PDA

View Full Version : Drilling a centered hole on a crappy lathe



beanbag
08-19-2010, 02:42 AM
The lathes I use are communal, so they are typically beat up, misaligned, etc. I have had inconsistent results trying to drill a concentric hole in a workpiece. I start with a center drill, and at least that spot appears to be centered. Then I use a drill in the chuck, and as usual, something will be a little crooked. I can center the tailstock, but the drill chucks are usually a little crooked. The drill usually chatters a bit at the start, but if I push a bit, it often goes in the divot and starts cutting. I recently did step drilling (drill a smaller hole first), and somehow by the end, my hole was .010 off center.

What extra precautionary steps can I take to make sure the hole is centered? Recently I have just taken to mounting circular stock vertically in the CNC and indicate and drill undersize, then use an end mill and a helical move to finish the hole.

doctor demo
08-19-2010, 03:02 AM
Try a boring bar, if the hole is critical.

Steve

The Artful Bodger
08-19-2010, 03:49 AM
My suggestions:

Try to keep the tool overhang to the minimum. To do that move the carriage as far as possible to the left, make sure the drill is as far into the chuck as it will go, retract the tailstock quil fully and move the tail stock up as close as you can get it.

Check the drill is sharpened correctly and not bent.

winchman
08-19-2010, 05:22 AM
Are you holding the work in a 3-jaw chuck? That could easily account for the hole being 0.010" off center.

beanbag
08-19-2010, 05:39 AM
no, I was using a 5C

murph64
08-19-2010, 09:20 AM
Was it your collet/holder or a community one? If it was communal, can you pick up your own and install it when you get there, and take it home when you leave?


Andy

Toolguy
08-19-2010, 10:28 AM
Try using a ball seat drill to start. That is a center drill with curved sides instead of straight, then angled. The drill bit seems to follow those better than the standard center drill.

Forrest Addy
08-19-2010, 10:35 AM
Drill the hole 1 1/2 diameter deep but a bit underized, bore to the drill size then contnue with the on-sized drill.

loose nut
08-19-2010, 10:45 AM
Center drills are OK for work that doesn't have to be accurate but they won't do the job right especially on an old beater.

For an accurate hole start with a spotting drill and then drill an undersized hole a couple of diam. deep. Bore the hole out to the finish drill size. Drill hole with plenty of withdraws to clear the chips. Don't force the drill, that will make it deflect. Easy peasy.

beanbag
08-19-2010, 03:52 PM
I was wondering, as long as the drill is going to be pointed crooked anyway, wouldn't it be better to have a floppy setup (tailstock ram extended, long drill)?

I don't seem to have that much trouble creating that initial divot because if I make the setup rigid (ram retracted), then even a crooked center or spotting drill will make a centered divot. The problem is when the drill starts cutting. I THINK, but am not sure, that the crookedness happened when I went up to a 1/4" drill from a ~3/16 drill as the second stage of step drilling.

BTW, most times, I don't have a boring bar small enough.

The spot drill I have is 90 degrees. That means that the drill flutes will catch first when I drill normally. Is that right?

oldbikerdude37
08-19-2010, 06:02 PM
get a 5 gallon glass jug, shove all your 1$ bills in it till its full. it wont be long and you can buy your own lathe. My buddies do that when they want a new bike and a lathe is cheap next to a new bike.

ldbent
08-20-2010, 01:18 AM
Many machinists use screw machine length drills. These are considerably shorter and more rigid than jobbers or mechanics length drills. If you're not using a length/depth stop in the collet adjust the stock so that it just protrudes from the collet. Is the end being drilled faced off squarely? Even in a communal lathe centering the tail stock would be worthwhile-no one would bother readjusting it unless they were turning tapers by offsetting the tail stock. With many chucks bottoming the drill in the chuck will prevent the chuck from fully tightening and slippage will occur.
Thanks,
Larry

Fasttrack
08-20-2010, 01:48 AM
Woah ... wait a minute here, guys. His problem isn't with the setup, it's with the equipment. I feel for ya, Beanbag. I run across this all the time and I'll tell you the best solution I've found:

Buy your own drill chuck and adapter. You won't regret it. Those old drill chucks see a lot of abuse and they can get worn out where the scroll doesn't center things properly, the adapter is bent (yes I've seen it happen), etc. I can't tell you the number of times I've had the exact same problem. I finally broke down and started carrying my own drill chucks with me. One MT3 and one MT4. That covers all the communal lathes I use. I don't bother with the ones that are there already. They suck.

