In article ,
Rob McDonald wrote:
(DoN. Nichols) wrote in
:
I accidentally mailed this (to a munged address), but I also
mailed it from a system which has no direct access to the outside world
(which I used for newsreading, among other things), so I was able to
recover the article and go back into usenet news to send it properly.
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.
Yep. At least probably better than from Home Depot. :-)
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.
O.K. The easiest to get is the pipe tapping high-sulfur oil from
Home Depot or just about any other hardware store. Molly-Dee I order
from MSC (one day delivery from them, and I seldom get off an order
smaller than $100.00 once I have decided that I'm going to call them. :-)
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.
For a 40 TPI with a 3/8" OD, I come up with only 1.22 degrees,
so your side relief should be sufficient here. With the 1/2", it is
even less.
[ ... ]
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.
O.K. One thing to consider, however, is that too small an
infeed results in the tip burnishing and sometimes work-hardening the
workpiece on one pass, and then digging under after one or more extra
passes.
Unless the depth of the thread is truly critical, I would check
depth of subsequent passes, and try to adjust the feed so you can take a
full depth pass (whatever works for you) for the final pass. Following
it with another pass at the same setting will burnish the finish a
little, if there was no spring to catch up to. (I'm used to my 12x24"
Clausing being rigid enough for pretty much any cut that I am willing to
risk.
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.
That "formed" assumes that your threading tool has a tip rounded
or squared off just right to meet the spec for that thread. Mine
(except for the final column) assumes a sharp point on the threading
tool.
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).
O.K.
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.
Those both help -- though there are the various interfaces
between the carriage and the tool (cross-slide ways, pivot for the
compound, and compound ways) all contributing to give. Have you pulled
the compound out at the pivot and made sure to clean out any chips
before re-installing it? IIRC (and based on my old 6x18
Atlas/Craftsman), the Atlas design has a boss projecting from the
cross-slide, with a dovetail turned on it, and the compound has two
setscrews pushing in bronze or brass angled pushers to contact the
dovetail. The Clausing, by contrast, has a hole in the cross-slide, a
cylindrical plug on the bottom of the compound, and two holes 180
degrees apart going through the flange on the compound and T-bolts in a
circular T-slot surrounding the hole. A much more rigid mount. Since
Clausing and Atlas combined at some point (I hear stories saying that A
bought B, and others saying that B bought A), hopefully the 10" has the
more rigid design.
Also -- what size of tool stock did you use for the threading
tool? I generally go for at least 1/2" on my Clausing (where the
quick-change toolpost and holders will accept up to 5/8" shanks.)
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.
O.K. That sounds pretty good.
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."
And remember -- if the diameter is too small, a live center can
get in the way of the the tool and toolpost, so you may want to pick up
a half-center (actually a bit more than half) to give you more elbow
room. *That*, you will have to use with a lubricant on the center, as
there are no live half centers. :-)
The ideal lubricant for that is a lead compound, which is of
course very difficult to get these days. Older machines often had a
cavity in the tailstock holding some, with a dauber to transfer it to
the center. Lacking that, I would suggest an extra pressure lubricant
for the center -- perhaps a lanolin based case sizing lube from a store
which deals in reloading equipment.
An alternative is a follower rest.
That is on my wish list :-)
Good. Keep your eyes open.
Next -- what shape are the jaws in? A common wear pattern on
[ ... ]
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.
I presume that I don't need to mention being careful to keep the
abrasive grit from reaching the bed of the lathe. Cover with a cloth or
newspaper or aluminum foil. At a suggestion -- the newspaper wet with
oil will help to keep the grit on the paper.
[ ... ]
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.
O.K. The trick for this is:
1) Remove the chuck.
2) set up a dial indicator mounted on the headstock and measuring the
register of the spindle.
3) Take some wood dowel (broomstick is the common suggestion, if
it will fit into the spindle), and push down and pull up, noting
the change in reading of the indicator. Someone who owns a
plain bearing lathe should chip in here -- but I think that the
reading should be on the order of 0.001" deflection when dry
(e.g. after sitting overnight), and much lower when the spindle
was just being run, and thus has a good film of oil.
Lacking a wood dowel, I would consider aluminum or brass rod, so
it is unlikely to damage the spindle.
You can find web based information on pouring a new Babbitt
bearing in place, and whatever scraping may be needed.
[ ... ]
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).
Or -- use one of the on-line metals places. They cost more than
a local place -- but you don't have to get lots more than you happen to
need. Hmm ... I wonder whether MSC has any? Let's check the web site,
since that catalog is too big to use while I've got a keyboard in my
lap.
Nope -- The only 12L14 listed on their page is machinable
expanding collets, so I guess that the online places are your best bet.
Not too many industrial places stock the 12L14.
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.
Which is why I listed all the possibilities that I could think of.
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.
[ ... ]
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.
O.K. Good luck,
DoN.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. |
http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. |
http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---