Is a lathe lead screw & thread dial gear really a worm gear and worm wheel?
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  1. #1
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    Default Is a lathe lead screw & thread dial gear really a worm gear and worm wheel?

    The Reed Prentice did not come with a thread dial so making one is on the long, satisfying list of things to do. I'm not ready to start the build yet but i like to think through a project long before i start so i opened the good book and tried to slueth out worm gear design. I also read many of the old posts on building thread dials which convinced me i was overthinking this a bit.

    In the end i figured out what i need to do; 4 tpi lead screw with smallest fractional thread the machine will cut is 5 3/4 tpi so i need a gear that will engage at a point that will turn the 3/4" fraction into a whole number. This means an integer multiple of 16 teeth with a circumference an integer multiple of 4 inches. So, either 16 teeth, 1.25" dia or 32 teeth, 2.5" diameter.

    But, in studying the machinery's handbook, i couldn't seem to fit my lead screw dimensions into any of the standard worm gear tables. This leads me to believe that, although similar in appearance, an acme screw and a worm gear are not the same thing. I'm concluding that worm gears come in standard dimensions with tooth geometry designed to carry specific loads under specific conditions and that acme thread screws are not designed in quite the same way.

    I know this may be a bit of an academic question but i'd appreciate the input from the experts if for no other reason to scratch this particular itch.

    By the way, this'll be a nice practice project for the universal horizontal mill and dividing head. I may even get fussy and grind a tool bit so i can use the shaper to cut the final tooth profile.

    Mark

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    It is first an indicator. It has no WORK to do other than spinning a marked indicator disc

    Lacking any sort of LOAD, most anything that reliably SPINS THE DIAL will be acceptable

    In simplest terms, it will work fine with a ..250" CIRCULAR PITCH gear of say 16 teeth - forget the angles that would be required by a worm and worm wheel - just make it THIN enough so such things are far less of a problem if they ARE NOT THERE

    Made one from HARD MAPLE once - worked fine

    You have to make CP gears, they are not laying around

    If it were .250 CP, it would need to be 1.273 pitch dia. for 16 teeth (16 times .250 divided by "Pi")

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    I took a stock Boston Fiber Gear and made a gear for a thread dial many years ago. Worked perfectly. It was for a 4-pitch lead screw and I want to say it was a 12 diametrical pitch by 16 teeth. Not quite .250 C.P. but function fine. As John said, it don't work to get the job done, just there for the ride as an indication device. KenS.

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    Some years ago,we scrapped a big ,very old lathe with a long threaded screw along the back,that was in fact a worm,and meshed with a worm wheel which drove across the saddle into an apron ,and thence to a rack gear.......i thought it was odd,but later in England I saw a similar preserved lathe called "Whitworths patent"......The was no lead screw ,but looked like there was......the lathe weighed 20 tons approx,and had a faceplate /chuck that weighed near exact 1 ton on the crane scales......no hollow spindle,solid .

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    An acme thread produces linear motion when turned in a nut, so flat sides can have full contact. A worm and gear both rotate, so the contract areas move around and a worm gear had teeth designed to best manage this.

    Worm and worm- gear are analogous to a rack and pinion, the rack has straight teeth but it generates curved teeth on the pinion gear.

    All of which is unnecessary to the creation of a thread dial. It's fun though.

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    The OD of the 32 tooth gear on my Lodge & Shipley is 2.8"

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    As John said the gear does no work, has no load and is simply an indicator to time the lead screw with the spindle. Making a worm gear to fit is not easy but something that will indicate the position of the lead screw to the relation to the spindle is not hard to do. I'll bet a spur gear will work it will just have to be the right size. If it had to handle a load a spur gear would fail but it needs not do more than rotate a dial! Granted a worm gear to match the lead screw is best but not having the proper gear then a gear that will time the spindle with the lead screw is all that is needed.

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    Quote Originally Posted by johnoder View Post
    It is first an indicator. It has no WORK to do other than spinning a marked indicator disc

    Lacking any sort of LOAD, most anything that reliably SPINS THE DIAL will be acceptable

    In simplest terms, it will work fine with a ..250" CIRCULAR PITCH gear of say 16 teeth - forget the angles that would be required by a worm and worm wheel - just make it THIN enough so such things are far less of a problem if they ARE NOT THERE

    Made one from HARD MAPLE once - worked fine

    You have to make CP gears, they are not laying around

    If it were .250 CP, it would need to be 1.273 pitch dia. for 16 teeth (16 times .250 divided by "Pi")
    Thanks John, I figured as much. My computations cmae up with the same answers as yours. I realize there's no need to fuss with this. If i do so it'll be for the practice.

