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Thread: understanding gear cutters.

  1. #1
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    Default understanding gear cutters.

    I would like to try help my friend make some gears for his southbend 9 lathe.I have a couple of questions .
    1)Can a flycutter get the job done in steel.?
    2)Can brass be used for these gears ?
    3)If a flycutter is not good enough would a hob type gear work?I dont think I can get my head around making a button cutter to make a gear.

    4)I dont get why you need so many gears to cut gears and dont understand why they come in sets of eight.
    5)To cut the whole range of gears does it mean you need a set of eight gears for different dp. How many variations are there to dp.
    Does this also apply to metric.
    What would the best way be to go about making some of these gears.
    Before you suggest buying them I live in darkest Africa.

  2. #2
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    I don't have a lot of experience so take my answers with that in mind.

    1. Yes. It will take a cutter with the correct shape.
    2. Assuming you mean the change gears for thread cutting, there is so little stress on them that 3D printed gears in plastic will work. Brass would be great but more expensive than necessary.
    3. The shape made by a button cutter creates the proper shape to the gear called an involute.
    4. The gear cutters are only correct for one size of gear but are "close enough" to correct to be useable over a limited range of sizes. The set of 8 covers all sizes.

    Gears don't care if they are metric or Imperial, they need the same tooth shape.

  3. #3
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    Just to be clear, there are dozens of DP sizes for involute gears ranging from 1 up 100 or more, the lower the number the bigger the gear tooth profile. In order to cut the whole range of gear teeth from about 12 T up to a rack, needs 8 cutters. Each cutter will cut a range of teeth, No8 will cut 12 and 13T, No 5 will cut 21-25T, No 2 55T to 134T and No.1 135T to rack (straight). You can see that each cutter is an approximation, but it is one which works. For lightly loaded relatively low speed gears such as lathe change gears, you can stretch these ranges a little without ill effects. I wouldn't have any hesitation about cutting a 50T change gear with a No.2 (55-134) cutter. Lathe change gears tend to be 14,16,18 or 20DP normally.

    A further complication is that the geometry of involute gears changed around the time of WW2, or a few years later. Before that time, the Pressure Angle was 14.5 degrees, much after it tended to become 20 degrees. A cutter for 20 degree gears will have this fact marked on it. A cutter for 14.5 degrees may not, because thats all there was at the time.
    So, the older the machine the more likely the gears are to be 14.5 degree, later ones, 20 degree. I suspect that Southbend will have used 14.5 degrees. People will tell you that you can't run 14.5 degree and 20 degree gears together. Strictly they are right, the geometry is different. In practice, for lightly loaded low speed gears with reasonable clearances, like change gears, you can.
    I strongly recommend that you read a book called 'Gears and Gear Cutting' by a chap called Ivan Law, I think its available on the net. This will tell you all you need to know about cutting gears with fairly simple equipment.
    'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger

  4. #4
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    1. Get a copy of Ivan Laws book on gear cutting (Workshop Series #17 Gears and Gear Cutting, Pisst it can be found roaming free on the web) , still in print I believe. It will tell you every thing you need to cut gears in a home shop including how to make button cut gear flycutters.

    2. Why use brass. Cutting gears in steel is not a big deal. Use free cutting steel, its better then brass for that. I cut some change gears using the button made flycutters out of Unknowium steel and they worked out great.

    3. If you are going to cut module (metric) gears then you can get cheap cutters from Banggood that are OK. Be aware though for some reason they number their cutters in the reverse order to the norm IE: a number 8 cutter is labelled #1 etc.
    The shortest distance between two points is a circle of infinite diameter.

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  5. #5
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    Thanks for the help guys. Dont you need a surface grinder to clean up the top of the button cutter. ?At least that is what you see most guys doing. That method where they use a hob looks simpler. What do you think. From what I understand it also eliminates the need to make eight different gears or am I incorrect.

