Aluminium FEA optimalized CNC router

[njhussey]

If you've got the Z blocks over the Y blocks how the heck are you going to do one set up??!

[PotatoMill]

Magic I suppose. I know there is a wizard over at cnczone. The setup was mainly for testing out forces and deflection, to do many iterations and test concepts I did not go trough all the details. Actually I think i'm going to use normal blocks on one side and the wider carriage on the other side. I'm going to post a image of a better assembled design soon.

[PotatoMill]

Little update on the Z and Y cartdesign, after a lot of iterations i'm now converging on a better solution for the design. However there are still changes to make and improve upon, and I hope on some feedback on my design.

A image of the whole thing. With the 100x200mm t=6 steel square tube in the front.

New setup on the stepper motor for the Z axis with a belt and pulley system. The Stepper is also mounted so it's easy to tighten the belt. It's also easy to pluck out of the machine.
The end of the ball screw it fixed with a custom double ball bearing fixture. Using 10mm thick SKF 6201 ball bearings.

Njhussey, after some back and fourth I managed to make space for the Mounting of the Z and Y blocks over each other.

Here is the square tube see-trough so it's possible to see the mounting of the spindle and the ball screw mechanism. I had to make a cut in the square tube from the top to make space for the ball screw. This did not however weaken the rigidity of the Z cartsignificantly.

[PotatoMill]

Update: Pictures is not good for viewing a design so I have placed it on GrabCAD for everyone to view in 3D. https://grabcad.com/library/z-cart-cnc-1it's currently only the Z and Y cart since i'm working on changing the gantry and the base.

[routercnc]

It's an interesting design and if you really want to do the Z axis that way then I would add a reinforcement panel on the front outer face of that RHS box to spread the load of the spindle out to the edges (I assume the spindle is mounted just on the front face and not sandwiched between the front and rear faces?). Say a 8-10mm thick aluminium plate as wide as the RHS and enough height to pick up on the 4 spindle mounting bolts. Otherwise the loads will go into the weaker membrane panel and not be transferred to the stiff edges. If you simplified your loading condition on the analysis this will not show up.

Same could be applied to the rails, I would add a thin strip of say 6-8mm thick steel, by 30mm wide on the inside of the RHS directly where the rails mount and screw the rail bolts right through into it.

Also in post#9, first picture, the upright bracket holding the ballnut to the Z axis is too thin. It needs to be approaching the length of the ballnut and then bolted down in 4 places. It should be a 'boss' not a 'bracket'. Also the plate it sits on is on the small side if you are trying to maximise performance. This needs a better connection to the upright Y axis plate. As drawn it will twist when loaded. Maybe a similar plate parallel to the current plate but above the ballnut would work trapping the ballnut in-between 2 plates.

Keep exploring, it's good to see something different every now and again.

[PotatoMill]

It's an interesting design and if you really want to do the Z axis that way then I would add a reinforcement panel on the front outer face of that RHS box to spread the load of the spindle out to the edges (I assume the spindle is mounted just on the front face and not sandwiched between the front and rear faces?). Say a 8-10mm thick aluminium plate as wide as the RHS and enough height to pick up on the 4 spindle mounting bolts. Otherwise the loads will go into the weaker membrane panel and not be transferred to the stiff edges. If you simplified your loading condition on the analysis this will not show up.

Same could be applied to the rails, I would add a thin strip of say 6-8mm thick steel, by 30mm wide on the inside of the RHS directly where the rails mount and screw the rail bolts right through into it.

Also in post#9, first picture, the upright bracket holding the ballnut to the Z axis is too thin. It needs to be approaching the length of the ballnut and then bolted down in 4 places. It should be a 'boss' not a 'bracket'. Also the plate it sits on is on the small side if you are trying to maximise performance. This needs a better connection to the upright Y axis plate. As drawn it will twist when loaded. Maybe a similar plate parallel to the current plate but above the ballnut would work trapping the ballnut in-between 2 plates.

Keep exploring, it's good to see something different every now and again.

The spindle mount is bolted from the front and the rear of the large square steel tube. However I'm not satisfied with it because the mounting of the plate can maybe drag the clamping around the spindle apart. I also need to make tramming possible.

The nice thing with the steel square tube is all the space for mounting parts. If I eventually want to add a 3:1 belt reduction on the spindle, for steel. (whisfull thinking)

Plates on the inside for the rails sounds like a good idea.

The upper ballnut connection is made of 3 layers of 10mm aluminium plates, where the lower one is bolted to the large steel tube. The grabCAD model should show it better. Or I'm misunderstanding something.

Thanks for the comments it have helped me removing a lot of weak points.

Edit: I see I forgot to add the mounting holes for the backside of the spindle attachment on the square tube.

[PotatoMill]

Now the whole thing begins to come together again. I have done some more FEA on the whole frame. I will just add that they are approximations and gives a rough overview. Joints and mounting points will add to the deflections.


Base:
I did some test with and without the side supports. The side supports make the base a lot more rigid. However, without the side support the deflection on the base is still insignificant compared to the gantry and the y and z carts. I also looked at how steel would increase the rigidity, but since the deflection on the base already was insignificant on the total frame, the increased stiffness from the steel was very small on the whole frame. The base is about 57kg with aluminum and 105kg with steel.

Gantry:
The gantry have a lot to say on the stiffness of the frame especially torsion that add to deflection on loads in the x axis. The gantry is changed from a frame with a lot of 60x60mm square tubes to a single 300x200mm t=6 rectangular tube. The stiffness of the new setup is about the same, but it makes the assembly a lot easier. Which means there is less areas to make mistakes the rigidity of the machine will suffer from. I had to go for steel on the gantry to get it rigid enough, an aluminum rectangular tube was just not enough. Casting aluminum could be possible, but that will again make it more complex and prone to errors. The gantry is still movable on its own, but it is now at 50kg. The Y and Z cart is at 20kg so in total the gantry and Y and Z cart will be 70kg.


The design on GrabCad is now updated https://grabcad.com/library/z-and-y-cart-gantry-cnc-1. However, the ball screws for moving the gantry is not finished yet. And I see that mounting of the rails for the Y cart makes it impossible to attach the Y cart so I have to find a solution for that. Moreover, it would be great if someone have a good suggestion.

[PotatoMill]

I'm currently considering making a stationary gantry design. Since the weight of the gantry pluss the z and y cart comes inn at 80kg (mostly in steel).
The cons will be;
-Longer machine
-Longer rails
-Potensial for sag in the bed

Pros
-Less weight to move for the steppers
-Simpler design
-Less twist on the z axis, the largest factor for displacement

The reason i'm posting this is because it probably a lot more pros and cons than I can come up with, that I hope someone here will add.

[PotatoMill]

So I have been working on redesigning the whole machine. I'm currently going for a stationary gantry design.

Due to the fixed gantry design the heaviest moving part is now the base plate with 30kg. I'm going to use 20mm HIWIN profile rails on all axis.
There will be one lead screw on each axis, no need for two on the X any more due to the stationary gantry. Here i'm going to use 1605 ball-screws. To drive the axis i'm currently looking at using servos in the 200-400W range. There is not the same amount of information on servos av steppers so I hope someone here have some knowledge on servos. Seems like JMC produce the cheapest servos and drives, but I cant find a lot of data on them. If there servos from JMC are good enough i'm probably going to use them. BST automation also seems to have a ok selection on servos, but again there is little documentation to find.

[Lee Roberts]

BST automation also seems to have a ok selection on servos, but again there is little documentation to find.

If you send them a msg Fred will be more than happy to help and send you what you need.

.Me