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Thread: How to check gear teeth
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22nd Sep 2018, 01:12 AM #1Senior Member
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How to check gear teeth
I'm looking at buying a cutter to make the 2 missing gears for my lathe.
One's 30tpi, the other 32tpi. I'll try to work out some way to cut them using the lathe or rig up some other method.
How do I measure the tooth profile of my existing gears so I know I'm ordering the right cutter ?
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22nd Sep 2018, 08:21 AM #2Most Valued Member
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What lathe is it?
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22nd Sep 2018, 08:22 AM #3Philomath in training
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Start with measuring the OD as accurately as you can, then use the formula DP=(n+2)/OD, where n= no. of teeth and OD is in inches.
If you get a DP pop out that is very close to an integer (say +/- 0.05), the gears are probably DP gears. If the number has some decimals to it, then the chances are that it is a MOD gear. In that case the number you want is MOD=(25.4/DP). Usually DP gears have a pressure angle of 14 degrees and MOD gears are 20 degrees. Most home schemes for measuring these rely on things like Plasticine and really only confirm things so can probably be skipped.
Another confirming piece of information is the machine they go on. MOD gears are typically the metric way of doing things, so if the machine is new (probably metric graduations), the gears are probably MOD. If older, then likely to be DP.
Michael
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22nd Sep 2018, 10:54 AM #4Senior Member
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22nd Sep 2018, 12:27 PM #5Senior Member
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120 tooth is 152.4mm diameter
40 tooth is 52.54mm diameter
Teeth pitch appears to be about 4mm.
What does this label tell you ?
P1030768.jpg
So would one of these in M4 #5 be the right thing?
https://www.ebay.com.au/itm/8pcs-M0-...torefresh=true
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22nd Sep 2018, 01:57 PM #6Most Valued Member
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No.
A 1.25 module cutter would be the preferred option.
If your lathe is very large which it appears not to be it would require a gear of 488 mm OD for the 120 and an OD of 168 for your 40.
So your 30 would be 128 OD and your 32 would be 136 OD.
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22nd Sep 2018, 02:14 PM #7Senior Member
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22nd Sep 2018, 02:21 PM #8Most Valued Member
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It is the module of your gears.
You can add 2 to your tooth count and then multiply by the module and see how close that is to your measured OD of your existing gears.
You will sometimes find that it won't work out to be exactly as you have calculated, this could be because of wear or just to suit the manufactures desire.
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22nd Sep 2018, 02:44 PM #9Senior Member
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22nd Sep 2018, 02:48 PM #10Most Valued Member
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Search gear formulas
Sent from my T85 using Tapatalk
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22nd Sep 2018, 03:49 PM #11Most Valued Member
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To help understand gearcutting better, I suggest that you get hold of a copy of "Gears and Gearcutting by Ivan Law".
It's a Workshop Practice Series book number 17, to help I've inserted a link to Ebay, where there are copies for sale.
https://www.ebay.com.au/sch/i.html?_...n+law&_sacat=0
I found the series a big help and written to easily comprehend.
KrynTo grow old is mandatory, growing up is optional.
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22nd Sep 2018, 07:01 PM #12Most Valued Member
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Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.
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22nd Sep 2018, 07:09 PM #13Philomath in training
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There are formulas around to tell you this. I've set up a spreadsheet to do it that I use and as Kryn says, Ivan Law wrote a book. However, to save you some time if the gears are M1.25, a 30t gear should be 40mm OD and a 32t gear should be 42.5mm OD. Depth of cut (in gear cutting referred to as D+f) is 2.7mm.
Chinese manufactures seem to have the numbering backwards, so you do need a #4 or #5 cutter, but you are better looking for one that states the right number of teeth to cut. The standard cutter to cut these two will cut from 26 to 34 teeth. Someone here may have one you can borrow too if it is just to cut those gears.
Michael
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22nd Sep 2018, 07:45 PM #14Senior Member
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This is the ebay link for those cutters :
https://www.ebay.com.au/itm/8pcs-M0-...torefresh=true
If you go to the 10th picture, the one I need must be the one on the bottom left hand side...M1.25, 20deg, #5, 26-34 ?
I notice some are marked with prob the cutting depth like the bottom right hand side in that same picture for 17-20 teeth shows M1.25, 20deg, 2.75, #3.......so is 2.75 the cutting depth for that one ?
Also is involute type correct ? If so, I might go ahead and order one of these since they're cheap enough.
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22nd Sep 2018, 07:51 PM #15
Hi Phaser,
I have some PDF's on gear cutting if you want them. About 50Mb total !Best Regards:
Baron J.
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