# Cross feed scew TPI question...



## Ststephen7 (Apr 1, 2018)

OK... I'll do my best to make this understandable.

My 1959 Sheldon 10" has quite a bit of wear on the cross feed screw AND a lot of wear in the nut.  I'm currently using a 3D printed cross feed nut (which is working great by the way), however I'd like to make my own someday soon, out of brass/bronze/cast.

I'd also like to make larger dials for my lathe, however...  Sheldon, for whatever reason, decided to use 8 tpi on the screws.  So... the dials have 125 graduations!?  I was thinking I'd use the saw blade trick to make my graduations, however... I don't believe a 125 tooth saw blade exists.

I think I could make my own new nut.  I found a 1/2" LH acme tap on ebay for less than $30 (pretty reasonable I thought).  However, using the tap will probably end up with still having too much backlash due to the wear on the screw.

Cutting the threads on the nut myself might enable me to get a better fit and reduce the backlash.  However... This is my first lathe, and cutting internal LH 1/2" acme threads is probably a lot of experience down the road.

So... now the question:

Since I should make a new screw AND a new nut, is there any reason not to just make them both with 10tpi?  I would then HAVE TO replace the dial, however I can easily find 100 tooth circular saw blades, so I think I could do that.  Are there downsides for changing the tpi from 8 to 10 that I'm not seeing?

Thank you,
Steve


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## benmychree (Apr 1, 2018)

Just more cranking to get from point A to Point B, and less bearing surface equals faster wear.  I wonder how precise the tooth spacing on a circular saw would be?


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## MrWhoopee (Apr 2, 2018)

IMHO, 10 tpi would be a better choice, .100 per rev. is much easier to count off. Not sure how it would provide less bearing surface as there would be more threads engaged in a nut of the same width. Saw blades are very precise and are a popular choice for indexing without an indexing head.


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## benmychree (Apr 2, 2018)

MrWhoopee said:


> IMHO, 10 tpi would be a better choice, .100 per rev. is much easier to count off. Not sure how it would provide less bearing surface as there would be more threads engaged in a nut of the same width. Saw blades are very precise and are a popular choice for indexing without an indexing head.


Less bearing area??? the threads are shallower for one thing!  It should be obvious that a coarser thread has more bearing area than a finer thread; as far as counting off is concerned, how may rules in common use have 1/10" graduations?  I have one, but very rarely use it; my 4R graduation with 8the, 1/16the , 32nds and 64ths is the standard for most all industries, except possibly the aircraft industry of WW-2, and I commonly use tm scale for rough measurements in the lathe and other machine tools.


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## MrWhoopee (Apr 2, 2018)

benmychree said:


> Less bearing area??? the threads are shallower for one thing!  It should be obvious that a coarser thread has more bearing area than a finer thread; as far as counting off is concerned, how may rules in common use have 1/10" graduations?  I have one, but very rarely use it; my 4R graduation with 8the, 1/16the , 32nds and 64ths is the standard for most all industries, except possibly the aircraft industry of WW-2, and I commonly use tm scale for rough measurements in the lathe and other machine tools.



While I agree that a coarse thread would present more surface per thread, a finer thread would present more threads in a given length.
As far as counting, I was referring to counting revolutions as the dial is spun. It is easier to count multiples of 100 than multiples of 125.
In addition, 100 divisions are somewhat easier to read than 125 on a dial of the same size due to the wider spacing.

"The potential benefits of fine threads are:
1. Size for size a fine thread is stronger than a coarse thread . This is both in tension (because of the larger stress area) and shear (because of their larger minor diameter).
2. Fine threads have also less tendency to loosen since the thread incline is smaller and hence so is the off torque.
3. Because of the smaller pitch they allow finer adjustments in applications that need such a feature."

http://www.boltscience.com/pages/faq.htm


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## f350ca (Apr 2, 2018)

It seams 8 tpi was more common on older machines when possibly more was laid out with fractional rulers. My shaper down feed is 8 tpi, had to make a 50 hole dividing plate to do the graduations with the dividing head. I'd go 10 tpi , much easier to keep track of.

