Sag in Lathe spindle test bars

[lalatheman]

Connelly in Machine Tool Reconditioning page 113 Says
"The effects of sag in test bars is often overlooked and unless allowed for becomes a major source of error in allignment tests.
A solid bar 1" dia 12 inch overhang
has natural sag of .00038
He also states a "properly " drilled out similar bar sags only .00016 and says the drilling information is readily available in reference works but doesn's say where.
Since I'm wanting to make some test bars i'd like to make them the best I can .
Instinctively I would think s series of stepped holes like from 1/4 to 7/8 or 15/16 along the length would be close.
He indicated the open end would be closed up and center drilled so bar could be ground between centers.
I would guess there are software programs dealing with sag in an evenly loaded cantilevered beam that could figure this ?

David
A

[Forrest Addy]

Connelly is sure right about sag and not just in proof bars either. Precision features extended horizontally but unsupported are subject to sag be they milling machine tables, tailstock quills, boring mill spindles, proof bars, or 64 year old whangers.

Make your test bar the same size as the big end of the spindle taper if you possibly can. Don't reduce the size for any reason. Drill the ID 80% of the finished OD for half the overhung length and 60% to 3/4 full length. A center plug in the end is always a nice refinement. Anything more refined is pointless and wasteful.

Personally, when I need a froof for a spindle axis, I 4 jaw chuck a hunk of pipe that just clears the cross slide, plunk a short 2 x 4 against the end with the center to dampen the ringing. and turn the pipe the full length to barely clean up. Then I relieve the center 1/16" on the dia leavingh 1/4 to 1/2" wide land at each end. I then cut the lands to the same diameter using a file and fine wet-or-dry to hold them within 0.0005".

A largish pipe is way stiffer than a proof bar but it's not as pretty. I usually set up a lathe with the pipe trick and used the proof bar for the customer's show and tell.

[This message has been edited by Forrest Addy (edited 08-17-2005).]

[Mcgyver]

look for an app, check out the freebee beamboy

http://www.filesclub.com/download/beamboy_91879.html