Ball Screw Mounting Question.

[Willyb]

Hi Guys

I am getting to the point of installing the Ball Screws in my new Milling machine that I am converting to CNC.

(1) First off, what is the best type of Bearing (Bearings) to use to mount the Ball Screws? Ball Bearing or a Tapered Roller?

(2) The Mill (see picture) is a round column bench type. What size of Ball Screws would you recommend to used in this Mill? 5/8” or 3/4”

(3) Not sure if there is room to install 3/4” Ball Screws in this Mill? Has anyone ever installed 3/4” Screws?

Thanks for the help.

Regards
Willy

[balsaman]

Willy, have you seen this thread? http://www.cnczone.com/forums/showthread.php?t=7180

I used tapered bearings, 5/8 ball screws, yes there is room for up to 7/8" ball screws I would say.

[trubleshtr]

What type of machine did you buy? and where?
I personally like angular contact bearings in pairs, to handle both thrust and radial loads.
are you gonna use setppers or servo's??

[Willyb]

Originally Posted by balsaman Willy, have you seen this thread? http://www.cnczone.com/forums/showthread.php?t=7180 I used tapered bearings, 5/8 ball screws, yes there is room for up to 7/8" bass screws I would say.

Hi balsaman

Thanks for the info on the thread. Do you remember the P/N of the taper roller bearings you used? I will likely end up using 5/8" Ball Screws because they are readily available. What did you end up with for backlash with your Ball Screws? Thanks for the help.

Willy

[Willyb]

Hello trubleshtr

I purchased this Milling machine from Busy Bee in Barrie ON. It is the B1977 model.
http://www.busybeetools.com/
Yes, I will also likely be using angular contact bearings in pairs because of the ability to adjust the preload. I am going to be using Servo Motors with a 2:1 belt drive. Are you presently converting a machine?
Thanks for the help.

Regards
Willy

[balsaman]

I used some odd sized bearings because I had them. I think they are something like .6" ID so I had to make sleeves to fit my screw which I turned down to .5". There is NO backlash in the bearings because I preloaded them. I am guessing a couple thou. backlash in the ball nuts. I can turn the screw maybe 1 or 2 degrees before I can feel the tightness of the axis moving. Will be able to measure better with a dial indicator one it's running.

Eric

[ESjaavik]

Use angular contact ball bearings.
http://www.ntnamerica.com/Knowledge/Angular_Contact.htm
http://www.ntnamerica.com/Engineerin...00/angcont.pdf
http://www.ntnamerica.com/Engineerin...ar_Contact.pdf

Also see the manufacturer's pages of high quiality ball screws. Their recommendations is angular contact ball bearings wherever I looked.

[trubleshtr]

I looked at that same model at the Dixie/401 busy bee location. Did you get it before the sale ended? I held off buying due to space......
I am currently building a machine from extruded aluminum, and waiting delivery of a 6' 5/8" ball screw, which I will take to work to machine the ends accordingly. I will use angular contact bearings, but I want to see the ball screw first before deciding on which size and arrangement.
I am also in the process of building my power supply (ESjaavik knows all about it thanks for your help)

[balsaman]

It's still on sale until March.

Eric

[balsaman]

How are angular contact bearings better than tapered roller bearings in this application?

Eric

[trubleshtr]

I would not say they are Better or worse, tappered bearings are very good for radial and thrust loading, but usually the thrust is good in only one direction, unless used in pairs, and pre-loaded.
Angular contact bearings are also good for radial and thrust loading (as the name implies) but are generally better for high speed, low noise/vibration and lower friction torque. (also in pairs to handle the thrust)
In general: ball bearings are good for lighter loads and higher speeds, where roller type bearings are better for heavier loads and lower speeds.
It really is realative to what your machine is doing, loads/stresses are different from machine to machine as are their intended application.
I think small cnc machines could use either types, given the hobby machines characteristics.(not fast, not heavily loaded)
You can get ball bearing types for cheaper, and if you where to stack 2 ball type bearings together vs. two tappered roller bearings together, you would probably find the angualr bearings take up less room on the shaft and would require smaller housing.
just my 2-cents from what I have learrned/observed as a millwright.

[Swede]

For very light and small machines, paired STANDARD radial bearings can be preloaded with accurate spacers between the outer races, but will not handle the loads that angular contact bearings will do. Really precise AC bearings come as ground pairs, designed to be stacked without spacers to deliver a predictable preload. Cheaper AC bearings, like the SKF 7200BEP, do need a spacer between the outer races. Most AC bearings have a relatively low contact angle. I think ballscrew-designed AC bearings have steeper contact angles, which are better for the higher thrust loads generated by the system.

Here's a drawing of a homemade block for paired AC bearings. If I were to do it again, I'd experiment with different thicknesses of spacers such that when the inner races are in contact, the outer races will be spaced to deliver a correct preload. Right now, as this drawing exists, the preload varies as the ballscrew nut is torqued down, and if torqued too much, the preload is excessive.