[NPT]

I'm still sketching, but I've been intrigued by cartesian parallel machines for a while. I've been mulling over a 2'x4' 2.5 axis quarter sheet plywood drill/cutter. XY would be parallel, but the Z-axis plunge would be in series on the carriage. Perhaps even the manual plunge of the router itself. Since the cartesian parallel mechanism I've seen require only free sliding along the axes and are driven via rotary joints - I was thinking of a large diameter steel tube with sliding bushing. This machine would be for cutting plywood mostly.

Since the tubing stiffness is largely a function of diameter, a potential source of large diameter polished stainless steel tubing is exhaust tubing. A 60" length of 3" dia 1.5mm wall tubing is around $45 shipped on ebay right now. Subtracting the carriage out, it might barely make a 48" working area. Bushings that big are another story. 3" bronze bushings would be crazy expensive. I do have a 7x10 mini lathe, I could bore something else to size. If it's stiff enough, I could certainly go smaller - but the tubing is cheap enough that I was exploring "as large as cheaply available" first.

Anyone who's worked with this stuff have an idea about its stiffness? Would it work? Smaller? Bigger? Is it actually straight and round with decent tolerances?

[wizard]

I kinda doubt that exahuast tubing is consistent enough to allow for a sliding bushing. Maybe large radius roller bearings that are sprung loaded in some way.

Im interested in why you want this parallel machine? Is it to save space?

[NPT]

Partly because I'm intrigued. But I figured it would be stiffer and cheaper - trading an expensive large precision sliding joint, with a more robust rotary joint. Yes, they're still free to slide, but the carriage can span a moderate length of the tube.

[RCaffin]

Tubing would be floppy (deflect) and would ring (vibrate). Not so good.

Cheers
Roger

[wizard]

Partly because I'm intrigued. But I figured it would be stiffer and cheaper - trading an expensive large precision sliding joint, with a more robust rotary joint. Yes, they're still free to slide, but the carriage can span a moderate length of the tube.

Stiff and cheaper. I really doubt that. If that was so you would see high adoption in industry. At work we have a variety of robots, from different companies and I don't think a one of them would be useful for general purpose machining like most router designs do. The robots often are relatively massive just to move parts around (think pick and place) much less handle variable forces on the joints.

The gear boxes for these robots are not cheap either. They are often cycloidal drives that have to resist significant forces from the cantilevered arms. I have a hard time seeing how they would be cheaper than a set of linear rails and lead screws if designing to the same stiffness.

In any event maybe we need to clear up exactly what you are thinking about building here. When i first read the positing I was thinking Watt linkages and as such realized that that had no application. So are we talking what is some times referred to as a "Delta" machine or robot? I'm not even sure how these machines became grouped with parallel machines. In any event we have deltas at work, again doing pick and place and I can't honestly see them as a ideal solution for a router. Some people have built 3D printers with mixed results. There are a number of arrangements possible so details manner here.

So when you say robust rotary units I'm imagining a machine that is say more limited than a robot that has three columns with slider that are driven up and down with traditional lead screws. If you are thinking about this sort of machine then there might be possibility in your approach. I would tend to think that heavier tubing would be easier to work with when it comes to fixation at each end of the tube. In place of tubing I've seen 3D printers that use T-Slotted extrusions with linear rails mounted on them. Maybe the biggest problem with a sliding bushing approach is that the stainless tubing will be soft, in fact it is likely annealed for bending. This will lead to a very short lifespan. Working with a variety of Swagelock type tubing, which is very nice stuff by the way, has me doubting the wisdom in using thin walled stainless tubing here; I just don't believe it is hard enough to resist rather quick wear from the sliding bushing and don't believe exhaust tubing would be much better.

If you are looking for cheap i'm not sure what the best approach would be. Maybe 4140 tie bars are in order. It would be somewhat expensive to have them machined but you might be able to find a salvage solution. It isn't uncommon to find perfectly good tie bars in old equipment being scrapped, think injection mold machines, presses and the like. The big problem is weight, a solid 2" diameter bar, say 5 feet long, will give you a work out. This does bring up a question though, how do you expect to keep the bushings from rotating on any round guide bar??? This just came to mind as it would be a huge problem if I understand the mechanism you want to implement.

