Lead screw question for Harold

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shootnride
Posts: 298
Joined: Tue Jun 05, 2012 12:31 am
Location: Sacramento, Ca.

Lead screw question for Harold

Post by shootnride »

Harold,
I have what is probably kind of a dumb question, but I've directed it to you because I know that you also have a Graziano lathe. In conversations with other hobby machinists concerning threading on the lathe, the topic of lead screw pitch has come up several times. When asked about the lead screw pitch on my lathe, I tell them that it is designated as a 3/8" pitch by Graziano which usually gets me the "deer in the headlights" stare. I understand that means that the carriage will advance .375" per revolution, but it doesn't correlate to anything "standard" in Imperial or Metric terms. It is 2.66 TPI or 9.525 mm. I am always at a loss to give any explanation about how this thing works. Just wondering how (or if ) you would explain Graziano's thinking concerning this 3/8" pitch lead screw.
Thanks
Ted
Some people raise the IQ of the room when they enter.........others when they leave.
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Harold_V
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Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Lead screw question for Harold

Post by Harold_V »

Ted,
Interesting question. I'm not sure I have the answer, as I've never really given it much thought, but what comes to mind is the fact that both Imperial and metric threads can be directly dialed by the throw of the proper levers. Modular as well as Whitworth pitches are also provided. What I suspect, but do not know, is that they have chosen a lead that allows for all pitches to be dialed. None (or precious few of them) are actually dead on for lead, as chosen gear/screw combinations yield minor error, which is cumulative. If the nut and screw have a long enough engagement, the threads may not be functional, assuming the nut was tapped with a proper pitch tap. I've had that experience in an unusual situation. It's generally not a problem because of the limited length of engagement of the nut. No problem at all if both screw and nut are generated by the same setup instead of the B thread being tapped.

If, by chance, you happen to discover the thoughts of Graziano, please post accordingly. I'll also try to see what the manual has to say, but that will have to be after today, as we are about to depart for our monthly shopping for provisions. That used to occur weekly, but not anymore, thanks to the virus.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
Russ Hanscom
Posts: 1955
Joined: Wed Mar 15, 2006 11:10 pm
Location: Farmington, NM

Re: Lead screw question for Harold

Post by Russ Hanscom »

The pitch of the lead screw should not have any impact on the ability to cut assorted threads.

My Harrison, which has an 8 tpi lead screw, will cut english, metric, modular, etc threads depending on which levers are in which positions. As noted, some of the pitches are not exact, but I did the math on metric, and the error is on the order of.0004% so it takes a relatively long engagement for thread error to cause binding.

A .375 " pitch seems amazingly course. There must have been a reason.
AllenH59
Posts: 479
Joined: Wed Mar 16, 2011 1:26 pm
Location: Prince George BC Canada

Re: Lead screw question for Harold

Post by AllenH59 »

My lathe has a 4tpi lead screw.. also cuts metric and standard..
shootnride
Posts: 298
Joined: Tue Jun 05, 2012 12:31 am
Location: Sacramento, Ca.

Re: Lead screw question for Harold

Post by shootnride »

Gentlemen,

Harold, I've kind of suspected pretty much what you've said. I figured that some Italian engineer spent a lot of time trying to come up with what he thought was the optimum combination of gearing in conjunction with the lead screw to provide the widest range of the most accurate thread cutting. Whether Graziano's formula is any better than any other manufacturer, I have no idea. I've just never seen any information about any other manufacturers using a 3/8" lead screw.

Russ,
I agree, the .375 pitch is a very coarse thread (2.66 TPI) and I hope there was a good reason for it.

Thanks for the responses gentlemen.

Ted
Some people raise the IQ of the room when they enter.........others when they leave.
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