Newbie- Supported Rails Design: Y & Z considerations

[pippin88]

Hi guys,

Getting most of the bits together for my CNC build.
I have:
Ballscrews: 1100mm, 600mm, 300mm
SBR16 (16mm fully supported rails): 1100mm, 600mm, 300mm - 2 rails of each for x, y, z with 2 trucks on each rail.
Spindle is a 1.5kw Chinese jobbie with VFD
Steppers: 280oz Probotix.

I'll be doing a moving gantry build.

Currently mulling over the Y and Z axis design.
I'm trying (as we all are) to maximise the travel while getting good rigidity. Obviously the shorter the travel (on the z at least) the easier it is to get rigid.

I'm planning on my mounting my Z rails in-line with the Y trucks (in fact bolting through the rail flanges, straight through an alu plate into the bolt holes on the Y trucks) - thus the horizontal distance between my Z rails will be the same as between my Y trucks.

Current thoughts:
1. Vertical distance between Y axis rails should not affect travel but the greater the distance the most resistant to sideways forces from cutting. Therefore I should have the Y rails 300mm apart vertically (full length of Z rails). This will mean a high gantry top, but the distance from the bottom to table will be equal to the working travel.
2. Horizontal distance between Y trucks: The larger the distance the less total Y travel available, but the greater resistance to sideways forces from cutting.
3. Vertical distance between Z axis trucks: The greater the distance the more resistant to cutting forces, but the less Z travel available.

Am I correct in these thoughts?
Is there a (feasible) way to work out reasonable figures / measurements for each of these, or is it guesswork and trial and error?

Large Y Vertical Distance & Large Z truck vertical spacing = shorter Z travel due to Z truck spacing:


Large Y Vertical Distance & small Z truck vertical spacing = longer Z travel:


Large Y Horizontal distance = more resistance to cutting forces but less Y travel:

[Walky]

I have the same rails as you, at almost the same lenghts (1000 x 700 x 300), so here are my thoughts:

Originally Posted by pippin88 1. Vertical distance between Y axis rails should not affect travel but the greater the distance the most resistant to sideways forces from cutting. Therefore I should have the Y rails 300mm apart vertically (full length of Z rails). This will mean a high gantry top, but the distance from the bottom to table will be equal to the working travel.

My gantry bridge is 120mm high (the aluminum square tubing, I mean), so that is the actual distance between the external sides of each rail support. It works pretty nicely for my needs, but I bet more would be better (sadly, that's the biggest size of aluminum piece I could get). I think 200mm would be nice, but 300mm seems a little excessive considering it's a moving gantry: when changing directions, for example, it could "bounce" because of the inertia (that's a lot of weight up there).

Originally Posted by pippin88 2. Horizontal distance between Y trucks: The larger the distance the less total Y travel available, but the greater resistance to sideways forces from cutting.

That's up to you, My Y-Z aluminum plate is about 100mm wide and it does the job for me, but I bet it could be better if it was about 150mm, but I like the extra travel.

Originally Posted by pippin88 3. Vertical distance between Z axis trucks: The greater the distance the more resistant to cutting forces, but the less Z travel available.

I wouldn't use anything less than 150mm (between external sides of each block), but as you said more is better.

If you're only going to work with soft materials (wood, acrylic, etc), I bet 200 x 150 x 150 is enough. If you're going to cut aluminum in a regular basis, a few mms more won't hurt, but I really don't know how much of a difference it would make at this point.

It's really up to you to decide depending on your travel needs. For the X-Y travels consider some extra for clamping, and for the Z axis consider some extra for spoil board and to compensate for different tool lenghts.


About that Z-Y mounting plate you're talking about, I use that approach but didn't mount the Z rails directly to the Y blocks, I'll explain why:

If you mount them together, in order to adjust the Y blocks (the adjustment screws on top of them) you would need to untighten them from the plate, which defeats the point of actually adjusting them. You need to have all blocks firmly attached to the plate before adjusting them (that's why you need to drill extra holes in the plate, between the block mounting holes).
So, it is just a matter of giving the Z rails their own threaded holes on the plate, and then you can remove them without loosening everything up.

The X blocks distance is important too, here I would go for as much a distance as possible: It will make your machine a lot stiffer and reduce side play.