In the end maybe the idea of a profile rail mounted to square tubing or extrusions isn't all that bad from a price standpoint.

[NPT]

Actually they would be free to rotate. This is the style of mechanism I was thinking, only in two axes with wider linkages to give the head good torque resistance.
https://www.youtube.com/watch?v=Y_LtfBViEyQ

[RCaffin]

Hum ... in my experience, you would be lucky to get an accuracy of under 1 mm like that. The arms flex.

Cheers
Roger

[wizard]

Actually they would be free to rotate. This is the style of mechanism I was thinking, only in two axes with wider linkages to give the head good torque resistance.
https://www.youtube.com/watch?v=Y_LtfBViEyQ

I dont see that being useful for machining ever! That is personal opinionwithout any analytical effort but nothing about that design inspire feelings of rigidity. Im trying to imagine where it would be useful. It strikes me as something dreamed up for a thesis project with no real application.

I could be completeky wrong here of course and somebody will come up with a perfect use for the mechanism. However i really doubt that the use would involve machining anything as the extend arms imply a lack of rigidity.

The other issue is one of cost. For router type operations i cant see this competeing with conventional router designs. A common theme with most router designs seen in this forum is that they are built with minimal machine shop equipment. This type of machine on the other hand would require a far better equiped shop. Beyond that it looks like the active arms are supported only by the nuts riding on the ball screws. That right there seems to be nuts to me.

I dont see a good reason to go with this design unless the reason is to explore the mechanics of the machine. That includes a bit of software development to support the kinematics.

[Arjay]

Actually they would be free to rotate. This is the style of mechanism I was thinking, only in two axes with wider linkages to give the head good torque resistance.
https://www.youtube.com/watch?v=Y_LtfBViEyQ

It looks like it would easily deflect. As a simple test, put your hand on the center section while it is in motion and push down. I expect it will flex with little effort.

[ger21]

All the weight is (and forces are) hanging off of the screws, which will easily bend and flex.

[NPT]

That's an illustration of the concept, not a prototype. Geometry can spread things around quite a bit. I'm thinking about mocking up a proof of concept with a 2 3/8" galvanized fence post rail this fall. (After a few other projects wrap up). With tubing, the stiffness comes from diameter, not the wall thickness. (That mostly gets you dent resistance). A Sarrus linkage can be vastly stiffer than that thing, and it can be redundant too.

[Arjay]

I believe we all know that tubing gets its strength from the diameter, but having a stiff length of tubing won't matter if the tubing surface isn't flat and parallel enoughfor some sort of carriage to ride on.

[wizard]

That's an illustration of the concept, not a prototype. Geometry can spread things around quite a bit.

That would still leave you with overhung arms with free joints at the ends. Also note the need for the nuts to rotate plus translate on the leadscrews. I dont see a way around this so i dontfind the design encoursging.

I'm thinking about mocking up a proof of concept with a 2 3/8" galvanized fence post rail this fall. (After a few other projects wrap up). With tubing, the stiffness comes from diameter, not the wall thickness. (That mostly gets you dent resistance).

Actually wall thickness gets you a couple of important features. Dent resistance might be one of them but manufacturability is a big one. In a conventional router wall thickness gives you screw holding ability.

In your application the big issue is likely end fixation. With very thin walls you may be stuck with welding plugs into the tubing. The ability to hold threads, especially mechanically stressed threads is very limited in thin walled tubing.

Of course the better equiped your shop is the less important these issues may be to you.

A Sarrus linkage can be vastly stiffer than that thing, and it can be redundant too.

[skrubol]

I think you'll need to show us a sketch of what you are thinking to do. The two illustrations you've shown have no sliding linkages, so I'm not sure how the exhaust tubing would apply.

[RCaffin]

I understand the kinematic design all right, but there must be very good reasons why such designs are not used in practice.

And there are reasons of course. The biggest one is that everything long has elasticity or flex. The long thin arms will deflect under load. Maybe not by much - only 1 mm at the tip. But for a CNC, that is fatal.

Cheers
Roger