Making a lathe test bar

[Suwannee Tim]

I have a PM 1236 lathe and I want to test and adjust the alignments so I need a test bar. I bought a piece of shaft, 1 inch diameter, 3 feet long, Thompson made said to be straight within 0.001 per foot cumulative. I need to center drill it as accurately as possible. I know how to indicate the bar in on the 4 jaw chuck but I have no idea how well the tailstock is aligned so how do I get a hole exactly in the center of the end of the bar?

 

[ttabbal]

You could center drill from the tailstock, then widen it out with a boring bar.

 

[Suwannee Tim]

You could center drill from the tailstock, then widen it out with a boring bar.

Yes but if the tailstock is not closely aligned and I have no reason to believe it is closely aligned will it not drill a hole that is off center? But if I am following you boring after center drilling will produce a concentric hole even if the center drilled hole is off center, is that correct?

 

[ttabbal]

That's the idea, the boring bar would clean up the drill hole and make it concentric. I've seen it done, but I haven't tried to verify it myself. You might need to use the compound to match the angle of the center you want to use while testing. Might want to wait and see if a more experienced member thinks of a better way.

 

[benmychree]

If the stock is 4 jaw chucked close to the chuck jaws, and indicated closely, the center drill will crowd to center and make a closely concentric center, if a recentering tool; is used to further true it up, it should be quite concentric. How is this bar to be used?
Also if the bar is held with the far end projecting some distance and held in a steady rest, the result will be the same.

 

[rwm]

Check out Bobs instructions:

Test bar

I checked my tailstock alignment on my mine by using 2 mt3 dead centers and making sure the tips meet. Is this not sufficient?

www.hobby-machinist.com

Robert

 

[Winegrower]

I took a 3/4” diameter 24” long rod, nothing special, held it in a 4 jaw chuck with minimal stickout, adjusted for minimum runout and center drilled. Did the same to the other end, optional but seemed appropriate. Then I pressed a couple of about 2” diameter, 1” thick aluminum donuts on either end, about 2” in. Then center one end in the 4 jaw, other in tailstock center. Make a couple of light passes to clean up, and now you have a pretty good test bar where you can make a very light cut on one donut and without changing the tool position make the same cut on the other donut. If the two diameters match, your lathe is perfectly aligned. My guess is it won’t be, but you can get the error to a few tenths with some fiddling around.

Any feedback? Would appreciate any critique. It was cheap, didn’t take much time to make, but plenty of time messing with tailstock alignment, and let me see up close some of the surprising error sources when you are pursuing tenths.

 

[mickri]

One thing about using a test bar is that the alignment is only good at the length of the test bar. The alignment could be off closer to the headstock or further away from the headstock. Mr Pete/Tubalcain has a video where he took a MT3 and a MT2 Jacobs drill chuck adapters (wrong terminology?) and turned the Jacobs chuck ends to the exact same diameter.

With one in the headstock and the other in the tailstock you can check the alignment at any place along the length of the bed. This is on my project list.

 

[mikey]

I have a PM 1236 lathe and I want to test and adjust the alignments so I need a test bar. I bought a piece of shaft, 1 inch diameter, 3 feet long, Thompson made said to be straight within 0.001 per foot cumulative. I need to center drill it as accurately as possible. I know how to indicate the bar in on the 4 jaw chuck but I have no idea how well the tailstock is aligned so how do I get a hole exactly in the center of the end of the bar?

I understand what you're asking. I agree with @ttabbal that the most accurate way to do this is by turning an accurate 60 degree taper inside the hole. This assumes your compound is accurately set and you have a tool that can access the hole and cut the material. Thompson shafting is case hardened but should be machinable in the core.

My question is what do you intend to do with such a bar? If you plan on putting it between centers to check tailstock alignment then you are accepting that 0.001" straightness tolerance. To me, that is too much run out to be acceptable. Using it in a jawed chuck or collet to check headstock alignment will not be terribly accurate for the same reasons, nor can you fine-tune the leveling of a lathe with such a bar.

In my opinion), the most accurate test bars are cut by the lathe being tested. I'm sure the other guys will be along to correct me.

 

[mmcmdl]

All correct answers above . Indicate it , center drill it and bore it concentric . Thompson rod is soft in the middle and won't give you any problems . All larger shaft work is started this way btw . The centers need to be true especially when going on to be cylindrical ground at a later process . Pulling out tapers over 12 foot shafts used to be a pretty fun experience years back for me , but I wouldn't want to do it now .