[RCaffin]

Hi katran

Just dreaming ...
Did you consider using plastic bushes instead of double ball bearings behind the chuck?I am just wondering as the chuck would not be spinning fast at all. I have been using nylon bushes on ground 20 mm steel shaft for a tool&cutter grinder I made, and that went fairly well. Of course, the loads would be higher on a 4th axis, but if the ID was something like 50 mm, the surface pressure on the plastic could not be very high.

Cheers
Roger

[RICHARD ZASTROW]

Just so the plastic bearings are configured to eliminate axial movement of the shaft. Maybe flanged bush bearings?

Dick Z

[handlewanker]

Hi, I would think that for a situation where both an axial and radial position is extremely important, and given the choice and all things being equal, twin angular contact ball races or tapered roller bearings win by a million miles over either plastic or bronze bearings.

Bronze bearings would work as well as plastic on a highly polished metal surface, both with lube, but the maintenance to keep the bearing free from any axial or radial displacement means you have to go the extra mile or two not only in the making but in the maintenance too.

Angular contact bearings can be pre-loaded to give a rock solid positioning while still moving freely, but plastic is not so forgiving as it tends to migrate with a slow steady force over time.

The 4th axis could be designed with plastic bearings to meet the requirements, but why go to alternatives when no problem exists.

A typical case of mixed bearing configuration, for no apparent reason, exists in the Vertex BS-0 dividing head, where the literature clearly states that angular bearings (plural) are a feature of the design for their model, but in actual fact there is only one angular contact bearing and that is behind the chuck with the tail end of the spindle being in a cast iron plain bearing with a plain bearing thrust washer and nut to apply the pre-load to the angular contact bearing.

You would have to go into the design deeply to decide if this was the only way the spindle could be mounted, but for the best layout the twin angular contacts, either both at the front or spaced one at each end of the spindle, wins every time, and the cost of the second angular contact bearing is not a factor for consideration for an alternative design.

BTW, anyone who can machine plastic successfully to get ideal bearings must have a reason to want to work with a difficult material when a better solution is at hand.
Ian.

[RCaffin]

Hi Ian

I would think that for a situation where both an axial and radial position is extremely important, and given the choice and all things being equal, twin angular contact ball races or tapered roller bearings win by a million miles over either plastic or bronze bearings.

Well, this is certainly the conventional thinking. But I note that the old cast iron ways and bronze nuts (with huge backlash) have given way to linear bearings and ball nuts (with much less backlash), and in some cases these in turn have given way to engineered plastic surfaces on the ways and even engineered plastic nuts (with essentially zero backlash). It is interesting that it is the high-end machines which have been getting most of the make-over, not the cheapies.

plastic is not so forgiving as it tends to migrate with a slow steady force over time.

Well, yes, but how much and how fast they deform is another matter. LMWPE would not be my first choice, nor would PVC (just as examples). But something like PEEK, or even acetal, can give very fine performance, not to mention ones like Vespel, Torlon or more propietary ones like Frelon and DryLin.
More technically, while many cheaper thermoplastics (eg PE) will flow under pressure and at slightly elevated temperatures, some of the more crystalline plastics are far more resistant - due of course to their crystalline properties. It is a mistake to lump all plastics in one bin!

The 4th axis could be designed with plastic bearings to meet the requirements, but why go to alternatives when no problem exists.

Ah, but there are problems with ball races: brinelling for a start. Plain bushes have a much higher shock load capacity since their contact area is relatively huge in comparison.

for the best layout the twin angular contacts, either both at the front or spaced one at each end of the spindle, wins every time

Easily asserted, but is it really true? Since engineering plastics are creeping into some parts of machine tools, giving superior performance, what other bearings could bear (pun, sorry) investigation?

BTW, anyone who can machine plastic successfully to get ideal bearings must have a reason to want to work with a difficult material when a better solution is at hand.

It's not that hard to do with the right plastics and the right tooling. Some people will say that aluminium is easier to machine than steel (for instance), but that is a ridiculous generalisation. Compare a nice free-machining steel (eg silver steel) with 1000-series aluminium for instance: the latter has all the lovable consistency of damp chewing gum and is death on cutters through heat and clogging.
On the other hand, I can machine (good) acetal or PET to about 20 microns on a production basis with sharp HSS tooling.

