a really good shop vac might draw 5 inches of mercury. The table in my
article will show you the holding force, but it's basically 1/6 atmosphere,
right? or about 2.5 PSI of force - if you figure you need 100 pounds of
force to hold your object (that's just a number pulled out of nowhere, but
it is probably plausible), then your shop vac would be OK with anything
whose diameter (remember area = pi times radius squared) is greater than 7
inches or so
try this table (view it in a fixed pitch font) - hopefully I made no errors
in the spreadsheet
Force for indicated diameter in inches
in mercury psi 1 2 4 6 8 10
1 0.49 0.3848451 1.53938 6.157522 13.85442 24.63009 38.48451
2 0.98 0.7696902 3.078761 12.31504 27.70885 49.26017 76.96902
3 1.47 1.1545353 4.618141 18.47256 41.56327 73.89026 115.4535
4 1.96 1.5393804 6.157522 24.63009 55.41769 98.52035 153.938
5 2.45 1.9242255 7.696902 30.78761 69.27212 123.1504 192.4226
6 2.94 2.3090706 9.236282 36.94513 83.12654 147.7805 230.9071
7 3.43 2.6939157 10.77566 43.10265 96.98097 172.4106 269.3916
8 3.92 3.078760801 12.31504 49.26017 110.8354 197.0407 307.8761
9 4.41 3.463605901 13.85442 55.41769 124.6898 221.6708 346.3606
10 4.9 3.848451001 15.3938 61.57522 138.5442 246.3009 384.8451
11 5.39 4.233296101 16.93318 67.73274 152.3987 270.931 423.3296
12 5.88 4.618141201 18.47256 73.89026 166.2531 295.561 461.8141
13 6.37 5.002986301 20.01195 80.04778 180.1075 320.1911 500.2986
14 6.86 5.387831401 21.55133 86.2053 193.9619 344.8212 538.7831
15 7.35 5.772676501 23.09071 92.36282 207.8164 369.4513 577.2677
16 7.84 6.157521601 24.63009 98.52035 221.6708 394.0814 615.7522
17 8.33 6.542366701 26.16947 104.6779 235.5252 418.7115 654.2367
18 8.82 6.927211801 27.70885 110.8354 249.3796 443.3416 692.7212
19 9.31 7.312056901 29.24823 116.9929 263.234 467.9716 731.2057
20 9.8 7.696902001 30.78761 123.1504 277.0885 492.6017 769.6902
21 10.29 8.081747101 32.32699 129.308 290.9429 517.2318 808.1747
22 10.78 8.466592201 33.86637 135.4655 304.7973 541.8619 846.6592
23 11.27 8.851437301 35.40575 141.623 318.6517 566.492 885.1437
24 11.76 9.236282402 36.94513 147.7805 332.5062 591.1221 923.6282
25 12.25 9.621127502 38.48451 153.938 346.3606 615.7522 962.1128
26 12.74 10.0059726 40.02389 160.0956 360.215 640.3822 1000.597
27 13.23 10.3908177 41.56327 166.2531 374.0694 665.0123 1039.082
28 13.72 10.7756628 43.10265 172.4106 387.9239 689.6424 1077.566
29 14.21 11.1605079 44.64203 178.5681 401.7783 714.2725 1116.051
30 14.7 11.545353 46.18141 184.7256 415.6327 738.9026 1154.535
"Bill in Detroit" wrote in message
...
William Noble wrote:
if you recall that one atmosphere is 14.7 PSI, this chart will help:
http://www.britishmetrics.com/html/pis-bar.htm
and if you further reacall that one atmosphere is 30 inches of mercury
(aprox), then it should all be obvious. You don't need a vacuum less
than .07 bar or so, that's 70 milibar, or 7X10E5 greater than the value
you have below. You REALLY REALLY REALLY do NOT NOT NOT want a high
vaccuum pump.
you may find my article on vacuum pumps and chucks helpful - you can
download it from my web page (www.wbnoble.com, click on vacuum pumps) or
you can download it from the tips section of www.woodturners.org - it's
the same article both places.
bill (not in Detroit)
Thanks, Bill. I don't think I've needed to know either of those numbers
more than once or twice since graduating from High school in 1970. And
possibly stretching all the way back to Jr. High.
I see various units offered in the catalogs, your offerings, and the other
offerings on the internet and I was trying to puzzle out just what sort of
unit would have more than enough suction without sucking all the money out
of my pocket or requiring an underground bunker for noise abatement.
Out of curiosity, how far away from what I need would a good shop vac be?
I mostly want to finish turn the bottoms of bowls in the 6-10" range with
flat edges that have already been sealed.
Hmmm ... maybe I could put a barometer in a wooden box with a plexi face
and test for myself. That should be good enough to give me a ballpark feel
for what's going on.
Bill
--
I not only use all the brains that I have, but all that I can borrow.
Woodrow Wilson (1856 - 1924)
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