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View Full Version : Roller pins on the gib?



Rotate
04-29-2003, 08:56 PM
I was wondering. Are there lathes/milling machine, where rows of tiny roller pins are used as gib (rather on the gib since it would still need to be adjustable)?

Albert

Thrud
04-30-2003, 12:16 AM
Albert
Not quite sure what you mean, but items like boring heads can be fabricated with precision shafting as the gib itself rather than a dovetail.

BTW - I loved your answer in the "ages" thread!

brunneng
04-30-2003, 02:03 AM
I think you're talking about some sort of linear bearing where instead of having flat metal on metal sliding contact(gibs) you have flat metal of bed underside in contact with a rolling contact surface. Like a flat timken cage. Right?

I was wondering about using small ball bearings under heavy spring tension on a mini-lathe. Just how much tension would be needed? Probably lots to resist cutting forces.
Maybe have them adjustable to set the hard movement limit to a couple thous and let the tension take up the rest and allow for any minute variations on thickness of bed from wear or poor quality(supppose I could scrape the ways on my cheap mini-lathe).

Why not make a lathe saddle where it's supported on rows of bearings and clamped by bearings. No sliding contact anywhere, no wear... ever... yee haw! no good?

kap pullen
04-30-2003, 11:28 AM
Carlton, and Cinncinati used to use something called tackoway bearings on their machining centers and drill presses.
The hardened rollers were in a recirculating track like the balls on a ball screw.
These were small replacable bearing units and very pricy.

One of those rollers bust, and it would chew your ways to hello and back.
Most use thompson ground shafting or some kind of plastic (tufnol?) now for ways I think.

Devlieg on one model used Belvue springs to preload their gibs.
Never supposed to have to adjust your gibs.
That idea never cought on.

kap

[This message has been edited by kap pullen (edited 04-30-2003).]

Paul Alciatore
04-30-2003, 01:41 PM
Consider:

A single ball in a ball bearing has essentially a single point of contact with the race. A roller has a line of contact and is therefore able to bear heavier loads. A flat piece of metal against another flat piece of metal has a very large bearing area and therefore has the best load carrying ability of the three.

Sudden shocks can shatter a ball or even a roller. There's scalling from the locally high pressures of balls rolling over the race or shaft. And what happens when chips or grit gets in there? As for the "no wear" thing; FORGET IT! Roller bearings will need to be replaced far more often than metal slides.

All in all, there's very good reason for using a flat metal to metal slide in a machine tool. Even slides with round holes that travel on solid shafts are more resiliant than ball bearings in a tool that will take repeated heavy loads over a long life (but not as solid as dovetail slides of a lathe bed). A ball or roller bearing would need to be grossly oversized to work well in such an enviornment. Yes, I know there are machines that use them but what condition are they in in 10 years? In 20? In 50?

Ball and roller bearings are great for high speed shafts and for places where friction is a primary concern. The primary concerns in a machine slide are solidity and long life.

lynnl
04-30-2003, 03:44 PM
Paul, that sure sounds like good sound rationale to me.

Are linear ball or roller bearings readily available in small sizes, say 5 or 6" long, for back and forth motion? I'm thinking of something along the lines of those conveyor panels like you'd see at a loading dock, for easy movement of cartons and materials, or infeed/outfeed tables... except more precise of course. I don't recall ever seeing such things, other than as drawer slides, etc. And I wouldn't call those really precise.

[This message has been edited by lynnl (edited 05-01-2003).]

Rich Carlstedt
04-30-2003, 11:49 PM
Yes, such a roller unit was used years ago to upgrade grinders.
A lot of shops had B&S #2 surface grinders and to lower the friction, you got a roller unit that looked like a ladder with rollers every inch or so for the flat ways, and a second unit that looked like two ladders next to each other with a common center leg ( 3 legs total)and the center leg allowed the two roller units to pivot around the center leg, so it could be placed on a V way with rollers on each side.
Since the V way used a different thickness compared to the flat, its rollers were larger in diameter.

Teflon coating came out after that and killed the concept...of course keeping grit out was a problem

darryl
05-01-2003, 12:21 AM
I have to agree, there's no load carrying capability like solid metal on solid metal. Needle bearings in a recirculating track would probably give the next best rigidity, but how do you preload that? Stick to the dovetail slides, use way oil, and be done with it. I like the idea, though.
I actually did use drawer slides for the spindle travel on a homemade micro drill press. I preloaded by making the assembly a slight press fit into the housing. It works smoothly without noticable play, but of course, the drilling action is downwards, and there isn't much in the way of side forces. There is a certain ammount of roughness that can be felt when raising or lowering the spindle, as Lynnl said, drawer slides aren't perfect.

[This message has been edited by darryl (edited 05-01-2003).]

Rotate
05-01-2003, 12:33 AM
I agree that flat metal surface of the gib provides the most support, but what's supporting the gib? Usually several relatively small screws which are used to adjust the gib. It's as though the gib is floating on stilts.

Albert

Thrud
05-01-2003, 02:26 AM
Lynn

You can buy linear bearing mounted in flanges or pillow blocks as well as press fit models to use with hardened shafting.

Princess Auto has pillow blocks for 1/4" shafts in their surplus/hydraulics section for about $3/ea.(CDN)

Paul
All modern linear bearings can be ordered with rail wipers even for the profiled rails to reduce wear on the bearings and rail. They vary in performance up to the most expensive ones used in High Precision units. They only induce slight drag in the system.