I few weeks ago, I decided to get a CNC router to cut foam, carbon fiber and maybe some aluminum. I had also decided not to build one myself, because I knew router building often end up as the new hobby instead of the hobby you originally built the router for. Then someone directed me to this forum and all my plans changed.
I am now planning to build an aluminum-framed router that will eventually have capability to cut different ferrous alloys. In addition, I hope I can do some high-speed tool-paths in aluminum. In the early stages of the design, I planned to use aluminum profiles for the build, without any need for welding, but profiles are expensive and did not have the stiffness I wanted. Then I went on to steel, but that got to heavy. So now I am going to use 60x60mm t=4mm aluminum square tubes welded together. I have to practice my aluminum welding some more before I start with the build. I have done FEA on most of the build, where I started with requirements of a frame stiffness of 10-20 N/μm (10'000 N/mm - 20'000 N/mm). Deflection is proportional with force so I have checked the stiffness with lower forces and then multiplied up to 10'000 N and 20'000 N. I have done many iterations on the frame and the FEA images under is only the most relevant for the current design.
Planned working area is 1000x500x250mm
The plan is to use 20mm profile rails and 16mm ballscrews on all the axis. With dual 1000mm drive one the X-axis. I'm going to use nema23 stepper motors but i'm still uncertain on the torque requirements I have to do some more calculations there. Though it seems like most of the steppers have a very high corner speed and to utilize all the torque and speed of the motors they should be geared down about 3:1. Any thoughts on this?
Base assembly (500N loads)
Stiffness; 223 N/um. I tried some different setups for vertical loads on the base. 5 vertical supports on the sides gave 4.8 times more stiftenes on the worst case compared to 3 vertical supports.
Stiffness; 53 N/um. No need for cross bracing on load in the X axis it's more than stiff enough.
The weak link where the forces in the Y axsis, this was fixed with some side bracing
Stiffness; 51 N/um. The side bracing gave 4.7 times higher stiffness. Success! (I tried a lot of different potential setups before deciding on this one)
Gantry (100N loads)
The whole weight of the gantry including the Z-axis came to 32kg, i'm going to see if I eventually can cut the weight down. I had hoped for something in the range of 20kg.
Single aluminium tube thick gantry. Were within 10 N/μm but not 20.
Stiffness; 32 N/um. Double thickness improved the stiffness 2.7 times, and the weight gain was small compared to the gain in stiffness.
Z - axis (100N loads)
The Z -axis ended up on 12kg. I have looked for ways to reduce the weight but often the stiffness suffers.
I'm going to use a 2.2kW water-cooled spindle. However if it is going to be use for ferrous alloys I have to gear it down to reasonable speeds. The plan is to use a belt reduction, but it's probably going to be a later update.
Under is some pictures of the results on the Z-axis. This is clearly the weakest link on the whole machine. It's no point making the the rest of the machine stiff if this part has 1/10 of the rigidity of the rest og the machine. The two vertical bracings on the front improved the stiffness but it's still not inside my requirements. The best solution to this that I have seen is Routercnc's MK4 design. However I dont have a CNC machine to build a CNC machine and I don't want to over-complicate my first build. It would be great if someone could come with input and suggestions on the design before I start ordering stuff, or it could become very expensive.
Stiffness of the Z-axis in the X direction (worst load direction) 3.9 N/um.
Stiffness; 0.85 N/um
Last edited by PotatoMill; 26-04-2016 at 08:35 AM. Reason: Wrote inn the load on the different setups
Have you done an analysis of resonant frequencies with the gantry and z in a variety of positions?If you will not be swayed by logic or experience simply pick the idea you
like best, but ask yourself why you sought advice in the first place and,
for a simple life, perhaps consider not doing so in future
Nice pictures. 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.
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 . . .
[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.
Last edited by PotatoMill; 26-04-2016 at 08:25 AM.
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.
If you've got the Z blocks over the Y blocks how the heck are you going to do one set up??!Neil...
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.
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.
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