Greg


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## benmychree (Apr 2, 2018)

I would agree to disagree ---- I have been working at the machinist trade for nearly 50 years now, I think I know what I am talking about.


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## Chipper5783 (Apr 2, 2018)

Steve, you'll need to make a new screw.  A worn cross feed lead screw is a very common issue.  However, the wear is not going to be even.  The far end of the screw is probably hardly worn at all (like new) - but mid point will have significant wear. When you make the nut, it has to work on the entire screw.  Yes, just a new nut will correct part of the problem (since the nut is worn too), but there will still be quite a bit of backlash when operating in the mid-point.

10 tpi or 8 tpi?  Not a big deal either way.  I have two lathes - one with a 10 tpi cross slide lead screw, one with a 8 tpi.  They both work just fine.  I suggest you not make extra work for yourself.  Fix what is broken and get onto the next project.  Of course making new dials is a fun challenge - but save that effort for something that actually adds value.


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## 12bolts (Apr 2, 2018)

As posted earlier, the thread wear on your leadscrew wont be uniform along its length. Make a new leadscrew, and a nut to suit. Cutting a LH internal thread is actually easier than a RH because the tool progresses out of the hole instead of into it. Much less stress that way. Aside from that, everything is still done the same. If you are intending to make new dials then I would change to a 10 TPI to suit a sawblade indexer. 100 division dials are the norm


benmychree said:


> I would agree to disagree


Millions of UNF fasteners would disagree..............

Cheers Phil


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## HMF (Apr 2, 2018)

This is one of those things where experts may have to disagree and re-focus on simply helping the OP.  He now has several options that he can pursue.


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## Mark_f (Apr 2, 2018)

I have been machining for 45 years and have made several new lead screws and nuts. I have also converted a few 125/rev to 100/rev without any problems. I spent my last 11 years of working as an engineer for a machine tool manufacturer. While ALL  the opinions here have some truth to them, some more than others, which ever way you decide to go, if you keep it lubed and clean, you probably won't live long enough to wear the new one out. Using .125/rev is a matter of getting used to it. Many older machinists are used to .125/rev as many old machines were made that way.  This is of course MY opinion as a master machinist and engineer. Those who know more may disagee. 
One other thing. If you have power cross feed, changing to 10 tpi will change your power crossfeed rates to a little slower rate. So in the end, it is a matter of choice.


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## Ststephen7 (Apr 2, 2018)

Thank you for your replies!

I believe what I'm hearing, at least from the majority, is that making the new screw and nut 10 tpi (as opposed to the original 8 tpi) will be not negatively affect the way the lathe works in any real way.

In order to be accurate with it I'll have to make a new dial with 100 graduations, and a wanted to make a larger dial anyway, so we're good there.

I did not remove it when cleaning up the lathe, and from the drawings it looks as if the power cross feed gear is a separate part (pinned to the screw), so the power feed should remain the same.   If it is part of the screw, I will not be making a new screw until I get a mill and learn to make gears.

Now to find some appropriate steel and start the project!


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## Mark_f (Apr 2, 2018)

So, you have power crossfeed. If you make a new screw 10 tpi, your cossfeed rates will be 20% slower. I did this for a Friend's lathe and it really doesn't present a problem for him as the crossfeed is mainly used for facing, but it would be a problem if you need to feed at a specific rate with the cross feed.

On the South Bends, the screw and gear are all one piece. Many cut the old screw off, drill and ream the gear, then loc-tite and pin a new screw in the gear.

I don't know what kind of lathe you are working on but there are write ups available on replacing the screw and making large dials with thrust bearings for the south bend lathe. They can be easily adapted to othe lathes.


