Do you all do any sort of bed leveling for CNC? I’m working on a 3’ x 6’ cutting area build, so I’m expecting flex to be an issue… Is there any reason that hooking up a height probe to determine the Z axis adjustment for the bed would not work? I would think the matrix-based height mapping found in most every firmware would work well for this.
It is always best to just set the machine up properly. Even with 3d printers. Software based fixes are never as good as proper assembly.
Sorry, I guess I wasn’t being clear… I want to account for flex of the gantry rods. I plan to do some complicated 3D joinery on large pieces, so getting the height correct is crucial (I recognize I might not be able to do this, but I want to try). I am planning to use a total of 6 supports (2 for the 4’ axis, 4 for the 7’ axis), so I think the flex I most need to consider is the gantry, since there’s no good way to add support to that.
If there’s a way to mitigate this issue with proper assembly I’m all for it… But it seems like the only way to do that would be to assemble a smaller build, or make some modifications to this one.
7’!???!!
I don’t think you are going to be satisfied with this machine at 7’
How much z travel?
what sort of tolerances are you expecting? Have you read the FAQ about tolerances?
What you are looking for is a closed loop system to somehow calculate z offset. The z deflection with differ depending on bit load. You can’t auto compensate for that in any inexpensive way.
Well, 7’ total… 6’ cutting length. If I can get tolerances similar to a dovetail jig I’ll be happy. I’m hoping to do hidden finger tendons and other fancy stuff on here: http://mkmra2.blogspot.com/2014/08/cnc-cut-wood-joinery.html
Z travel is 4" right now… Thinking of dropping it to 2" or even 1" and getting some carbon fiber tubes to help with the weight.
That’s kinda what I was afraid of regarding the bit load… I’ve never used a CNC before, so a bit of this is me finding out how the cutting aspect works. My hope is that having the other axis be shorter (4’ total, 3’ cutting area) would mitigate some of the flex issues.
I understand that everyone wants to build it big, just in case, but your are starting at the very extreme of the machine. If you have no CNC experience your are making it extremely hard on yourself to learn. Your CAM skills will have to be exact for it to work at that size, I mean perfect.
You really really should build it much smaller and learn it.
I can’t imagine you will use a 6’ finger joint very often. I have a construction background and I think the biggest I ever used was about 5" "and that was large) for drawers.
I am not trying to discourage you but conduit is a few dollars. Build it small and learn how to use it, then if your really need to try and scale it up and see what happens.
I am in the middle of building a large format machine but it is slow going do to holiday sales.
Fair enough, though that large scale work really is what I’m interested in. I’ve been considering dropping down the size, but I’ve been reluctant partly because I don’t have anything to cut these SS tubes with 
I’ve been following your thread on the large format machine… Like I said in the other thread, I’d be happy to beta test it 
Also I’m pretty handy with OpenSCAD, if you want to make it parametric.
But yeah, I guess I’ll go ahead and drop down the size on this one. Since I’ve got your attention, would you say a 12 inch x 12 inch cutting area with the 25.4 stainless tubes would be rigid enough to do some more “serious” aluminum cutting?
“Serious” means nothing to me, And I can’t give specs since you will be building your own machine. See the FAQ’s for tolerances. In construction 1/32" is ridiculously accurate, in engineering serious is a dimension followed by a temperature it is expected to be at while holding that dimension.
All my machines are about 24" square with 3-6" Z axis. I have a few aluminum videos.
I’m not saying it can’t be done 7’ could be easy I have no idea. If it is 12" in the other direction I see no reason it won’t work. I will say 7’x7’ will not work.
So try it, make it 7’ by the absolute smallest you an in the other 2 directions. That is the point of this machine, make it your way. Just offering as much help as I can before you get frustrated.
Fair enough. When I said “serious” I should have said aesthetically pleasing at arbitrary cut depths. My primary use cases were a.) large pieces of furniture, and b.) small intricate carvings that will require accurate depth (simple V-carve won’t do). So I eventually plan to have a large furniture-making router and a small CNC for carving… I had planned to make a more traditional small CNC using the big MPCNC, but maybe I’ll just do it the other way around…
Also, I just want to say I’m impressed so far by how well everything fits together… That’s not exactly the norm for DIY projects. Thanks for creating this and making it available!
Your welcome, it’s fun for me and hopefully for all the users!
I think you are asking questions that will answer themselves when you use it.
I think you can see why I can’t/don’t give you exact numbers to any of your questions (it will always vary, machine size, material, bit sharpness, machine flex, experience, weather, planetary alignment), and you can’t tell me how accurate you need it (your not sure what a good dovetail actually takes, I am sure you have just always made it as perfect as you possibly can). I think you will find for wood working (depending on size it is) it can be much more accurate that you are used to. I have to cover my ass and assume you or anyone that reads this is expecting .00001" precision/accuracy. I can tell you at first with wood working it will seem slow, cutting 6’ at 3" depth on a table saw only take 30 seconds…it would take forever on this relatively. When you learn what this is best for (different in each users case) it will be awesome.
I think you might want to check out some 3D stuff first. Stuff you couldn’t really do by hand. Imagine carving a face in each corner of a picture frame. The frame is going to be faster to build with other tools, but could be cut with a cnc. Then you can drop it in and carve 4 identical faces in the corner of the frame, something you can’t really do by hand even if you had all the time in the world you wouldn’t get them as perfect to each other as the cnc will. Or like you said some crazy joinery, but it you are going to make 10 boxes with a bunch of 6’finger joints you might just want to cut the template with the cnc and use a hand router to do the production work much faster.