Square ways opinion

[ozzie34231]

I'm a home shop guy and have a chance to buy an old mill with little apparent wear. It has ballscrews with near zero backlash in X and Y.
It does have square, (opposed to dovetail), ways.
Opinions welcome, please.
Ozzie

[RICHARD ZASTROW]

Square or "Box" ways, if sized properly, can carry heavier loads then any other type of the same size. That's the good news.

The bad news is they are more prone to "stick-slip" and are better suited for much slower motion speeds. There are exceptions like air bearings on box ways etc., but I doubt your mill has them installed on it.

There are "plastic" surfaces that ride on the ways such as Rulon etc. that are an improvement over metal on metal surfaces.

All said, for a typical milling machine, IMHO box ways are fine.

Dick Z

[ozzie34231]

Thanks Dick.

[handlewanker]

Hi all, I wonder if anyone has considered having the square slideway design but using air bearing technology to carry the weight of the table etc.

I bought a KO Lee cuter grinder some time back and it came with an end mill attachment with air bearing spindle.....very smooth on 80 lb PSI.

With that factor in mind it occured to me that the same design could be applied to a CNC mill slideway set-up with zero friction.

The first slide, the X axis is probably the longest one and so carries the least weight per sq inch of area, table, vice/dividing head, cutter loading pressure etc and the second slide, the crosslide, carries the combined weight of the table cross-slide, vice/div head and cutter loading, all on a short slide length.

Automobile engines operate by pumping SAE 50 viscosity oil at about 50 lb PSI continuously to keep the white metal bearings seperated from the steel crank shaft, bearing journals etc, so why couldn't the same system be used for mill slides?

Pumping oil to bearings means there will be a loss factor that in engines just splashes back to the sump and gets pumped around again...no loss, it's also messy if not contained in an enclosure like a sump.

However, air bearings just exhaust to air, no splashing oil etc, provided the bearing surface area is sufficient to carry the load, also the air bleed off guarantees the dirt will be automatically eliminated from getting into the bearings.

A shop compressor normally carries about 100 PSI in a tank and air bearings use air on a contiouous basis, but the bleed off is quite small.

To do the sums means accessing the bearing area to carry the load and then applying the air pressure in PSI.

100 PSI for a given slideway bearing area may not be enough to keep the metal ways apart under load without metal to metal contact, but if the air useage is small there is no reason to not use air at say 200 PSI or more from a small direct on line compressor without the need for a storage tank.

It would also be completely feasible to use air at 1000 PSI (if the design warranted this much) using a direct on line compressor, as there is no tank to worry about as a potential bomb, just a high pressure pump, probably of the multi cyl wobble plate type.

This strategy is similar to the design used in steam engines where a monotube boiler running at several thousand PSI only has a small amount of fluid in the coiled boiler pipe at any one moment in time, that serves for the boiler.

I would hesitate to go for a low pressure/higher viscosity hydraulic bearing system due to the recovery and reuse of the media used, but if a water based fluid mix (coolant, soluble oil/water) were used it might make the recovery from a semi sealed system easier to design, especially if a few leaks caused fluid loss, but with proper designed end seals the loss might not be a factor.

Surface area X air pressure X cutter load would be the deciding factor.

Slideways could be hard cromed with moving slides having white metal surfaces, no lubrication apart from some very thin oil misting to prevent galling if contact were made under load.

This may have already been tried but I haven't seen any reference to it.

One thing is apparent, the surface of the slides does not have to be made for sliding contact as the seperator being pressurised fluid or air is to prevent this, and ordinary mild steel running on mild steel might very well be an option, and I wouldn't rule out plastic slides with Nylon bearings too.
Ian.

[RICHARD ZASTROW]

Ian, been there, tried that. (air bearings) Actually, when properly applied, they're great. However, you need near a clean room type shop.

In a typical machine shop environment there's too much oil etc. around. Due to minuscule clearances required, air bearings jam tight with water condensation or oil in the air supply. A little oil on the ways and air bearings bind up.

Oil suspended (hydrostatic or hydrodynamic) way system have been used for many years on boring mills, grinders, planers etc. for the last hundred years, so that obviously works. A little messy though.

Dick Z

[handlewanker]

Oh drat....I thought I'd discovered a hitherto little known device......everthing's been invented at least a dozen or so times before it seems.

I'm of the opinion that if'n you can't beat 'em join 'em.

I don't suppose using coolant, soluble oil and water mix would solve the problem of leakage seeing as the mix would be present in the machine anyway, that way you wouldn't have to worry if'n the seals round the sq sections let out a bit of fluid, perhaps make the slideways actually leak the fluid at the edges as part of the system.....OK.. only thinking out loud.

I had in mind the situation that occurs in a car engine where oil at mild pressure of only 50 lb sq in keeps the white metal away from the crank pins both radially and on the faces under load, while allowing the oil to bleed away via the big end face clearance.

In a mill the problem with oil would be the contamination from coolant etc...very messy...as anyone knows when just the way lube oil drips down into the coolant tray and has to be scooped off with a skimmer box device.

The problem of having a sq section slideway and trying to make it ride on a wave of pressurised oil and at the same time maintaining pressure between the slide faces probably puts off most designers.

The air bearing spindle I have has the spindle bearing surface round in a cylinder, so that makes clearances preset to achieve min air useage and good sealing.

I only realised when you mention it that the air supply needs to be really free from water vapour and contaminants, so that is one issue I'll have to adress when I get it up and running.

When it comes down to it the simplest way is usually always the best, all things being equal, and retrofitting and old mill with new technology is usually a lot of bother to get really right as opposed to starting from scatch and using say Hiwin type slideway design and ball screws straight off the shelf.

It would take a lot of courage to butcher an old mill to refit with ball screws and Hiwin type slides.

It all comes down to the type of work you intend to do, and the lure of a cheap old mill can cost you in the end.
Ian.

[RICHARD ZASTROW]

Maybe you could use cutting oil as the way oil??? LOL

Actually, I really don't see air or oil suspended way system as a worthwhile modification to a milling machine. How much of an improvement would that be for the effort/expense required to accomplish it?

Now, if you were to be building a high speed machining type milling machine HMMMMMMM

Dick Z