martin54 wrote:I usually have more frequent problems with the Z axis ballscrew and loose set screws. If I were to build another machine I would take a serious look at using R&P on that axis
What sort of problems do you have with the z axis Ballscrew Ted, funny because personally I would prefer ballscrews to rack & pinion on a machine, especially the z axis. Personal choice I guess for a lot of people.
It is not so much that I have lots of problems with the Z axis ballscrew, other than the usual coupling set screw issues most people have from time to time. Like I said earlier I am hesitant to goop any of the loctite products in a small set screw hole cause it is so easy to crack or strip them out in aluminum.
Way, way back, before I am became a propeller head software writer I was a mechanical engineer. One of the things I designed was large flat sheet laser cutters for the steel rule die business. Back in those days gantries were pretty much unheard of and moving tables were typical. We built big honking 4 X 8 tables using 2 inch diameter ball screws tensioned between mounting blocks to help reduce the S curve whips inherent in spinning long cylindrical shafts. That, and a significant amount of power is required to just turn the screw due to their weight and inertia.
What I do not like about them these days is maintaining them. On my first machine I did a "ball screw" type design but the nuts were not the recirculating industrial type. They were delrin with a spring gizmo to reduce backlash. Replacing those is a rather tedious process if you ask me. The other thing I noted is that I could not achieve very high rapid speeds or cutting speeds. Those screws were double starts so getting the machine up to speed was problematic. Five start on an x or y axis results in some loss of accuracy. I never got that machine to rapid above 150 IPM and the cutting speed top end was about 90 IPM.
When I decided to build a new machine I took a serious look at R&P. Maintenance is easier in my opinion. To replace anything in the drive system all that is required is removing one bolt, loosen a turnbuckle and the whole assembly comes off the machine. There are no issues with creating vibration by spinning a long slender shaft at higher speeds. Since the rack is steel and the spur is aluminum the rack does not incur any significant amount of wear. Also, since the rack is upside down and outside the cutting area it does not accumulate debris. Here is a pic of what it looks like from the work area side:
The drive system for the axis you can see on the right side of the pic is outside that aluminum extrusion.
I like that the spur gear is pulled into the rack by a stiff spring to eliminate backlash. On my last machine those little delrin nuts had been cross slit with a spring wrapped around to provide anti backlash. But I think the inertia of the gantry could easily open that spring causing a bit of overtravel.
Is R&P perfect? Oh heck no. On large hi speed industrial machines it has been replaced by either helical drive R&P or linear drive motors.
On my current machine I can achieve rapids of 800 IPM and easily cut at 250 IPM. I posted a video a while back of the machine cutting at 800 IPM. Here is a link:https://www.youtube.com/watch?v=Pt8n43_YRjI
There is no way those little plastic nuts would have handled those g forces.
If I were to build a super accurate machine I would probably use ballscrews and sacrifice speed for accuracy. But those would have to be ballscrews with recirculating bearings.
Well, that is my thinking on the subject, of course, it is nothing more than my opinion.