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retep
08-18-2004, 01:01 AM
I've been working on building decent CNC mounts for my Sherline mill. I took a look at the design of the "official" mounts from Sherline and noticed that the bearings were what I assumed were angular contact bearings. Also other designs for homebuilt mills that I've seen, notably the one at 5bears.com, used angular contact bearings.

Well, to make a long story short I couldn't find any supplier of angular contact bearings in my area so I used thrust bearings instead.

My question is, given that it seems to me that thrust bearings are both cheaper and more easilly available why does everyone use seem to angular contact bearings instead? On something like a CNC mount the load is almost a pure thrust load. So why use angular contact bearings where 50% or so of the "design load" is commited to radial load?

In any case I've completed one of the mounts and have installed it on my y-axis. It works wonderfully! Far less friction and smoother than the simple metal-on-metal design I had been using before, and for that matter, better than the stock manual handwheel mount design Sherline provides.

Forrest Addy
08-18-2004, 02:18 AM
Well this looks like the start of another nice long contentious bearing thread.
A machine tool spindle has to run on a nearly perfect axis for which the bearing selection and design has to be made with particular care. The spindle has to resist thrust and radila forces without perceptable play or the error will affect the work.

The Bridgeport milling spindle we are all familiar with uses angular contact ball bearings for reasons of precision and cost effectiveness.

There are a number of choices for spindle bearings. Plain bronze bearings were used in machine tools to great advantage long before the developement of ball and roller bearings having sufficient accuracy.

Nowadays we have ball and roller mearings in a huge variety of sizes, services, and grades of precision. Plain radial ball bearings have radial internal looseness to permit their assemble and thanks to the because of the races they do not have sufficient axial stiffness to work effectively as a thrust bearing. Therefore plain vanilla radial ball bearings are limited to the less glamorous roles in machine tool design supporting the transmission and power shafting.

The predomiment bearings used in machine tool spindles are angular contact ball bearings and tapered roller bearings. The conical path of the rolling elements of these two bearing types lend great rigidity both axially and radially to oppose the cutting forces transmitted to the spindles they support.

retep, I'm not sure what you mean by "thrust bearing" but if you mean like this:

http://kianho.com.sg/thrust_ball_bearing_S.gif

it's not a good choise all by itself as a spindle bearing. They offer little to no radial restraint.

If you wish to design a spindle you really need angular contact bearings or tapered roller bearings. If the local bearing outfits give you a blank look, there are many on line bearing websites who will sell you anything in the line of spindle bearings you desire:

http://bearingsdirect.com/cgi-bin/autocart.cgi?CAT=36

This website shows precision grade ABEC-7 with are quite expensive but preferred for precision spindle service.

You should have local bearing suppliers - at least in your nearest large city. Look in the Yellow Pages (even now the best resource for finding stuff) under headings like "Bearings" and "Mechanical Drives."

These are professional supply houses. If you know what you want you can spit out the order, pay your money, and take your goods home that very day. If not don't expect them to help you design your spindle except a suggestion or two at first. They're likely to have $40,000 per month customers meaning the little guys like you and me have to accept our lack of importance in the real world of commerce.

You do need to study the subject of bearings to make a knowledgeable selection of the many options available to solve your particular problem. No-one's going to spoon feed you the info you need one on one. Most major bearing manufacturers (Fafnir, SKF, Toyo, etc) have tutorials on their websites. Find them and do some studying.


[This message has been edited by Forrest Addy (edited 08-18-2004).]

Forrest Addy
08-18-2004, 02:23 AM
Accidental double post

[This message has been edited by Forrest Addy (edited 08-18-2004).]

JCHannum
08-18-2004, 07:43 AM
The comments on spindle bearings are appropriate. However, if I understand, the bearings referred to are the bearings for the feedscrews for table motion. Thrust bearings are a good choice in this application.
On the other hand, a bearing house that can supply thrust bearings should also be able to provide angular contact, or any other type bearing.

spkrman15
08-18-2004, 08:04 AM
Ok i looked at the two different pictures of bearings. Can you describe the exact differences between thrust and angle bearings? I just couldn't see it

Rob http://bbs.homeshopmachinist.net//smile.gif

J Tiers
08-18-2004, 08:54 AM
The typical thrust bearing has two flat washers and a set of balls or rollers held in a pancake shaped holder. Clearly no radial restraint.

