdon, private communication). They modified the basesof their pneumatic orbital sandersby adding an extra row of dust extraction holes positioned down the centre of the sanding base. Workers using the sander found that wood dust emissions were visibly lower than those from a standard sander, when edgesand corners of components or thin sections were being sanded. We have attempted to carry out work of quantitive type aimed at finding an optimal dust extraction pattern.
MODIFICATION
OF THE SANDER
BASE
Several approaches to altering the hole pattern of the sander base are possible, depending on whether the requirement is simplicity in the processof modification or a more fundamental assessmentof alternative patterns. The easiestmethod of introducing extra holes is simply to drill them in the existing sander base.The baseof the sander used in tests has four holes along the length of each long edge, as shown in Fig. 1, which line up with channels in the metal base of the sander itself. The rubber basefastensup flush to the sander so that any dust that passes through the holes travels along the channels and into the four inlets through which the air is drawn by the internal fan. The drawback of adding holes in this way is that their position is limited to the parts of the base lying over the extraction channels. Alternatively, channels can be milled in the rubber base itself, connecting any additional holes to the existing holes and giving a route to the fan inlet apertures. This method allows additional extraction holes to be positioned centrally along the length of the sander base, and it is an improvement over the previous method, but it is still limited in the number and position of holes that can be added. By far the most flexible method of altering hole pattern and size is to use a completely redesigned sander base, and this was the method chosen for our studies. Removal of the existing base and its replacement by a new one would be a complicated feat of engineering, but the effect of hole pattern can be investigated by attaching a new baseplate to the existing one, after the removal ofits rubber cover. This makes the sander slightly unwieldy, which would be a problem in practical use,but not in our simulation. Holes could be drilled almost anywhere in the new base. The number of hole patterns that could be investigated is extremely large, but this can be limited by restricting the choice to patterns that have the symmetry of the base itself, which has two axes of symmetry passing through the centre, one parallel to each side. A pattern that did not have this symmetry could be optimal only if the sander were used in an asymmetrical fashion; and there is no reason to expect that this is the case. The size of hole can also be varied from the 10 mm dia. hole used in existing equipment. Since the extraction systems to be investigated are likely to have more rather than fewer holes it is reasonable to consider smaller rather than larger holes, and so a diameter of 5 mm was chosen as the alternative. The abrasive area of the sander is reduced by the presenceof holes, and holes larger than 10 mm in diameter might affect
158
A. Thorpe and R. C. Brown
IO I
0
0
0
0 0
24
0
00 19
r
000
0
0
000
0
0
000
0
0
000
0
0
000
0
0
000
0
0 0
000
0
0
000
0
0
000
0
0
0
0 Pattern
0
0
0
Pattern
1
I
2
Fig. 2. Modified sander basesof the two basic patterns, with all holes uncovered (the basesshown have 5 mm dia. holes; bases with an identical pattern of 10 mm dia. holes were also used).
the quality of finish on the sanded wood. Holes much smaller than 5 mm in diameter might need to be so numerous as to be impractical and would probably clog too easily. For the investigation, four milled aluminium baseswere made, with 5 and 10 mm dia. holes in two different patterns, as illustrated in Fig. 2. The baseswere covered with 5 mm thick neoprene rubber with holes punched in the appropriate positions. Each basehad a large number of holes, and could give rise to many different hole patterns if sets of holes were selectively sealed. EXPERIMENTAL
METHOD
The performance of the orbital sander with modified sander baseswas investigated using automated apparatus situated inside a large recirculating dust tunnel as described in previous work (Thorpe and Brown, 1994).The sander was set to full speed (setting number 6) and was fitted with 50 grade sandpaper (coarse grit); and the apparatus moved it at a speedof 0.3 m s- ’ with a total downward force of 50 N, in a reciprocating motion across the specimen,with a stroke length of 0.8 m. The specimen was, in most instances, a 5 cm dia. rotating dowel of beech, since this gave a good simulation of an edge whilst preventing the development of flats. Final tests were carried out on flat wood to ensure that the previously high dust capture efficiency on
Dust extraction
with orbital wood sanders
159
such specimenswas not adversely affected by modifications that improved the capture efficiency when dowel was sanded. The tests were carried out in a uniform air stream with a velocity of 0.25 m s- ’ and, where sampling was carried out, the sampling instruments were placed downstream, as in previous work. In most assessments,however, the efficiency was calculated from a comparison between the massof wood removed from the test specimenand the massof dust collected in the filter bag. In previous work, during which an orbital sander was applied to a dowel, the latter had been parallel to and equidistant from the long edges of the sander base. This resulted in the dowel being as far as possible from the original holes, but in a real sanding situation the sander would be used in various orientations with the dowel making contact with any part of the sander base.This variable was accommodated, to some extent, by carrying out tests with the sander baseat a variety of positions and at two orientations: with the long edgeof the baseparallel to the dowel, and with the long edge perpendicular to it. In each casethe test was carried out at a number of positions. The choices made for the lines of contact between the dowel and the base depended both on the number of holes and on the hole pattern in any particular base.In order to minimize the number of tests, wherever possible the orientation and position which gave the largest distance between hole and dowel contact point was tried first, and if this did not give an acceptable capture efficiency no further tests were carried out using this hole pattern. When the best hole pattern was realized the sander was tested with this for a full 60 min whilst the gravimetric total dust concentration downstream was measuredwith an isokinetic membrane filter sampler. A sampling rate of 10 1.min- ’ was necessaryto ensure that a ‘weighable’ sample was obtained.
