[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:

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).

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.

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]

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]

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]

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...

[pippin88]

Z rails mounted to Z plate would also stiffen the Z plate.

Then I could use side braces on Y plate to stiffen that if needed - these would also enclose the rails and screw and protect from dust etc a bit.

Man there is a shedload to think about when designing one of these. I haven't even looked at the table and X axis properly.

[RomanLini]

This phpt of my machine gives you an idea of the geometry, the bottom linear rail and gantry bottom are only 65mm above the table, and with the tool right down there is practically zero flex anywhere in X or Z;

http://www.cnczone.com/forums/attach...7&d=1267255369

In the photo above the Z is not even fully down it is only about half way down.

For the Z re-mounting adjustment the router is on a 6mm alloy plate, which is just screwed to the 10mm acrylic plate on Z. So a few screws and it can be remounted any height.

I can only get jobs up to 65mm high under the gantry, but it will easily take a 4" (100mm) rotary axis under the tool as the rotary axis doesn't need to travel under the gantry!

[pippin88]

Attached is my working design at present. (Solidworks eDrawing file)
I haven't implemented ballscrews or motor mounts etc as yet.
Will add some bracing to gantry.

Will be using 18mm MDF (hoping to get the parts cut to size reasonably).
Eventually probably move to 12mm Alu plate for Z plate, Y plate and then gantry brace.

Thoughts?

[LazyMan]

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.

The main compromise with mounting the Z rails to the Z plate as appose to z bearings blocks to Z plate is that you would have to increase your gantry clearance height to maintain the same bit clearance height. It is much more cost effective and easier beef up your Z plate than it is to beef up your whole gantry. Take a look at shopbots. They essentially have Z rails attached to Z plate and look at the massive gantry clearance required to get their specified "nominal z cutting area." I have yet to find an industrial CNC router that has the z rails attached to the Z plate.

[louieatienza]

The main compromise with mounting the Z rails to the Z plate as appose to z bearings blocks to Z plate is that you would have to increase your gantry clearance height to maintain the same bit clearance height. It is much more cost effective and easier beef up your Z plate than it is to beef up your whole gantry. Take a look at shopbots. They essentially have Z rails attached to Z plate and look at the massive gantry clearance required to get their specified "nominal z cutting area." I have yet to find an industrial CNC router that has the z rails attached to the Z plate.

Check out the machines of CR Onsrud, Titan, and Zimmermann. These are massive machines with z axis assemblies topping 500 pounds plus. I would say the ShopBot is light industrial compared to these beasts. Bartuss1 is making a huge 5 axis configured with the rails on the moving part of the z.

You could beef the z plate up all you want; at the end of the day, the trucks will be closer together with the z plate dangling down, no matter what height it's at. So no matter what position the router is at you have a relativevly long torque arm. But I feel at the end of the day it's a clearance and travel issue. If you don't need a lot of total travel it's probably more efficient to mount the rails to the y plate since you can spread the trucks father apart. If you need more z travel I think it's better to mount the rails to the z plate. The closer the lower trucks are to the the collet, the stiffer the design should be. With the rails mounted to the z plate, you can even have the collet flush with the bottom of the z plate for max clearance; the spindle mount will actually be above the lower trucks.

[LazyMan]

Check out the machines of CR Onsrud, Titan, and Zimmermann. These are massive machines with z axis assemblies topping 500 pounds plus. I would say the ShopBot is light industrial compared to these beasts. Bartuss1 is making a huge 5 axis configured with the rails on the moving part of the z.

Okay I'm wrong there. Yes machines with longer z travel, particularly 5 axis machines do use the rails on z plate, but they also have massive gantry assemblies. On a 5 axis machine, bearing blocks on z plate would simply not be practical. Besides, I really don't see why the bearing trucks have to be spaced any closer in one design vs the other. Both methods have their advantages and compromises. The main problem for me is that if one wants to cut through 3 inches of material with a 3 inch bit, the minimum gantry clearance would have to be just over 6 inches to clear the cutting tool with the rails on z plate configuration but it would only have to be just over 3 inches for blocks on z plate configuration. However, if one has no intention to use long bits than this becomes less important. Either way, I did not mean to suggest that z rails on z plate was wrong way to do it, I was just throwing ideas out there because I went through the same design considerations myself.

[louieatienza]

Okay I'm wrong there. Yes machines with longer z travel, particularly 5 axis machines do use the rails on z plate, but they also have massive gantry assemblies. On a 5 axis machine, bearing blocks on z plate would simply not be practical. Besides, I really don't see why the bearing trucks have to be spaced any closer in one design vs the other. Both methods have their advantages and compromises. The main problem for me is that if one wants to cut through 3 inches of material with a 3 inch bit, the minimum gantry clearance would have to be just over 6 inches to clear the cutting tool with the rails on z plate configuration but it would only have to be just over 3 inches for blocks on z plate configuration. However, if one has no intention to use long bits than this becomes less important. Either way, I did not mean to suggest that z rails on z plate was wrong way to do it, I was just throwing ideas out there because I went through the same design considerations myself.

Yes I've mulled over this quite a bit myself. Yes I agree that you can make the gantry sides shorter if you mount the rails to the y plate, but then you have to do one of two things to get the same travel; either make the y plate taller, or move the trucks closer together. Either way, you need about 3 inches of z plate below the lower truck. So it becomes a question of whether it's stiffer to beef up the z plate, or makes the gantry legs taller...

[pippin88]

The main problem for me is that if one wants to cut through 3 inches of material with a 3 inch bit, the minimum gantry clearance would have to be just over 6 inches to clear the cutting tool with the rails on z plate configuration but it would only have to be just over 3 inches for blocks on z plate configuration. However, if one has no intention to use long bits than this becomes less important.

Maybe my brain is fried from night shift, but I can't see how this works.
Travel is dictated by the total length of the Z rail minus the outer spacing of the Z trucks. In my case 300mm-150mm = 150mm travel (6 inches).
Where exactly that range of travel sits compared to the gantry is dictated by where I chose to put it in relation to the gantry, which isn't dictated by whether the rails sit on the Y plate or Z plate.
I want my travel to allow me to hit the table with the shortest router bit (so that I can always cut fully through with any bit). That gives me travel of slightly greater than 6 inches above the table surface.
If I want to cut a 3inch deep pocket with a 3 inch bit then I'll need to get to the top of my travel. If I've placed the low point of my gantry at 3 inches I can clear the 3 inch work and have 3 inches travel above the gantry bottom (and therefore get the 3 inch bit over the 3 inch work) - cool, but I'll only ever be able to clear 3 inches of work.
If I put the gantry bottom 6 inches above the table I'll be able to clear the same piece or a nearly 6 inch high piece.
Where the travel lies is simply dictated by the bottom edge of the bottom component of the linear motion (in Z rails on Y, it's the bottom edge of the rails; In Z rails on Z, it's the bottom edge of the bottom truck sitting on the Y plate)