Cutting Small Threads- Single Point

[90LX_Notch]

After I posted that I had successfully cut an 0-80 thread, Dave (Steamer) asked me to list my method.

I cut it as if it was any other thread with one exception, lots of spring cuts. The other big consideration was a very sharp tool.

The brass rod was held in Dremel collet in a holder I had made. (My three jaw chuck is bell mouthed which would have caused the rod to wiggle and not thread properly.)

Set the compound at 29.5 degrees.

Set the tool off of the chuck face. photo 1

Set the tool on center. photo 2

Zeroed the cross slide to the part.

In order to know when to stop feeding the compound I determined what my final cut depth will be as follows: I subtracted the minor dia. from the major dia. and halved it. I

then set an indicator and fed the compound until that number was reached on the indicator. That showed where to feed to on the dial of the compound.

To get the thread to run out at the end, I set a feeler gage on the bed. When the carriage hits the feeler gage it begins to bow. (Photo 3) At this point I simultaneously

disengage the half nuts and back the cross slide out. (YouTube video) The trick to making this method work is to set some type of reference pointer to cross slide. Photo 4

shows a piece of wire pointing to 0 on the protractor scale of the compound. This is safety measure to insure that the cross slide is set to the original zero and not a

revolution off. To get the length right, I used an indicator and ran the carriage the required length. At that point I set the feeler gage. Then I did a couple of dry runs

and made adjustments until I disengaged at the required length. This has proved to work remarkably well. The required length for this part was 1/8 on one end and 5/16 on the

other. I came in at .124 and .310 respectively.

After that I just cut it like any other thread except for added spring cuts. The spring cuts- after a few thousandths of feeding on the compound I would keeping making passes

at that setting until the tool didn't cut anymore.

I hope this makes sense.

[ame]http://www.youtube.com/watch?v=m64EPNxTQkw[/ame]

 

[90LX_Notch]

I goofed with the pics here they are.

 

[90LX_Notch]

Here's a video of it being cut. Notice the rod deflect when the tool starts.

[ame]http://www.youtube.com/watch?v=QwHSZBZnt3s[/ame]

Some pictures of the finished thread. I just can't get a clear picture of it.

 

[Herbiev]

Great thread. Definately deserves a karma point :bow:

 

[Ned Ludd]

The use of a feeler gauge is inspired, well done that man. :bow:
Now, when do we see you single point the nuts? :big:
Ned

 

[90LX_Notch]

Thank you Herbie.

Ned, you're too funny!

 

[steamer]

Ah yes very clear! I can commend you on your job well done!


With my lathe ( a 12" logan) for real fine pitches, I dispense with the compound feed all together, and I especially do NOT disengage the half nut. I can cut to a shoulder because I thread in back gear, and I run three phase, so instant reversal is pretty easy..


I don't disengage the half nuts as I have found that they may not repeat with these real fine threads...when the pitch is .008 or so...a .001 is a lot!

Of course...its a big lathe to do this with....

Well Done!

Dave

 

[90LX_Notch]

Dave,

Good point about the inaccuracy of the half nuts. At this point, I can't tell how accurate the thread really is. To the naked eye it looks very good. Hopefully, in another month I'll be working again for the summer and for shi*s and giggles I want to throw it up on the comparator.

-Bob

 

[GRAYHIL]

Why not support the thread with a centre? as per rough drawing.

The r/h diam can be left any size to accept the centre for both turning the profile and thread cutting.
Graham

 

[90LX_Notch]

Graham,

I had considered that. The problem is the size of the thread. The rod is prone to flexing. The rod is .062 dia. bar stock; the minor dia. for 0-80 is .0447. I don't have a center drill that would have been able to make an accurate center on the end of the bar. I think what is really needed is a follow rest which is on my "to do list". But, that might have rolled the crown of the thread as it was being cut.

Bob

 

[steamer]

Well, to Graham's point , you could use a FEMALE support with a straight bore that just clears the OD of the thread diameter. That could be a small round piece of steel with a nice, well centered clean hole in it.

It would fit over the thread blank during turning, and could be moved out of the way by retracting the tailstock quil.....just a thought

Isn't a lathe a wonderful device!?

Dave

 

[90LX_Notch]

Dave,

Now that's a good idea. If I ever have to cut a small thread again I will try that.

-Bob


This was one of those things where I didn't have a die and just decided to give it a shot fully expecting failure. My lathe is a 200.00 clapped out Atlas/Craftsman 618. I was shocked when it worked and the thread form looked good. The pictures don't do it justice. So at that point I just decided to go with it and make the valve rod for Poppin. I would love to hear what George's approach would be.

 

[GRAYHIL]

Steamer
Yes an internal centre and u/cut to inside dia of nut is a much better idea.
Graham