    I have found a piece of v155 steel lying around. It says it needs 1100c or 2150f to heat treat. My furnace only gets to 900c. Do you think map gas can take it to the right temp.I wonder what v155 is the equivalent to. The bohler rep seemed to think silver steel or o1 is not suitable for this as it loses its edge too quick. What do you think.?

  6. #6
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    For change gears on the lathe using a flycutter with a single tooth that is ground from some HSS would work just fine and be a lot less work and expense. You'd use it the same way that you would use the gear cutting set. Namely in a mill where the gear blank is sitting on a mandrel held by some manner of dividing head and the flycutter is sitting at the center height.

    You'd make the flycutter's "tooth" out of something like 1/4 or 5/16 square HSS and use a good magnification source to get the shape such that it fits the tooth of a similar size gear really well. So a bench grinder for roughing then small stones or diamond burrs in a rotary tool for the final fitting to where the fit is as good as you can get by eye in concert with your good magnifying glass.

    So if you have a dividing head or a rotary table you can set up in vertical mode on your mill you can make the gears with a minimum of additional tooling. Note that since you are making the single flycutter tooth to fit a similar size gear that you are essentially making it with the proper involute curve at the same time. You're basically making one tooth of a gear cutter. So if the gear cutters have 6 teeth you will need to feed the cut at 1/6 of the feed rate that you would have gotten away with when using a proper gear cutter.

    Meanwhile making gears with a hob means making a hob or buying a suitable hob from someplace. A lot of hobs I see in home shop videos are cut as a spool and then cuts are made for some manner of clearance. But these cutters have the same limitation as making our own home shop taps. Namely that the teeth don't have any proper top and side clearance. So they can work but they drag in the cuts a lot and that means light passes and lots of lube. Meanwhile our flycutter with the single tooth that was ground and finished with all the proper side and front clearance angles can cut somewhat more aggressively.

    I have not made any gears myself. But I did make a sort of gear a while back. It was a setup to cut 40 "V" notches around the edge of a threaded collar to accept a position locking toothed pin. So not as critical as a gear. It was actually easier to do than I would have expected. And that along with knowing that change gears do not need to be the world's most precise fit eases my OCD'ism enough that I'd dive in and have a go with a single tooth flycutter.

    Note that there's a few different styles of hob cutting out there. One uses a simple hob with separate rows of teeth. It's then used in a mill just like you would use a gear cutter. But it forms the involute as it cuts. So you still need a mill and some manner of dividing device to accurately space the teeth. Then there's a spiral cut hob which is used with a special holder for the gear blank that allows the blank to spin freely. In use the blank is offered up to the cutter at an angle that matches the spiral angle. The cutter both cuts the tooth and rotates the blank around at the same time. But that requires a lot of special gear in preparation before you cut the first gear.

    Out of all three of these options by far the least time to dive in and get cutting is going to be the single tooth flycutter ground to fit a similar size range of gear. It might mean making two or perhaps three different cutting teeth to suit your range of gears. But factor that against making a hob and mounting the hob on an arbor.

    Having made some home made taps and dreaming about making a proper shaped hob I've always thought that I'd opt for using a larger diameter such as 1". I'd cut either three or four teeth sets in the hob. But I'd set up some way to hold the blank in my lathe offset slightly from center and give myself some way to index it so I cut three or four "skims". I'd then cut away the gullets to form the front faces at the positions that leave the teeth with some side clearance thanks to the way the gullets are cut.

    If that sounds like something you would like to try and can't figure out the geometry let me know and I can make a sketch.
    Last edited by BCRider; 08-16-2019 at 12:16 PM.

  7. #7
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    Quote Originally Posted by plunger View Post
    1)Can a flycutter get the job done in steel.?
    Yes, presuming that you have the ability to grind the correct shape, as well as inspect it for correctness.

    Quote Originally Posted by plunger View Post
    2)Can brass be used for these gears ?
    Sure. You can use anything you want, within reason.