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    I haven't had the excuse to make any thread dial yet. However, if I were to make one, I'd choose a relatively hard material (perhaps bearing bronze) that is less prone to embed grit.
    More than one perfectionist gashed the blank cutting grooves at the same angle as the lead angle of the lead screw. Then they cut a "hob" replicating the diameter, profile and pitch of the leadscrew and "hobbed" the thread dial gear by letting it free-spinning by the hob engaging the gashes.
    As John and other have said, you do not need any perfect mesh with the teeth and, if you use a spur gear, it is better to keep it rather thin.

    Paolo

    PS If you attempt hobbing the free-spinning blank without any tooth already gashed, you get a mess with fewer than expected teeth, since the hob will drive the blank faster than it should. A friend of mine tried this, before I found him a thread dial for his Heavy 10.

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    Marka understand you will need a worm gear hob to make a worm gear. If you intend to buy the hob be sure your sitting down! They are a bit pricey! Then the device holding the gear being made must be timed with the spindle. Dividing head can be used as Paolo is referring to, the blank must be precut (gashed) then the head disengaged from the driving internal worm and allowed to free wheel letting the hob rotate the precut worm gear your making and complete the tooth form.

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    Quote Originally Posted by Paolo_MD View Post
    I haven't had the excuse to make any thread dial yet. However, if I were to make one, I'd choose a relatively hard material (perhaps bearing bronze) that is less prone to embed grit.
    More than one perfectionist gashed the blank cutting grooves at the same angle as the lead angle of the lead screw. Then they cut a "hob" replicating the diameter, profile and pitch of the leadscrew and "hobbed" the thread dial gear by letting it free-spinning by the hob engaging the gashes.
    As John and other have said, you do not need any perfect mesh with the teeth and, if you use a spur gear, it is better to keep it rather thin.

    Paolo

    PS If you attempt hobbing the free-spinning blank without any tooth already gashed, you get a mess with fewer than expected teeth, since the hob will drive the blank faster than it should. A friend of mine tried this, before I found him a thread dial for his Heavy 10.
    i'm thinking i'll gash the blank with the mill using a slotting cutter and then refine the tooth profile with the shaper and a properly ground tool bit (no hobbing required). Again, for me, it's as much about the practice as the outcome. If i screw it up (not pun intended), i'll just start over. At some point in the future i will need to repair or make a new work wheel for the power downfeed on an old camel back drill press but that's a battle for another time.

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    Quote Originally Posted by marka12161 View Post
    i'm thinking i'll gash the blank with the mill using a slotting cutter and then refine the tooth profile with the shaper and a properly ground tool bit (no hobbing required). Again, for me, it's as much about the practice as the outcome. If i screw it up (not pun intended), i'll just start over. At some point in the future i will need to repair or make a new work wheel for the power downfeed on an old camel back drill press but that's a battle for another time.
    Your not going to cut the tooth profile with a shaper. The profile is helical. The hob is like a screw both the gear being made and the hob must be turning at the same time. The blank will have to turn as the ram moves forward plus the down feed will have to cut a radius if done on a shaper that has no provisions to cut like that you will be unable to make a worm gear.

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    I might have to make one for my Greaves Klusman lathe as the one on it has a broken tooth. I have not investigated whether or not it skips when turning but if it does I plan on 3D printing a pattern and casting one in either Babbitt which I could do in my shop or in brass.

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    On my Harrison the gear is a thin brass or bronze disc and doesn't have full depth engagement with the leasscrew, the clever bit is that there are several different gears with different tooth counts, you change the gear for different tpi threads.

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    The Southbend has a trapezoidal acme-looking spur gear, with a straight 4-degree angle to the teeth. If you made the disc thin as noted by Peter, it wouldn't require any angle to the teeth. The angled teeth and thick gear allow a lot of slop with positioning and retracting the gauge.

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    Quote Originally Posted by Froneck View Post
    Your not going to cut the tooth profile with a shaper. The profile is helical. The hob is like a screw both the gear being made and the hob must be turning at the same time. The blank will have to turn as the ram moves forward plus the down feed will have to cut a radius if done on a shaper that has no provisions to cut like that you will be unable to make a worm gear.
    I understand the geometry. The leadscrew is about 1 7/16" diameter and i'll make the thread dial gear maybe 3/8 or 1/2 inch thick. As others have said, since this transmits no power to speak of, the fit is not important and no need to mess with a hob for something like this.

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    Quote Originally Posted by marka12161 View Post
    I understand the geometry. The leadscrew is about 1 7/16" diameter and i'll make the thread dial gear maybe 3/8 or 1/2 inch thick. As others have said, since this transmits no power to speak of, the fit is not important and no need to mess with a hob for something like this.
    Oh OK that will work for the threading dial since there is no load simply a means to turn the dial so as to time the engagement of the half nut when threading. But it will not work on the Drill press power feed. Thickness will not help, you can make it 1-7/16" thick and you will still have a very, very lite duty gear drive. For power feed the helix of the tooth must match the helix of the screw.


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