An alternate type has what looks like a regular ball bearing setup, but will come apart if pulled lengthwise. The balls are not trapped in grooves

A regular ball bearing of the radial type has two grooves, one in the inner and one in the outer race. The balls run and are trapped in those grooves. When loaded up with balls, the grooves won't allow the pieces to come apart, because the balls are partly inside both the inner and outer races. But you can see about 60 to 70% of the ball diameter from the side.

An angular ball bearing is like a radial one, but with the contact point between balls and races cocked. The inner race instead of being just "under" the balls, is under and to one side. The outer is "above" and to the other side. If you put a ball on the inner race only,from one side you would see only about 30% of the ball, from the other you might see 90% of the ball.
Therefore, when pushed on, one direction the balls have quite a bit of race holding them,similar to a thrust bearing. In the other direction, they have less than a radial bearing.
They have something like 70% of the thrust capacity of a thrust bearing, and 60 or 70 % of the radial capacity of a radial bearing. They can be made with different percentages, which are described as the "angle" of the load.

retep
08-18-2004, 09:35 AM
<font face="Verdana, Arial" size="2">Originally posted by JCHannum:
The comments on spindle bearings are appropriate. However, if I understand, the bearings referred to are the bearings for the feedscrews for table motion. Thrust bearings are a good choice in this application.
On the other hand, a bearing house that can supply thrust bearings should also be able to provide angular contact, or any other type bearing.</font>

That's what I expected too! However while angular contact bearings were available in quite large sizes I couldn't find any available in smaller sizes, around 1/4 ID.

Unfortunately as well I'm lacking a credit card so ordering stuff online can be tricky. Rather deal locally except if I can pay via paypal.

JCHannum
08-18-2004, 09:44 AM
It looks like the smallest single row angular contact is the 100 series, 10mm or 0.3937" bore. It would be possible to sleeve to a smaller bore, but the thrust bearing will serve the purpose.

Allan Waterfall
08-18-2004, 10:30 AM
<font face="Verdana, Arial" size="2">Originally posted by J Tiers:


An alternate type has what looks like a regular ball bearing setup, but will come apart if pulled lengthwise. The balls are not trapped in grooves

</font>

Not a good idea to pull them apart,will likely damage the balls or the grooves.

If you do need to dismantle them, the correct way is to place the outer on a ring and place the lot on a hotplate,the outer will then expand enough to let the inner and the balls drop out.

This is a reverse of the way they are assembled,at least it was when I last worked at "RHP Precision Bearings"

Allan

Forrest Addy
08-18-2004, 11:33 AM
D'oh!

Dumb me. I went back and looked at retep's post and there's it is in the last paragraph "y-axis." Well, now there's more stuff on mine about spindle bearings as if we needed more.

1/4" sized angular contact bearings are probably hard to find. There are instrument bearings in that size range but I don't know about the angular contact design. Thrusts in those little mills are very low. I would think you could oppose a pair of radial bearings to take out the axial lash although stiffness would suffer.

I like the pair of ball thrust bearings combined with a regular ball bearing. It's a bulky combination but the axial and radial restraint is there.

J Tiers
08-18-2004, 01:06 PM
<font face="Verdana, Arial" size="2">Originally posted by Allan Waterfall:
Not a good idea to pull them apart,will likely damage the balls or the grooves.
Allan

</font>

Um.....that is AXIAL THRUST bearings I was referring to, and they do indeed come apart, since the plane of the races are perpendicular to the shaft. They are just like the washer type, except they have the grooves to locate the balls radially, and an outer housing to discourage particles from dropping in.

They generally come apart under gravity, but you better catch the parts.... A few have the housing "spun over" to hold the assembly together, and so don't come apart.


ANGULAR contact bearings would be as you say.