Re: Jonny's CNC portal mill
Posted: August 11th, 2016, 3:52 am
For those who are designing their own CNC mill as well:
The most important thing that I've learned about designing CNC linear systems:
The good thing about precision linear rails is that they have play tolerances in the 5-15 um range, maybe a bit more for the usual cheaper quality stuff. The bad thing about that is, that you will have to align these rails in the very same accuracy range - because there is virtually no play, which means no room for error. Say you have a 0.1mm parallelity error between two rails of the same axis. For one, the axis will not run smoothly, and might even get stuck. And the other thing is, the balls inside the carriage will be worn out quickly and the carriage will get stuck and break eventually.
Now, it's easy to design stuff in CAD 100% accurately aligned. But you have to be able to build that in reality. In my case, I have two 1000 mm long rails that are 710 mm away from each other. How will I get these two perfectly aligned against each other? Note that the frame in between is made up from many different aluminum extrusions, which doesn't help at all
This is the main reason why I don't like those "raised gantry" type machines. It's even more complicated to get the rails aligned up there.
Professional machines solve that problem by just making one large cast iron base, and precision-milling the mounting faces for the linear rails, effectively creating nearly perfect reference planes. I decided to try and build my frame able to be aligned instead of face-milled. That will be much more difficult, but I don't need access to a huge CNC work center that can work with such a large piece.
So, what's my plan to get the thing aligned?
I start off with two beams. These are 40x80 mm Misumi HFS8 alu extrusions. They are strong like a tank, and completely impossible to deform with human strength. I put these together, but not tightened 100% yet.
View rotated horizontally by 90° clockwise
Now I add the build platform piece by piece (30x90 mm extrusions) into the frame.
Part flipped around 180°
The build platform parts are linked together using these 2.5mm steel brackets to ensure that they do not bend upwards or downwards.
Add all the platform beams and links, and then tighten everything down, making sure that the angle between the 40x80 mm beams is 90° and the frame is not warped in it's horizontal plane.
Part flipped back 180°
And then add the next 40x80 mm beam (not tightened 100% yet)
Now, these two 40x80 mm beams will hold the X axis rails (added as an illustration). So they need to be aligned perfectly parallel to each other, or my portal will get stuck or be hard to move on one or the other end of the machine.
What I plan to do here is to fix the left 40x80 mm beam and build platform to the rear 40x80 mm beam in a good 90° angle. Then adjust the right 40x80 mm beam (later when fixing it to the frame) until the two opposing beams are perfectly aligned.
I will measure that using a dial gauge (0.002 mm resolution) mounted on an aluminum extrusion beam: But first I need to add the last 40x80 mm beam, in order to be able to fix the previous one.
Now, the problem about these extrusions is, they are cut with 0.5mm length tolerance (which is already pretty good, but it's not perfect).
Because of this, I made the 30x90 mm extrusions 0.5mm shorter than the 40x80 mm ones. I will then add 0.1mm thick shims to get them within 0.1mm of the 40x80 mm beams.
Then add the final beam. The right side (in this view it's actually the rear side) is still not fixed 100%, because now it's time to do that alignment and secure the frame in place.
Now, the sides marked with red arrows have to be aligned perfectly. The other sides marked blue do not. The shims I added earlier are not needed to make the frame parallel, but to prevent the beam from deforming by more than 0.1mm when tightening down all the screws.
And of course the whole frame can't be warped in its horizontal plane, which is why all of this has to be done on a perfectly flat surface. I know, my reference plate is a bit too small for that, but I have to make do with it. I will also try using a water bubble to check the levelling.
All in all, the plan is not perfect and it will be difficult to do it. But I hope this will work...
/edit:
On most other machines one would first assemble the whole portal, then use the portal as a reference for aligning the rails. I will probably have to leave one of the X rails loose until the portal is mounted and then do that alignment with the portal as well. But for this to work, I need to have the reference faces correct. Other machines would actually leave the whole frame loose and tighten it around the portal, aligning the frame "into" the portal. I could do that as well if nothing else works.
/edit2:
By the way, one very important thing to consider is that during all of this, the ballscrews are not mounted yet, and have to be adjustable to whatever final result there will be. Make sure to consider that as well, because it's just as bad to have a misaligned ballscrew than having a misaligned rail.
The ballscrew has to sit loosely and be aligned by moving the axis up and down all the way, then tighten the ballscrew mounts piece by piece until everything runs smooth. In my case, the ballscrews are mounted with steel brackets that sit on the 40x80 mm beams. These brackets can be shifted to align the screw and then tightened down where they are.
/edit3:
One more thing for me to think about:
Once that frame is all tightened down, it must be strong enough to never deform. The question is - will it be?
I think it will help a lot when I cast 50 kg of concrete into the whole underside of the frame. It will fill all these notches and hold everything together. Once that has dried out, the frame should be solid and super tough. Will talk to my brother about that concrete, he is a construction foreman..