HOLE
PATTERNS
USED
IN THE
TESTS
The patterns used in the tests are illustrated in Fig. 3, and in Table 1. Patterns B, F and G, are derived by blocking holes in array 1, and patterns A, C, D, E, H and J are obtained from array 2. All following referencesto basetypes are given in capital letters, and all referencesto sander position and orientation are shown by numbers. In many instances both 5 and 10 mm dia. holes were usedin the samepattern. Table 1 gives both the number of holes in each pattern and the area of the holes in the sandpaper as a percentage of its total area, since this might affect the quality of the finish of the wood.
MEASUREMENT
OF AIR VELOCITY
THROUGH
THE
EXTRACTION
HOLES
The resistance of the paths taken by air entering the dust extraction system of the sander may vary according to which hole is entered, and so there will not necessarilybe uniform airflow among the holes. Severeimbalance will have an adverse effect on the performance even if the pattern of extraction holes is sound. The air velocities through the extraction holes, shown in Table 1, were calculated from the measured total
160
A. Thorpe and R. C. Brown
extraction flow rate and the area of extraction holes in the sander base and are, therefore, mean values for each pattern. The variation in air velocity among the holes in a given pattern was quantified by mounting the sander with the basefacing upwards and measuring the velocity through each hole using a Dantec model 54N50 hot wire anemometer with an omnidirectional probe. The probe was placed 3 mm above the sander base, becauseif it were closer it could be damaged by the rapid vibration of the sander. The air velocity was measured with the sander running at full speed,and with a filter bag attached. Two sander bases were investigated in this way: the standard sander base with eight IO-mm dia. extraction holes, and base C with 18 5-mm dia. extraction holes. With the standard base the velocity was highest through the middle four holes and significantly lower 1
0
0
0
0
0
0
0
0
0
0
0
0
0 *
0 h
-3
i-
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
h
n”
h
*
-3
h
h
h
h
-2
0
0
0
0
0
0
0
0
-4
-2
0
0
t
0
0
’
0
0
0
0
0 0
0
0 -2
0
0
0 0
0
0
0
A
*
r\
0
0
0
0
0
0
0
0
0 m
h 0
-2
0
0 0
0
0 c
D
Fig. 3. (A-D).
-1
Dust extraction with orbital wood sanders 1
2
1
161
2
-
3
0
t t
-4
-
c.
h
-
0
0
c
-3
0
8
-4
-
-3
0
0
0
0
0
0
0
0
-1 i
-L F
E 1
0
2
0
0 -6
0 -3
-1
-5 e
A
h
n
0
0
H
0
-1
0
0
0
0
0 G
-1
0
0 J
Fig. 3. (E-J). Fig. 3. Hole patterns used in the tests, achieved by selective blocking of holes in patterns 1 or 2, Fig. 2, and showing positions of contact with the dowel.
through the outer four holes, the mean velocity and standard deviation through all the holes being 2.22f0.41 m s-r air. The velocity varied much less with hole position for base C, being 2.12f 0.14 m s-l. All of the velocity measurements were made with the sander operating under zero load, and the air flow rates when the sander is used in practice are likely to be lower than these values. The position of the probe during these measurementswas less than one hole diameter from the sander surface, and the measured mean air velocity lay between the mean entry velocity and one-tenth of this value, as expected.
