Re: [CAD_CAM_EDM_DRO] CNCing the knee axis...

Hi Kim,

I cnc'd the knee on my BP clone several years ago after posting the same
kinds of questions and doing research online. On my first cnc'd mill, a
Rong Fu 31 type mill drill I cnc'd the quill. With R8 tooling this was
a complete (almost) disaster if you ever wanted to run a program
incorporating a tool change. You never have a "spare" 4" to clear the
tool out of the spindle. This has never been a problem on the BP with
the knee cnc'd.

I've had excellent results, with any play being in the .001" range or
less - being essentially negligible for my purposes. (Other things
change by similar amounts as the machine warms up over a long run.) I
fitted two 75lb gas springs (~$15 ea from McMaster) one on each side of
the knee and tightened the knee gib a bit. I also shimmed the bevel
gear by a couple of 'thou to reduce gear backlash (at the expense of a
bit more gear wear). I also fitted a ballscrew to raise/lower the knee.
I have pics posted in the photos section of CCED under BP Retro, I
think. (I'm having trouble logging in at the moment to check if they're
still there.)

I run the knee at about 100IPM max with zero problems. (X and Y rapid
at about 200IPM.) My Z motor is a big Baldor 4030 series with a 2:1
reduction and tons of reserve power.

The following website gives a good explanation of the issues and
justification: http://www.cncauto.com/knee11.htm

Cheers, Peter

Kim Lux wrote:

>Hi people.
>
>Has anyone ever CNC'd a BP clone by controlling the knee instead of the
>quill ?
>
>We are about to retrofit a BP clone and are thinking of controlling the
>knee. We've already CNC'd a square column milling machine with a heavy
>head. We counterweighted the head to make it lighter. It works well.
>We are moving it at 100 IPM with ballscrews and Ametek motors, direct
>drive.
>
>Our CNC retrofit would replace the acme screw and 90 degree drive with a
>ballscrew and belt drive. We'd like a 2:1 ratio (motor speed to screw
>speed) to retain some Z axis speed.
>
>I've got two concerns with controlling the knee:
>
>a) does the knee remain square to the machine when not locked ? Ie is
>the knee designed to be an axis feed, ie moved up and down without being
>locked ? (my friend tells me no...) Our mill does not have a gib
>adjustment on the knee.
>
>b) how would one counterweight it ? We used actual weights and a cable
>system to counterweight the head on the square column , but the knee is
>going to be heavier than that head was. I'm thinking of constant
>pressure air cylinders ?
>
>Thanks for your thoughts and advice.
>
>