The most important thing that I've learned about designing CNC linear systems:
The good thing about precision linear rails is that they have play tolerances in the 5-15 um range, maybe a bit more for the usual cheaper quality stuff. The bad thing about that is, that you will have to align these rails in the very same accuracy range - because there is virtually no play, which means no room for error. Say you have a 0.1mm parallelity error between two rails of the same axis. For one, the axis will not run smoothly, and might even get stuck. And the other thing is, the balls inside the carriage will be worn out quickly and the carriage will get stuck and break eventually.
Now, it's easy to design stuff in CAD 100% accurately aligned. But you have to be able to build that in reality. In my case, I have two 1000 mm long rails that are 710 mm away from each other. How will I get these two perfectly aligned against each other? Note that the frame in between is made up from many different aluminum extrusions, which doesn't help at all
This is the main reason why I don't like those "raised gantry" type machines. It's even more complicated to get the rails aligned up there.
Professional machines solve that problem by just making one large cast iron base, and precision-milling the mounting faces for the linear rails, effectively creating nearly perfect reference planes. I decided to try and build my frame able to be aligned instead of face-milled. That will be much more difficult, but I don't need access to a huge CNC work center that can work with such a large piece.
So, what's my plan to get the thing aligned?
I start off with two beams. These are 40x80 mm Misumi HFS8 alu extrusions. They are strong like a tank, and completely impossible to deform with human strength. I put these together, but not tightened 100% yet.
View rotated horizontally by 90° clockwise
Now I add the build platform piece by piece (30x90 mm extrusions) into the frame.
Part flipped around 180°
The build platform parts are linked together using these 2.5mm steel brackets to ensure that they do not bend upwards or downwards.
Add all the platform beams and links, and then tighten everything down, making sure that the angle between the 40x80 mm beams is 90° and the frame is not warped in it's horizontal plane.
Part flipped back 180°
And then add the next 40x80 mm beam (not tightened 100% yet)
Now, these two 40x80 mm beams will hold the X axis rails (added as an illustration). So they need to be aligned perfectly parallel to each other, or my portal will get stuck or be hard to move on one or the other end of the machine.
What I plan to do here is to fix the left 40x80 mm beam and build platform to the rear 40x80 mm beam in a good 90° angle. Then adjust the right 40x80 mm beam (later when fixing it to the frame) until the two opposing beams are perfectly aligned.
I will measure that using a dial gauge (0.002 mm resolution) mounted on an aluminum extrusion beam: But first I need to add the last 40x80 mm beam, in order to be able to fix the previous one.
Now, the problem about these extrusions is, they are cut with 0.5mm length tolerance (which is already pretty good, but it's not perfect).
Because of this, I made the 30x90 mm extrusions 0.5mm shorter than the 40x80 mm ones. I will then add 0.1mm thick shims to get them within 0.1mm of the 40x80 mm beams.
Then add the final beam. The right side (in this view it's actually the rear side) is still not fixed 100%, because now it's time to do that alignment and secure the frame in place.
Now, the sides marked with red arrows have to be aligned perfectly. The other sides marked blue do not. The shims I added earlier are not needed to make the frame parallel, but to prevent the beam from deforming by more than 0.1mm when tightening down all the screws.
And of course the whole frame can't be warped in its horizontal plane, which is why all of this has to be done on a perfectly flat surface. I know, my reference plate is a bit too small for that, but I have to make do with it. I will also try using a water bubble to check the levelling.
All in all, the plan is not perfect and it will be difficult to do it. But I hope this will work...
/edit:
On most other machines one would first assemble the whole portal, then use the portal as a reference for aligning the rails. I will probably have to leave one of the X rails loose until the portal is mounted and then do that alignment with the portal as well. But for this to work, I need to have the reference faces correct. Other machines would actually leave the whole frame loose and tighten it around the portal, aligning the frame "into" the portal. I could do that as well if nothing else works.
/edit2:
By the way, one very important thing to consider is that during all of this, the ballscrews are not mounted yet, and have to be adjustable to whatever final result there will be. Make sure to consider that as well, because it's just as bad to have a misaligned ballscrew than having a misaligned rail.
The ballscrew has to sit loosely and be aligned by moving the axis up and down all the way, then tighten the ballscrew mounts piece by piece until everything runs smooth. In my case, the ballscrews are mounted with steel brackets that sit on the 40x80 mm beams. These brackets can be shifted to align the screw and then tightened down where they are.
/edit3:
One more thing for me to think about:
Once that frame is all tightened down, it must be strong enough to never deform. The question is - will it be?
I think it will help a lot when I cast 50 kg of concrete into the whole underside of the frame. It will fill all these notches and hold everything together. Once that has dried out, the frame should be solid and super tough. Will talk to my brother about that concrete, he is a construction foreman..