Lathe Test Bar

aliva

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I want to check the tail stock alignment on my 12 x 36 lathe, just no sure how long the test bar should be. I've seen them anywhere from 12" and up.I'm using 1" superior shafting that has a constant diameter of
+-.001
I'm thinking the longer the better. my lathe can accommodate one 32" in length., center to center
Ive check the alignment by putting a center on the head stock and moving the tail stock about 4" from the head. the alignment is fine. Any misalignment at 32" would probably increase the closer I got to the head stock.
I normally don't chuck any thing that long, but is it practical to align at this distance.
Any thoughts would welcome
 
Its checking the alignment the headstock, if the bed is twisted or bowed using a test bar that is that long may not give you an accurate/adequate picture. The Connelly book gives more detail and if you google the Southbend procedure it will give more details. I am going to get an alignment bar for my lathe, its only 12" long but it is not for turning rings, it is used with a DTI to check the alignment of the headstock. Tim
 
+/-0.001" sounds pretty bad for test equipment, I am pretty sure you can get that degree of accuracy from a used automotive shock absorber shaft. I would only consider this to be "OK" for a quick and dirty check of alignment.

First I would do a two collar check to make sure the bed is not twisted. Abom77 has a nice video demonstrating the procedure. I use a 3"x12" aluminum tube on my 11x30 lathe. I would think this is probably about the right size for yours as well.

Then turn a shaft between centers and mic it to see if it is tapered or not. You can dial the taper down to under 0.001" pretty easily this way. I have heard folks here getting down to a tenth, I am happy under 0.0005"
You can get a pretty good degree of accuracy by mounting a shaft between centers and turning an inch near the head stock. put a dial indicator on the carriage and zero it on the work, flip the shaft and crank the carriage and see how far off your are.
Longer may be "better," but it may not be relevant. I would use a bar that is about the same length as where the head stock is mostly used, since this is the area you are most concerned with being in alignment.
 
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