Thursday, March 22, 2012

Centering Problem

No, machining, not politics.  If you want to center a hole in the end of a cylinder with a lathe, it is pretty easy: put it in the chuck, and make sure that you have it centered in the chuck using a dial indicator. Then use a center drill in the chuck in the tailstock, and voila!  The hole will be centered within .010" or better without any effort at all.

In this case, I have a foot long piece of acetal, and it is both too heavy and too slippery for the Sherline chuck to hold it in position, even before you start the motor.  I tried to locate the center hole using a micrometer by measuring the radius from several different points on the edge of the rod, and using where the micrometer scratches crossed, then used the drill press.  This was close, but not quite close enough.  Any other suggestions for how to do this?

UPDATE: Thanks for all the suggestions.  I had forgotten the most obvious--measure multiple radii across the circle, and where they cross, is the center point.  Using a right angle and then measuring the radii at a right angle from the two planes of the right angle should get it pretty accurate.  Rotating the rod and repeating will minimize errors.

11 comments:

Anonymous said...

Try this
Vhttp://www.woodcraft.com/product/2004363/8961/steel-center-finder.aspx

Clayton said...

The picture isn't terribly clear: how does it work?

Anonymous said...

If you place a round object against both sides of a square and draw a line bisecting the 90 degree angle it will bisect the circle. Turn the device about 90 degrees and draw a second line and the intersection is the center.

w said...

Look at http://www.groz-tools.com/index.cfm?md=Content&sd=PopPrintProduct_cat_A&ProductID=44

Anonymous said...

Place the cylinder against the two arms at 90 degrees to each other and scribe the center arm. Rotate 90 degrees and repeat. The intersection is the center.

Greg said...

Don't you have one of these? You should!

http://www.use-enco.com/CGI/INSRIT?PARTPG=INLMKD&PMPXNO=22508131&PMAKA=327-2994

I have one in 12" that is a 4 pc. set. I think I paid about $30 on sale.

Greg said...
This comment has been removed by the author.
Anonymous said...

Don't know if it would still work in this case (2 feet is awfully long) but the method I've heard for smaller cylinders is to put your drill pit in the chuck of your drill press point up.
Lower it to your vise and lock the base of the bit in. Open the chuck, raise it up and chuck up the cylinder. Now when you lower it, it will automatically be centered over the drill bit.

Anonymous said...

Is this the same as one of those compass-and-straightedge problems? If so, a line perpendicular to and running through the center of any chord will go directly through the center of the circle - two of those, and the exact center is known (i.e. draw chord, use compass from both ends to create "x"s, and the line connecting the "x"s runs through the center)

Anonymous said...

Does anyone know of a good reference for all the ways you can use a machinist combination square to measure things?

bdipo said...

I've heard for smaller cylinders is to put your drill pit in the chuck of your drill press point up.
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