I am not saying that plastic bushes would work; I am just asking whether anyone has tried them for slow speed use, given their increasing use elsewhere in high-end gear.

Cheers
Roger

[handlewanker]

Hi Caff, you're quite right on most points, with R&D anything is possible with anything you can name.

I'm not a great fan of plastics, but I appreciate their uses even if I don't fully utilise their potential......I have a couple of plastic gears on my lathe and they work well, but it was a matter of expediency at the time and a certain curiosity with the material being available.

A 4th axis is probably one area where you could have many choices, and provided the bushes met the requirements offered by a conventional bearing arrangement, why not use them.

When it comes to spindles and bearings, I'm a dyed in the wool conventionalist when it comes to choice of bearings.......the design of the housing would dictate what the bearing lay-out would be, and as labour and time are important, doing it one way means you only want to do it once.

I have the same attitude towards plain bearings generally, and I would never use a plain bearing if a needle, ball or roller could be used.

Old habits and methods die very hard in my school, and that tends to slant your imagination when it comes to being inventive.

The biggest leap of faith is at the moment......I'm going to be a complete unrepentant heretic and go CNC with a vengeance, the whole nine yards and a bit, even though I do not anticipate any production in the air.......embracing the "new faith" is a compulsive addiction that needs to be completely catered for or the end result is a cataclysmic failure.......imagine carbon steel cutters on most materials at high speed as opposed to a carbide one.

The same goes for any form of plastic for current design needs........maybe in the future the plastic revolution will fill the needs, but not as I see it for the present.

I couldn't tell one plastic from the next, so maybe I'm overlooking another technology through complete ignorance.

Nobody would dream of a lathe built from wood as a viable option, but they do make routers from Bamboo that work very well, provided the material doesn't get damp and start sending out roots and take root.....LOL.
Ian.

[691175002]

Plastic really has not infiltrated machine tools in the way you imply.

Plastic deforms very easily. Turcite/Rulon machine slideways require low loading per area, and use a single thin layer to reduce stick-slip friction.
A ground ballscrew/rollerscrew outperforms plastic nuts in every category except cost (especially in machine tools where rigidity is king).

I can certainly imagine a working plastic bearing, but it would use only a thin layer of plastic to reduce friction between large metal surfaces (just like how it is done for machine tool ways). A reasonably sized plastic bushing simply cannot hold the work-piece rigidly under normal cutting forces.

Plastic is like jello compared to most metals. Even the new engineering plastics can't compare to steel, and most of the really hard ones don't slide that well anyways.

[RCaffin]

Hi Ian

A 4th axis is probably one area where you could have many choices, and provided the bushes met the requirements offered by a conventional bearing arrangement, why not use them.

That is what I am wondering. Mind you, 'wondering', not 'doing', at this stage. Maybe 'wondering why not'.

I'm going to be a complete unrepentant heretic and go CNC with a vengeance

Enless fun. I did that too.

imagine carbon steel cutters on most materials at high speed as opposed to a carbide one.

Well, that's the funny thing. I have gone back to HSS for some materials as it works better. You see, the edge you can get on carbide, even the best, is not as good as what you can get on HSS. It's a function of grain size. On a micro scale, the surface of carbide is rougher due to the matrix structure. So aluminium sticks to carbide much worse than to polished HSS. Plastic machines better as the edge gets sharper. Much testing done to confirm this. So my lesson is don't use a cutter previously used on steel for Al or plastic.

Cheers
Roger

[RCaffin]

Hi 691175002;1367676

Turcite/Rulon machine slideways require low loading per area, and use a single thin layer to reduce stick-slip friction.

Well, yes, that is true, but so what? Fact is, high end machines are using the stuff. Yes, they do use big areas to keep the loading down, but the amount of wear (especially compared to old cast iron ways) is reduced fantastically. In fact, there are firms out there whose main business is to resurface old cast iron ways with plastic. Done properly the results are excellent. At the right surface loading a good plastic does not scrape away like cast iron, and holds tolerance better with far less maintenance.

