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On the Cheap: Making a Lathe Test Bar From Water Pipe

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vtsteam:
And mount between centers. Obscured, but yes there's a well centered 60 degree pointy arbor in the 4-jaw.

 

vtsteam:
So the cutting begins. I took about .003" (.08mm) per pass, as far as I could reach on the bar (the lathe dog, tailstock and tool holder prevented turning to the ends. But I'm making a tool I expect to use with all of these in place anyway for easy adjustments of the tailstock while working.

A true precision test bar for testing out the accuracy of a lathe would have the chuck removed and a center in the lathe spindle. Testing the lathe is not the purpose for the tool I was making.

Anyway, back to the skims. After each pass the bar diameter is tested with a micrometer at either end. If one end is larger than the other, the tailstock is adjusted to (attempt) to get rid of the taper.

I found this VERY tricky to do because on my lathe, set-over is determined by two screws on opposite sides of the tailstock base. To adjust you back one off and turn the other in with a slotted screwdriver. Well, they were super sensitive, and I was trying to correct, at first, a total taper of 3 thousandths over 6" of skimmed bar length. An eight of a turn jumped the taper 3 thousandths in the opposite direction!

After overshooting a number of times (and an equal number of skim passes), I had it down to a thou and a half. My final attempt was merely applying pressure to one screw, without loosening the opposite. I couldn't actually feel any movement of the screw, it just felt I had increased my tightening pressure slightly. I then locked the tailstock down again, and did one more skim of the test bar. This time the taper was only a half thou, and I called it done!

 

vtsteam:
So now I had a uniform diameter test bar (within the tolerances I can hold), but that doesn't necessarily mean that the tailstock is completely aligned with the headstock and ways. I wanted to see if it also agreed with a DTI on the carriage run along the ways, and if there was any variation in between.

I was happy to find that the DTI stayed zeroed all across the bar from headstock end to tailstock end. So I'm happy with the result. I do wish I'd got a better finish  on the test bar, but well you pays yer money and you takes yer choice, and this is a test bar made out of water pipe. For me it will be a useful tool.

 
  

vtsteam:
I turned down the ends to get rid of the rust, and then painted them, both so I would recognize it as a test bar instead of a miscellaneous length of something, and also to remind me not to use the absolute end diameters, since they weren't turned between centers like the rest. 

awemawson:
Have you come across “Rollies Dads Method”  for checking alignment? If not google knows all about it !

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