Working with communal machines can be a real PITA, especially when they are "student" machines. :(

darryl
08-20-2010, 03:06 AM
If the problem is an off-center chuck, you may be able to drill a reasonably accurate hole aligned to the spindle axis, but it will never be straight with the part. If the problem is with the way the bit is held in the TS drill chuck, then maybe you can make your own drill bit holder. Bore out a piece of scrap that is a close slide-on fit to the outside of the TS quill. Reverse it and face the end. Extend the TS quill about an inch and mount this adapter onto the quill. You may want to drill and tap for a setscrew to hold it in place. If you do this, put a brass disc into the hole before the setcrew so the screw itself doesn't dig into the quill.

Mount a stubby drill bit in the chuck, then slide the TS up to that. Snug it up just enough to take up play, but not enough that you can't slide it along the ways by hand. With a careful start you should be able to get a concentric hole drilled into the adapter by pushing the TS towards the drill bit in the chuck. When that hole is deep enough and sized to hold the drill size you want to use for your project, drill and tap for another setscrew to tighten down on the drill bit.

Now you have an adapter to fit the hopefully undamaged outside of the TS quill, with a drill bit in it that's parallel to the spindle axis. Anytime you mount this adapter, you do it the same way, eg with the setscrew straight up. This keeps the alignment of the bit the same as when you initially drilled the hole in the adapter for it.

You have the option now of running the quill out to push the drill bit into the workpiece, or leaving the quill clamped and pushing the TS along the lathe ways to advance the drill bit into the work. Maybe you would want to keep this jig to create the pilot holes for other larger holes which you might risk drilling using the drill chuck- as Forrest suggests, maybe a deep enough pilot hole would guide a drill bit well enough to keep it on center.

Then again, maybe your biggest problem is poorly sharpened drill bits. You mentioned going from 3/16 to 1/4 inch- a poorly sharpened bit could easily screw off sideways, then try to follow its own error after that. Try a different combination of pilot drill and final size drill. For a 1/4 inch drill bit a 1/8 pilot is probably enough.

J. Randall
08-21-2010, 02:46 AM
When using worn out chucks, tighten down on the drill using all the holes on the chuck body, the bit will run truer than it will just using on hole to tighten with.
James

J Tiers
08-21-2010, 10:31 AM
Unless you use a 4 jaw, YOU WILL NEVER GET IT CENTERED unless you shim the work. Even if you DO use a 4 jaw, a beater chuck may be work off-axis.

Best to do everything at one setup, so it is all concentric. Most any lathe that is actually usable will still spin in a reasonably true manner. if iot does not, off to the ladle with it, or fix it right (not gonna happen with one that is everyone's, and thus nobody's).

if you can't do all at one setting, you are stuck.

You have two problems.

1) getting the work concentric and on-axis (4 jaw, and/or shims, that's your option)

2) drilling concentric... WHICH DOES NOT HAPPEN.

Drilling is a low precision operation. If that is good enough,

a) use a short, sharp drill.

b) "tease" it into the work, letting it "bore" a concentric hole, which it will do if you do not force it.

c) a spotting drill will do that, it is a short, sharp drill, after all.

d) if you need precision, drill undersize and bore. If not, start off with your short drill, and if it is not long enough, move to a longer one to finish, once you have a good deep starter hole.

e) if you can keep a smaller drill on--center, drill undersize at the web thickness of your larger drill, to full depth, or nearly so, then follow with the larger

smalltime
08-21-2010, 10:47 AM
I would keep doing the CNC thing.

Less time and way less stress, and you get a good part at the end.

fishfrnzy
08-21-2010, 02:10 PM
For a short depth you can use a center-endmill to get a nice centered hole. Will work like a boring bar.

beanbag
08-21-2010, 03:08 PM
Getting the stock centered is not a problem. Either 4 jaw or collet.

One problem with a crooked drill chuck is that if the drill points off center, it will act like a boring bar and make the opening of the hole oversize. This is for when you;ve retracted the drill for chip clearing and are starting the next pass. I've gotten around that by first pushing the drill inside the hole and then turning on the lathe.

Mr Ron
08-21-2010, 05:37 PM
So much info you haven't given. Is the drill bit correctly sharpened? What is the material? What speed are you using? Clamp the tailstock spindle and push the tailstock by hand with the drill bit into the work piece to get it started. Then clamp the tailstock to the ways and unclamp the spindle. Use the handwheel to advance the bit. You might also drill a few thousands smaller and finish with a reamer.

beanbag
08-23-2010, 02:31 PM
So much info you haven't given. Is the drill bit correctly sharpened? What is the material? What speed are you using? Clamp the tailstock spindle and push the tailstock by hand with the drill bit into the work piece to get it started. Then clamp the tailstock to the ways and unclamp the spindle. Use the handwheel to advance the bit. You might also drill a few thousands smaller and finish with a reamer.

yes
any material in general
the correct speed or slower