Need opinions on BP ball screw bearings

[Dwane]

I have a question about the bearings on the ball screw of my mid 80's BP that has been retrofitted with CNC control. I took it apart on the x axis because of backlash. I was surprised that the bearing arrangement was exactly as in my 1957 BP with acme screws in it. On the servo side,there are 2 angular contact bearings mounted together for thrust (yes they are in the correct way). On the handle side, there is a single bearing that is allowed to move I guess to allow for screw expansion. The AC bearings had dried up grease in them (this machine had been sitting unused), but didn't look or feel bad after cleaning. After reassembly I'm still not happy with the feel. Does not feel crisp when moving handle, more springy. Has anyone experimented with other bearing arrangements, are other mills different? I would have thought that this would have evolved more than it has over the years, especially for machines with servo's. How about radial bearings together with regular thrust bearings? Tapered roller bearings on each side maybe? Anyone been there, done that? Sorry so long winded. I hope someone had patence to read this and reply.
Thanks in advance,
Dwane

[mactec54]

Hi Dwane

It sounds like someone has just converted a manual machine,The Bridgeport CNC machines all had ballscrews

Post some photos of your machine & we will be able to see what you have

[Dwane]

Mactec54
Thanks for reply, will try to get pics. It is a series I Bridgeport. This one doe have ball screws. I believe that the machine was retrofitted by Southwestern Industries with CNC control. I was referring to my old BP that has acmes, but otherwise the bearing arrangement looks very similar to this one. Are there differences with the axis screw bearings in the Bridgeports that came through with CNC?
Dwane

[mactec54]

Hi Dwane

Yes they are very different from a manual machine,Unless you have a EZ Trac, the Boss type machines, the X axes Ball nut rotates & the screw is fixed

[gus]

ballscrews 'feel' like crap. servos don't care.............

the best feeling machine is a nice 1962 short table bridgeport with a million miles on it

[Dwane]

gus,
I wouldn't care how the feel was if my generated circles were round. They aren't. How are the EZ trac mills set up, are the bearings different?
Dwane

[mactec54]

Hi Dwane

Yes they are more like a manual machine, There were a lot of different CNC kits put on these machines as well, you may be able to control the backlash in the control if it is not to much

[Dwane]

Mactec54
Thanks for the info, I was hoping that there was something mechanical that someone else tried that helped this kind of problem. I have used other knee mills with cnc, that feel a lot more solid than this one, and mill much rounder circles. I am not really sure if it is bearings, or ball screw. Getting about .003 lash when I put a .005 shim between 2 outer AC bearings. get .005 backlash with no shim. I am probably eventually going to retrofit a new control, this one is outdated. There are no backlash parameters that I know of.
Dwane

[mactec54]

Hi Dwane

If you still have the backlash in the bearings & the bearings are back to back(which they should be) then you should be putting the shim in between the 2 bearings on the outside edge of the bearing only this will put preload on the bearings

Someone may have fitted regluar bearings & not precision so you may need more shim, you will be able to feel the preload in the bearings before you put them back into the housing

[Dwane]

Mactec54,
That is the way that I placed the shim between the bearings. I cut a piece of plastic shim stock to match the bearing O.D.. Then I cut a hole through it big enough so that the inner races would not touch. I don't know how to check the preload without them being tightened in the housing though. These bearings are Fafnir m204kt4. I can't seem to find information on Fafnir nomenclature.
Dwane

[mactec54]

Dwane

I read your post after I had posted did not check were you had put the shim, the shim would want to be the steel type, plastic gives alittle when you compress it.

You check the peload with the bearings clamped together on the shaft of the ballscrew

clamping the centre (inside bearing races) will load the bearings

[Dwane]

Mactec54
I put the bearings on the shaft with a shim in place. I didn't feel much movement axially when I push and pulled it. While rotating them however they don't feel very smooth. I am now suspecting that they might be bad. I am considering replacement.
I appreciate your help
Dwane