Triple start threading

[duke851]

I would like to learn how to machine cut the outside threads and use a tap for the inside. I was going to purchase the taps from Silver Pen Parts but don't have a clear understanding of how the sizes work. one tap they have that I would want is given as 14 x .8 x 3. Does this mean it has three .8 threads .267 apart Or three .24 threads .8 apart?

Can anyone shed some light on this?

Thank you in advance

Colin

 

[mredburn]

The first is 14mm diameter the second is .8mm pitch (the distance between tip to tip) and the last is for 3 threads formed at once. One full turn of the threads will move you 2.4mm (3x.8mm)

If thats not clear think of a 3 lane highway spiraling down. The 3 threads are cut parallel on a spiral as the tap/ die cuts them.

 

[mredburn]

If you do a search Skiprat posted a thread on cutting triple lead threads on a lathe. He had a lot of pictures and info. It may be in the Library

 

[mredburn]

Yup it was in the library
http://content.penturners.org/library/techniques/skiprats_triple_start_threading.pdf

 

[mredburn]

Shown is a sample of the triple lead. The green stripe would be one of the three formed threads as it spirals down the tenon. Hope this helps I will probably put something like this as an explanation on the website.

 

[duke851]

Mike thank you for your quick replies.

I did read Skiprat's article and found it very helpful. I am obviously not grasping something though.

Correct me if my understanding is wrong.

In a normal single start thread say M10 x .75 the thread peaks are every .75 mm and 1 turn would move the fastener .75mm.

So to cut the thread I set the Lathe to .75

In the triple start M14 x .8 x 3 (This is where my confusion comes in) the pitch is given .8 but one turn moves the fastener 2.4mm?

So would the lathe be set to 2.4 to cut the three threads (hence peak to peak .8mm which is the value given as the pitch not what the individual thread is cut as?)

In Skiprats article he cut a 12mm diameter rod and sets the lathe to cut a 3mm pitch thread and cuts three(every 1mm). So would this be called a M12 x 1 x 3 and not a M12 x 3 x 3?

If so that makes sense and my problem is a misunderstanding of what the given pitch means.

If I have this correct the pitch is the distance between threads regardless of the number of starts. So what you set the lathe to cut is not what they give as the pitch but is the given pitch X number of starts!

Oh snap, my lathe doesn't give a 2.4!

Thanks again for taking the time to help

Colin

 

[mredburn]

Yes. You have it. The pitch in the metric system is the distance from one peak to another whether at the bottom or at the top. In the imperial (inch) system the pitch is the number of threads formed per inch.

 

[John Den]

The pitch of a thread is defined as the distance between peaks (incidentally equal to 1 divided by the turns per inch TPI).
In single start thread, as you say, the pitch also indicates how far a nut progresses with a single turn.
Things become a bit more complicated when multi-start threads are considered. The pitch is still equal to the distance between peaks. But they are also defined as having a lead. This is defined as the number of threads times the pitch and therefore for a triple start 0.75mm pitch (at whatever diameter) will have a lead of 3x0.75mm ie 2.25mm.
For 1 turn, the nut now moves 2.25mm.
To cut a triple start you will therefore have to set the lathe up to cut a thread with a pitch of 2.25mm.
BUT you will only have to cut each thread depth to that of a 0.75mm depth. so that they will not cut into the thread that sits alongside.
The starts of the 3 threads will start at positions that are a third of 360 degrees ie 0 degrees, 120 degrees, 270and 360 degrees
Hope this helps,
John

 

[duke851]

Thank you Mike and John, It was my understanding of how the pitch is defined in the multi start threads that was throwing me. You have explained it perfectly and I think I have a grasp of the fundamentals. Now I just need to practice.

Colin

 

[John Den]

Best of Luck mate!
Any problems maybe I can help
Regards,
John

 

[airborne_r6]

I have to complement you guys. You did a good enough job explaining this that I completely understand it and understand how to set it up on the lathe.

Now if I just had a metal lathe to go try it on.

 

[Paul in OKC]

The starts of the 3 threads will start at positions that are a third of 360 degrees ie 0 degrees, 120 degrees, 270and 360 degrees
Hope this helps,
John

Only correction I can give is it would be 0, 120, 240, not 270 on the spacing. (Although it make a unique thread!)
Easier if making your own thread as you can choose a thread evenly divided by three, so when you set the compound you can advance an even number for each thread, and not have to 'split hairs'!

 

[John Den]

OOPs and My unconventional method of cutting Mutistart Threads

Only correction I can give is it would be 0, 120, 240, not 270 on the spacing. (Although it make a unique thread!)

I'm a bit under the weather at the moment with an infection of some kind!!!!
Should read:-
"The starts of the 3 threads will start at positions that are a third of 360 degrees apart, ie 0 degrees 120 degrees and 240 degrees"

I realized it was wrong after a couple of days but, by then, the ability to edit my post had past!

Easier if making your own thread as you can choose a thread evenly divided by three, so when you set the compound you can advance an even number for each thread, and not have to 'split hairs'!

As far as advancing the compound is concerned I have discovered this method works on a nice tight decent lathe but on my ole, clapped out, Sieg Chinese lathe (Never really great in the first place) this resulted in 3 threads that were not really starting 120 degrees apart and not consistent in shape either. This resulted in male and female threads that didn't quite like starting in some orientations. As I used the taps, I had produced, to make their corresponding Dies, errors just unacceptably increased.


Thoughts around this problem showed that plus and minus ½ thou" of horizontal, error due to sloppy slides etc., resulted in a 4 degree error in starting the individual threads. So I looked for a way of reducing this and this is the way I do it now that gives me consistent results!


I tie everything down. I tighten up the end Gib screw on the compound slide and don't touch it till I've finished all three threads

I get much better results by using a collet chuck which I use with all its fixing screws present for triple threads:-

1. Lock the compound side solid with its end Gib screw.
2. Set the cut depth with the cross slide. (5 thou first cuts to 15 thou then 2 then 1 for the three final cuts)
3. Lock the cross slide with its last gib screw.
4. Cut the thread
5. Unlock the cross slide gib screw and withdraw the tool.
6. Reverse the motor and go back to the start.
7. Repeat 2 to 6 above
8. Cut to the correct depth of thread.
9. Repeat the last cut 5 times to allow for spring of the tool.
10. Undo the fixing nuts and remove collet chuck with the job still tight in the collet.
11. Move the chuck around to the next position of the 3 holes.
12. Repeat 2 to 7 for the next thread
13. Repeat 8 and 9 for the 3rd thread.

Cutting 2 start and 4 start threads becomes a bit more difficult but after some thought I arrived at this method that, maybe surprisingly, does work:-

1. Remove 2 of the 3 fixing studs from the collet chuck and set them aside.
3. Loctite the remaining stud into the collet chuck.
2. Using the collet chuck's one remaining stud insert it into one of the 4 fixing holes in the headstock back chuck mounting plate. Rotating the chuck as far as it will go clockwise to take up any slack in position before tightening its fixing nut.
3. With a Sharpy mark this as number one and the others likewise to 4
4. Proceed to cut the multi threads 2 or 4 as 1 to 13 above using every other hole for a 2 start and all holes for a four start.


I use a flat topped HS tool ground to a front angle of 60 degrees with clearance angles of about 8 degrees. (i.e No top rake or side rake)
With small cuts, as suggested above, the resulting threads are very good with minimal jamming, judder, although, with Drill Rod(For taps), a small amount of bur removal, with a Swiss file, on the crests of the threads is needed.

Regards,
John