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waynec
01-08-2005, 11:10 AM
I have an older PR machine with the ball screw Z axis.

I'm having intermittent problems with it binding, and loosing its Z location. Two questions-
1) Can I use graphite lube on the screw?
2) Can I upgrade the ball screw Z axis to a Rack and Pinion mechanism?

Thanks.

Wayne from WHite Salmon

Brady Watson
01-08-2005, 12:45 PM
Wayne,
I would actually keep the screw over the rack & pinion on the Z. You may need to replace the ball nut or at least check it out and make sure that it is not too worn. I imagine that ShopBot could provide you with a replacement or recommend where you could get one. Also, I forget if they have springs on the Z like the r&p tools. If so, check to make sure that the spring isn't beat as well...The springs do wear out after a while.

Many manufacturers that use ballscrews on their CNCs recommend Slick50 as a lubricant. You also might want to come up with a regular schedule for inspecting the nut and screw and cleaning the dust out of it. It *may* be possible to cover the cleaned screw with a flexible neoprene tube or something to keep the dust out.

-Brady

jthelen
01-08-2005, 06:14 PM
Wayne,

Now here is something I have experience with. I built my machine from scratch 4 years ago. I did R&P on the X and Y and a Ball screw (BS) on the Z. I am in the middle of Replacing my BS right now. (for the second time.)
First I would like to say when the BS is new and clean tit works great. Three things a BS hates. Misalignment, dust-dirt, and hitting the brick wall.

Dust-dirt: You need to keep the BS in the cleanest environment you can with bellows or some shielding from the dust. You can take the ball screw assembly off the machine and put the whole thing in parts washer cleaning fluid and spin the screw up and down it's length. This may or may not wash the dust out. As for lubricant I don't know if I would use anything the dust might stick to (oil) in a wood environment. I lost my first BS to dust and dirt. It would lock up on the lifts and brake loose on the next plunge. Of course it would go too deep as it didn't lift enough.

Brick wall: Hitting the immovable object (The table top) when jogging is hard on the BS as well as the router bearings. (Computers see .1 and 1 very differently) My Z axis will exert over 2500 pound of pressure before the stepper driver lets go. I have the little drivers that could so it will just keep trying after they let go. This has a tendency to flatten your ball bearings and pit your screws slightly. This too will lead to lock up down the road.

The lock up is always random. When I first had the problem and was trying to figure it out, I set up a dial indicator and had it go down to the zero point, move to a different spot and do it again then repeat for 3 hours. I could not get it to fail. (You have to put an expense piece of material under a spinning bit if you want it to fail.)

Both times when I had these problems the BS felt smooth when turning it by hand. (driver off) You have to disconnect it and feel it on its own. Dust will feel crunchy or sticky definitely not smooth.

Misalignment: This is what I am dealing with now. I had a slight misalignment and the side force over time this wears the ball bearing out and it won't hold the Z depth. You will see small differences in the depth after some lifts. Unless your Z speed is too high you might jam a very warn BS on a lift. This will feel sloppy. I had about 1/8-3/16 inch side play at about 4 inches from the nut. It still rolls smooth in your hand but it is shot just the same.

I am looking into rebuilding my ball screws as I am getting a collection of them.

Hope this helps,

Jim

workingdog
01-09-2005, 02:19 PM
I also have a old rack and pinion pr.
It sounds like it is just a matter of time before I start having problems with the ballscrew.
Q: What all would I have to replace on the Z to convert to rack and pinion??

Mike Windsor
01-09-2005, 11:31 PM
I have a pr machine , I upgraded the control box and found that I could cut a lot faster with the new drivers , but the limiting factor was the ball screw z axis when it came to speed and missing steps ( this was mostly alignment problems , and the whole ball screw instalation was a bit Rube Goldberg in execution ) . I didn't have time to mess with it so I just bought a new R&P Z axis from shopbot ( about $695 at the time ), I may find time to tune up the old ball screw Z and instal it as a second , but I'm very happy with the new Z setup the motor is three times more powerfull than the old one and it's never missed a step and the router mount is true to the axis so I can drill round holes , not ovals like you get when the router isn't true to the axis.

jthelen
01-10-2005, 12:48 PM
Mike, Z speed?
Do you do allot of 3d work?
If you have a 4x8 table then 600 Inches per minute is great but would you really want your Z with under a foot of travel to be going 600 IPM? Most of the time in 2D work your depths are under a inch. (maybe up to 3 for foam) If your Z is set at 30 IPM then it takes 2 seconds to go full depth of 1 inch. (plus your safe or clearence hight time) If you are drill a thousand holes that will add up but not as much as 300 verses 600 IPM on the X and Y.

In 3d work your controller will limit the X and Y speeds to the top cutting speed of your Z when cutting continues 3D motions. It has to or your part would not come out right. Here a higher Z speed would benefit you, but there are other factors to consider on a Z over X and Y.
Gravity will play a part. If you had your Z speed match to your 600 IPM X,Y speeds gravity would work against you. I did some over head rigging work for a Sound Company years ago in "Flying" public address systems for concerts. The nylon straps the held the cabinets up had to have a breaking point of at least 8 time the weight of the cabinets. This is because of the vertical load when stopping the weight when it was being lowered. (point of motion) This is similar to the Z axis motion, although I am sure the software ramps the speed down at the end of the travel.

I would be interested to know what the top Z speeds people are using on their Bots?

Jim

Mike Windsor
01-10-2005, 05:45 PM
Hi Jim : yes I do do a lot of 3D work and use visual mill 5 . The stock post processor in visual mill outputs moves and jogs as " MS " and " JS " thus setting the speed for x , y and Z axis at the same speed . I modified the post processor to allow me to create files with 1/3 and 1/2 Z move speeds in relation to the x, y speed . This helped with a lot of cut strategies , but in some strategies similar to the ones put out by Mill Wizard ( where the shape is created with up and down moves , as opposed to waterline cuts )Z speed is everything . Most of my files are cut at 1.5 IPS or 90 IPM with jogs and transfer moves at 2.5 IPS on all three axis . With the ball screw Z I never used a cut speed above .5 IPS or a transfer speed over .75 ---On a large 3D file this adds up quite a bit .

jthelen
01-10-2005, 11:31 PM
Mike,
I have Visual mill 4 and Rams. I always thought it was odd the VM did not allow you set a separate Z speed. I thought this is something they would have changed (fixed in my opinion) In VM 5. I use Rams for most everything now. I upgraded it to the "Gold" version and my only complaint is the simulation is slow and not as good as VM's.

Jim

stevem
01-11-2005, 09:25 AM
VM allows you to adjust Z speed separately for both insert and retract. Iím still using VM4