1911 LeBlond thread chasing dial gear question
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  1. #1
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    Default 1911 LeBlond thread chasing dial gear question

    I have a 16" LeBlond lathe missing the entire thread chasing dial assembly. And unlike most lathes, this one doesn't have any mounting hole, just a shouldered hole for the thread chasing shaft. It looks as though the entire assembly rests, suspended by the dial itself. Anyways, I have a 6 tpi lead screw with what looks to be an 18 dp thread. When I measured where the gear would sit on the shaft, it measured 1.5". But doing the math it showed that I either have to use a 24 tooth gear which measures 1.380 which is too small to reach the lead screw threads or 30 tooth which is too large/wide to fit. Assuming the gear has to be divisible by 6? What am I measuring/doing wrong?

    Jimimg_3017.jpg

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    Fimg_3019.jpgimg_3020.jpgimg_3017.jpgimg_3015.jpg

    Sorry, some of these photos are upside down. I am NOT computer literate and I'm sure some of these names will get corrected too.

    Jim

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    img_3018.jpg

    This is what I measured.

    Jim

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    Never mind the DP. Most likely to be circular pitch

    PD of gear will be very close to 1.495 assuming pitch dia of lead screw is 1.4166

    Circular pitch of gear will have to match pitch of lead screw - which is .166 if 6 pitch

    1.495 times "pi" is 4.697. Divide that by 24 and you get .1956 circular pitch - or a bit smaller than 16 DP

    I'd start with an over the counter 24 tooth 16 DP 14 1/2 degree pressure angle gear and see if I had to cut it down a bit to accommodate the helix angle of the lead screw

    Also emailed catalog scan as requested

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    The gear that meshes with the leadscrew is quite easily made freehand with an angle grinder,if thats all you have ....it simply meshes with the screw and turns the numbered dial......which is also easily made with no more than a drill and number punches.....or even a painted dial ,for a less long lasting solution.......making these devices couldnt be simpler ,yet lathe newbies inevitably call for CNC hobbing of the "cog" and professional engraving and enamelling of the dial.

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    6TPI leadscrew seems a bit small for a 16" lathe,check if its two start thread.........for instance ,a lathe that size would normally have 4 tpi ,or even 2 tpi,single start,which is what my 20" Swift has ,and the most recent scrapped Lang 20" had .....so recheck the pitch ......and Ive never seen the threading indicator with more than about 8 teeth.......30 is just silly.

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    Quote Originally Posted by johnoder View Post
    Never mind the DP. Most likely to be circular pitch

    PD of gear will be very close to 1.495 assuming pitch dia of lead screw is 1.4166

    Circular pitch of gear will have to match pitch of lead screw - which is .166 if 6 pitch

    1.495 times "pi" is 4.697. Divide that by 24 and you get .1956 circular pitch - or a bit smaller than 16 DP

    I'd start with an over the counter 24 tooth 16 DP 14 1/2 degree pressure angle gear and see if I had to cut it down a bit to accommodate the helix angle of the lead screw

    Also emailed catalog scan as requested
    Knew I could rely on you. Many thanks and I will pursue this. Thanks for the scan too!

    Jim

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    It is definitely a 6 tpi screw and no way in hell will any gear close to 8 teeth work. This gear has to have a diameter of 1.5"

    Jim

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    Thread dials are usually either "4 inch" or "2 inch" dials, with the 4 inch more common.

    The "inch" refers to how many teeth the gear has, a 4 inch has 4x the DPI, a 2 inch has 2 x the DPI. In your case, a 4 inch dial would have 24 teeth.

    The dial assembly is generally thinned or cut away in whatever manner is needed to make it fit, since the tooth count and diameter are fixed by the leadscrew dimensions and resulting tooth size. So the size of gear is accommodated by the design of the dial assembly.

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    Quote Originally Posted by jarhead jim View Post
    Knew I could rely on you. Many thanks and I will pursue this. Thanks for the scan too!

    Jim

    You are welcome

    These .08608" wires can give you a very close idea of the lead screw pitch dia.

    acme-wires.jpg

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    Quote Originally Posted by johnoder View Post
    Never mind the DP. Most likely to be circular pitch

    PD of gear will be very close to 1.495 assuming pitch dia of lead screw is 1.4166

    Circular pitch of gear will have to match pitch of lead screw - which is .166 if 6 pitch

    1.495 times "pi" is 4.697. Divide that by 24 and you get .1956 circular pitch - or a bit smaller than 16 DP

    I'd start with an over the counter 24 tooth 16 DP 14 1/2 degree pressure angle gear and see if I had to cut it down a bit to accommodate the helix angle of the lead screw

    Also emailed catalog scan as requested
    Jon, just got in the above aforementioned gear and it isn't the right one. The teeth are too far apart. I'll try to post a photo or two. What would be your second guess?