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## Richard King 2 (Apr 2, 2018)

save some time and frustration and buy both and machine the ends to fit your dials.  I usually cut off the turned side of screw and bore a hole in the threaded rod and press the new acme shaft into the old turned end and pin it.

http://www.greenbaymfgco.com/ACME-nuts.php
http://www.greenbaymfgco.com/ACME-threaded-rods.php


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## Richard King 2 (Apr 2, 2018)

Life is to short to argue about this, plus Nelson might give the participants the boot...LOL  Have a nice week


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## HMF (Apr 2, 2018)

Richard King 2 said:


> Life is to short to argue about this, plus Nelson might give the participants the boot...LOL  Have a nice week



Aww, come on Rich, I'm not *THAT* bad.  Seriously, sometimes we disagree, and, in the middle of the disagreement, we forget the original purpose, which is to help someone out with their problem.


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## woodchucker (Apr 2, 2018)

Ststephen7 said:


> 'd also like to make larger dials for my lathe, however... Sheldon, for whatever reason, decided to use 8 tpi on the screws. So... the dials have 125 graduations!? I was thinking I'd use the saw blade trick to make my graduations, however... I don't believe a 125 tooth saw blade exists.


You know, if you are making a screw and nut, you can do whatever you want. 10 tpi, 20tpi.. 8tpi, whatever.
If you.   20 tpi would be way nice...
We had a discussion a long while back about Kurt vises being regular 60degree screws, not acme. After that conversation. I occasionally wonder why a cross feed needs to be acme?  Maybe it doesn't. Maybe a regular screw would do the job..
At 20tpi, you would have a super nice precision crossfreed. Just thinking out loud.


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## magicniner (Apr 2, 2018)

benmychree said:


> Less bearing area??? the threads are shallower for one thing!  It should be obvious that a coarser thread has more bearing area than a finer thread; as far as counting off is concerned, how may rules in common use have 1/10" graduations?  I have one, but very rarely use it; my 4R graduation with 8the, 1/16the , 32nds and 64ths is the standard for most all industries, except possibly the aircraft industry of WW-2, and I commonly use tm scale for rough measurements in the lathe and other machine tools.



I find decimals more common on inch drawings than fractions and a dial which reads decimal divisions directly is easier to keep track of over more than a full turn, for me. 
But what you're used to is just what you're used to, it's not necessarily right, or best for everyone, I now use DROs on all manual machines and also have CNC machines too so measurements in Inch/Metric and dials catering to Fraction/Decimal make no difference any more


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## RJSakowski (Apr 2, 2018)

IMO, the characteristics describing a good lead screw are precision and wear resistance.  Strength is not one of them.  With the possible exception of knurling, it is rare to apply more than a fraction of a ft. lb. of torque.  

The Grizzly 602 has a .060" (actually, 1.5mm)cross feed pitch, so 60 divisions on the dial.  Making feeds of more than .060" requires some mental addition.  Not much of a problem as the micrometer is always handy to check progress.  The tailstock feed also has a pitch of .060".  This is more of a problem when boring holes to depth as it isn't convenient to actually measure.  The compound has a pitch of .040", for 40 divisions on the dial, again not a convenient number.

The Atlas Craftsman 6 x 18 has a tailstock pitch of .0625" so every other turn, I have to remember to add a thousandth.  The worst case is the mill/drill where the fine feed on the quill is .085"/turn.  Drilling a 3" deep hole while trying to do the math in your head is a challenge.  Small wonder that I wanted DRO's on the machines.


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## Boswell (Apr 2, 2018)

magicniner said:


> I now use DROs on all manual machines


I agree on the DRO's  Especially for us hobbiest. Changed my relationship with my lathe forever, no more mistakes keeping count of turns and there are several low cost version that sound like the work reasonably well.   So to tie this back to the OP question, no need to make any changes to the work leadscrew, just add a DRO.


