Aluminium FEA optimalized CNC router

[routercnc]

Hi Potatomill

Some good work there using analysis to explore the design. Some comments -

Although aluminium is 3 times lighter than steel it is also 3 times less stiff. Using steel in the base assembly, where the extra weight is of no consequence and is actually very beneficial, might negate the need to add the extra triangular bracing. Especially if you can go for high wall thickness.

Don't forget that the stiffness you are aiming for (10-20 N/um) is for the WHOLE machine, from tool tip to bedplate. If you design each sub-assembly to be 10-20 N/um you will get somewhat less stiffness in total because they will behave like a set of springs in series. In other words each sub-assembly needs to be significantly stiffer than 10-20 N/um so that when added together the total stiffness is in the 10-20 N/um range.

The Z axis is always a challenge with this style of machine. Try 20-30mm thick aluminium plate and see what that does. I suspect it will not give what you want especially at 450mm long. So another option is to use a piece of RHS (steel) with say width similar to the current plate width, depth 80-100mm, and length 450mm. Then house the spindle inside with cut outs or access holes to the spindle as required.

I think the limiting factor of the design, if you really are trying to get 10-20 N/um will be the torsional stiffness of the gantry as seen by the tool tip (i.e. Z axis and gantry modelled together with load applied at tool tip).

Good luck with it all . . .

[PotatoMill]

[QUOTEmagicniner;79528]Have you done an analysis of resonant frequencies with the gantry and z in a variety of positions?[/QUOTE]
Not yet but eventually I’ll get to it. I don’t know how accurate it will be because of all the simplifications i need to do on the analysis. Frames like this have a tendency to vibrate quite a lot.

I would advice you to make bed analysis as well. Consider the surface you will mount the hole machine. On floor or on table. Maybe you need adjustable legs. This will probably change all calculations you made so far.

I'm not completely satisfied with the base fixturing myself. I do not have the most ideal location for my machine so I have to move it twice a year. Through a door up some stairs. Hence, the plan is to put weight on it when it is in place. If I find a better location for the machine, I will change the design accordingly.

Although aluminium is 3 times lighter than steel it is also 3 times less stiff. Using steel in the base assembly, where the extra weight is of no consequence and is actually very beneficial, might negate the need to add the extra triangular bracing. Especially if you can go for high wall thickness.

I totally agree. Steel would be first choice if I find a good place for the machine. The Young's modulus for steel is certainly better than for aluminum.

Don't forget that the stiffness you are aiming for (10-20 N/um) is for the WHOLE machine, from tool tip to bedplate. If you design each sub-assembly to be 10-20 N/um you will get somewhat less stiffness in total because they will behave like a set of springs in series. In other words each sub-assembly needs to be significantly stiffer than 10-20 N/um so that when added together the total stiffness is in the 10-20 N/um range.

20 N/um is an optimistic goal. I had to do a lot of simplifications in the analysis. The worst case for the base is currently at 51 N/um and the gantry 33 N/um. As you pointed out, the twisting of the gantry, the deflection will increase with the length of the Z-axis. The base will not be as stiff either due to the fact that it's fixed to the ground in the analysis. I forgot to write it in the original post, but 500N loads where used for the base and 100N for the gantry. Adding it now. Deflection is about proportional with force, so I just multiply it up to find the stiffness in N/um.

The Z axis is always a challenge with this style of machine. Try 20-30mm thick aluminium plate and see what that does. I suspect it will not give what you want especially at 450mm long. So another option is to use a piece of RHS (steel) with say width similar to the current plate width, depth 80-100mm, and length 450mm. Then house the spindle inside with cut outs or access holes to the spindle as required.

I will explore this setup, and some other potential setups and post what I find out.

[PotatoMill]

Update on the design. I have finally managed to get sufficient stiffness in the Z cart. The solution was no surprise, maximizing second moment of area, and... steel. I also moved the Z blocks down as far as I could and placed all the Z blocks over the Y blocks. I am now planning to use a 100x200mm t=4mm square steel tube. The Z cart became twice as heavy, now 20 kg (44 lbs). However, about 20 times stiffer. The large cut on the backside is so I can get more space for the ball-screw and eventually remove the spacers for the Z blocks. I did test if the cut would compromise the design but it held up well to loads in all directions and torsional load.


Now that the Z cart is good, torsional stiffness in the gantry is the problem. I am also doing the analysis on the whole gantry with Z and Y carts. The connections between the blocks and rails is not correct. They are just bonded. I do not know of any good ways in SolidWorks to make them in an efficient way, without messing around with spring connections. Therefore, it is a gross simplification, but it should give an approximate overview of the deformation.

The image is with both gravity and a 100 N (22.4 lbs) load in the X direction, a potential worst-case load. It gives a deflection of 0.014 mm (0.55 thou). With some vibrations, joints, and the base, the deflection will be bigger. At this point, the structural frame is not the weak link any more, but ball-screws, rails, steppers, drivers, and so on. The stiffness is at 13N/um now. As I am a newbie on this, I wondered if this is a good point to settle?

The gantry, Z and Y carts weight about 40 kg (37.8 lbs) with a aluminium gantry. And 65 kg (143 lbs) with a steel gantry. So I guess steel gantry is not out of the question.

I also looked into changing the design on the gantry, by using two parallel beams and having the Y and Z cart mounted in the middle. Like Routercnc's MK4 router design. This had many advantages on the rigidity and weight distribution.

However, this would require rails on both sides of the Z cart and I cannot find any good way to prep the surface and mount them accurately. With the classic gantry design, I can use epoxy, but with the parallel beam design, the Y and Z cart becomes a problem. The design would also be a lot more complex with parallel ball-screws on Y. In addition, four rails on Z or parallel ball-screws here too. And I have enough problems to worry about already, and on a first build it's best to keep it simple.