You say you're using the 280ozin probotix steppers, are you using their Probostep drivers too? If so, I really recommend you to upgrade the power supply if you got the basic 24V one; I changed to a 36V-10A switching power supply from Ebay and the difference in speed is like night and day (you can safely adjust that supply up to about 38-40V, too).

[louieatienza]

You could mount the rails to the z plate instead, this way you can still get good travel while maintaining ridgidity...

[pippin88]

Walky:
Thanks for your input.
The adjustment screws in the blocks adjust preload yes?
Also if I did have one set of bolts holding Z rails to Y trucks I would have to get the alignment perfect first time, there wouldn't be any room to adjust.
There is a definite advantage though in that it maximises horizontal distance between the Z rails & Y trucks at the same time as preserving Y travel.

louieatienza:
Do you have a picture of that setup?
So Z trucks would be attached to Y plate?
I'm unsure how this changes travel at all and it means the stepper has to lift the weight of the rails as well.

[Walky]

Originally Posted by pippin88 Walky: Thanks for your input. The adjustment screws in the blocks adjust preload yes? Also if I did have one set of bolts holding Z rails to Y trucks I would have to get the alignment perfect first time, there wouldn't be any room to adjust. There is a definite advantage though in that it maximises horizontal distance between the Z rails & Y trucks at the same time as preserving Y travel. louieatienza: Do you have a picture of that setup? So Z trucks would be attached to Y plate? I'm unsure how this changes travel at all and it means the stepper has to lift the weight of the rails as well.

Since those are precision rails/bearings, it's important to adjust them with everything already in place since simply assembling after adjustment can leave one or more bearing overtightened because all parts are related and, if one piece moves when assembling, probably something else will too. The overtightening I'm talking about here is not directly (but can be indirectly) related to the adjustment screws, but to the angle position of each block relative to the rail. e.g. A slightly bent Y mount plate can force the bearings when tightening, the same with slightly offset mounting holes for the bearings/rails.

Another factor to consider is that some chinese rails have the mounting holes (on the aluminum extrusion) drilled by hand, so some of them might be slightly offset too. At least mine are like that.

The adjustment screw press the open linear bearing against the aluminum bearing block where it's housed, closing the bearings and making a tighter fit with the rail. Too loose and you lose some rigidity, too tight and the bearing gets too hard to move and maybe even start making marks on the rail (no good). There's a mid point you'll probably get just by feel when adjusting them.
personally don't like the idea of mounting the SBR rails on the Z plate, simply because it prevent you from adjusting the blocks with everything in place, since you would have to remove the rails in order to access the adjustment screw (to adjust them properly you need to have both the rails and the block in place). Don't get me wrong, it is still a viable choice and many people use it, but I think in this case it might no be the best choice because of the adjustment issue.

BTW, never ever insert or remove a block from a rail without loosening the adjustment screw; I've lost a few balls from linear bearings because the blocks were too tight when tried to place it into the rail and I forced it too much, a couple of balls just fell off because of this!).

Oh, and never remove the ball nut from the ballscrew, the balls will fall off and some say it's a PITA to put them back properly.

[louieatienza]

Originally Posted by pippin88 Walky: Thanks for your input. The adjustment screws in the blocks adjust preload yes? Also if I did have one set of bolts holding Z rails to Y trucks I would have to get the alignment perfect first time, there wouldn't be any room to adjust. There is a definite advantage though in that it maximises horizontal distance between the Z rails & Y trucks at the same time as preserving Y travel. louieatienza: Do you have a picture of that setup? So Z trucks would be attached to Y plate? I'm unsure how this changes travel at all and it means the stepper has to lift the weight of the rails as well.

I think it's a more ridgid design. Consider how close you have to mount the z trucks to get a decent amount of travel in the z. With the rails on the z plate and the trucks on the y plate, you don't have to make the y bridge as tall, and you can spread the z trucks father apart. Yes this could complicate things a bit, as far as where the stepper is located. I don't think the weight is too much an issue with the right stepper and screw. I used this idea in my design, and will be using it in my next two builds. The Solsylva and Momus plans also incorporate this design idea (I built a Solsylva and have the Momus plans as well.) Many commercial machines (Onsrud, Titan, Zimmermann to name a few) also incorporate this design.

I should note that if you do not need much z travel, then having the rails on the y plate might bve the easier way to go.

