I want to build a 4’ x 4’ mpcnc to cut hard wood and pine. My thought is to use 1” bar stock for outer rails and a heavy gauge steel for inner rails. Will the belts handle it at this size? What I think I have gathered from the forums so far is that you have to know cam fairly well I’m assume to set up shallow passes in hard wood other than that is this size too large to handle hardwood effectively?
The general wisdom on the forum embraced by both the machine designer and experience users is that for any work area over 3’ x 3’, Ryan’s other design, the LowRider is a better choice. It is designed to cut full 4’ x 8’ sheet goods. A Primo MPCNC with 4’ on a side will need gantry near 5’. Sag will be a real problem at these lengths, and that is a lot of mass for the steppers to be throwing around.
My thought is to use 1” bar stock for outer rails and a heavy gauge steel for inner rails.
There are many discussions on stiffness and how it might be improved on this forum. In general going to heavier materials has two downsides. First, is sag. While the heavier material is stiffer, it also must support more weight. In the end, beyond a certain point, heavier materials make the situation worse. The second problem is that the steppers must accelerate all the additional mass. That introduces a list of problems and generally leads to the need to cut slower. Ryan has worked hard in his material choices to hit the sweet spot between stiffness and limiting the mass. Given the plastic parts, I think he is on target and you will be disappointed in your performance with heavier materials.
is this size too large to handle hardwood effectively?
Size does not change much in how a router cuts through material, and Primo users cut hardwood all the time. Accuracy is the big problem with larger Primo machines.
Thanks for your feedback. I may just scale back to 3 x 3.
Just to be clear 3’ x 3’ would be actual workable area or physical outside dimensions.
Outside. It’ll have a smaller workable area.
Perfect thanks
A little extra info…i have a 2x4 primo (work area). The steppers don’t have any problem moving the 611 at the speeds we use, so that’s something. I also seem to have more sag in my table than I have in the primo, as I’ve noticed about a 20thou dip in the middle relative to the endmill, but the short direction tube is likely bearing a lot of the weight(and I’ve put braces on the rails, but they always seem to move easily, even when the router is directly over them).
What i DID have problems with was the flex in the long tube when cutting along the short direction. It would vibrate back and forth on moderately aggressive cuts. In the xy plane. I’ve epoxied a half inch emt into it because I’m too cheap to spend money on serious stainless, even just for that direction. It helped a bit, but I still get some charter. If it I get to cut too much. Easy to clean up with a finish pass, but something to be aware of. I posted a video somewhere before the epoxy.
So, not exactly the size build you’re looking for, but plenty of reason to think a 3x3 would be as rigid as a 2x4, or at least similar. On the other hand, if you want to cut 4x4 sheets, you could build a 2x4 and tile it. I’d feel comfortable doing that with mine as is, even better if it were the lighter, less flexy 660.