I can certainly imagine a working plastic bearing, but it would use only a thin layer of plastic to reduce friction between large metal surfaces (just like how it is done for machine tool ways). A reasonably sized plastic bushing simply cannot hold the work-piece rigidly under normal cutting forces. (My italics)

Oh yes, exactly. You use thin layers of plastic, not thick ones. The idea that there is anything wrong with this in comparison with 'reasonably sized plastic bushing ' is simply a misunderstanding of the material. You do not treat it the same as (say) a bronze bushing.

Fact is, engineering plastics are being used in high-end machine tools. Fact of life. And I don't think those guys are stupid.

Cheers
Roger

[wizard]

Plastic really has not infiltrated machine tools in the way you imply.

I don't know about that, synthetic bushings are very common in the custom machine world especially if a clean environment is required.

Plastic deforms very easily. Turcite/Rulon machine slideways require low loading per area, and use a single thin layer to reduce stick-slip friction.

Exactly. Igus makes such in their DryLin series. You can also look for what are called PTFE liner bearings. There are lots of options actually and plain bearings may hold up better in some situations

A ground ballscrew/rollerscrew outperforms plastic nuts in every category except cost (especially in machine tools where rigidity is king).

Cost is always a consideration. Beyond that sometimes it is easier to fit a plain nut in a tight location.

I can certainly imagine a working plastic bearing, but it would use only a thin layer of plastic to reduce friction between large metal surfaces (just like how it is done for machine tool ways). A reasonably sized plastic bushing simply cannot hold the work-piece rigidly under normal cutting forces.

What you have imagined is pretty common in the industry. Beyond that you can spread the load out.

Plastic is like jello compared to most metals. Even the new engineering plastics can't compare to steel, and most of the really hard ones don't slide that well anyways.

That is a bit over the top. Like all engineering projects the task at hand and current technology determines which way you go with a bearing solution. Old fashion oilite bushings can sometimes be a reasonable solution.

[handlewanker]

Hi Wiz, I have a DIY power hacksaw, using standard hand hacksaw 12" blades, built in 1982 for a special purpose job and the main swing arm had Oilite bushes in it.......they only lasted 31 years and now have to be replaced due to some ovality........needle bearings or ballraces would still be going strong.
Ian.

[RCaffin]

Hi Katran

Now for the really fun bit. I wonder whether you could do the following measurement for us?

Take a long stiff steel bar and lock it in the chuck centred ACROSS the face. A large steel ruler would be fine, flat side to chuck face.
Rotate the chuck so the ruler is horizontal. Power up the motor to hold it there.
Place a dial indicator (say) 300 mm from the chuck axis on the edge of the steel ruler, with room to travel. Record the indicator value.

Now, hang a 1 kg weight off the ruler at 300 mm from the chuck axis on the dial indicator side.
Question: how much movement does the dial indicator show?

Remove the weight.
Question: how much movement does the dial indicator show? ie, how far back does it go towards 'zero'?

Then, hang the 1 kg weight on the other side (at same distance from axis).
Question: how much movement does the dial indicator show?

Remove the weight.
Question: how much movement does the dial indicator show? ie, how far back does it go?

Repeat the cycle several times.
These figures will give us hard experimental data about the 'stiffness' or backlash in the toothed belt. Never mind the theory: what happens in real life? Enquiring minds want to know!
Note that loading and measuring some 300 mm out from the axis is a harsh test - no matter, we can scale the results.

Of course, repeating the whole deal with 2, 3, 4 kg would be interesting: do the movements scale linearly with torque?

Cheers
Roger

[louieatienza]

katran very nice rotary axis! I wish my shop was as clean as yours!

However I don't understand the fascination that some have here with timing belts, especially in a rotary designed for light milling in aluminum? Now tests have to be done to humor the peanut gallery? You folks have some great ideas; it's time to put pedal to the metal and see you implement them into your own builds! Nit-picking of why steel was not used instead of aluminum? katran built most all the machines in his shop, using the machines he made! Not everyone here has access to a powerful knee mill and lathe; you use the tools you got and build to your capabilities. Though I'm sure if katran built a cast iron mill, the steel chips would be flyin'!

