Sharp V threads

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Mr Ron
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Sharp V threads

Post by Mr Ron »

I thought I understood thread forms and how to single point them on the lathe, but after looking at various manuals and charts covering threading, I realize I don't have the whole picture. I am told to grind a thread cutting bit to a "sharp" 60° angle, but the tip may be rounded. When I look at thread illustrations, they show a flat at the root of the thread and the flat is defined as pitch÷8. That's where I'm confused. It would seem that I would need to grind a flat on the tip of the bit to conform to the thread form shown in the charts. I can see that a large thread like 1"-8 would need a flat .016", but on small threads like 6-32, is it necessary to provide a flat which would be .004"?
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GlennW
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Re: Sharp V threads

Post by GlennW »

Mr Ron wrote: I can see that a large thread like 1"-8 would need a flat .016", but on small threads like 6-32, is it necessary to provide a flat which would be .004"?
It depends on the thread type specified.

If it is a UNJ or a UNR thread, the root radius would be required.

For a standard UN thread, probably not as critical depending on the application.
Glenn

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dbstoo
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Re: Sharp V threads

Post by dbstoo »

The UN thread specifies a flat root to minimize cracking from stress risers. The standard says the edges of the flats may have a radius but the radius must start at the major diameter.

To do it right, each thread will have it's own tool with the tip matching the thread's profile. To make life easier, many threading tools are a modified sharp V that is approximately right for a handful of pitches that are close to the same size. They can make that claim by making a small flat on the end so you end up with a nearly sharp V root, but not quite. This works because the crest will always be flat and truncated so it will not notice that the root is a few thousandths of an inch deeper than expected.
earlgo
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Re: Sharp V threads

Post by earlgo »

After many years working in industry and watching in-house inspectors check threaded vendor parts, I know that the 'flat' at the root diameter of a thread is never looked at. Because of time and money constraints, only the thread go/no-go gage is used. It is a crap shoot whether the surface finish is checked, but obviously if it is severely 'bricked' a stern letter would be sent to the vendor. The drawing 'boiler plate' tolerance block mostly held a surface finish spec of 63 unless the drafter was awake.
There are some critical industries where this is not true, but in the places I worked that make commercial goods, it is.
If you are making threads on your own parts, you obviously can use the Mr. Ron standard and a pointed threading tool or anything else that floats your boat.
--earlgo
Before you do anything, you must do something else first. - Washington's principle.
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GlennW
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Re: Sharp V threads

Post by GlennW »

That sharp point can also be very delicate, so just lightly touching it with a fine stone can improve tool life and finish rather than having it chip off by itself.
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Harold_V
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Re: Sharp V threads

Post by Harold_V »

earlgo wrote:There are some critical industries where this is not true, but in the places I worked that make commercial goods, it is.
If you are making threads on your own parts, you obviously can use the Mr. Ron standard and a pointed threading tool or anything else that floats your boat.
--earlgo
I agree fully. I went from the missile industry to working in a shop that made mining and floatation equipment, and there was NO QC, very unlike the place I was trained, where every detail of every part was inspected. In the missile industry I witnessed expensive castings being discarded because a dimension was missed by a thou (their logic was you can't risk a million dollar missile by using a tens of thousands dollar machined part that did not meet specs), while in the mining industry, if a part would assemble, it was considered good and useable.

Harping on the subject of proper tool form and upholding proper pitch diameter in order to make proper threads is for the purpose of gaining an understanding of what is required to make proper threads by choice (not chance). There are industries where it's mandatory---and industries where it's simply not relevant. My logic is it's best to be informed of proper procedure, so when they are required, they're not a stranger. I can't imagine anyone would be sorry for knowing too much.

Harold
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Harold_V
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Re: Sharp V threads

Post by Harold_V »

GlennW wrote:That sharp point can also be very delicate, so just lightly touching it with a fine stone can improve tool life and finish rather than having it chip off by itself.
Yep. The very reason why the unified series includes the flat. It is no secret that the sharp form is delicate and easily damaged, and lends nothing to the strength of a threaded part. The truncated minor diameter of an A thread benefits further by lowering stress risers as well as gaining a small amount of diameter.

One thing I learned early on is taking a deep first pass tends to destroy the tip of a threading tool, in particular when it's quite sharp. There's considerable conflict of chip flow, so pressure at the tip is beyond normal pressure for equal cuts where chips can flow easily. Eliminate the sharp tip and you gain considerable strength.

Harold
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Mr Ron
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Re: Sharp V threads

Post by Mr Ron »

Another question came up. Are taps and dies designed to create that flat at the root and crest of the thread?
Mr.Ron from South Mississippi
dbstoo
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Re: Sharp V threads

Post by dbstoo »

A modern UN tap and die will, if used with materials that are the appropriate major and minor, leave a compliant crest and root.
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GlennW
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Re: Sharp V threads

Post by GlennW »

No doubt about it!

I just threw a new 5/16-24 high quality tap up on the Comparator for your viewing pleasure...
DSC00478.JPG
The dark area is the tap.
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!
stephenc
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Re: Sharp V threads

Post by stephenc »

I learned how to single point threads by trial and error . It was one of the first things I learned how to do when I got my lathe .
After reading this thread and the other two I've learned that I have been doing things the right way for the most part .
One of the things I've always done is leave the sharp point on my tool while knowing full well there was supposed to be a flat for perfectly correct threads .
So this afternoon I tried threading with a small flat on my tool , I didn't measure it or try for the perfect size flat for the 7/8-14 thread I was making .
I did no more the touch the tool to the grinder .. if I had to guess the flat was about .010

The very first thing I noticed was I had much much much better chip control .
The next thing I learned that really surprised me was instead of the usual 5-6 spring cuts I normally took I only needed two .
And visually the threads look better at the bottom with a flat instead of the sharp groove .
I've been converted ... no more sharp tip threading tools for me
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Harold_V
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Re: Sharp V threads

Post by Harold_V »

stephenc wrote:The very first thing I noticed was I had much much much better chip control .
The next thing I learned that really surprised me was instead of the usual 5-6 spring cuts I normally took I only needed two .
And visually the threads look better at the bottom with a flat instead of the sharp groove .
I've been converted ... no more sharp tip threading tools for me
Yep!
In regards to the use of a sharp point, if you'd examine the tip, you'd likely discover it well rounded (tip failure). That's a recipe for high tool pressure and skipping (intermittent) cuts. Yet another reason to not create sharp threads when not necessary (pipe threads are an exception). By providing the flat, you gain a serious advantage.

When I'm creating an A thread, I generally start with the major diameter on size (nominal). Once the thread is complete, I like to go over it with some abrasive cloth. The (reasonably) sharp crest is quick to deburr, and is readily reduced in size, which is a requirement for class two threads. Unless one goes crazy, even a class three will still be well within tolerance.

Harold
Wise people talk because they have something to say. Fools talk because they have to say something.
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