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

On Wed, 2003-09-24 at 09:39, Peter R wrote:
> Hi Kim,

Hi Peter.

> I cnc'd the knee on my BP clone several years ago after posting the same
> kinds of questions and doing research online.

Thanks for sharing your experience.

> 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 suspect that the little square column mill drills are going to sell
well because of that issue. You can move the head up and down as well
as the quill and they aren't much more expensive than a decent round
column mill drill.

> 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'm glad to hear that.


> 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.)

Keep me informed (privately if appropriate) if they are 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.

I've got a Baldor 3330 (1/2HP) right beside my desk. The encoder bolted
right on...


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

Thanks Peter.

> 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.
> >
> >
>
>
>
>
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Kim Lux <lux@...>