Subject: cnc mill retrofit

From: [email-masked] (Richard Hal Buxton)
Date: Fri, 17 May 1996 00:18:39 GMT

After much thought, I am ordering a Bobcad retrofit kit for a
South Bend, Bridgeport style, milling machine.  I don't forsee
any problems mounting the stepper motors for X and Y, but I am
thinking about Z.  Should I drive the quill or the knee?  The
quill appears to be the easier route to go, but I am hoping
that there is someone out there that has been down this road
before and can save me from missing something.  Any thoughts?
Rick

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From: [email-masked] (Jonathan M. Elson)
Date: 18 May 1996 05:32:36 GMT

Dave Ruigh ([email-masked]) wrote:
: Richard Hal Buxton wrote:
: > 
: > After much thought, I am ordering a Bobcad retrofit kit for a
: > South Bend, Bridgeport style, milling machine.  I don't forsee
: > any problems mounting the stepper motors for X and Y, but I am
: > thinking about Z.  Should I drive the quill or the knee?  The
: > quill appears to be the easier route to go, but I am hoping
: > that there is someone out there that has been down this road
: > before and can save me from missing something.  Any thoughts?
: > Rick

: After just completing a "low budget" (actually free) retrofit
: of one of our shop's Bridgeports, I'll comment on this one.
: While it's possible to drive the knee on this type of machine,
: the torque requirments would be extreme, probably beyond what
: an affordable stepper/drive could deliver, unless you're 
: willing to tolerate very low z feedrates with a reduction drive.
: I built a simple quill drive using a 5/8" ballscrew and 110 oz/in
: dc servo with a 2:1 reduction drive (.375 pitch timing belt).
: I removed the original depth stop mechanism and fabricated a 
: block that mounts to the same 3/8-24 hole in the quill as the
: stop. This connects directly to the ballnut. I've been able
: to rapid at 120-150 in/min with no problems. The key is to get
: the leadscrew as close to the axis of the quill as possible
: to minimise bending moment of the link. I used aluminum for 
: this link, and there is noticeable flex when it's driving
: up and down, steel would be a much better choice. I've also
: heard of people driving the quill via the drive pinion for
: the original handle, eliminating the need for a ballscrew.
: My machine has rather a lot of slop in this gear, so it
: wasn't an option.

Yup - this is exactly the plan for my machine.  The problem with it is
that I often switch from a jacobs chuck to a small end mill and back
again during the work.  While changing the tool loses the Z reference
anyway, I end up using the knee motion a lot.  Maybe this is only because
the knee is calibrated (sort of) with the dial on the crank, while the
quill is uncalibrated for all practical purposes.
I think I am going to try to have BOTH axes on an NC drive.  Also, for
exotic work, you may want to tilt the head, so that the quill is no 
longer parallel to the knee motion.
Don't forget the rotary table or dividing head for a 5th axis.

: The whole thing is somewhat crude, I just plopped an XY table
: from a salvaged wire bonding machine on the Bridgeport's 
: table (actually handy, as I have 2 X and Y axes). The table
: is the weak point, not really being rigid enough for heavy
: cutting, but fine for most of the small fixtures we make.
: It's a far cry from our old CNC (sold during downsizing 3
: years ago), with its 40" X 28" X 18" travels, Fanuc control,
: and tenth repeatability, but it beats the hell out of 'crankin
: handles.

: Dave