I do not cut metal, haven't cut any plastic and mainly hardwoods. I built my own design, similar to the Gatton CNC early design, 1" plywood with Angle aluminum rails, but with OpenBuilds components. Except for the X-axis, which is C-Beam, the rest is V-Slot. I measured deflection on the Y-axis and got a few thousands, but when I measured the deflection on the gantry I got the +/- movement when pressed. I have checked and rechecked the eccentric adjustment, the rollers are tight. I thought that I needed to widen the distance between the rollers so I made an outrigger that extended the fore and aft distance to about 300 mm roughly 120 mm on each side of the riser. No help there. Perhaps I should have used C-Beam for both sides of the Y-axis. I have the Y-axis V-Slots attached to the spoil-board with 5 inside/outside corner brackets, which stiffened up the side-to-side movement. When I do a cut with a 1/4 in end mill .4 in deep the tool (I believe the whole assembly) will deflect .11 at the start and the end but moving on the X-axis will cut a straight cut. I must find a solution as I will be unable to do inlays with this error. My Gatton, which I used for maybe 7 years was just worn out and I wanted a stiffer more accurate machine. I thought that OpenBuilds products would provide that for me, and I still think it will, I just need to find the solution. Any help will be very much appreciated. Ken Hall
While I haven't been able to work on it too much the last few months, I did some basic deflection FEA on my gantry because I noticed how much C-Beam can bend (a lot more than I had previously thought): M4: 1510SS Heavy Mk.I - adding beams is heavy, and required me to go to NEMA 34 steppers on Y, but everything is a compromise in machine building. You can also fill the channels with various compounds- concrete, mineral epoxy, self-etching engine block filler, etc.- that provide some shear resistance inside the columns themselves. Otherwise, it's about machining strategies and your roughing/finishing setup.
Thank you for that information This was my original attempt to solve the column movement problem, spreading the force over a greater distance. Unfortunately, I did not have the means to put compression from the bottom, so the original wheels still controlled the deflection left and right up to .01" I am not even yet at the tooltip deflection, although I did see a deflection of about .011" when plunging a 1/4" endmill at 25 in/mm and 18000 rpm, had the same deflection at the beginning and end of cut into 1/2" ply, cut depth .4" and horizontal cut of about 3 inches. Here is a sketch of the design and a couple of images of what it looks like from the outside and inside. I am convinced that if I can get the columns rigidly upright that I will be able to do inlays in hardwoods accurately enough. The x-axis is a C-Beam, all others are V-Slot. I measured deflection in the middle and at the far end of the Y-axis and only had about .002" I don't think that is a major part of the deflection at the top of the X carriage.
These are very short carriages, which may be contributing to the long-lever instability. Those wheels have to be spread way out (and probably add 2-4 more of them per axis), probably to 200mm or even 300mm long carriages with diagonal bracing. Or go to linear rail, which would allow you to have relatively short carriages with very high rigidity, though then your column attachment method really matters a lot since there's less room for gussets. Generally the more rigidity you need, the less working area you'll be able to have.
Yes, that was my thoughts about the wheels needing to be spread out more. Unfortunately, I do not have a machine shop and am not a mechanical engineer so I am just winging it. I was convinced that the closeness of the wheels was the problem. I have ordered larger plates and additional wheels, hopefully, that will bring the problem into a more acceptable area. The Sphinx that they had as a kit had a much larger carriage and many wheels on each side of the X-axis. It appears that they no longer carry that unit or the plates that they used. Thank you for your insight.