There are many commercial machines out there that do use belts for power transmission and gear reduction, slinging infintely more weight than this rotary axis.

[handlewanker]

Hi Caff, that's a very interesting test trial......foot pounds of torque by any other name, but a definite indication of the ability of the unit to resist back deflection with a load.......if it goes one way it'll also go the other, so only one deflection under load is really needed.

I would think this test could be a benchmark for all 4th axis tests as it's so easy to apply and give tangible meaningful results without resorting to strain gauges or other expensive paraphernalia.

It would give an indication of the ability of the 4th axis system's drive train to resist the back or forward pressure generated by a cutter when either climb or hook milling, and also the strength of the stepper motor to hold position with a static load.

Belt creep also contributes to another form of backlash, but this is only when the belt has driven a number of teeth forward under load that migrates the teeth in the pulley groove against the tooth drive face and when it reverses and drives back the creep changes to the back of the tooth face.....it takes a number of steps to achieve this.

So you can go from A to B in a clockwise direction under a load and get a potential loss of steps at B, but when you drive counter clockwise back to A the load reduces, the position is back at zero and then belt tooth migrates back when the load resumes once again, so you get a double loss of steps once you go back to A from B, and this will be maintained until A is reached, whereupon when the drive goes clockwise again the loss of steps will once again be double.

This is a situation I hope to prevent with a geared set-up.
Ian.

[handlewanker]

I've just been doodling once again on my Ipad with a hydraulic motor direct drive for a 4th axis, and this would be totally backlash free as in impossible to have backlash when the oil pressure is acting in both directions at once.

This is something like a direct stepper motor drive, but with tons of torque in both directions simultaneously, all the time, making it impossible for the forces of any cutter to move it in either direction, but this would take a huge amount of amps to achieve and also a huge cooling system for the stepper....highly impractical, although if the amps were adjustable to the load, it would work.

The hydraulic motor can quite small for tons of force and can also be held stalled in any position, and with an encoder give accurate positioning in both directions without missing a step or encodec line.....air over oil comes to mind with a sensitive valve system.
Ian.

[johnsattuk]

I've just been doodling once again on my Ipad with a hydraulic motor direct drive for a 4th axis, and this would be totally backlash free as in impossible to have backlash when the oil pressure is acting in both directions at once.

Oil is compressible ie: springy

[handlewanker]

Oil is compressible ie: springy

Well, I never found my car's brakes to be "springy".

Air over oil is springy if you don't have check valves each way, but fluid under compression by itself is not springy.
Ian.

[johnsattuk]

Rule of thumb - 1% per 1000psi volumetric-ally

http://www.machinerylubrication.com/...-fluid-perfect

[mactec54]

I'm not sure why you guys have been posting all this none related stuff in Katran's thread

Start your own thread & spill your BS there, as you have messed up Katran's 4th axes build thread

[handlewanker]

Hiiiiyyy Mac, belt drive not being the very best solution generates a desire for a better solution....etc.

Sure, the build by Katran is neat and tidy and I think even he would tire of the superlatives describing the build quality now it's up and running, and eventually it gallops off into the sunset as another thread dies never to be revived again, meanwhile back at the ranch the faithfull of the 4th axis god debate all the other possibilities that could be an alternative drive to the general design Katran is using.

He's already been encouraged to keep the third bearing for the spindle end support, so it does pay to chew the rag sometimes, and we all can benefit from a general discourse as long as it stays more or less on topic.

I'll be going into the 4th axis build scenario soon so I want to get all the info pertaining to a successful solution that has been used to date.....ie, plan twice, cut once is my motto.

Katran has already made his choice in the drive method and will tolerate any inadequacy in the drive mechanism if or when it occurs, but some of us are leaning on the side of perfectionism and would like to have a better way.

Did you have something to say, or are you just saying something?
Ian.

[RICHARD ZASTROW]

Play nice, guys.

Dick Z