Re: Max feedrate of ball screws?

Seth,
Like you I am a serious woodworker and understand the necessity for high feed rates, plus the desirability of being able to work with other materials.

I first looked at leadscrews and discounted them. They dont like dust and even with the best dust extraction systems we will still get some flour hanging around. The leadscrew shaft speeds necessary to achieve high feedrates mean we must use large diameter leadscews to avoid shaft vibrations and whip which will aggravate resonance problems and may limit the speed regardless of all else. Or devise a way of mechanically supporting leadscrews midpoint without impeding the mechanics travelling past the supports. Large diameter leadscrews also bring into consideration shaft balancing, support structures and inertia etc.

Then I considered rack and pinion. Dust being a major factor here but possibly mounting a rack and pinion so the rack is inverted and the pinion below it may work.

After seaching all the archives (which is very difficult and time consuming)I found some good information on chain drives which are more than adequate for woodworking and are described in previous lists as being at least as accurate as rack and pinion and very tolerant of dust and grit. I seem to recall accuraccies of .001" are achievable. Which is accurate enough for a lot of metalworking as well as more than ample for woodworking. Of course we can still microstep to finer cuts for smooth surfaces but the repeatable accuracy may be limited to about .001". One of the old lists described in some detail an old type industrial gantry router (with excellent reputation) constructed with chain drives but I havent been able to find it again.

My machine will operate in your desired speed range. I have a counter shaft across the machine with a #25 pitch sprocket each end of my Y axis. The #42 Y axis stepper drives the shaft from the centre of the shaft to equalise torsional deflections of each end of the shaft. I feel for my purposes #25 pitch chain was the most practical method, it has a tenslie strength 860 -1000 pounds so I expect it will operate at 10 -15% of its strength.

It may be more practical considering the economics to have a woodworker and a metalworker as separate stand alone machines. Lets face it multipurpose wood working machines are ok but cannot compare to separate dedicated machines and it may apply to cnc also despite its greater versatility.

btw my router is a little larger than yours about 4'6" by 2'6" actual workpiece are but with 3 separate z axis (and 2 more planned).Just like me its a little odd

Regards
Bob Thomas

--- In CAD_CAM_EDM_DRO@y..., "dgmachinist" <scsm@m...> wrote:
> Greetings!
>
> I don't suppose anyone out there could tell me the max feedrate
> of ball screws? The screws that I have in mind are .625 dia., .200
> lead. I need them to be 60" long. (I'm building a 2'x4' router)
> They will be supported at each end with the nut moving back and
> forth. (like every typical gantry) Since the X axis is 60" long, I
> decided to go with dual screws, motors, etc.
> I see all of the "commercial" routers are boasting speeds of 400+
> IPM. How do they acheive those rates?
>
>
> TIA
> Seth