Lathe Machinist Question

[danski26]

I have a mandrel chucked between centers and I am getting .003 runout on the mandrel. This is the best that I can get! I have tried the set up without the action and lathe dogs thinking the off balanced setup might be making the runout but nope.......still best i can do is .003. The mandrel is from brownells. I would like to see near .0005 on a mandrel ground between centers. Is this too much to expect? Or is my setup wrong?99123

[nhrifle]

You should be able to get it better than that. Look up what I know to be called a "bump center". Basically a square piece of stock with a bearing on one end. Put the mandrel between centers with light pressure. Place the bump center in your tool post with the bearing to the mandrel. Turn the spindle at a moderate RPM and slowly bring the center into the mandrel. The bearing will bring the mandrel to under a thou runout.

Otherwise, maybe the ends have been center drilled slightly off. You could face the ends and redrill them true.

[str8shot426]

I would say you're expecting too much. That mandrel was machined on a different lathe. How true are your centers on the head and tailstock. How accurate is your indicator.
All these things can add up to .003" easy.

If you are just looking to square the face of the receiver, I wouldn't worry about it, but if you have the clearance you could turn it around in a 4 jaw and dial it right in.

[douglasskid]

I agree with str8shot426, check the centers on the head and tail stocks for proper alignment.

[uscra112]

Is the mandrel bent? Take the action off and test just the mandrel. If the runout is worst at one end, the center at that end may be off, or the center itself isn't running true. If it's worst somewhere else, the mandrel ain't straight. I spent years testing machine tools for accuracy. Several lathe tests use a mandrel like that, (although usually larger diameter). First task was always to check the mandrel itself on a known good surface plate to see if it was straight. Test mandrels were ground between DEAD centers to be sure they were true. That one was probably made on a lathe.

Afterthought: If you have runout at one end, reverse the mandrel. If the runout switches ends, it's the mandrel centers. If it does not, it's the center on your lathe.

If I paid good money for a mandrel like that, I'd expect at worst .001" runout. I can make my own, better than that!

I see you are using a live center at the tailstock. If you have one, try using a dead center to see if it makes any difference. Your RPM for a setup like that should be 60-120 anyway, so heat won't be a factor. Grease it with Never-Seize.

[MBTcustom]

The lathe center alignment has nothing to do with it.
Make sure to indicate the center in the faceplate for concentricity, and indicate the live center also. If they are both good, then mount the mandrel only between centers with light pressure, and without the dog. If the mandrel checks out, put on the dog and recheck, if that checks out, put on the action and recheck. Build your setup one piece at a time and keep reading those indicators.
Just looking at your setup there, and assuming that the brownells fixture is correct (you should have less than .001 RO even on a lathe turned mandrel) I suspect that you may have bent the mandrel with your dog screw. Take it loose and remeasure. Remember that a 1/2 bolt is capable of exerting over 600lb of pressure with just a small turn of the wrench which is enough to warp the hell out of any .700 shaft ever made.

If you take the dog screw loose and that doesn't fix it, put your indicator on the centers like I told you, without tearing down the setup and see if they are running true. If they are running true, but the mandrel isn't, check to make sure you don't have a piece of crud in the mandrel centers. If they are clean, then I'd say you have a problem with Brownells.

[6bg6ga]

I would suspect poor quality control and a bad piece from Brownells. Not their fault its just what they ordered in. In short no one has any pride anymore and simply ships junk out the door. As goodsteel pointed out too much pressure will equal a bent piece and that is your runout.

[JMtoolman]

Another thing you might wish to check. Put the bar between the center on the lathe and the center of the tailstock. Place your indicator on the carrage and run the carrage back and forth while indicating on the center line of the bar. This will tell you if the tailstock is not in allinment with the lathe center. If it is not in alignment, use the adjusting screws in the tailstock to bring the tailstock back into alignment. You will have to lightly, very lightly lock the tailstock down so the adjustments can be made, then check, remove the calmping pressure, reapply the tailstock lock, and check again. It takes a number of times checking and adjusting, lock and clamp, check, until you get it aligned right. You should be able to get less than .001 in the length of the bar. Best of luck, the toolman.

[country gent]

Indicate the centers also clean the tapers mounting them good. If the centers arnt running true nothing else will off of them. At work we would instal the head center and then take a light cut on it to make it true to the spindle. Check the live center and if all is running true then the mandrel. If its still out the centers can be slowly scrapped into perfect alighnment with a small 3 corner scrapers point. Find hih point and lightly scrape that side of center holding the angle. May be a light wire burr on the edge of center .

[akajun]

Goodsteel laid it out for you, check the inside of the spindle first, then the headstock center, then the mandrel itself, then with the reciever, then the dog. Also I would ditch that live center for a dead center in the tailstock, lube it with centersaver or white lead which you can get at an art supply store. Make sure you are not bending it by putting too much pressure with the tailstock. Also marking the mandrel and the headstock center with a sharpie. Get a runout reading, mark the high spot, then rotate the mandrel a bit and repeat, is the high spot the same?

