Hi..

It looks like everybody is making cnc routers with ball bearings for the linear motion.

But why ?
Isn't it possible to do without ball bearings for linear motion ?

Flat metal on flat metal with a lubricant in between ?
The bridgeport and sherlines mills do it this way.
Is this more difficult for hobbyists ?
Or are there other problems ?

Without ball bearings would be a lot cheaper.

You would need more motor power, but how much more ?

With ball bearings the contact area per ball is very small.
All the balls together still have a small contact area.
Without the ball bearings the pressure would be spread over a much bigger area.
Which would cause less wearing out.

Could this be done with ordenairy steel on steel ?
Or should this be a special metal ?

Which lubricant would be best ?

How flat should it be ?
Should it be highly polished ?

Is there a name for this methode ?
Would make it easier to search the internet for it.

I hope you can enlighten me with your knowledge.
I find it a interesting question, which wil not leave my mind for a while :-)

Thanks.
Vroemm.

....Flat metal on flat metal with a lubricant in between ?....

As you point out this is used on many machines including some CNC machines. The lubricant can be way oil which has additives for extreme pressure and also to make it more sticky so it does not run off the ways so quickly. But on manual machines just plain mineral oil will work. Grease can be used but eventually the additives that give grease its thick consistency build up and make a mess.


Could this be done with ordenairy steel on steel ?
Or should this be a special metal ?

Steel on steel no.

Sliding bearings, whether they are machine ways or simple sleeve bearings on a shaft (which is really a sliding bearing going in a circle) are normally made using to different metals one harder and/or stronger than the other. There are some exceptions: Brass will slide okay on brass, ductile cast iron will slide okay on ductile cast iron and hard chrome will slide okay on hard chrome all with correct lubricant.

Steel on brass is okay, steel on ductile cast iron may be okay, plastic such as Delrin, UHMW polyethylene, teflon and some of the graphite filled nylons will slide okay on steel. Turcite is a plastic that is used to slide on steel or cast iron ways.


How flat should it be ?
Should it be highly polished ?


Everything must be flat and straight but not polished or perfectly smooth. You might have seen on commercial machines with cast iron or hard chrome ways that there are swirl patterns in the surface. The purpose of these is to have very tiny cavities which retain oil. When the surface is too perfect the oil can simply get scraped off and then there is metal to metal contact.

Good info from Geof.

As for the additional motor power, in my experience with cast iron dovetail ways vs. THK HSR 20mm linear bearings, same load and same speed... probably 5 times more motor power required for the cast iron ways. Maybe more... I've never done a direct A/B comparision.

Friction is a killer for CNC machines on a budget. It's like comparing zero to a lot. A 500 pound router gantry on linear bearings can be pushed with a finger or two.

You are correct though that ways can be more rigid than bearings. Search "box ways" on google for more info. Do note that that you'll mostly find comparisons among high budget commercial machining centers, not homemade machines.

Hi Vroemm.
Just to add to your thoughts - consider air bearings. If you were thinking of large machines, then yes, you would need a sophisticated set up. In fact for a machine requiring tight tolerances it would be quite impractical.
However, if you were considering a small, lightweight machine to work foam, for example, then why not.
At school we used a very simple set up to do mechanics experiments with air supported slides. Friction free.

John

The method you're describing is called a plain bearing, and it is VERY widely used. It appears on some very expensive CNC machines, as well as all older manual machines. Plain bearings are more linear than roller element bearings, but have higher static and dynamic fiction. They are also mechanically quiet when compared to a rolling element bearing. There is a lot to designing plain bearings, just as any other bearing.

More importantly, a good plain bearing requires more effort on the implementer's part to do properly. A good plain bearing requires a deal of handworking to become a quality, long-lasting element. Even in the most expensive CNC machines that are used, the handwork (scraping and lapping) will be advertised as testament to the quality of the tool. A roller element can be fully machine-produced without detriment to quality.

Also mentioned, aerostatic bearings (air bearings) are actually the current kings of accurate rotary and linear motion. For heavy loads, hydrostatic bearings are the item of choice. Both these options require far more engineering that either plain or roller element bearings if you are going to use something that isn't a pre-existing solution.

Worth a read I think. http://www.moglice.com/newsite/pages/straighttalk.html

i have been working in a shop rebuilding tool grinding machines. on just about all machines we rebuild now we use a surfacing material called "turcite". first we machine all the surfaces straight and flat. then we add the turcite to one side and scrape and flake the cast iron side. the turcite also gets the scrape and flake treatment. with a little way oil between the two surfaces they slide like butter. the turcite is a cinch to scrape flat, and the machines last a lot longer than the old cast iron to cast iron surfaces. when it does were out, all you need to do is take a flat scraper and peel it off and resurface it with another layer.

metal scraping, although very time consuming to do, is not beyond the capabilities of the home shop. the basic tools required aren't that much, although you can spend some pretty big bucks if you want it to go quicker ( http://www.dapra.com/biax/scrapers/index.html ). I think that they also have some videos on how to do it in one of the adds in home shop machinist.

Worth a read I think. http://www.moglice.com/newsite/pages/straighttalk.html

Thanks for the link. It was very useful.