# Internal tool for external threads?



## 4ssss (Oct 4, 2018)

It would, but the tool would probably flex


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## dpb (Oct 4, 2018)

Would it flex more than when cutting internal threads, and if so, why?


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## P. Waller (Oct 4, 2018)

dpb said:


> Without getting into a debate about the best single point threading methods, my question is this:
> Will using a right hand internal lay down threading tool, with full profile inserts, cut correct external threads, when used on the back of the part, with the lathe running in reverse, feeding left to right?


Yes
There are some differences between the internal/external thread forms but these are marginal, if the parts are being made for a Customer that has the capacity to measure these features then the answer is no.
If the Customer can not or does not measure at that degree of accuracy have at it, the parts are unlikely to return with a red tag and 5 pages of printed Non Compliance data.
Good luck.

I have done this in the past, do not tell anyone (-:


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## T Bredehoft (Oct 4, 2018)

Seems to me, the tool in the picture above was made to cut on it's lower left face.  If cutting as above, it's cutting on it's lower right face. It doesn't have the correct relief on that cutting edge.   But Yeah, it'll work.


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## P. Waller (Oct 4, 2018)

T Bredehoft said:


> Seems to me, the tool in the picture above was made to cut on it's lower left face.  If cutting as above, it's cutting on it's lower right face. It doesn't have the correct relief on that cutting edge.   But Yeah, it'll work.


Not a threading tool in this case just a triangle insert boring bar, it is the only picture that I have of using a boring tool for OD turning on the back side of a part. 

The programming requires all X axis positions to be -0.000 moves in this case, I probably could have set the tool offset on the back side of the part but this often opens another can of worms with these lathes.

Recently I tried to program a gang tool holder in conversational mode, this machine simply would not run this way, had to FingerCam the entire operation line by line. This is cumbersome and time consuming at best.


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## BaronJ (Oct 5, 2018)

dpb said:


> Without getting into a debate about the best single point threading methods, my question is this:
> Will using a right hand internal lay down threading tool, with full profile inserts, cut correct external threads, when used on the back of the part, with the lathe running in reverse, feeding left to right?



Yes !  I do that all the time.


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## mattthemuppet2 (Oct 5, 2018)

the question would be "does the cutting edge have enough relief to clear the flank of the thread". If use, you're golden. If no, the flank of the thread will push the tool out of the cut and you'll get a crappy finish.


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## BaronJ (Oct 5, 2018)

Hi Guys,

That picture above is me cutting a 4-48 thread,



This is a M30 X 1.25 cut in the same way, lathe in reverse and away from the chuck.


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## T Bredehoft (Oct 5, 2018)

I guess it works.  If I were doing it at work, I'd put it on an optical comparitor to be sure.


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## BaronJ (Oct 5, 2018)

Hi Guys,

Actually looking at the first picture from post "4" that insert is somewhat different to the one that I'm using.
I can't find the part/type number for mine at the moment.  When I bought the holder, it was supplied with a box of ten new inserts.  So far I've not had need to use any of them.  About all I can remember is that it was described as a full form 60 degree internal threading insert.  You do have to get it bang on center hight through.


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## P. Waller (Oct 5, 2018)

dpb said:


> Without getting into a debate about the best single point threading methods, my question is this:
> Will using a right hand internal lay down threading tool, with full profile inserts, cut correct external threads, when used on the back of the part, with the lathe running in reverse, feeding left to right?


Do you know what a Full Profile Threading tool is?
It will form the full thread profile as described by the published thread data, one tool for each lead not a simple 60 Deg. included V

A 16 TPI tool will not make a full profile 18 TPI thread. A full profile tool will require a different tool for every thread lead.
Metric threads also employ a a 60 Deg. included angle but have a different root and crest profile.


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## dpb (Oct 5, 2018)

Yes, I know about full profile tools.


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## 9t8z28 (Oct 13, 2018)

Hey that looks like a QCMI Circle Machine carbide boring bar.  Nice!   





P. Waller said:


> Yes
> There are some differences between the internal/external thread forms but these are marginal, if the parts are being made for a Customer that has the capacity to measure these features then the answer is no.
> If the Customer can not or does not measure at that degree of accuracy have at it, the parts are unlikely to return with a red tag and 5 pages of printed Non Compliance data.
> Good luck.
> ...


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## Hit-N-Miss Tom (Oct 15, 2018)

dpb said:


> Without getting into a debate about the best single point threading methods, my question is this:
> Will using a right hand internal lay down threading tool, with full profile inserts, cut correct external threads, when used on the back of the part, with the lathe running in reverse, feeding left to right?





9t8z28 said:


> Hey that looks like a QCMI Circle Machine carbide boring bar.  Nice!


Just a side note; if one has a screw on chuck there may be a danger in running it backwards. Maybe not so much at threading speeds, but if it loosens some threads are history.


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## BaronJ (Oct 15, 2018)

Hi Tom,

Yes I've heard the same stories !  The chuck on my Myford is a screw on one, I've done all sorts of things running in reverse and never had a problem.  I'm not saying it couldn't happen, its never happened to me.


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## dpb (Oct 24, 2018)

Follow up:
The first test of this method was to cut a 1 1/16 x 18tpi thread.  Using .032 wires, I needed a pitch measurement of 1.072”
I used a Carmex 16IR18UN insert.  
The process went very smoothly, cutting at 250 rpm.  Unfortunately, by the time I reached the needed pitch diameter, the major diameter was only 1.050”, which is under spec for either a 2a or 3a class fit.  
This was done on a piece of scrap.  I cut the actual part, using the same method, with a partial profile tool.  Worked great, came out on spec.
Thanks for the replies.


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