[cwheel]

Goodsteel gave great advise. Biggest offender is often the ball bearing center in the tailstock. Cure, use a hardened stationary center in the tailstock with lube. Second biggest offender is someone using a hardened center in the headstock and not recutting it first. Softer center goes in the headstock, and it gets cut every time it is removed, compound @ 30degrees. Third offender, to tight of a lathe dog screw, tighten just enough to hold work piece being cut. Tip = put a copper penny between the tip of the screw and the work or mandrill. After all of this, mount the mandrill between the centers and don't over tighten the tailstock, tighten just enough to remove any end shake. If their is any run out after these steps have been taken, it's the mandrill, and with .003, I'd send it back.
Chris

[codgerville@zianet.com]

I agree with goodsteel. Best advice yet.

[leftiye]

If your mandrel has off center drilling (center holes), chuck up one end at a time and indicate the outside of the mandrel true. Then re-cut the angles of the center drilling. Use the compound feed set at the correct angle.

[trainfever]

I'm just curious since you already have a lathe, why didn't you just turn your own mandrel? I'm actually about to start work on turning my own mandrel for a small ring 98 that I am rebuilding.

[Stockcarver]

Headstock center must be witness marked and ground true with a tool post grinder.

Ball bearing tailstock center are known to have run out.

I have sets of centers ground and witness marked for each lathe. They live in protective plastic tubes with caps when not mounted on the lathe. I indicate the centers with a tenths reading indicator when they are installed in the spindle.

Piece of cake!

[MBTcustom]

Headstock center must be witness marked and ground true with a tool post grinder.

Ball bearing tailstock center are known to have run out.

I have sets of centers ground and witness marked for each lathe. They live in protective plastic tubes with caps when not mounted on the lathe. I indicate the centers with a tenths reading indicator when they are installed in the spindle.

Piece of cake!


Or you could do like I do and forget the faceplate and precision center all together. Just chuck a 1/2" piece of somthinorother in your three jaw chuck and turn a 60 degree point on it.
There.
Perfect precision.
Drive your dog off one of the chuck jaws and get back to work. If you pull down your chuck in a job shop to put the dusty faceplate on there just so you can get a 60 degree point, you're going to get a royal chewing from the foreman.

Just a 60-ish degree point whooped in there with your compound will give you instant sub-.001 alignment, and when you are done, you throw the piece of steel back in the rack, or back in your toolbox. I've got a 3/4" piece of lead steal that I use for a center 1000 times or so. It started out 12" long (the boss sure didn't begrudge me that) and now it's whittled down to about 6" long from truing it up for a center. Another 10 years, and I'll have to beg for another piece I recon.

The live center is the nutcracker. If that's wrong, you're going to have to tear it apart and see what bearings are in there, and pray to God you can still get them. A good live center is an expensive piece of equipment if you really expect to get less than .001 TIR out of it.
Push come to shove (pun intended) you can use the dead center, but that's a bit of a bummer. I used one once thinking its accuracy gains would outweigh the negatives.
Wrongo.
Shaft heated up, center pressure increased, burned the end of the shaft. I cooled the shaft down, now there's wiggle and tool chatter. I tightened the tailstock to eliminate chatter and added more center oil. The shaft heated up, center started smoking. The dead center went flying across the shop followed by a blue streak of language momma told me never to use, installed live center, harmony was realized. Foreman just shook his head and said "you'll get your head out of your butt one of these days kid. It doesn't always work like you read it in the books does it!"
NO SIR, LOL!

[Red River Rick]

I would chuck that mandrel (you have enough there) in a 4 jaw independent chuck and dial it in to within a half thou. Then, support the opposite end with the tailstock and see what kinda run-out you get.

FWIW

RRR

[danski26]

Thanks for all the great insight gents. I will be working on this over this coming weekend when I have time to devote to it. I will update when I have it licked!

[WallyM3]

Like Red River says: 4-jaw at the head stock end, and I use a Royal at the tail stock (that's been verified through many parts).

[danski26]

Well the first thing I did was check the runout on the mandrel using my surface plate and last word indicator. The runout was almost nothing .0001. Next I looked at the live center, again almost no runout. On to the dead center in the headstock. Hmmm....runout on this Chinese piece of **** is .002 and its hardened so I can't even true it with the tools I have.

Ok, just for grins I chucked the mandrel in my three jaw with the tailstock on the other end with the live center. Runout is .0001. That s way good enough for me! I made the truing cut and all is well in the world......for now!

Thanks for the help tracking down the problem guys!

[robroy]

The problen with the dead center may have been crud in the spindle bore. Don't give up on the center till you check that. Glad you got things running true enough.