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## MrWhoopee (Apr 2, 2018)

I agree that a DRO is an very worthwhile addition to any lathe. I have just installed a cheap DRO (~$185) on my SB Heavy 10, and I love it! However, as a matter of principle, I would repair the worn cross-feed first. I did that, and reduced the play on the dial from .140 (!) to .014 (.007 lash). Mine turned out to be loose components and a missing thrust bearing instead of wear. I also agree that the purchase of a precision acme screw would be preferable to making one.


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## projectnut (Apr 2, 2018)

One thing that hasn't been mentioned is the degree of precision needed when cutting threads for a lead screw.  Most precision lead screws and nuts have an accuracy of .007" per foot.  Do you feel you can achieve these tolerances on your first attempt at cutting acme threads?  Do you know if your machine in its current state is even capable of holding the tolerance?  If either are in question I would take Richard King's suggestion and save time, expense, and frustration and purchase the necessary parts.

As for the tap available on eBay, do you know the thread tolerance.  A tap used for a lead screw nut should probably have an H1 tolerance or less.  H1 is the base dimension + .0005".  The most common taps are H3 which is the base dimension +.0015".  I would ask the vendor to give you the information before making a purchase.  If you use an H3 tap you'll introduce even more backlash.  If you're looking at a "made in China" tap I wouldn't trust that it's anywhere near the tolerance advertised.  A similar size tap from McMaster costs almost $85.00.  While they aren't the least expensive I'm not sure you can buy a quality tap for the price you mentioned.

As for the 10 tpi vs. 8 tpi, I would just replicate what's currently on the machine.  I have one machine (a Sheldon) with an 8 tpi screw, and another with a 10 tpi screw.  Both work fine and it's easy to switch from one to the other.


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## machPete99 (Apr 2, 2018)

I am not any expert on leadscrews, but I would probably lean towards buying the leadscrew and making my own nut single point style.
That way if machined carefully you probably can have a better fit than off the shelf.


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## P. Waller (Apr 2, 2018)

Produce a screw that meets your personal needs.
The only correct way is the one that works for you, One may produce a lead screw with any lead, you must make the dial accordingly.


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## basalt (Apr 4, 2018)

When I modified my bench lathe crosslide I made 2 nuts and mounted them back to back with a small space in between. Now I can loosen them and with a small hole in the top of the crosslide, reach in between with a small screwdriver and force them apart to adjust the backlash. Works like a dream.


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## RJSakowski (Apr 4, 2018)

basalt said:


> When I modified my bench lathe crosslide I made 2 nuts and mounted them back to back with a small space in between. Now I can loosen them and with a small hole in the top of the crosslide, reach in between with a small screwdriver and force them apart to adjust the backlash. Works like a dream.
> View attachment 264147


The Grizzly G0602 uses a split nut and a pointed setscrew driving into the split adjusts the backlash.


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## epanzella (Apr 6, 2018)

Obviously staying with 8TPI would be easier as the dials would not have to be changed. That said, If you plan on changing the dials anyway, like others have said, one hundred thou per revolution is a lot easier to keep track of and the dial is easier to fabricate. You could also use a 50 tooth sprocket and mark every .002.


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## Silverbullet (Apr 6, 2018)

I've never trusted dials , my tool box consists of dozens of precision measuring instruments. With all the mag bases and clamps ill stick with those over dials and worn parts. If my lathe had DRO I would trust that ,, maybe ,,,. As some know when your young you learn the way to make good parts . I've set up and ran jobs on machines that the dials just weren't readable at all. In the jobing shops I worked if the parts were out of tolerance you didn't work there very long. And being young married with two babies in five years I needed the job. An old British ,,,really Scotsman,,, taught me to never ever trust the machines . Use indicators and job blocks not the dials . Just my take on dials I don't use them.


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## Mark_f (Apr 6, 2018)

I use dials all the time unless I need a tolerance closer than .001". Never had a problem. In fact , I learned using the dials except for like I said..... Closer than .001". It is a matter of choice.


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## KBeitz (Sep 5, 2018)

My Bridgeport has the split nut...
But what we need to come up with is a spring loaded nut that will automatically
adjust for wear on the screw....


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