    Quote Originally Posted by plunger View Post
    3)If a flycutter is not good enough would a hob type gear work?I dont think I can get my head around making a button cutter to make a gear.
    If you cannot make a button cutter, you assuredly can not make a Hob. Button cutters are among the simplest to achieve.

    Quote Originally Posted by plunger View Post
    4)I dont get why you need so many gears to cut gears and dont understand why they come in sets of eight.
    One needs cutting tools. Not gears. And you will need to study a little more so that you DO understand why. The bottom line is that the eight cutters are APPROXIMATIONS of the correct shape and cover a RANGE of tooth numbers. Each, themselves being approximations. Not truly correct forms.

    Quote Originally Posted by plunger View Post
    5)To cut the whole range of gears does it mean you need a set of eight gears for different dp. How many variations are there to dp.
    Yes, if depending upon Form Milling cutters one needs the whole set of eight to cover every tooth count possible. There is Diametral Pitch, Circular Pitch, Module, Axial Pitch, and some other more specialized variations.

    Quote Originally Posted by plunger View Post
    Does this also apply to metric.
    Yes.

    Quote Originally Posted by plunger View Post
    What would the best way be to go about making some of these gears.
    That depends completely upon what result you are expecting and demand.

    Quote Originally Posted by plunger View Post
    Before you suggest buying them I live in darkest Africa.
    So? Who cares? If you are going to purchase tools, one can safely conclude that they will be shipped to you. As such, a purchased gear could be, as well.

    Start with looking at the Gear section of Machinery's Handbook and the Ivan Law book. From there decide exactly what you expect and demand from the gears. That will largely dictate how you need to proceed.

  8. #8
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    To be fair to the gear cutters each cutter is the ideal shape for the gear that is roughly in the middle of the range for each cutter. The more towards the limits of the range given for each the less perfect it is.

    Having said this the amount that each is away from ideal at the limits is darn small. It's only an issue if you're making gears that are intended to run VERY quietly and with minimal friction loss. As has been mentioned frequently in this thread and the other thread we can get away with a slightly loose engagement than what we'd get with perfect gears and perfect engagement for minimal noise and maximum smoothness and the gears will run just fine.

  9. #9
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    Quote Originally Posted by Zahnrad Kopf View Post
    Yes, presuming that you have the ability to grind the correct shape, as well as inspect it for correctness.



    Sure. You can use anything you want, within reason.



    If you cannot make a button cutter, you assuredly can not make a Hob. Button cutters are among the simplest to achieve.



    One needs cutting tools. Not gears. And you will need to study a little more so that you DO understand why. The bottom line is that the eight cutters are APPROXIMATIONS of the correct shape and cover a RANGE of tooth numbers. Each, themselves being approximations. Not truly correct forms.



    Yes, if depending upon Form Milling cutters one needs the whole set of eight to cover every tooth count possible. There is Diametral Pitch, Circular Pitch, Module, Axial Pitch, and some other more specialized variations.



    Yes.



    That depends completely upon what result you are expecting and demand.



    So? Who cares? If you are going to purchase tools, one can safely conclude that they will be shipped to you. As such, a purchased gear could be, as well.

    Start with looking at the Gear section of Machinery's Handbook and the Ivan Law book. From there decide exactly what you expect and demand from the gears. That will largely dictate how you need to proceed.
    Thanks for answering so clearly.I do see banggood does have cheap cutters .It would simplify buying the right cutter. But our exchange rate and dismal 3rd world postal system would ensure this project wont happen this year . It sounds like a fly cutter is the way to go.I ask about brass because I would think it would be easier to cut but I am not sure if it would easily gall and cause chips and possibly problems later on.I dont know if I could grind a piece of hss to the correct shape.I will have to try.
    I have successfully made a hob to cut the worm gear out of pbronze for my bandsaw.

  10. #10
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    If "Best" way is defined as least expensive, then 3d print the gears.

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