# Involute Gear Pitch for ACME Thread



## bretthl (Feb 23, 2019)

I'm trying to figure out which involute gear pitch works with 8 and 10 TPI ACME thread?  By "work" I mean for low pressure application similar to a lathe thread dial or other low torque application.


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## Asm109 (Feb 24, 2019)

.1 and .125 inch CIRCULAR PITCH gears.   NO diametral pitch gears will be correct.

.1 cp gears are  pi *1/.1=  31.4159 DP
.125 gears are  25.1327 DP


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## bretthl (Feb 25, 2019)

8 TPI ACME pitch = 1/8 = 0.125
10 TPI ACME pitch = 1/10 = 0.100

Circular Pitch = Pi*D/N

For a 32 Diametrical Pitch gear with a pitch diameter of 1" then

CP = Pi/32 = 0.0982"

That should be close enough for 10 TPI ACME?

Nothing works for 8 TPI ACME, at least with inch gears.  Metric perhaps?  What size gear is used on an 8 TPI lead screw thread dial?


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## RJSakowski (Feb 25, 2019)

You don't get to pick your circular pitch, diametral pitch, AND pitch diameter.  Normally, one would select the desired lead (and starts, if applicable) for the worm, the desired gear reduction.  The lead for a single start worm will be equal to the circular pitch of the mating gear.  For a single start worm with a lead of .1 and a 32:1 reduction, the pitch diameter will be 1.019".  For one with a lead of .125", the 32:1 reduction will require a gear with a pitch diameter of 1.273".

Because of the geometry of the worm, the O.D.  will depend on the width of engagement of the gear with the worm  Minimally. it should be 1.08" and 1.35" for the two cases cited.


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## bretthl (Feb 25, 2019)

What I want to do is this with a 10 ACME threaded rod, turn the rod, turn the gear, low torque -


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## PMartin (Mar 17, 2019)

Your gear will need to be quite narrow to even work at all. Worm wheels (your gear) have teeth that have an angle that matches the angle of the thread helix. You won't have that with a regular spur gear.


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## BGHansen (Mar 18, 2019)

PMartin said:


> Your gear will need to be quite narrow to even work at all. Worm wheels (your gear) have teeth that have an angle that matches the angle of the thread helix. You won't have that with a regular spur gear.


Some one on forum posted using a tap as the work and cut a worm gear.  Then turned the worm to match the tap.  As PMartin noted, the worm gear would ideally have a radius matched to the worm for more tooth contact.

I'll do some searching and see if I can find the thread that had the tap/worm technique.  You might be able to find an Acme tap to match your Acme rod.

Bruce


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## bretthl (Mar 18, 2019)

PMartin said:


> Your gear will need to be quite narrow to even work at all. Worm wheels (your gear) have teeth that have an angle that matches the angle of the thread helix. You won't have that with a regular spur gear.



You are right ... and I need to do it right.  I have a PM935 mill and Z axis lock is crap.  There are two lock screws/handles located on the left ways.  Locking them is getting tedious.  My initial thought was to replace the handles with small gears that interface with a vertical Acme threaded rod with a handle at the top.  One light twist of the rod to rotate each way lock screw (timed of course).  Time is money and messing around under the table to tighten lock screws is a poorly conceived shortcut on an otherwise perfectly mediocre machine.

I am open to ideas.  The Bridgeports seem to have the Z lock activated by a handle on the front of the knee.  Can this be rigged on my machine?  If I had access to Bridgeport I could answer that.

I'm not going to let this go.  I am really annoyed over this feature and I will fix it, somehow.


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## BGHansen (Mar 18, 2019)

Here are some older posts with info on making a worm gear.

Bruce









						calculating effective gear diameter
					

The project is to make a worm gear set to drive an instrument, a telescope. I have settled on a worm gear design. I have a piece of Acme rod for the worm drive and am planning on cutting the gear with a tap. I am ready to cut the first test gear and after watching several videos I realized the...




					www.hobby-machinist.com
				











						Dividing And Indexing Head Project
					

A while back, I built an indexing head from aluminum and steel. While not an extremely heavy duty device (which it was not intended to be) it works well. BUT, I have decided to build a dividing/indexing head from steel as a more advanced and heavier duty unit. I would like to use a worm wheel...




					www.hobby-machinist.com
				











						Quick & Dirty Worm Wheel making
					

I was trying to make a substantial speed reduction to a small, but very powerful 230 Watt motor to drive a small auger.    The motor specs are impressive  Mabuchi Motor 12v Rs-775sf 230w 19500 rpm  Model    : RS-775 18V        Weight    : 11.9 oz Operating v    : 6v - 20v    Length of motor    ...




					www.hobby-machinist.com
				











						Make A Worm Gear
					

Did you know you could make a worm gear if you had the old shaft?  Well me either until I saw a post by another member on here a few weeks back (can't find it now) briefly showing how he cut four slots in a threaded shaft and mounted it in the lathe.  He then mounted a piece of round stock on...




					www.hobby-machinist.com


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## bretthl (Mar 19, 2019)

This is very helpful, thank you.


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## gatz (Dec 15, 2019)

Can only guess how the 2 gib screws are situated on your machine as I'm not familiar with the PM935 mill.  
The Bridgeport uses a single lock which has a taper that bears against the knee gib when the front handle is rotated up.

"Tming" 2 screws to advance evenly is not going to be easy.  Using indexable (or adjustable) handles with external threads to match what's on your mill might get the job done if they are_ linked_ in such a way so that they tighten up evenly. The linkage(s) would have to have an "evener" (think of a single horse evener , but in reverse) to allow some leeway between the handle positions. A lever with an eccentric or screw/nut assembly could then be attached to pull the center of the evener to employ the locks.
Check out some indexable/adjustable handles on McMaster-Carr or on Amazon, to name a few sources.

gatz


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## bretthl (Dec 29, 2019)

gatz said:


> Can only guess how the 2 gib screws are situated on your machine as I'm not familiar with the PM935 mill.
> The Bridgeport uses a single lock which has a taper that bears against the knee gib when the front handle is rotated up.
> 
> "Tming" 2 screws to advance evenly is not going to be easy.  Using indexable (or adjustable) handles with external threads to match what's on your mill might get the job done if they are_ linked_ in such a way so that they tighten up evenly. The linkage(s) would have to have an "evener" (think of a single horse evener , but in reverse) to allow some leeway between the handle positions. A lever with an eccentric or screw/nut assembly could then be attached to pull the center of the evener to employ the locks.
> ...




There are two indexing handles that tighten against the left column gib.  They are a pain in the ass to work with because you have to reach clear around and under the table.  I don't know why the Bridgeport lock feature was not copied on this mill.

I have not given up on this project just other priorities for now.


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## benmychree (Dec 30, 2019)

If the vertical gib is adjusted correctly, it is not necessary to apply the lock(s) for most work


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