Machining Question

Fritz Maurer · Jan 23, 2018

I have a disc of aluminum 1-1/4 " thick x 18" O.D. that I would like to lay out and drill for a SAE bell housing, to mount a pump on. Not sure of bell housing number, it's on a John Deere 4239T. How do I find the center of the disc, then, how do I determine the bolt circle? I was planning on doing this on a Bridgeport with a rotary table. Thanks, Fritz

Crazy Horse · Jan 23, 2018

I started to describe one of these methods that I knew about and then realized Mr. Google was going to do a much better job.
Here ya go ...... I guess the exactness depends on the lines you draw, etc.
Finding the center of a circle ....

dr sportster · Jan 23, 2018

Bolt circle lay-out .
lay out

Crazy Horse · Jan 23, 2018

Actually, this is the method that I knew already, those other methods are new to me. With this one, you can use a compass to
build your perpendicular bisectors (like they do in the diagramatic video in this link) ..... but you can also do it with a
right-angled plastic triangle and a ruler (and no compass). Just measure the chord you have drawn in and find the halfway
point, then use your plastic right-angled triangle to construct your perpendicular bisector line. Do the same to the other
chord and where those two bisectors intersect, that'll be the center of the circle. Once again, as accurate as your
measurements and techniques. There are probably a number of other ways as well, not sure about that.
Circle center .... another method

Crazy Horse · Jan 23, 2018

Hi Doc .... what exactly is your chart calculating for the person who plugs in the measurements? Somehow that escapes me. And the XY offset of center also blows by me. I'm no machinist so maybe there is something I'm missing even in the original question that Fritz asked.

Janicholson · Jan 23, 2018

With a rotary table and the disk centered on the table, put a scribe in the collet and position the apparent center under the quill
close to the visual center. Just touching the disk, rotate the table. This makes a circle with the scribe. move the table to
make a smaller circle until there is only a point. Now the Y coordinate and X coordinate are zeroed. Reset the DRO to X0 Y0 move
the table X+ to the radius of the bolt circle and mark the hole position and note the DRO readings. Rotate the table the correct #
of degrees and mark holes Repeat as needed to set the pattern. Check using an existing plate to assure intention and drill. Jim

Ellis Kinney · Jan 23, 2018

Use a center finding head for a Starret square. Ellis

IanC · Jan 23, 2018

This link has a very good explanation and dimensions for SAE housings. https://www.sbmar.com/articles/understanding-bellhousing-and-flywheel-measurements/

Poppa Noel · Jan 23, 2018

Janicholson has it right and its quick

Machining Question

Fritz Maurer

Well-known Member

Crazy Horse

Well-known Member

dr sportster

Well-known Member

Crazy Horse

Well-known Member

Crazy Horse

Well-known Member

Janicholson

Well-known Member

Ellis Kinney

Well-known Member

IanC

Well-known Member

Poppa Noel

Member

Similar threads