162
A. Thorpe and R. C. Brown
Table 1. Variation in sander capture efficiency with size, and pattern of extraction holes and extraction flow rate
Hole pattern
Dowel position
Hole diameter (mm)
A A A B B B B C C C C D D D D E E E E E E E E F F F F F G G G G G G G H H H H J
1 2 3 1 2 3 4 1 2 1 2 1 2 1 2 1 2 3 4 1 2 3 4 4 1 2 3 4 1 2 3 4 5 6 6 1 2 3 3 1
5 5 5 5 5 5 5 10 10 5 5 10 10 5 5 10 10 10 10 5 5 5 5 10 5 5 5 5 5 5 10 5 5 5 10 5 5 5 10 5
PERFORMANCE
Number of holes
% of base taken by holes
32 32 32 25 26 26 26 18 18 18 18 15 15 15 15 13 13 13 13 13 13 13 13 14 14 14 14 14 10 10 10 10 10 10 10 10 10 10 10 I
4.1 4.1 4.1 3.3 3.3 3.3 3.3 9.1 9.1 2.3 2.3 1.6 1.6 1.9 1.9 6.6 6.6 6.6 6.6 1.6 1.6 1.6 1.6 7.1 1.8 1.8 1.8 1.8 1.3 1.3 5.1 1.3 1.3 1.3 5.1 1.3 1.3 1.3 5.1 0.9
AS A FUNCTION
Flow rate (1. min-‘)
Air velocity through holes (m SK’)
Capture efficiency (%)
166.9 196.5 197.8 174.9 181.4 196.5 187.8 165.5 212.2 197.4 192.8 187.8 185.2 161.4 170.9 167.9 194.1 216.8 219.1 177.5 223.7 176.2 173.5 196.5 189.1 180.1 158.7 161.4 166.9 200.2 178.8 162.8 172.2 177.5 196.5 161.4 180.1 186.5 243.3 111.5
4.43 5.22 5.26 5.72 5.93 6.43 6.14 1.95 2.50 9.32 9.11 2.66 2.62 9.15 9.69 2.74 3.17 3.54 3.58 11.61 14.63 11.52 11.35 2.98 11.48 10.94 9.64 9.80 14.19 17.02 3.80 13.84 14.64 15.09 4.17 13.72 15.31 15.86 5.16 21.56
71.8 68.6 61.3 67.0 70.7 54.2 61.0 60.6 53.2 68.6 52.8 58.3 52.5 69.9 44.5 62.7 57.0 67.3 61.0 80.3 60.7 70.0 68.9 49.3 70.6 65.7 67.3 65.3 76.4 60.8 55.2 67.9 82.7 63.4 46.8 75.1 61.0 36.3 46.5 39.5
OF HOLE
PATTERN
The results of the tests are detailed in Table 1, and it is clear that many of the patterns give good dust capture efficiency in someor all orientations. In summary, base A showed good capture efficiency in all sander orientations and positions but is probably unacceptable becauseof the large number, 26, of extraction holes. This is also the casefor baseB. BasesC and D would probably be acceptablefrom the point of view of the number of holes, especially if they were 5 mm in diameter. However, the capture efficiencies,although promising, did show a marked dependenceon sander orientation
Dust extraction
with orbital wood sanders
163
and position. Base E was very promising, especially with the smaller holes, where efficiency was high at all orientations and positions. BasesF and G had a relatively poor capture efficiency with large holes but good with small. BasesH and J were poor. Since baseG has the fewestholes, this should be least likely to affect the finish of the wood adversely. The fractional area taken up by the 5 mm holes is also less than onethird of that with a standard sander base. Bases with fewer holes than this did not perform as well and can, therefore, be ruled out as possible alternatives. As a check, base G was tested again with flat wood and gave a capture efficiency of 88.3%, indicating that the hole arrangement does not adversely affect its performance in this situation. All of the patterns gave consistently better capture efficiencieswith 5 mm dia. holes than with 10 mm dia. holes. The reason for this is not obvious. The air velocity at the entrance of a 5 mm dia. hole will be four times as great as that at the entrance to a 10 mm dia. hole, provided that the volume flow rate is constant. At distances significantly larger than a hole diameter the difference will be small. However, the larger area of the 10 mm holes should result in more particles being under the direct influence of the airstream. Inside the sander these small holes would be expected to cause a slight reduction in the transit time of the dust to the collecting bag. TESTS
OF OPTIMUM
HOLE
ARRANGEMENT
AT A NUMBER
OF FLOW
RATES
