In article ,
Rob McDonald wrote:
I hope somebody has the patience to read through this :-) I am trying to
include all the details because I don't know what I am doing wrong.
I recently tried to cut a 3/8-40 thread on a piece of 1018 HRS. First
pass was smooth. With later passes the surface seemed to get rough,
1018 may not be the best choice -- and depending on where you
got it, it could be *very* bad. Home Depot "steel" is rather poor for
machining in any case. Very gummy, which could contribute to your
problems. And if it is of poor consistency, tearouts are more likely.
(See also the cutting oil comments later.)
then before reaching final depth small sections of the thread tore out...
so I tried again:
I don't see you mentioning *any* lubricant while cutting the
threads. I use lubricant even when threading 12L14 (about the nicest
threading steel that I have worked). I usually use one of three
lubricants, depending on the degree of difficult expected:
1) The high-sulfur pipe threading oil sold by almost everyone,
including Home Depot, packaged by Rigid (among others).
2) Molly-Dee (light oil with molybdenum disulfide).
3) Sul-Flo -- *very* high sulfur cutting oil -- best applied with
an acid brush. It has so much flowers of sulfur in it that it
looks more like mustard. It stinks when cutting, but you
produce beautiful threads.
Note that I usually use carbide insert lay-down threading tools,
typically TiN coated for the larger sizes.
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.
smooth. It is mounted directly in a turret type tool post, (i.e. the tool
bit is horizontal, not sloped upward as in a rocker type tool holder).
O.K.
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?
I set up the threading tool on center height and perpendicular to the
surface.
Good.
I set up the QC gearbox for 40 TPI, spindle for slowest speed (about 40
RPM), compound slide at 30 deg.
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.
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)?
First pass was just barely grazing the surface to ensure that there were
no surprises (i.e. mistakes in my setup), zeroed the collars.
O.K.
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.
I measured the OD occasionally. It increased by 0.001" after about three
passes, i.e. it appears that a "burr" is being raised - is this
normal? I didn't expect it. I touched up the OD with a file before
proceeding.
I touch up the OD with a file after *finishing* the thread. No
point until then.
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:
================================================== ====================
izalco:dnichols 23:27 threads -i 40
For a thread of: 40.0000 TPI
the pitch is: 0.0250"/thread
| Thread Style
Format | Sharp-V | Trunc | Formed
================================================== ===================
Single depth: | 0.0217" | 0.0189" | 0.0135"
Double depth: | 0.0433" | 0.0379" | 0.0271"
29.5 deg. angle feed: | 0.0249" | 0.0218" | 0.0155"
Note: Sharp-V is top and bottom sharp. (Not often the best choice.)
"Trunc" is top truncated only.
"Formed" is both top and bottom truncated or rounded.
Select whichever suits your needs and available tooling.
================================================== ====================
The pitch calculation is for convenience in using a small CNC lathe
which I have. "Single depth" vs "double depth" is because some lathes
have handwheels calculated in diameter removed, instead of radius. My
program calculates for 29.5 degrees, not 30 degrees, as that is the
angle which I use. You seem to be getting shallower cuts than my
program suggests for a pointed tool and truncated crests.
However, one other thing comes to mind -- how tight are the gibs
on your lathe? If they are too lose, 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.
Lock the carriage, and press sideways on the toolpost. See how
much it rocks. If it is any preceptable amount, you need to tighten the
gibs to prevent this. While it is possible to set the point of the tool
over the center of the ways, to minimize the sideways thrust, as you
crank the compound, you will exit that sweet spot.
The forces can also cause the tip of the tool to dip, placing it
below the centerline of the lathe.
Finally on one pass I got a rather rough appearance to the thread at the
right hand end, although the rest of the thread still appears to be
smooth.
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.
I would support the end with a live center, if it allowed me to
get in close enough to do the threading -- otherwise, with a hardened
half-center and proper lubrication. (The half-center is ground on one
side, so the tool can get closer to the center of the end of the
workpiece.
An alternative is a follower rest. It provides brass wear
points behind and above the workpiece, and travels with the carriage, so
it supports directly behind the cut. The positions of the wear points
counters the two most likely directions of deflection. It is
absolutely necessary when threading a long shaft, as even a tailstock
center won't support it in the middle.
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.
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. If
this is the case, it *might* be possible to make new jaws with tighter
way grooves, but it would be quite difficult -- especially for someone
who is at your current stage of machining indicated by your question.
So -- a replacement (new) chuck would probably be the best bet. Bison
seems to make high quality chucks for reasonable prices. (They are from
Poland.) I have a nice 6-1/4" one on my 12x24" Clausing. I suggest
getting one with two-piece jaws, so you can replace the top jaws if they
ever become worn, or so you can fit "soft" jaws and bore to hold a
special project. I don't need that too often, but it is really nice to
have when I do need it.
In case it's relevant I am doing this on a 10" Atlas lathe.
Another set of thoughts?
1) What kind of spindle bearings? Earlier ones have bronze
bearings which are adjusted by removing shims and tightening,
and then reaming to the proper size for the spindle.
Newer ones have ball (or roller) bearings, and until they die,
all that is needed is to keep them well lubricated.
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).
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).
Larger closer to the chuck is more likely to be bed wear.
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.
Use a proper threading lubricant. (*Not* motor oil -- one of
the ones which I mentioned above will do -- though Sul-Flo is quite
difficult to get in small quantities. A club member who *really* wanted
some bought a minimum quantity, and then sold it by the gallon to other
club members to share the costs. :-)
2) Any ideas why I ge 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.
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.
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