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View Full Version : Spotting Center Holes On A Test Arbor



JoeLee
12-30-2012, 05:38 PM
I need to make a test arbor to check the accuracy of my L and R tail stocks for the T&C grinder and it will also be used to check the center height of the workhead to the RH tail stock.
I have a piece of 1" ground shaft about 3" long. I have to spot or center drill each end so I can put it between the centers.
The only way I can do this is in the lathe with a center drill. The hole should be on center even if there is slight run out in the lathe chuck. Is there a better or more accurate way of doing this???
I believe that the point of the tail stock center, at least the LH tail is ground slightly off center. The shank of each center is within .0001 of each other so I know the bores of each tail stock are good like as in a matched pair. They must have ground the points in a collet that was slightly off center as most are.

JL................

Scottike
12-30-2012, 06:58 PM
My thought would be to support the work in a 4 jaw or soft jaws on a three jaw and
your steadyrest. Drill your center hole undersize and then clean it up with a small boring bar set to 30deg.
that should give you a trued center hole.
I just did something similar, ground the boring bar from a broken 3/32" end mill & used a centerdrill w/ a 1/8 pilot.
Just my opinion, others here may have a better solution.

edit: Forget the steady rest, I misread your post and was thinking 1"x 3 ft not 1"x3"!

Mcgyver
12-30-2012, 07:35 PM
you have to grind it after the centres are made. adjust table until dia is to a 1/10" each end. I'd use a longer bar.

oldtiffie
12-30-2012, 07:49 PM
For each end:
- true up (minimum TIR) in a 4-jaw chuck;

- drill with centre dril;

- (re) bore the 60 degree taper.

Bored taper will be true to lathe axis even if a bit out with repect to the test bar OD.

Grind parallel between centres - use the universal tqable to adjust any taper to parallel. Ground cylinder is now true to the bored centres.

A difference in tail-stok centre heightof say up to 0.002" is insignificant - covered previously.

I'd advise you to consider a test bar with a taper to match the taper in your head-stock spindle as well - to set the head stock parallel to the table longitudinal axis.

GillMcLane
12-30-2012, 08:26 PM
That is covered in this video-
http://www.youtube.com/watch?v=JT0zZuLO-84&feature=relmfu

Gill

loply
12-30-2012, 08:34 PM
Center drill it then bore it out at 30/60 degrees to take out the inaccuracy introduced by the tailstock.

Use a 4-jaw and make sure the work is well centered.

oldtiffie
12-30-2012, 08:40 PM
Typical test bars (mostly for lathe) - most are 8>12" long and about 1" diameter.

http://www.google.com.au/search?q=test+bar+for+lathe&hl=en&tbo=u&rlz=1R2IRFC_enAU360&tbm=isch&source=univ&sa=X&ei=zN3gUOfqO-WSiQeG-oHwAw&ved=0CDAQsAQ&biw=1920&bih=818

http://www.google.com.au/#hl=en&tbo=d&rlz=1R2IRFC_enAU360&sclient=psy-ab&q=test+bar+for+lathe&rlz=1R2IRFC_enAU360&oq=test+bar&gs_l=hp.1.2.0l4.1549.3320.0.7615.8.8.0.0.0.0.511.2 488.2-5j2j0j1.8.0.les%3B..0.0...1c.1.Yes6TpLY7Hk&pbx=1&fp=1&bpcl=40096503&biw=1920&bih=818&bav=on.2,or.r_gc.r_pw.r_qf.&cad=b

http://www.ebay.com.au/itm/LATHE-ALIGNMENT-TEST-BAR-3MT-HIGH-PRECISION-GROUND-BAR-/110657010235#vi-content

JoeLee
12-30-2012, 10:28 PM
I made the test bar, I know it's a bit short especially for the lathe but it tells me what I need to know which is the difference in height between the two tail stocks and the work head center. I just center drilled each end, put it between the centers and rotated it........just like the guy did in the vidio. I get 0 run out, no need to bore it, I have a good tight tail stock on the lathe. I don't know if I would want to grind the entire length of the test bar to the center holes, seems like a lot of work. Turning the holes to true center would be the easier way to go if needed.
Oldtiffie........ the work head alignment we talked about in my previous post is done. I worked the climb out of the spindle housing base.
Now I'm trying to bring the tail stocks in.

JL...............