I have built my first CNC machine based on a design from many influences. So far I have been very happy with its performance on timber, however the main reason for the build was to be able to cut aluminium. The problem I have been having is the cutter has a tendancy to vibrate, and sometimes jump across the cut. From what I can tell, this is because the axis deflects rather than forcing the cutter into the cut, and then it sort of "catches up". Sorry my description isnt the greatest. I am looking for any ideas on how to strengthen the interface between the X & Z axis, and the Z axis itself to try and increase rigidity in this area. At the moment, the design has a 1m wide X axis, which faces away from the router. The Z axis is floating - the lead screw nuts are fixed to the gantry plates, and wheels keep the Z axis in place. I am open to any and all ideas from the community!
Cutting a Z axis mount plate about 2" taller with 2 sets of wheels at the bottom and one set at the top would help some but probably not solve everything. The problem likely goes beyond the mounting plate and to offer any more suggestions we need to see photos of the machine you've built in order to help identify any weak spots.
I will get my model sorted to the current design, but basically it has OX style plates between the X and Y axis, with 1m C beams for X & Y.
Okay here is a couple more snapshots of the design. The model is missing some wheels and things, but you get the idea. The Gantry is connected to 12mm thick aluminium plates, which are also the Y axis plates. Currently only 2 wheels top and bottom of the Y axis, but I intend on putting three in there, and adding an internal plate on the opposite side of the C beam to support the wheels. I suspect most of the deflection is coming from the wheels guiding the Z axis.
Ultimately, the leverage induced by the Z axis reaching so far down to get the bit to the bed is overpowering and bending the 5mm wheel axles. The plate mentioned above will help resolve this but only to a limited extent. The best solution is to not make the Z-axis reach so far. So in addition to the plate mentioned above I suggest adding a 4 or 5 inch stack of MDF to get the aluminum up as close as possible to the fully raised bit thus greatly reducing the leverage on the Z axis and subsequently the forces on the wheel axles.
Thanks Rick. The height of the gantry is a design feature. The frugal side of me doesn't like having 100mm of Z travel which cannot be utilised. I assume you are meaning the wheels on the Z axis? I could potentially make some sort of support to stop these bolts bending.
You need to learn the difference between frugality and pride. A frugal person doesn't waste aluminum. Pride is all that is keeping you from doing what I have suggested. I am only suggesting it for when you are cutting aluminum. The rest of the time your design feature can remain intact. As far as stiffening the wheels there is not much that can be done. Doubling them at the fulcrum point is really the only feasible solution. Lengthening the back span will help but the best solution is reducing the length of the lever arm.
I guess I need to look at an alternate solution then. I guess a custom Z axis with linear rails is where I need to head to achieve what I want. I understand reducing the gantry height will reduce the leverage, but I would rather design a solution to solve the design requirements.
Alternate solution: Build a different machine. It doesn't matter what you do. As long as you have that much leverage, anything short of a tank will not satisfy. You build according to what you have in mind. If you are only cutting foam, then that is no problem. once you h=give it anything hard (wood, aluminum, even plastic), you're going to have a bad time. Rick told you what you need to do. There's no point in continuing this thread if you won't consider their suggestions.
What is "that much leverage"? The intention is to have no more than 100mm Z travel. Just because one person says it can't be done, doesn't mean it can't. There are a lot of very clever people on here, all with their own ideas. I am hoping someone may suggest something I haven't thought about, which will be a cheaper and easier option. I am currently working on a potential solution using SBR rod and bearings to replace the Z axis wheels. For the record, the machine as it currently stands runs excellent with plastic and wood.
Here is a quick model of an idea I had. I only had a model of open topped bearing blocks, but would use closed tops. I would expect that with SBR16, any leverage would be transferred to the gantry, making this the weakest point.
You said it yourself. "The height of the gantry is a design feature." Once you reach a certain point of strength in the joint, the c beam itself may not be able to stop the torsional forces. What do you want to cut with the machine? Your main post says aluminum, but do you plan on putting 4" thick aluminum through it?
By design feature - I should have said design requirement. I dont intend on cutting 4" thick aluminium, but I do have a project lined up to deep pocket 2" thick. I want to have the extra clearance for some projects I have in mind. I understand the C beam may no longer be able to withstand the torsional forces. If that turns out to be the case, I will look at strengthening that. The idea is for this machine to build itself into a competent hobby milling machine.
Here is another iteration of the design, with additional linear guides on top and bottom of the X axis. I think this would be a very simple modification, which would add a significant amount of torsional strength to the whole mechanism. What do you think?
The ones on the X axis won't be of much account because of their length but the ones on the Z axis should help a bit but I would still worry about flex in the round bars. As a suggestion, you might consider the rectangular bar type glides, anchoring them to the sides of the Z axis C-beam with the glide blocks attached to the plate. This gives a much more direct flow of forces out of the system and the C-beam will help stiffen the guide bars thus taking even more flex out of the system.
Thats an interesting idea - will see how it goes. In regards to the X axis - I was thinking about doing the linear rod as well as wheels. I could bolt linear (rectangular) rails to the top and bottom of the X axis C beam, but then the torsional strength of the C beam would be the limiting factor. I am trying to find information about the material properties of chrome bar to work out how resistant it is to bending, but I am struggling to find it unfortunately.