[pippin88]

There are two options I see to maximise the distance between rails whilst still maintaining the access to the central adjustment holes in the trucks (without unmounting the trucks):

1. Y carriage made up of two plates that are bolted together. Y trucks are bolted to back plate at widest distance. Z rails are bolted to front plate at widest distance. To adjust the Y trucks you have to take the front plate (and the whole Z axis) off.
2. Mount the Z trucks to the Y carriage (instead of rails) as louieatienza mentioned. A right angle allen key should be able to get in between the plate and the rails to get into the adjustment screw.

I'm still hesitant to mount the Z rails on the Z carriage (rather than the trucks) - it's a lot more weight for the stepper to move. My spindle is already pretty heavy.

[RomanLini]

How much Z travel do you need? I always think you are better off with 2" Z travel and fantastic rigidity, so you can cut aluminium etc and 99% of your cutting will probably be flat sheet anyway.

The alternative, running like 8" Z travel sounds great like you are getting more machine for your money, but at the cost of a machine that will always have much worse performance... And if 99% of your cutting is flat sheet then that doesn't sound smart to me.

I built my machine for 65mm Z travel with great rigidity, but with the added feature that my Z plate that holds the spindle can be unscrewed and re-mounted up or down in a minute (on a grid of holes with 10mm spacing). This lets me use a tool much longer then 65mm or cut material much higher than 65mm, as long as total X tool movement is less than 65mm.

And up to this point I've only moved it a couple of times, because as I said 99% of your cutting is on flat stock 1" thick or less.

[pippin88]

Thanks for the input RomanLini. Limiting Z travel is definitely food for thought - as you say, very little of what I'll tackle will be very high (though I'd like a good six inches to make sure I can get most jobs done )
Do you have some photos of that adjustment?

Now starting to learn Solidworks - it's great. I was previously using AutoCad, not being aware it's primarily geared for 2D work.

I'm currently working with a design of:
Y Trucks 155mm from outer edge to outer edge in horizontal plane
Y Trucks 205mm from outer edge to outer edge in vertical plane
Z Trucks 155mm from outer edge to outer edge in horizontal plane
Z Trucks 150mm from outer edge to outer edge in vertical plane

[ger21]

One problem I see is that when the spindle is all the way down, you have about 150mm of the plate that 's free to flex from front to back, with only the spindle to stiffen it up. I'd make some side plates to stiffen up the Z carriage.

You might also be better off to shorten the Z carriage by 50mm and let the spindle hang down below it. It should be more rigid that way.

Similar to how I'm doing mine. http://www.cnczone.com/forums/955216-post229.html

[louieatienza]

You should consider the thickest material you plan to cut. For example if you need to cut a block 2" thickness max, then you need a bit with at least a 2" flute, and the bit woulf probably stick out 2-1/4" past the collet nut. Then you have to consider the safe Z height (for rapids) over the workpiece, say another 1/4". And home maybe another 1/4" above that, and that's tight. So in this case you'd need at least 4-3/4" clearance below the collet nut to cut 2" material, and that doesn't give you a heck of a lot of room to work with... and that desn't even take into account a spoilboard, or clamps/fixtures...

In the above example, if your workpiece is the same size as your work envelope you may not evevn be able to insert your bit unless you do it before affixing the workpiece. So there's another consideration...

Then you have to consider if you'll use a dust boot,and how/if it can be adjsutable...

[louieatienza]

Originally Posted by pippin88 There are two options I see to maximise the distance between rails whilst still maintaining the access to the central adjustment holes in the trucks (without unmounting the trucks): 1. Y carriage made up of two plates that are bolted together. Y trucks are bolted to back plate at widest distance. Z rails are bolted to front plate at widest distance. To adjust the Y trucks you have to take the front plate (and the whole Z axis) off. 2. Mount the Z trucks to the Y carriage (instead of rails) as louieatienza mentioned. A right angle allen key should be able to get in between the plate and the rails to get into the adjustment screw. I'm still hesitant to mount the Z rails on the Z carriage (rather than the trucks) - it's a lot more weight for the stepper to move. My spindle is already pretty heavy.

I think you might be underestimating the power of your stepper/leadscrew combo. Maybe unless you plan to do a lot of 3D moves at high speed, you'd consider the weight of the z. But having more mass at the z can be advantageous, since it would have a higher "inertia" or need a higher force to be deflected...