centering question

I have recently acquired a Sherline lathe, and I am learning how to use
it. I have run into a problem that I sure those of you with more
experience (or practically any experience) can help me solve. I needed
to center a 3/8"-16 threaded hole in a Delrin cylinder.
The first couple of times that I did this, it worked great. I put the
workpiece in a 3 jaw chuck, faced each end, and then used a center drill
in the tailstock to make a small hole. Then I used a 5/16" twist drill
in the tailstock to enlarge the hole, and then used a 3/8"-16 tap. The
first two times that worked well, I was doing this to a cylinder that
was only about 3/4" long.
Then I tried to center a hole in a 3" long cylinder, using the exact
same mechanism--and it didn't work. The hole was way off--about .05"
difference from one side to the other.
Possible sources of difficulty:
1. I know that twist drills are intrinsically less accurate than center
drills. Should I look for a 5/16" center drill to make the pilot hole
for the twist drill? Or should I start with a twist drill the diameter
(or slightly smaller) than the pilot hole of the center drill, and
gradually move up in size?
2. Is it possible that that the 3 jaw chuck can't hold a piece of Delrin
that long without high speed rotation causing it to wiggle slightly
off-center? Is the solution to drill the center drill and/or twist
drill at very low speed?
Clayton E. Cramer
Reply to
Clayton E. Cramer
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You need to isolate your problem. Your problem could be that your lathe isn't aligned correctly, or it could be that the ways are right but that the tailstock needs to be aligned to the headstock. Could also be that your chuck isn't true. Start with this: put a center in the headstock and another in the tailstock, and scoot the tailstock right up so the centers almost touch. Do they look perfectly aligned?
It isn't easy to drill a hole .050" off center in a rotating piece. It's much more likely that 3" out from your headstock, the workpiece was .050" out of center. Really think about it, asking yourself the question how could a piece be centered at the headstock end and .050" out 3" towards the tailstock. Thinking like this is how you really learn about a lathe.
You may find the book "How To Run A Lathe" by the South Bend Lathe Works to be useful. Many, including me, have.
Clayt> I have recently acquired a Sherline lathe, and I am learning how to use
Reply to
Grant Erwin
What diameter is this part? I assume too large to fit through the chuck and spindle center holes. If forced to do this work that far from the chuck, you would want to use a steady rest to support and center the outer end of the workpiece.
You can put the part in the chuck and slowly rotate the chuck by hand while the drill bit is very close to the end of the workpiece. If it .025" out of center, it would be very easy to see this with the naked eye.
Reply to
Jon Elson
According to Clayton E. Cramer :
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What is the diameter of the Delrin cylinder? Perhaps 3/4"? Even with steel, you don't want to support something purely in a chuck which extends as much as four times its diameter. And Delrin is a lot less rigid than steel.
What you need to do to start is to set up a steady rest to support the free end, and keep it from bending. (I know that the Sherlines have steady rests as options -- but I don't know whether you have one.)
Once the Delrin is supported, start with a center drill, and follow up with a drill bit appropriate as a tap drill for your 3/8-16 threaded hole to be.
If it were much larger, I would suggest drilling it undersized, and using a boring bar to enlarge it to the proper size for tapping. I don't know whether you have a boring bar small enough for this, however. But a boring bar can bring a hole back on center even if the drill wanders a bit when starting -- as long as the starting hole is not too far off center for its size.
Good Luck, DoN.
Reply to
DoN. Nichols
For a piece that long on a Sherline, you need to use a steady rest.
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Reply to
Sounds like the chuck. To really center work you need a four jaw chuck with independent jaws and an indicator/dial gauge to center the work. Bugs
Reply to
Three things: After making the center hole, use a smaller drill for the initial pilot hole, say 1/8" and make sure the pilot drill registers on the conical sides of the center drill hole. Use 2 or more other drills, graduated in size, up to the final size.
Run slowly when working plastic. Plastic will soften under the generated heat causing drills to wander in the hole and seek the wrong path.
Use stiff drills such as screw machine drills, esp. for the pilot holes. You will appreciate the extra clearance on your Sherline lathe. Jobber length drills are just too long for Sherline equipment, esp. in the sizes around 1/4" and above. Try this for the 1/8" pilot hole. Shorten the drill by cutting almost all of the shank area off. This will give you a much stiffer drill for the initial pilot hole. Drills can be cut by nicking them on a grinder and breaking at the scored spot in a vice. Use a rag around the drill and strike it with a hammer.
Bob Swinney
Reply to
Robert Swinney
Sounds to me like you didn't center the end of the piece of plastic. The jaw does center that end to about a thou or so but the far end could be almost anywhere. Invest in a nice dial test indicator and stand for it and use it. Used machinery stores will often have such tools for a lot less than what the retail is and will do fine for the home hobbiest. Pros often don't go theere because they can afford the full retail on a new one.
-- Why do penguins walk so far to get to their nesting grounds?
Reply to
Bob May
Not Delrin, Bob. It's not a thermal plastic. It responds very well to high speed machining, and is, indeed, the best way to go, assuming the part isn't too long. My feeling is that's the case in this instance, due in part to the miniature equipment in use.
