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  1. #1
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    Default 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 ?

  2. #2
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    What lathe is it?

  3. #3
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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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    Quote Originally Posted by pipeclay View Post
    What lathe is it?
    Hafco AL 350 A

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    Quote Originally Posted by Michael G View Post
    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
    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

  6. #6
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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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    Quote Originally Posted by pipeclay View Post
    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.
    Ok cool, so how does 1.25 relate to my gears ?

  8. #8
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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.

  9. #9
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    Quote Originally Posted by pipeclay View Post
    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.
    Ah ok well when I try that with the gears mentioned earlier, it works so that's pretty cool.
    So how is the diameter of the gear blank and depth of cut calculated ?
    (can't hardly tell I ain't done this before).

  10. #10
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    Search gear formulas

    Sent from my T85 using Tapatalk

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    Quote Originally Posted by phaser View Post
    Ah ok well when I try that with the gears mentioned earlier, it works so that's pretty cool.
    So how is the diameter of the gear blank and depth of cut calculated ?
    (can't hardly tell I ain't done this before).
    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.
    Kryn
    To grow old is mandatory, growing up is optional.

  12. #12
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    Quote Originally Posted by KBs PensNmore View Post
    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.
    Kryn
    Agree. I have this book and it's pretty good for beginers. It explains all the critical dimensions etc. etc.

    Simon
    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.

  13. #13
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    Quote Originally Posted by phaser View Post
    So how is the diameter of the gear blank and depth of cut calculated ?
    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

  14. #14
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    Quote Originally Posted by Michael G View Post
    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
    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.

  15. #15
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    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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