Thread cutting….what went wrong…?

[Paul Relf-DaviesParticipant @paulrelf-davies37806]

[Paul Relf-DaviesParticipant @paulrelf-davies37806]

Good morning.
I had a little problem thread cutting yesterday. I wanted to cut an M5x0.8 thread on a piece of steel rod, actually a turned down M10 bolt.

I turned the bolt down to 5.5mm diameter and cut the thread with a tailstock die holder and generous use of cutting fluid.

On inspection, the bolt had about 3 good threads up close to the shoulder on the bolt ( the last threads cut) and the rest of the piece was completely smooth…?

It appears that the die cut the threads, then cut them off.

I was obviously doing something wrong here!

Any ideas?

This is the 1st time I’ve cut threads on the lathe (with the die holder) by manually turning the lathe. Previously, I be used power at the slowest pully ratio and in back gears, with no problem at all.

Cheers

Paul

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Paul Davies on 10/04/2016 10:54:39:
Good morning.
I had a little problem thread cutting yesterday. I wanted to cut an M5x0.8 thread on a piece of steel rod, actually a turned down M10 bolt.

I turned the bolt down to 5.5mm diameter

.

Paul,

The first question must be : Why 5.5mm ?

MichaelG.

[Anonymous]

The second question is: where did the die come from and what does it look like, ie, a picture.

Andrew

[Neil WyattModerator @neilwyatt]

Posted by Paul Davies on 10/04/2016 10:54:39:

I turned the bolt down to 5.5mm diameter and cut the thread with a tailstock die holder and generous use of cutting fluid.

The work should be slightly under size, to allow for metal extruding as the die forms the thread.

If that isn't a typo and it was a high-tensile bolt, I'm sort of surprised the die has any teeth left on it!

Neil

[Ian S CParticipant @iansc]

Another question, what grade bolt was it?

To make a 5 mm thread, you use a 5 mm rod, you can go a few thou under size, but why the heck would you go over size?

Ian S C

[Russell EberhardtParticipant @russelleberhardt48058]

With an oversize starting diameter the cutting force is too great for the first few threads to pull the die along properly.

Russell

[Paul Relf-DaviesParticipant @paulrelf-davies37806]

Oh dear.. I think a moment of dawning illumination has happened….!!

When tapping a female thread, one drills the hole under-size…Since I was doing the opposite I turned the rod a little oversize, when (of course!!) I should have turned it to to the final diameter….D'oh

Shows what can happen when you are in a bit of a hurry!

On the plus side….I won't make the same mistake again!

TY All!

P.

[Steven VineParticipant @stevenvine79904]

Been there. As you say, lesson learnt.

I've even turned a rod down to tapping size, and confused the hell out of myself.

The mind does wander.

Steve

[Ed DuffnerParticipant @edduffner79357]

Hi all,

I hope Paul doesn't mind me asking some related questions.

When cutting single-point internal and external threads on a lathe for parts that will be screwed together, how much tolerance or clearance should there be between the two threads?

Should material be removed from both threads or from just one of them to stop them binding and stay on spec?

Is the thread 'gap' predetermined by the bore of the internal thread and the minor diameter of the external thread?

Ed.

[John HaineParticipant @johnhaine32865]

Ed, there are two ways to answer this.

The first is, go and look at the standards that define the tolerances, if you want to be purist.

More practically, it's easier to make an external thread and adjust it to size taking tiny cuts off, so making your own a good approach is to make the internal thread first as close to theoretical size/form as you can, then make the external thread and shave off tiny amounts to just fit the internal thread you just made.

It also depends on the purpose of the thread. For fasteners, given that you have a range of acceptable tapping drills for a given thread giving different engagements, you can afford to take a bit more off in the interests of an easy life.

[Ed DuffnerParticipant @edduffner79357]

Thank you John, that makes sense!

Ed.

[MWParticipant @mw27036]

It's okay, i once turned a diameter down to 24.5 as opposed to 25.4

Michael W

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