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(DoN. Nichols) wrote in
:

In article ,
Rob McDonald wrote:

... I recently tried to cut a 3/8-40 thread ...

1018 may not be the best choice -- and depending on where you
got it, it could be *very* bad.

I got it from a local steel supplier (Elma Steel) it could have come from
anywhere.

I don't see you mentioning *any* lubricant

I used a cutting lubricant I had on and, but I doubt it's as good as any
of the ones you mentioned. It is time to invest in some good cutting and
threading lubricants.

I ground a new 60 degree HSS threading tool with 12 deg. relief on
both sides and 10 degrees side and back rake, and stoned the top and
sides

O.K. Though ideally, there should be more relief on the
left-hand side than on the right-hand side to accommodate the helix
angle of the thread.

Good point, I hadn't thought of allowing for the helix angle.

I turned a 0.500 diameter section on a short bar. It is held in a
three jaw chuck, (no tailstock support).

0.500" for a 3/8-40 thread? Or is this just a test pass with a
larger diameter before going to the final diameter?

Test piece after the initial failure.

Hmm ... there are debates about the angle, but the ideal
according to some is 29-1/2 degrees -- so it does a light cut on the
right-hand flank with each pass, along with the heavy cut on the
left-hand flank.

OK

O.K. Did you turn a runout groove at final thread depth, so you
don't have to wind the cross-slide out very quickly (and at precisely
the same point each time)?

Yes.

I started by feeding in the compound 0.002", reducing progressively to
0.0005" for the last pass.

Hmm ... for 16 TPI, I start out with 0.010" per pass, and reduce
to perhaps 0.002" for the last pass or two. I also make two or three
repeats at the final depth.

Knowing that I had trouble already, and that my Atlas lathe is not very
rigid I wanted to minimize deflection/vibration.

I seem to be reaching the final thread form before I have cut as much
as I thought I needed from the Machinery's handbook, (0.01534" deep,
0.0177" on the compound at 30 degrees).

Hmm ... my threads program suggest the following for 40 TPI:

This value: Formed Single depth: 0.0135" corresponds to the value for an
internal thread in the handbook. I am using the value specifed for an
external thread.

However, one other thing comes to mind -- how tight are the gibs
on your lathe? If they are too loose, the compound or the carriage
will rock to one side under cutting loads, thus possibly tearing out
the thread, even in the presence of adequate lubricant.

The gibs are tight, so I don't think this is the problem (but I'm going
to double check them anyway).

The forces can also cause the tip of the tool to dip, placing it
below the centerline of the lathe.

The turret tool holder is quite rigid, and the tool is clamped very short
to minimize this.

O.K. How long is "short" compared to the diameter of the
workpiece? If there is more than 4 times the diameter sticking out of
the chuck, you will have deflection -- to an amount dependent on the
cutting forces involved.

Nowhere near 4X. Test piece is 1" diameter protruding 1-1/2" from the
chuck, the last 1" is turned down to 1/2" diameter with a groove at the
shoulder - this simulates the configuration I would be machining on the
real part.

I would support the end with a live center,

I'll try that. The fact that the problem is worst at the end of a
cantilevered workpiece makes me think "deflection."

An alternative is a follower rest.

That is on my wish list :-)

Next -- what shape are the jaws in? A common wear pattern on
chuck jaws is to wear at the tips (where short workpieces are clamped)
more than at the back. So -- when you clamp something using the full
length of the jaw faces, that workpiece will be tightly clamped only
at the back, and can deflect somewhat at the front. Check the
workpiece for deflection with force to see whether this is happening.
A solution, if it is *just* the faces of the jaws, is to set them up
under load, and run a toolpost grinder (or even a toolpost mounted
Dremel with a grindstone) in a little at a time, until you get the
full length of the jaw faces cleaned up.

This is a definite possibility. The jaws are in lousy shape and I have
been considering grinding them as you suggest.

However, if the jaws have been tightened too much at the tip, too
often, you will have wear in the ways in the chuck, and in the jaws.

I'll look into this, I don't remember them being loose this way but it is
an old chuck. If so would definitely replace rather than repair.

..I suggest getting one with two-piece jaws,...

Thanks for the suggestion.

In case it's relevant I am doing this on a 10" Atlas lathe.

Another set of thoughts?

1) What kind of spindle bearings?

It is a babbit bearing headstock. I must admit I have had concerns here,
since it is an older lathe which is known to have spent part of it's life
in a low production environment. I lack the skills to confidently
determine it's condition.

2) What is the condition of the bed near the chuck? A worn bed
will allow the whole carriage to rock as it approaches the chuck
(usually the most worn area), and this could be the cause of
your problem -- or at least one cause. In particular, with the
square bed which the Atlas line used, you could also have wear
in the width of the bed, so the carriage can move forward and
back with force near the chuck (the most worn area).

I will look into this.

Do you get taper when turning close to the chuck?

Larger at the free end is likely deflection of the workpiece
(either bending or worn/sprung chuck jaws).

Yes, this is what I get, possibly related to the battered chuck.

1) What can I do to reduce/eliminate the "burring"

File when you are done. This is normal -- especially with soft
gummy steels like you are using. Get some 12L14 to see what heaven is
like. Even with 12L14, I file after cutting to clean up the crests.

That was my first choice, but the local supplier doesn't stock it. I may
pick up an aassortment next time I'm in the city (I'm a 2 hour drive
north of Toronto).

2) Any ideas why I get the rough thread, especially at the right end?

At the very end suggests deflection of the workpiece, though it
could be play in the gibs of the cross-slide or the compound. Or a
combination of them all.

I suspect you are right, possibly several things, none of which are bad
enough alone to be problem.

So -- you now have a checklist of possible causes. Someone else
may well think of something which I forgot to mention, so look at
whatever other followups appear, too.

Good Luck,
DoN.

P.S. Now that I have spent this time typing, I suspect that at
least
*one* other followup will have shown up. There were none when I
started typing.

Thank you very much for taking the time DoN. As you say, I now have a
list of possible causes to investigate. I have been "improving" this
lathe one step at a time as I find problems and learn how to deal with
them. This will be the next step.