I would like to have a test bar for my lathe. A small local outfit near me has 2 ceneterless grinders (btw, both of them are for sale). I could probably talk him into grinding me a precision 1" or 1.5" diameter by, say,
12" long. Question: Will this do me any good if it doesn't a concentric center hole for the tailstock? Am I just as well off turning my own bar an the lathe, checking for taper etc.
He will be closing his shop soon, and I thought if there is any benefit to owning a precision ground bar, I'd go ahead and get one.
If you do the centre drilling at each end, then the grinding process should make it centre to the holes, assuming you are holding the bar on the surface grinder by dogs and centres...
Or, at least that is what I have worked out by looking over the shoulders of people doing it, could be wrong of course.
Have a look at the Southbend "How to run a Lathe" book - - it describes how to align the tailstock by taking 2 cuts at each end of a bar and measuring the difference,, ie the run out. Theres copies floating around the net in .PDF format, but its good to buy a reprint as a reference to keep on the bench. (And thats probably generational, feel MUCH happier with printed books rather than "context sensitive help pages" on the PC)
And yes, they have precision ground bars at school, kept in their own wooden cases lined with felt - makes setting up SOOOOOOO easy, ie after taper turning etc. So, if you can get one, go for it...you can never have too many tools or calibrated known accuracy references...
Depends what you are trying to achieve with the test bar.
A/ To set the headstock pointing accurately down the ways, a test bar needs to have a taper ground at one end to match the taper in the lathe headstock. You can then fit it to the headstock taper, and with a dial gauge see how well things track.
B/ To adjust the tailstock to be in line with the headstock you need a test bar with accurate centre in each end, preferably recessed in a shallow axial boring to protect their edges. Fit centres to head and tailstock and again use the dial gauge to see how things sit as you adjust the set over.
If you start with a rough bar of steel with centres in each end and put it through a centreless grinder, the centres will not end up co-axial with the outer surface of the bar. If you grind first it will be nigh on impossible to put centres in to the degree of accuracy that you want. Also beware that centreless grinders have a habit of producing lobed work, where any diameter measures correctly but the work is not actually circular - like an English 50p piece.
You need to rough turn your bar, centre both ends, harden it, polish the centres, then grind between centres on a cylindrical grinder to the best accuracy you can. Last operation is to make a nice varished wooden box lined in green felt to keep it in !
Someone will correct me but I don't think a centerless grinder is what you need. A centerless grinder balances rounds on a knife edge with a combination of grinding wheel and a regulating wheel to keep everything in balance.
Forget the centerless grinder, you need a bar that is ground between centers on an OD grinder. Find a shop with an OD grinder that can accomodate the test bar size that you need. Have a bar turned or turn it yourself to about 10 to 15 thousandths oversize and have it carburized and hardened. ( if you are real careful you can leave this step go otherwise it will nick very easily.) After carburizing send it to the grinder and have him finish it... It might be easier to have him do the whole thing.
A centerless ground bar will be round but you also need a set of center holes that are concentric with the od. The centerless will be close, the OD ground bar will be right on center.
Greetings Ivan, If you can get the round bar cheap then it's easy to put in accurate centers yourself. Support the end of the bar in the steady rest while chucking on the minimum at the other end. Drill and bore the center. ERS
O.K., I'm thoroughly confused. I read the referenced article and could not understand the portion about the tailstock spindle wiggle. He just finished taking a light cut on a test piece and mounts the indicator at the back of the stock to check along the length for taper. Could somebody please explain why indicating the back of the stock would give readings that are different from a front mounted indicator. The writer states the following:
"With an indicator on a magnetic base and vertical rod, I set the indicator contact at center height and I take a reading along its length but from the rear of the work. If I take it along the front edge it will of course show a perfect reading as the front edge will always be cut parallel to the carriage travel."
I miss the logic. It's probably me, but I need a better explanation.
If the tail stock is offset toward the front, the indicator on the front will follow the tool path but the piece will be tapered - smaller at the tail stock, while the indicator at the back will show the difference over the length of travel - twice the offset of the tail stock. Gerry :-)} London, Canada
If the indicator is placed on the front of the bar just turned, there is no difference from the path the cutting tool just took. If there is an offset of the tailstock (or a loose tailstock that deviates due to cutting pressure), the bar will in fact be a cylindrical taper, but that won't show on the front, only on the back.
Said another way. A tailstock that is offset toward the front of the ways (toward the front way) will cause a taper to be cut that is smaller at the tailstock end, if the tailstock is offset to the rear of the ways (toward the rear way) or if it cannot resist the cutting pressure and deviates then the workpiece will be larger at the tailstock end of the piece.
The taper will not show from the toolpath, unless the tailstock is very very loose and perhaps totally unserviceable. But he taper will be very obvious from the backside of the workpiece. In fact it will be twice the offset or deviation value.
Said yet another way. Consider the results from turning between centers, dog driven workpiece, with the tailstock offset. The toolpath is a constant value from the front way, it is rigid on the crossslide/compound. The result is only obvious on the opposite side of the taper.
I used to install/setup/runoff new machinery. When installing new machines, the accepted procedure was to measure diameter of the test piece as it will show the deviation even when the offset is very small, the indicator may have trouble indicating such small deviations. We had to certify our setups to tenths. If the diameter was different from one end to the other by .0003 then the tailstock was off by .00015
Can't you just use a micrometer to measure the diameter of the bar at the headstock end and then the tailstock end of the cut? That's what I did with my old Clausing lathe when I got it. Had about .001 difference on a cut about 12 inches long if I remember correctly.
Yes Garrett you can, then step the tailstock over until you get a true cut. Use a mag base and a dial indicator to step the tailstock the correct amount, then recut and recheck.
Your Clausing was good if your error was that small, easy to fix.
A test cut can provide the information, in effect you are making our own test bar.
We had to report deviation per foot found, then corrected deviation per foot. A test (whether tramming a bar or making a cut) had to be for a predetermined distance, held between centers, turned by a lathe dog.
A "test" bar also comes in handy though when checking for vertical deviation, ie the tailstock and headstock not being at the same distance above the ways. Place the test bar between centers, tram the top, then scrape or shim as necessary.
And a test bar can be held in a four jaw chuck, zero'd for runout, then tram the sides to determine that the chuck is pointing exactly level and exactly down the center line of the lathe.
Proper sequence is important, to make sure you are chasing the correct error.
Wish I could remember the name of the published standard that we used to carry and had to meet. Been some years though. Bet someone here can come up with it.
I do remember some particularly picky ones that we had to make sure part of the machine wasn't in the sun when making the tests, the thermal expansion would throw us off. Make the tests on a cool cloudy day, then leave before it warmed up and they started using it.
A fellow has been running an ad in Home Shop Machinist for several months offering test bars. Has MT taper ones also for checking for head and tailstock problems. Or he offers to do custom work. IIRC, the cost for the standard stuff was in the $100-150 range, hardened and ground.