Re: [CAD_CAM_EDM_DRO] Re: Z axis

I drive the knee on my CNC mill (BP Clone) and can report good results
although it took a lot of fiddling to get things set up correctly. I
posted questions on alt.machines.cnc and received many of the same
helpful suggestions described earlier - i.e. gas springs and/or
emergency brake to stop the knee's weight backdriving the ball nut. I
really appreciate having 16" of vertical travel under computer control.

My machine started life with an acme screw for Z but when I drove the
knee up and down at my stepper's top speed (yielding 24 IPM on Z axis)I
got horrible gib vibrations which I couldn't really eliminate with gib
adjustment. (It would just return at different speed.) And I still had
a backlash problem if I ran at full speed - so I installed a ball screw
& nut to see if that would solve the problem.

I also purchased two $14 gas springs (75lb each) from McMaster Carr (p/n
9416K403 - mounting fittings are extra)and installed them at the front
of the knee. I'd probably suggest a pair of slightly stronger gas
springs (say 125lb) but a combined 150lb of gas spring stops any
movement of the knee - even with the stepper motor removed. (Though I
wouldn't stick my hand under it without wood blocks to support it...)

I can post a couple of pics tomorrow if anyone is interested.

Cheers, Peter

p.s. I've had an excellent experience dealing with Rockford Ball Screw.
They are very well organised.

p.p.s. I've also been dealing with Flashcut for a couple of years and
love their little controllers (I have 2 cnc'd machines) and PC software
- they just don't break!



Jon Elson wrote:

> mjf462001 wrote:
>
>
>>--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
>>
>>>mjf462001 wrote:
>>>
>>>
>>>>Hugh,
>>>>
>>>>Thank you for the link. I am still not sure weather
>>>>I should drive the quill or the knee. When I get around to making
>>>>
>>up
>>
>>>>my mind I will at least have some pics to reference to.
>>>>
>>>If you use the machine to drill holes, or if you have some slack
>>>in the vertical ways for the knee, like mine, then I think you
>>>
>>should
>>
>>>go with the quill. My knee sags out, toward the operator, when you
>>>lower it. I can't tighten the gib because there is a worn spot on
>>>
>>the
>>
>>>ways.
>>>
>>>Jon
>>>
>>John
>>I saw a pic of Dan's setup. The way he drives the knee, I would say
>>it looks pretty rigid!
>>
>>Without having to do the math I wonder how much torque a guy needs
>>so he wouldn't have to gear down (slow down) the z axis drive.
>>The mill is new and without any wear.
>>
>
> I'm not saying it isn't rigid, just that you are moving a LOT of iron to
> poke
> a 10 gram drill bit into a workpiece. On a Bridgeport Series-I size
> machine,
> you are talking about moving about 1000 Lbs of iron up and down every
> time. It certainly will work, but it may cause much more rapid wear

on the

> knee slide. You should note this is the most highly stressed way on the
> machine, as the weight is completely off to one side of the slide.

Worse,

> it puts the load on the dovetail, rather than on the flat!
>
> I think it is unreasonable to try to CNC the knee without a ballscrew
> retrofit. And, after a ballscrew, you'd need a safety brake to prevent
> the table from dropping when you go to E-stop, or lose power. A
> reliable counterbalance, either weights and pulleys or gas springs,
> would help out, but then you'd definitely need a ballscrew, or the
> backlash would show up, without the knee weight.
>
> Jon
>
>
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