Previous work (Thorpe and Brown, 1994) has shown that the dust extraction efficiency of sanders is higher when they are used with external extraction systems, which gave a high flow rate, than with integral extraction systems,for which the flow rate is much lower. It is useful to put this observation on a quantitive basis, and to this end baseG, the optimal pattern, was tested with dowel at position 2, for a range of flow rates, provided by a vacuum cleaner with flow control. The results, illustrated in Figs 4 and 5, show that the effectivenessof the dust control system increases as the extraction air flow rate is increased. Figure 4 shows results obtained by comparing the weight increase of the dust collection bag with the weight loss of the specimen of wood being sanded, and Fig. 5 shows results obtained from measurements of airborne dust concentration. There is more scatter in the latter results, and the two methods of estimation suggestthat approximately 90% efficiency is obtained at a flow rate of 60&900 1.min- ‘, but that little benefit is obtained by the use of higher flow rates than 900 1.min-‘. When an integral extraction system is used the flow rate will drop as the filter bag becomesloaded, owing to increased resistanceto air flow, and this also results in a drop in capture efficiency. This indicates that the bag should be replaced regularly. In contrast, when flat wood was sanded (Thorpe and Brown, 1994), the loading of the integral filter bag was found to have very little effect on the capture efficiency until the bag was practically full, when the flow rate dropped almost to zero and the capture efficiency fell very quickly. CONCLUSIONS
The addition of extra holes in the standard sander baseof an orbital sander results in an increased efficiency of dust capture when the sander is used on dowel or edges.
A. Thorpe and R. C. Brown
164
0
200
400
800 Flow rate (Vmin)
800
1000
1200
Fig. 4. Dust capture efficiency of sander with base G at position 2 as a function of extraction flow rate, estimated from weights of wood removed and dust collected: 0, experiment; -, by-eye fit.
f
1.5-
z 5 ‘i: e E $
1.0-
E zi 2 0.5 -
0
200
400
800 Flow rate (Vmin)
800
1000
1200
Fig. 5. Airborne dust concentration as a function of extraction flow rate for sander with baseG, at position 2: 0, experiment; -, by-eye fit.
The degree of improvement and its variation with the orientation and position of the dowel depend critically on the hole size and hole pattern. A pattern of holes has been found which gives a much increased capture efficiency for all orientations and positions of sander baserelative to the axis of the dowel, and the sander fitted with the improved basecontinues to work well on flat wood. For this and other patterns, bases with 5 mm extraction holes consistently gave better capture efficienciesthan baseswith the samepattern of 10 mm holes. Even with. the optimized sander base the capture efficiency depends critically on flow rate. If the flow rate
Dust extraction with orbital wood sanders
165
decreasesowing to clogging of the filter bag or if the speed setting on the sander is reduced, the capture efficiency drops dramatically. Likewise, an increase in flow rate results in a much improved capture efficiency.
Acknowledgements-We would like to thank the manufacturer who lent us the sander used in the experiments, and the staff of Amdega Ltd, Darlington, for the loan of a sander with additional extraction holes. We are also grateful to our coieagues at HSE, in particular Mr K. A. Dobson, Mr R. Boland, Mrs J. P. Rhodes, Mr R. Johns and Mrs P. M. Waldron, of the Field Operations Division, and Mr P. A. Roberts of the Research and Laboratory Services Division.
REFERENCES Hampl, V., Topmiller, J. L., Watkins, D. S. and Murdock, D. J. (1992)Control of wood dust from rotational hand-held sanders. Appl. occup. Environ. Hyg. 7,263-270. Regnier, R., Braconnier, R. and Aubertin, G. (1988) Efficacite des dispositits, de captage integres aux machines poratives. INRS 33 9-354. HSE Translation 13698 (1990). Thorpe, A. and Brown, R. C. (1994) Measurements of the effectivenessof dust extraction systems of hand sanders used on wood. Ann. occup. Hyg. 38,279-302. Thorpe, A. and Brown, R. C. (1995) Factors influencing the production of dust during the hand sanding of wood. Am. ind. Hyg. Ass. J. (in press).