Measuring (probable) Acme TPI

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neanderman
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Measuring (probable) Acme TPI

Post by neanderman »

One day, I'm likely to try making a replacement crossfeed rod for the Dual drive.

A very basic question: is this 8 TPI?
20171105_212758-01.jpeg
It doesn't appear to be a standard thread per my 17th ed of Machinery's Handbook as it's 0.750 major diameter.
Ed

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choprboy
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Re: Measuring (probable) Acme TPI

Post by choprboy »

Looks like 8tpi to me. 3/4-6 seems to be "standard", but McMaster and others carry 3/4-8 Acme rod and nuts.
spro
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Re: Measuring (probable) Acme TPI

Post by spro »

It is eight.
earlgo
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Re: Measuring (probable) Acme TPI

Post by earlgo »

The 12" Atlas Models 3980,-81,-82,-83, and 3990, -91 are also 3/4-8 Acme on the lead screw.
--earlgo
Before you do anything, you must do something else first. - Washington's principle.
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neanderman
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Re: Measuring (probable) Acme TPI

Post by neanderman »

Thanks, gentleman.

When the time comes, I'm sure I'll have more questions.
Ed

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tornitore45
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Re: Measuring (probable) Acme TPI

Post by tornitore45 »

Lead screw do not have to follow a standard.
The diameter and the lead have no strict relation like fasteners.
The diameter is chosen based on strength, flex and wear consideration'
The lead must give a practical dial. A 6 TPI would imply a dial with 166.666 division
Mauro Gaetano
in Austin TX
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neanderman
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Re: Measuring (probable) Acme TPI

Post by neanderman »

Mauro, I fully understand the rationale for the thread pitch; the size is what I found curious.
Ed

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spro
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Re: Measuring (probable) Acme TPI

Post by spro »

What you may find curious is machining off the ends to fit. Ideally, this is done close to the chuck. At least you can get a center drill point. Suspending a long section of 3/4" Acme outside the head, calls for a fixture. It doesn't have to be massive but has to be secure. We don't want it whipping around at speed.
I guess there will be no whipping involved if it is machined at the tail end. One still needs the accurate center.
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neanderman
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Re: Measuring (probable) Acme TPI

Post by neanderman »

Good points, spro.

Given the diameter of the threads is 3/4", and that is the maximum diameter, I assume I could start with a piece of 3/4" drill rod, cut the threads and then turn the ends down to size?
Ed

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Re: Measuring (probable) Acme TPI

Post by spro »

Apologies, ED. I misread what you were doing. I was thinking that you were doing a lead screw and as it is a standard size, purchasing a length of it. Whipping would be an issue due to the length, so an outboard support at the left side of the head might be required.
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neanderman
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Re: Measuring (probable) Acme TPI

Post by neanderman »

Overall length I'd just shy of 28", so I can turn it between centers and I have a follow rest.

But, yeah, something as long as a lead screw hanging out the end of the headstock would be asking for some whipping!
Ed

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spro
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Re: Measuring (probable) Acme TPI

Post by spro »

That's a honker of a cross slide screw but that's a big lathe. I wonder why you don't purchase the ready threaded length to fit. Turning and trueing the ends is a class job in itself.
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