    Jimimg_3054.jpgimg_3056.jpg

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    This is the one I ordered. This also measures 1-5/8" across and I'm pretty sure it needs to be 1-1/2".

    Jimimg_3057.jpg

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    A normal 16 DP 24 tooth spur gear will have an OD of 1.625 and a PITCH DIAMETER of 1.500

    The usual method of finding the DP of gear would be to measure OD, and count teeth. You ADD TWO to the count and divide by the mic OD

    SO 26 divided by 1.625 = 16 DP

    As in my post above - this 1.50 PITCH DIAMETER has to work with the PITCH DIAMETER of the lead screw - with sufficient slop to accommodate helix angle of screw


    Quote Originally Posted by jarhead jim View Post
    This is the one I ordered. This also measures 1-5/8" across and I'm pretty sure it needs to be 1-1/2".

    Jimimg_3057.jpg

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    Quote Originally Posted by johnoder View Post
    A normal 16 DP 24 tooth spur gear will have an OD of 1.625 and a PITCH DIAMETER of 1.500

    The usual method of finding the DP of gear would be to measure OD, and count teeth. You ADD TWO to the count and divide by the mic OD

    SO 26 divided by 1.625 = 16 DP

    As in my post above - this 1.50 PITCH DIAMETER has to work with the PITCH DIAMETER of the lead screw - with sufficient slop to accommodate helix angle of screw
    Here's a couple pictures of the lead screw. Maybe this will help. I'm at wits end.. Definitely 6 threads per inch, measuring 1.185 OD. The 16dp gear I got, the tooth won't even fit in the groove of the lead screw no matter how you tilt it. img_3059.jpgimg_3062.jpg

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    From my post above

    see if I had to cut it down a bit to accommodate the helix angle of the lead screw
    Sounds like you need to whittle it down - which will make teeth thinner. Maybe you can make it usuable

    Another approach might be 18 DP - but this is non standard - at least at such places as Boston Gear
    Last edited by johnoder; 10-26-2021 at 04:06 PM.

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    Here's a gear that should work. May have to adjust the bore to fit.

    South Bend Metal Lathe Thread Dial Gear 13", 14 1/2", 16", 24" 3D Printed NEW | eBay

    Ken

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    Quote Originally Posted by jarhead jim View Post
    Here's a couple pictures of the lead screw. Maybe this will help. I'm at wits end.. Definitely 6 threads per inch, measuring 1.185 OD. The 16dp gear I got, the tooth won't even fit in the groove of the lead screw no matter how you tilt it.
    Ah, yes, that pesky detail.

    The leadscrew is an acme thread. That is not necessarily fitting a gear tooth, it is not quite a rack.

    It's not far off from a 14.5 degree rack, but the "teeth" are not as big. The gear tooth is not half the pitch tall, the acme screw thread is.

    So tooth count is as described, but tooth FORM may not be.

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    Quote Originally Posted by JST View Post
    Ah, yes, that pesky detail.

    The leadscrew is an acme thread. That is not necessarily fitting a gear tooth, it is not quite a rack.

    It's not far off from a 14.5 degree rack, but the "teeth" are not as big. The gear tooth is not half the pitch tall, the acme screw thread is.

    So tooth count is as described, but tooth FORM may not be.
    I'm gonna run a 10 or 12 dp cutter through the gap of the teeth of the gear. But looking in my machinery's handbook I'm a little confused on what plate to use and indexing.... page 947 says #39 plate and 1 -26/39ths turn Is that correct? 40 to 1 gear ratio here.

    Jim

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    Ok, I made the teeth thinner, didn't do a bit of good. Turned the gear down to a smaller diameter next and that didn't any good either. When you put the gear up to the lead screw and walk it along, the second tooth hits the leading edge of the next tooth hard enough that it wouldn't go in without pushing the gear out. Is it possible this should be an 18 tooth gear? If so, it won't touch the lead screw when attached to the shaft of the thread chasing shaft. The teeth need to be closer, but still have an overall outside diameter of 1.5 inches. Is this possible? Maybe a 30 tooth gear? But then the math doesn't add up beings the lead screw is 6tpi with an overall outside diameter of 1.185.....

    Jim


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