[4]

Thread cutting help needed

[3]
[10] Like what you see?
Click here to donate to this forum and upgrade your account!

Investigator

Active Member
Registered
Joined
Jun 8, 2016
Messages
248
Likes
117
#1
I need help threading on the lathe. Just for info, I have a Logan 12" lathe, it is a turret lathe but it has a threading dial. It has a dual tool post cross slide, and the previous owner had added what appears to be a compound from a 10" Logan. So I have an AXA QCTP on the compound. The issue to keep in mind is I have no accurate way to set the compound angle. So, I'm threading with the cross slide.

But that isn't the issue, that was just background.

The question is this.... I need to thread 2 mating parts 1"-20 UNEF, one set of interior and one set of exterior threads. This is a 2 part muzzle device with a taper for mounting. The drawing I have calls out different diameters for the threads than what is in my Machinery Handbook. I think the drawing was 'simplified' by the person I got it from,and I'm not sure what dimensions to use.

So after rambling, what are my major and minor diameters for both the interior and exterior threads?
 

Attachments

Asm109

Active Member
Registered
Joined
Aug 29, 2017
Messages
196
Likes
217
#3
You don't have to set the compound to an exact angle to use it to cut threads.

If you have the compound set at 0 degrees ( same direction as cross slide) you will cut equal amounts of material on each side of the cutter.
As you swing the compound to the right, you will cut more on the left side of the cutter and less on the right side.
At 29 degrees you are just taking a skim cut on the right. At 30 degrees you are cutting nothing on the right.
Go over 30 degrees and you get total failure and you no longer cut a proper shaped thread.
Take a crude protractor and swing the compound over 25 or so degrees and you get a lot of the benefit of using the compound without risking failure.
 

JimDawson

Global Moderator
Staff member
H-M Platinum Supporter ($50)
Joined
Feb 8, 2014
Messages
7,138
Likes
5,385
#4
Nominal diemensions The minor diameter of the OD threads is 0.9387, the major diameter is 1.000 For the internal thread the ID is 0.9387 and the major diameter is 1.000

Now having said that, since these are mating parts, make the internal thread part first, then hand fit the external threaded part. The actual OD of the external thread part will be about 0.995 or maybe a bit less after you get done fitting, and the minor diameter might also be a bit less also.
 

benmychree

John York
H-M Supporter - Gold Member ($25)
Joined
Jun 7, 2013
Messages
2,427
Likes
1,820
#5
I think that it is lots easier to see when the external thread comes to a point "almost a point", then use it to fit the internal thread, where it is not so easy to see the (almost) sharp point. Generally, I would make a plug gage to the proper pitch diameter, using thread wires or a thread mike, then thread the ID, then the OD using the ID part as the gage; If I did not want to make the plug gage and did not plan to use a plug gage in the future, I'd likely go with my first stated mode of threading the OD to a sharp point first, and using that to gage the ID part.
There is more than one way to skin a cat ----
 

jocat54

Active User
H-M Supporter - Gold Member ($25)
Joined
Dec 25, 2011
Messages
513
Likes
300
#6
I use a piece of wood cut at 29 1/2*(my miter saw is very accurate) and set it to the face of the chuck and the angle cut set to compound--fast and easy.
Some one on here gets the credit for it. Don't remember who it was though.
 

Investigator

Active Member
Registered
Joined
Jun 8, 2016
Messages
248
Likes
117
#7
What part of the compound do you use as a gauge surface? Got a pic of it?
 

BtoVin83

Active Member
Registered
Joined
Aug 30, 2017
Messages
161
Likes
162
#8
Anything parallel to the to the travel
 

seasicksteve

Active Member
Registered
Joined
Oct 29, 2014
Messages
146
Likes
93
#11
That set up is a bit cumbersome but will produce the desired result. If you were so inclined you could make a triangle that had a 30* angle the base of the triangle would be placed against the flat surface of the chuck the altitude would run parallel to the bed ways and the hypotenuse would be used against the side of the compound to set the angle for threading. The 30* angle is arbitrary you can make it whatever angle you prefer for threading
 

P. Waller

H-M Supporter - Sustaining Member
H-M Platinum Supporter ($50)
Joined
Mar 10, 2018
Messages
570
Likes
373
#13
It is a small lead thread, ignore the compound as it will not improve the operation.
Feed from the cross slide straight in and thread away, infeeding at an angle IS NOT REQUIRED with such threads regardless of how many y-tube video stars tell you that it is the PROPER WAY.
 
[6]
[5] [7]
Top