Dieing a long thread AKA What happened?

[Chazz]

I'm doing the cross slide upgrade on my 9 x 20 Lathe from the stock 6mm to 7/16" x 20TPI. I got a 7/16" x 20 TPI LH Die and the screw has a taper in it? The starting end is the right size but it tapers to the point that the new 'nut' seizes up about 3/4 of the way down the screw. (BTW The diameter of the rod was turned consitent)

The die is adjustable and I re-chased the threads about 5 times and the screw now appears fine, but, having only tapped\died(Sp?) 1~2 inch screws\holes up to now, I've never encountered this phenomenon.

So what should a person be looking out for whilst doing longer screw threads?

Thanks,
Chazz

 

[spuddevans]

Could it be that when using a die on a long thread that the swarf gets choked up a bit in the dia, forcing it apart very slightly and therefore not cutting to full depth for the whole length? ???


Tim

 

[wheeltapper]

Hi
Are you sure the thread is tapering?
the thing about cutting a long thread with a die is, the thread tends to 'creep' and over a distance the pitch can go out.
it doesn't notice with a narrow nut but if the nut is any length it will bind somewhere along the length.

what should be done is to screw cut the thread first to get a constant pitch and tidy it up with a die afterwards.

Roy

 

[Ken I]

Agree with Wheeltapper - screw cut first - you may need to use a two point (traveling) steady to do this. It does not need to be to full depth as the die will follow it when finishing.

If you measure the thread you have cut you will find it out of pitch over a distance (try a number of sample distances). Recutting will not fix it.

Alternatively buy a decent piece of rolled thread (high tensile - unplated - is generally better quality) threaded rod as your starting point stock.

Flank finish (on good stuff) is generally far better and longer wearing than anything you can produce with a die and its generally not terribly expensive.

Ken

 

[Chazz]

Thanks Guys, I would have liked to start with cutting in the lathe but I need the cross slide first. :-\ (I modified the bed for the 'new' nut and the 'old' nut will no longer fit).

The pitch seems ok as I made a Go Nut with the same tap I used to make the cross slide nut and it actually ran up and down the screw like a wing nut.

Because I did it in one go (as opposed to an inch at a time and let things cool down) I was thinking maybe the rod was expanding more and more as I kept cutting the threads wit out stopping?

I don't think swarf was an issue as the threads are actually quite clean, I actually had to take a small needle file to the tops as they were quite sharp.

Thanks,
Chazz

 

[wheeltapper]

You say you made a 'go' nut.
was it the same length as your leadscrew nut?

you won't notice if the pitch is off with an ordinary thickness nut, but once the nut is any length there will be a mismatch in pitches.

Roy

 

[MachineTom]

I'll guess that you don't have a set of thread measuring wires. Here is something that can get you close. Take a #60 size drill bit, lay the shank end in the thread and measure with a known mic, no calipers allowed, I get .4572 on a known good thread. Measure along the length in several places, see the variance in size. Make sure there is not any Fuzz on the threads to throw off the diameter.

When using an adjustable die, the die holder screw should be 90° to the die adjusting screw.

 

[Chazz]

MachineTom - "When using an adjustable die, the die holder screw should be 90° to the die adjusting screw." You see, I do learn somthing new every time I come here!

And no, I do not have thread wires (yet) nor have I ever used them. But I know what is on my list for next payday swag order. ;D As I said being a rookie to machining, all my tapping and dieing has been by hand on 1" ~ 2" screws, nuts etc. NEVER anything this big, hence the head scratching.

TNX
Chazz

 

[tel]

If you were cutting a pretty tight thread to start with, I would suspect that the die got warm enough for a little expansion to occur, now, contrary to what a lot of people think, when you heat a bit of metal with a hole in it, not only does the metal expand, but the 'ole gets bigger as well - thus providing for shrink fits.

 

[Lew Hartswick]

If you were cutting a pretty tight thread to start with, I would suspect that the die got warm enough for a little expansion to occur, now, contrary to what a lot of people think, when you heat a bit of metal with a hole in it, not only does the metal expand, but the 'ole gets bigger as well - thus providing for shrink fits.

I would guess youre right on there. A 7/16 thread is removing quite a bit of metal and
is going to generate a fair amount of heat over a long threading op.
On the other hand I've done single point threading and had similar effect. :-(
...lew...

 

[peatoluser]

it could be that when cutting a thread with a split die all the cutting forces are transmitted to the (usually) 3 adjusting screws on the die stock. As these are often small grub screws (cup end or point) and are softer than the die, they can flatten out a bit allowing the forces to open out the die.
Check out the state of adjusting screws. I,ve never used a die to cut a long length of bar , but when i,ve used one to cut longer threads on a number of bolts i invariably have to tighten the screws to maintain correct thread diameter.

 

[Chazz]

Thanks Tel & Peatoluser et al, I think a bit of both were the culprits + my inexperience. Once my cross slide is up and running I will use the lathe to 'start' longer threads as suggested.


Thanks again for the insight.

Regards,
Chazz