Reply to
Harold and Susan Vordos
I started with a sherline... I suspect the biggest source of error is that you have a long work piece support by tiny jaws on the sherline 3 jaw chuck. ie you have a 3 inch piece support by 1/4 inch jaws. This will not force the work piece to be straight. In this manner the end of the work piece probably has a lot of runout. If you can face the end of the work piece (next to the chuck) so that it is square it will help a lot. The faster you run the part, the more the spinning force will encourage it to move off center. The thiner the piece, the more it will bend. A steady rest will help a lot. Screw machine length drill bits will help too. You might have better luck with the 4 jaw. Do you have the key in the headstock with the headstock preloaded by pushing the chuck as you tighten the screw? See the sherline manual.
Reply to
Chuck Sherwood
You stated that on the "first" pieces, you faced them off on each end, then center drilled and bored. Okay so far. If you faced off the end of the bigger(longer) piece the little nipple left would be centered and your center drill should just touch the center of the nipple when getting ready to center drill it. If not, something wrong possibly with the piece being too long. Do you have a steady rest? Ken.
Reply to
Ken Sterling
Thanks to all for the many suggestions. This is a 2.375" diameter, 3" long piece of Delrin. Yes, it is a bit slippery, and I suspect that the rotation is causing it to slip a bit.
Yes, I have faced both ends, and when I do so, the workpiece length is accurate enough that I can say that opposite sides of the cylinder are the same length within a few thousandths of an inch.
The 3 jaw chuck seems to be accurate enough that I can't blame it; using a micrometer, its runout is at most a few thousandths of an inch--not enough to explain the problems that I am having--especially because I can turn short pieces (1/2" long) and get acceptably accurate centering--good enough that my cheap micrometer is less accurate than the centering discrepancies.
Sherline does make a steady rest--but it only goes to 1.75" diameter, so that's not an option.
Would it make more sense to mount the drill chuck in the headstock, so that what is rotating is the drill, not the workpiece? Of course, that means that I need to find a way to mount the chuck in the tailstock instead, and I don't know if a Sherline lathe can do that.
Clayton E. Cramer
Reply to
Clayton E. Cramer
According to Clayton E. Cramer :
Hmm ... if the facing is not totally square (easy to have happen, as the first face is probably a saw cut), then it will cause the workpiece to tilt in the second holding so it is parallel to the first, but not square to the axis of the workpiece.
How do you measure the runout with a micrometer? A dial indicator is the better tool for this.
And lacking a steady rest, what I would do is start the spindle turning slowly (IIRC, the Sherline has a variable speed motor), and measure the runout at the free end of the workpiece. Loosen and tighten the chuck as you make adjustments until it is at a minimum. Keeping the speed slow, use the center drill to make a center in the end while it is running slowly so it does not slip in the chuck. Then bring a live center up to support that while you face as much of it as you can manage before you hit the center.
Then swap ends, and you should have less trouble getting it centered, as the faced ends will fit on the steps of the reversed chuck jaws (they must be reversed to hold something that big in diameter). Center drill this side too (after making sure that the runout is minimal, support with a live center and face as much as you can. Then withdraw the center and face the remaining part, and reverse the workpiece again to complete the facing of the first side. (Hold it centered with the live center while you tighten your chuck this time.)
At this point, you can drill it and tap it. Do you need the threaded hole to go through the full length? If not, it is better to drill and tap each end separately, instead of drilling it all the way through, because drill bits tend to wander drilling deep holes. That could be the cause of your error if it was in the end held in the chuck, instead of the end facing the drill bit and tailstock.
Again -- how are you measuring the centering with a micrometer? (Unless you have one of the multi-anvil ones used with a round anvil to measure the distance from the hole to the outside diameter.
Thus you have to go through more steps, as described above.
It depends. I know that the tailstock on my old Unimat had an external thread which matched the spindle nose thread -- but that is rather uncommon.
And I don't think that will really help you. Again -- do you need the threaded hole to go all the way through? That is a long distance to try to drill without a gun drill. If you can drill each end separately, that will probably be better.
Enjoy, DoN.
Reply to
DoN. Nichols
actually, Harbor Freight, on sale, has indicator stands cheaper than I've seen them used ($6 if I recall correctly)
Reply to
You probably need to concentrate on holding the delrin more accurately before you do any machining. Perhaps the chuck jaws are not staying parallel to the part's surface as you tighten them.
At any rate, given a set of chuck jaws in reasonably good condition, and fitting in the chuck okay, an inch or two of the delrin in the jaws should provide plenty of stability to machine. It's often interesting to indicate a piece just snug in the jaws, and then after tightening. (Surprising how much they move sometimes)
Good luck!
Reply to
Have you seen a sherline chuck? There is no way he can get an inch of contact period. Holding anything over 3/4 inch means he is grabbing only about 1/4 inch. Been there done that and bought a bigger machine. chuck
Reply to
Chuck Sherwood
So now the OP's 'been there, done that'. He thought his problem was drilling/tapping, and it seems you agree he more likely had a workholding problem.
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