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KWGeorge

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#1
Hello all those with more experience threading on a lathe than I (probably everyone!) So the one thing I never took the time to learn or got around to was threading. Well now I have a project that needs to be threaded on the lathe so I thought I would try and learn finally. I ran my first real cutting test yesterday and the results are functional. It screws in well and is not a sloppy fit however it is not right at least I don’t think so. I was hoping you more experienced guys could help me diagnose the issues.

In the attached picture you see a 5/8” 11tpi end results and what you also see is the left half of the thread matches the angle of the cutting bit while the right side not so much. I had the compound set at 30 degrees validated with an external protractor and the cutting tool at 90 degrees to the work. So, was my feed to aggressive? Did I not have the right angle on my compound rest?

Thanks much!
 

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Latinrascalrg1

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#3
Set the compound to 29°. When you advance into the cut do so by using the compound so that its only cutting on the front (left side) edge to the tip but not the rear (right side) edge. What does the cutting bit look like at the tip, Is it a 60° triangular cutting edge?
 

Dave Paine

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#4
Been there, done that.

The scale on my lathe only goes to 55 deg, hence I foolishly assumed the 29.5 deg I could see on the scale was the one to use. Wrong. For some reason Grizzly decided to end the scale at 50 deg instead of 60 deg.

The 29.5 deg mentioned in other replies is an angle from the face of the chuck. My scale on the compound is with respect to the bed ways.

Since I have no scale to read and I wanted a consistent method to set the compound, I just cut a block of wood to 29.5 deg on my table saw, which has a decent mitre gauge. I epoxied a rare earth magnet in the face.

I then wrote the angle on the wood to remind me this was a jig and not a piece of scrap.

I now just attach the wood to the compound then swivel it so the block is flush with the chuck face.

Angle_block_to_set_threading_angle_7616.jpg
 
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KWGeorge

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#5
Well I think you guys fixed my issue and educated me at the same time! I really appreciate that and all the very quick responses!

Stupid me thought that I just needed to set the compound to the 30 degree mark on the cross slide as I show in the picture. I really did not understand the responses about the difference 1 degree (or a half of a degree) would make that kind of difference until it was mentioned that the setting was relative to the chuck! Dave Paine picture hit me strait in the head and I chuckle at myself. Oh well I am old but still learning.

IMG_4520.jpg

In the second picture I used a face plate to reference against and as you can see I was WAY off originally but you already knew that :) My markings only go to 45 degrees so I think I will make opposing punch marks for future reference.

IMG_4522.jpg

Thank you guys so much for helping me.
 

Rooster

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#6
Greeting's KW, after looking at your first pic i think you invented a new thread form. ( LOL )
 

ttabbal

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#7
I'm a heretic and don't use the compound for threading. 90 degree tool angle, ground with the 60 degree angle to match the thread form. Feed with the cross feed. I also go upside down and in reverse to thread away from the chuck. Works for me on a PM1127. Only downside I can see is higher tool forces. Made cutting really small parts harder. I'll have to try the compound for that next time I set up a small part. That was a 5-40, which I don't see myself making often.

Now that I have said that, I'll wind up having to make a pile of them.... :)
 

epanzella

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#8
I too thread with the xfeed. That said your compound needs to be 30deg (or 29.5) from perpendicular to the part, not off the bed centerline. There have been a lot of people on here making threads like that by following the graduations on their compound. There will be more!
 

KWGeorge

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#9
One more time, thanks fellas :)

My second try using the advice given here on some 5/8" Cold Rolled.

IMG_4530.jpg
 

Bob Korves

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#10
It appears you have addressed most of the problems. Another important thing is to make sure that the tool is set up for making a symmetrical thread of the correct angle, with the center line of the "V" 90 degrees to the work. The commonly used tool for doing that is called a fish tail.

Here is an old thread that shows how to use it properly:
https://www.hobby-machinist.com/threads/the-fish-tail-60-degree-thread-gauge.8048/
 
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Dave Paine

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#11
Second time looks very good.
 

P. Waller

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#12
For small lead threads say < .125" I never use the compound just plunge straight in with the cross slide, this has worked very well for the last 30+ years working in a machine shop.

CNC lathes may be programmed with an approach angle that behaves like a compound, I often do not set this value so the tool moves straight in like a cross slide, have never noticed a difference in threading performance at small leads.
 
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