should have thought of this. while I’m looking at the gallery, help me understand the working height.
for example, if I know I’ll ONLY be using 3/4 inch MDF. I should go with 3/4 working height?
thag would be 3/4 + 2.87 (as it’s the minimum stated on the cut calculator)
ok, so from what I see in the gallery, heres what I know I’ll be using this machine for: Everything.
Jokes aside, I’ll be using it mainly for wood and plastic. But I really like the fact that you can almost do everything with it. Im a graphic designer by trade, geek at heart, DIY lover. “printing” t-shirt with it sparks something in me, but also I like working wood. I have a speaker buildng project on the table, and a couple of logo sign (wall hanging decoration) to do for a couple friends already.
And I also think of a laser engraver for the (near) future.
My thought process for 4inch working height was: 2.87 in minimum + 2x 3/4 in =4.37in
picked a common speaker baffle (front face) thickness, and mutiplys by 2 because sometimes speaker has 2 baffle, so 1.5inch thick
So between 4in and 4.5 in of working height. Does that makes sense ?
The trick is, and the reason Ryan and I have trouble recommending high Z, longer Z can leave you with less rigidity. The consequence is that you might end up with more failed projects, and slower milling than if you kept it “tight”.
It’s totally up to you, but I would rather you build it small, get your beak wet, and then build it to accommodate a speaker build when you’re ready. By then, you will know what limits you have.
You won’t find many bits 4" long either.
You can also change the size relatively easily.
But, wood and plastic aren’t the toughest materials to mill, so until you do metals, I would say you’d be fine with a 3.5 or 4 in. Z. Remember that it gets weaker by the square of the distance, so a 2x increase in length is a 4x decrease in rigidity. The really long Z axis are really only suited for 3D printing.
Somehow I never realized it was quadratic. Yikes. Now I see, for a given load, twice the length means twice the torque (tilting away from vertical) so twice the angle of tilt. Twice the angle of tilt at twice the length is 4x the displacement at the tool.
At first I thought, that can’t be right… but after a little thinking I convinced myself that it is indeed correct
Where would the pivot point be on the gantry? At the center of one of the tubes depending on the direction of the force? I’m asking because there is some distance you can’t do anything about. You’ll always have the distance between that pivot point and the lowest bearings in the middle assembly that are holding the Z rails. Let’s say that distance is 2". If you go from 2" to 4" workspace height, then the lever arm length increases from 4" to 6". In that case, tilt would not be 4x, but “only” 2.25x.
Also, when I push on the end mill manually with the steppers turned on, I can see the roller bearings on the frame roll a little. This is caused by the belts stretching a little I think. When I let go of the end mill, the rollers go back to where they were. So part of the force on the end mill is used to stretch the belt instead of tilting the end mill. Belt stretch is dependent on the length of the belt, not on the workspace height. I have no idea how much tilt vs belt stretch there is in practice.
In practice, you can always bolt on a little bit more spoilboard to raise the height
I have the feeling that the length of the legs does not matter much. I did use stainless steel for the legs, but I can see or feel no flex at all in the frame itself when I push or pull it. Slightly longer legs would not be an issue I would expect.
If everything is working well it will use very little force. Where rigidity matters is the occasional bite. You need it to just chew through. If your belts are moving, the zip ties might be too long. If they are over 1" long, even if they are tight they will flex a little. The belts will stretch a little, but generally, if they are tight, they will not stretch much more with forces on the tool.
That’s all mostly based on my theories. I don’t know how much is measurably true.
Ok, zip tie story time. I know this is controversial on the MPCNC, but I’m convinced the design is not great on this point. I originally built mine with the standard parts. I kept the zip ties relatively short, and also used another zip tie to “squeeze” the zip tie into an 8 like shape (Ryan wrote about it somewhere in the build instructions I think). If anything, I over tightened the belts.
Everything was hunky dory. Then I got my SKR Pro with TMC2209’s and I tried sensorless homing. It didn’t really work, the drivers didn’t notice the stalls from crashing on the corners, so they kept on going and kept on skipping steps in the corners. But, more importantly, the belts were oscillating up and down like crazy, but only on the Y axis and not on the X axis.
On the Y axis, the (adjustable) zip ties were on the same corner as where I was trying to home. The steppers were pulling on the zip ties, stretching them, and allowing the rest of the belt to sag. This was causing the oscillation. On the X axis, I was homing on the non-adjustable zip ties, and they wouldn’t stretch, so no oscillation.
I upgraded to these tensioners and the problem went away. Tensioning and temporarily removing the belts is now a piece of cake. If the corners would be slightly redesigned, these tensioners could be even better (especially the ones on the top). I don’t know why they aren’t the default design.
Sorry for taking a simple question and making it seem very complicated. My advice: unless you want to mill aluminum, go for 4" if you think you’ll need it. See if it works within your tolerances. Unlikely, but if not, cut off 1". The plastic corner pieces do not require you to be very accurate with the cut. My disclaimer: I’m also quite new to this.
And there’s also the option of a drop table (a table with a removable middle section) if you only sometimes need more than your machine will give. With the spoilboard in, my table is 6" or 7" from the bottom of the outer frame tubes (depends which one you measure from). If I pull the spoilboard out I have a tray insert that gives me another 5 inches or so. With that tray removed I have about three feet to the floor!
This is to compensate for material thickness, not to give me three feet of z travel. LOL
Peter I think you are thinking in exactly the right direction.
People get fooled when it comes to tension because tighter does not mean stiffer, but it’s a common mistake to think it does. When people conflate the two, they can seriously over-tighten (esp easy when it’s turning a screw) which leads to other problems. This is why the standard design doesn’t invite super tight belts.
The “right” tension is just enough to ensure neither side of the belt ever goes slack over the range of loads. Normally this is set for cutting loads but sensorless homing is going to be a different requirement.
I would hope that sensorless homing would detect a skipped step regardless of the stiffness of the load, but thats another topic.
I suspect tight inside corners might suffer from low stiffness even when taking light cuts. If the cutting action causes the tool to deflect into the workpiece, it can get into a “runaway” condition and cut more than intended. This is not just with climb milling. Conventional milling can also run backwards over the section that has just been cut.
Having said that, it’s really an edge case and for most materials you dont need to worry about it.