I need to make a gear, How to measure the old one?
This gear has a pitch dia. of about 1.562 and 40 teeth. what is a good way to tell if it is 14.5* or 20* preasure angle? I don't have a drawing to work from once again. Where can I get a cutter?, msc does not list one for the correct pitch dia.
A quick way to check pa is to roll the gear through modelling clay, and cut the clay through the impression and measure the angle of the teeth if the clay 'rack' you have made. I suspect that the gear is module form (possibly 1 mod)and will most likely be 20 degrees.
Peter
I culled this method from somewhere / some-one who appeared to be a reliable authority:-
"As long as you know the 'DP' of the gear the following will give you a good guide to the 'PA'.
1) Take a measurement over any number of teeth with whatever equipment you have
2) Multiply the cosine of the 'PA' by 3.1416 and the divide by the 'DP' add this to your measurement
3) this should be the measurement over one more tooth if its not the same, change the cosine of the 'PA' and try again
4) example 10dp 14.5pa 30 teeth, the meas over 3 teeth .776,cos.986147 multiplied by 3.1416 and divided by 10 = .304 add .776=1.080 over 4 teeth.
Where it says measure over any number of teeth, that means just put a caliper, etc, over a few teeth where the faces of the caliper are tangent to the gear flanks."
Not tried it myself but the geometry involved looks reasonable at a cursory glance. Its easy enough to sort out what the likely error are if your measurements aren't quite right. The difference in cosine of 20° and 14.5° gives around 10% variation. With things like this when you are not sure what's going on it helps to set-up a spreadsheet and see what happens when you change things.
Clean the old gear of oil and place it on a flat bed scanner. Then take it's picture with the scanner. Compare the images electronically with digital racks. These can be hand drawn or drawn with CAD. You will soon find a rack that fits the gear and then you'll have everything you need.
When I had to measure a gear once the easiest way I found once I got it close to what I thought it was was to go to www.rushgear.com. I believe it is them who have a application where you can download 2D files of the gears. Than print it out with your computer in 1:1 scale. Find 2 or 3 that are close and lay the gear out on top of them and see how well it matches.
Once you have established the data, you can buy second hand in the UK from Tracy Tools, new from several Indian makers. You just need to get the exact definitioin of what you want and then I may be able to help.I assume that you only want to make one gear, so I can hob it for you given the right data (or the old gear to work from)
Peter
I need to make 6 of them right quick. By the end of the week if possible. They will have a strange drive tube on them, a real strange deal, motorsports stuff
With the OD you give, there's a good chance it's a 1 module gear. (fwiw: since 1 module is 25.4 DP, the PD should be 1.5748")
afa: pressure angle...
Check the base pitch -- Clive pointed you in the right direction on this.
Use a vernier and check over something like 4 teeth, and 5 teeth... and look at the difference. It's important to make sure the vernier jaw faces are on the flanks (profile) of the teeth on both measurements (not touching the root of the tooth or the tip).
If it's 20° pressure angle, you should see about .116" difference between measuring over 4 teeth and 5 teeth, (or between 5 teeth and 6 teeth).
If it's 14.5° pressure angle, you should see about .1197" for a difference.
If it's 25° pressure angle, the difference should be about .1121".
If it's anything relatively modern, it's unlikely that it's 14.5° PA (unless based on a very old design).
Here is a DXF of a 40 tooth, 1 module 20° gear (you'll probably need to right-click on that link and do a "Save As" to save the DXF on your computer).
hey precisionmetal, thanks for the help. It is a mod 1 20* PA. I checked with Newman tools, out of stock price 368.60! Machine Shop Discount Supply, in stock 50.00 to my door.....
It sounds like you're completely on top of this, but just be sure that the milling cutter you purchase is for the range of teeth that "40" falls within.
"Where it says measure over any number of teeth, that means just put a caliper, etc, over a few teeth where the faces of the caliper are tangent to the gear flanks."
I cannot seem to visualize how to make this measurement. Can anyone explain it another way?
This would be measuring over a span of 4 teeth. Then measure over 5 teeth and the difference is the base pitch.
Note that it's not always 4 and 5 teeth. Depending on the pitch, number of teeth, etc., it might be 6 and 7 teeth, or 3 and 4 teeth. Doesn't really matter as long as the faces of the vernier (or a thin flange mic) are absolutely/clearly on the profile of the teeth in each measurement.
Last edited by precisionmetal; 06-02-2009 at 11:05 AM.
Reason: spelling correction
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