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

[vtsteam]

I don't know about you, but I hate using good clean bar stock for something I may use very occasionally if I don't have to. Steel prices have increased so much that I hoard what steel, hot and cold rolled, I have for use in important projects.

The one thing I find as scrap everywhere in abundance is water pipe. That's a material of preference for me if at all possible. I've even built a steam engine with it -- remember the Pipe and Bolt Mod-Up a few of us did here some years ago?

Pipe steel isn't always great -- it varies in how well it cuts and finishes. But it's plentiful, and, well, a perfect finish isn't always needed. Anyway, I was in need of a rough and ready test bar to reset my tailstock after setting it over when cutting tapers.

To use, it can be set between centers. Then you mount a DTI on the carriage and test at the headstock end, then move the carriage to the tailstock end and adjust the set-over until the DTI agrees there.

But how to make if you aren't sure of your tailstock's position to start with. Well, here's how I did it.

First acquire two pieces of prime quality steel: an old piece of half inch water pipe, and a rusted paint encrusted bit of slightly bent 5/8" rod.

 

 

[vtsteam]

Next chuck the round bar and clean it up a little to make some short plugs for the pipe.

 

[vtsteam]

They will be a very loose if not sloppy fit in the pipe depending on how deep you go, but it doesn't matter much because in my case I've decided to weld them in place. But a more refined sensibility might have chosen slightly larger or better rod, and carefully fitted it to the bore. A press fit, some bearing lock compound, or even epoxy, or solder could have been used. Or a pin through both. There are many ways to skin a cat! (Never liked that expression. Why not a potato?)

In my case I chose to weld them. Unfortunately somewhat after sunset, and outdoors, so my auto-darkening welding helmet both flashed me to start, then totally obscured what I was welding. Nevertheless I managed to deposit a few blobs of metal in the approximate right area by guesswork alone.

 

[vtsteam]

This is the step where you try to bring some sense of order to the chaos you've created by cutting away as much evidence of your ham-handedness as possible.

 

[vtsteam]

Center drill it.

 

[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 !

[vtsteam]

Thanks Andrew -- I kind of remember the name. Reread about it at your suggestion. That's for aligning a lathe on stand etc.

This test bar is for re-centering a tailstock after setting over to make a taper.

Example, I have set the tailstock over to cut a Morse taper in something and I need return it to center accurately and quickly.

Now that I've made it, very simple: Set the test bar between headstock and tailstock centers and using a DTI in the carriage, directly adjust the tailstock over until the reading matches the headstock end reading. A matter of two minutes and accurate to half a thou.