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pepecon
12-11-2004, 07:16 PM
I read with interest the article "Make a lathe test bar" by James W. Hauser in the december 2004 issue of Machinist Wokshop.I was hoping to be able to make one for my lathe. Unfortunately, I do not have a friend who will accurately grind the ends for me. How does one do this on his own lathe. The test bar is used to test if the lathe runs true. Can both ends of the bar be turned at the headstock, since this would have the least runout. ?

Forrest Addy
12-11-2004, 07:56 PM
Test bars are great for a factory or a busy rebild shop. For home use they're a waste of time to find and keep. One thing they don't tell your is the amount of droop in their long slender but solid length. Uncompensate test bar droop causes spindle misalignment in the vertical plane (makes the spindle axis rise toward the tailstock) is a critical consideration when it comes to aligning the tailstock.

Whenever I needed a test bar to level and align a lathe I made a one from a piece of Sch 40 iron pipe that would clear the cross slide and about 1" longer than the swing. The pipe is light for it's length and thus it droop but little.

If you chose to make one, "death-grip" it in the 4 jaw chuck (NOT the three-jaw; the grip is too insecure) and dial it in for best clean up on the OD.

Leaving one 1/2" wide unmachined band next to the jaws and another at the unsupported end I'd rough machine a 1/16" deep relief between them. Chances are the pipe will want to sing to you. Grab a short piece of scrap lumber and use the live center to hold it against the end of the pipe. The wood won't really steady the pipe but it will stop it from ringing like a bell.

Next, fine tool machine the end band and the band next to the chuck to a common size and polish them lightly so they're smooth and within 0.0005" of a common size.

Dial-in both ends to ensure they're concentric and note next to each band its size to four decimal places. You now have a test bar suitable for proving spindle alignment on the finest lathe so long as you allow for any small discrepancies. By running a dial indicator across the bands in a horizontal and vertical planes you can level and align the machine right on the money. Your method has recognition and approval by NMTBA and NIST approval whereas a factory built test bar has to be separately certified according to some stringent rebuild specs.

You can re-use a rusty pipe test bar any number of times so long as you dial it in accurately. If you have to machine it, it's useful life is limited to 3 or 4 re-cuts.

So. Got a question about your spindle alignment or a leveling problem? Use the rusty pipe method. You can make one in about the time it took me to write this response (26 minutes) if you already had the pipe handy.

[This message has been edited by Forrest Addy (edited 12-11-2004).]

J Tiers
12-11-2004, 08:25 PM
Forrest, the one in the mag is one to put between centers to aline the tailstock. Won't suffer from droop the same way.

Of course, it isn't useful for much other than alining the tailstock on an otherwise straight lathe. If one tried to use it for bed alinement, it would not work. You have to know either the bed or the tailstock is correct first.

Thrud
12-11-2004, 09:42 PM
It will droop in the middle - you just don't have the equipment to show it or prove it perhaps.

A hollow test bar will droop significantly less as it is more rigid than a solid section - Ideally it should taper internally from the supported end to the free end (on bars designed to be used this way).

Guero
12-12-2004, 05:57 PM
Forrest, I'm saving your instructions to disk. That's the most common sense and practical method I've heard of yet for lathe alignment. Thankee much.