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Discussion in 'CNC Mills/Routers' started by Craig, Mar 29, 2017.
Discussion in 'CNC Mills/Routers' started by Craig, Mar 29, 2017.
Building a couple of CNC's based on the Crawlbot
Craig published a new build:
Read more about this build...
Craig, what will happen if there is a wood chip, or a piece of the cutting material you are using, in the way of the skateboard wheel? I think the wheel will climb on top of the chip (if you are lucky) and will raise the z axis with it and then go down again. Any thoughts on how you can prevent this from happening?
Or if a fore/aft load is too high, won't it just slip?
This CNC will have a vacuum system attached like my others. I have never really had a problem with large chips when milling, but its not to say it wont happen! I don't think it will be an issue if the correct tools, speeds and depth cuts are used. I have mainly produced just fine saw dust in my other projects from various timbers, but again i have the vacuum system attached to pick it up. But if it was an issue, I have seen machines set up with weather strip brushes that could run around the wheels to address those sorts of problems. Thanks for the question!
When you mean slip, do you mean lift up? Or will it slip in the Y axis ?
On the Y Axis, there are bearings that run along the edge of the MDF (2 delrin wheels attached under the Y Rail on either side) When I pull the Y Rails (the ones with the skateboard wheels) in tight on opposite edges of the MDF board, it not only glides smoothly along the Y, there is also no twist. (Providing the MDF you are buying from your supplier is perfectly square) But I am only placing these stresses onto the gantry manually at the moment (but quite forceably i might add). The real test will be when the motors are connected and its milling something. You may see in the photos that I have reinforced the edge of the MDF with aluminum angle for the delrin wheels to run on.. I found that when I manually ran the gantry up and down the MDF at first, the delrin wheels started to almost immediately wear into the exposed edge of the MDF (which was expected) which would mean constant re-adjustment as it wore down the edge (a flaw I suspect in the Crawlbot) The aluminium angle is a cheap as well as permanent alternative to this. The GT2 belts will then run from the motor, (there are 2 motors on the Y axis at either end) down to the small bearings and then attached at either end of the board. (More photos to come on this setup soon as the belts arrived today) This is what will run the machine in the Y-Axis
If you mean the lifting of the Z axis, without a lower wheel or bearing running underneath the board, that I have yet to test. Now the Crawlbot does not have bearings/wheel underneath and rely's on the weight of the unit to keep it down, but of course you would have to be still very weary of cutting depths and speeds, tooling and feeds etc etc. The "Low Rider CNC by Visious" has based his design on the Crawlbot, but introduced adjustable lower wheels to his design, to ensure the Z Axis does not lift. In saying that, if the lifting becomes to much of an issue that I cannot overcome, I have already designed the adjustable bottom wheels to be attached.
Hope I have answered that for you (or was I off track?)
Since you are running on the outside rather than the inside, why not go with the wider more industrial HTD belt system or a lead screw?
On my larger gantry, I will be running a 10mm or even 15mm wide belt on the outside (but haven't quite decided on belt type, pulley size etc as of yet). Currently on my smaller version, 6mm will be fine but have to be sure that the belt is the wired reinforced type. (Test by tightly bending it, it puts a kink in it) otherwise it is only good for 3D printers or similar! I am currently running the same GT2 belt on my other CNC and have not had any problems with stretching, accuracy etc. But again, it is totally up to the user, and what ever people feels works best for them. I can only base my opinions on experience.
The idea of my larger CNC is so that I could easily pack it away when not in use, so a leadscrew (especially when i make my table over 3 metres long) may be costly and not as portable, as the motors would have to be attached to the table. Again, it is up to the user and the configuration they desire. The great thing about using V-Slot for the Y-Ends, is that you could easily attach a 8mm lead screw nut block to the outside onto the extrusion with a leadscrew running down the outside instead of a belt is defiantly an option for a permant setup, and still be saving money on materials for the Y-Axis track.
I meant, "the wheels are spinning, but the machine is not moving due to excessive cutting load."
Coming along nicely. Can't wait to see it come to life...
LOL, did the thing crawl under the bed?
Have you taken a look at this design called the "LowRider" which was also inspired by the Crawlbot?
LowRider CNC -Full Sheet CNC Router- by Allted
Will not be any different to any other machine that runs a belt system. The wheels are only a guide, the movement and torque is from the motor and belt. You also have to understand what the machine is capable of depending on the setup and I think the only real way to learn this is from experience (as well as researching as much of the internet of course). Any machine that you apply to much cutting force outside of it capability is going to miss steps. On my other desktop CNC I built (similar to the ShapeOKO machine), I am running 60oz nema17 motors with 500w spindle. For example: Cuts MDF at 1mm depth, about 15mm/sec (The MDF I get is high density) with 2 flute carbide bits. So the only time I have experienced heavy cutting load, is when the CNC is feed rate is too high (or too low), cutting too deep in a single pass, and/or the tool is blunt etc etc. Again, you have to know what the machine is capable of doing with the type of router being used. My first experience with CNC's was about 18 years ago with glass, and man you quickly learn about feed rates on those things.
Yes, I made reference to it in an earlier reply. I have been following the project closely and only come across the Crawlbot design, because the Low Rider made reference to it. The Low Rider has introduced bottom adjustable guide wheels (unlike the Crawlbot) possibly because the Low Rider machine would be significantly lighter and the Crawlbot relies on weight to keep the machine down. Like the MPCNC, great design.
Craig - Nice work - I like the Crawlbot concept and can't wait to see how your machine performs.
Love the idea of being able to clamp a gantry to a standard sheet of MDF, I would like to build something inspired by these designs but that doesnt need belt on sides, Im thinking of actively powering wheels to move down the sheet
Unfortunately in my setup, the forces of the router will be too much for just motorized skateboard wheels to rely on movement in the Y-axis during milling. The only thing I can think of is that a rack and pinion may be of an advantage to your idea.
Hmm I was thinking of Tank tracks , Above and Below the Cutting Board
What are you using for spacer between skate wheel and rail?
I used "shf8" 8mm rod holders not only to add strength, but also as a spacer if I ever decided to run the motors and belt on the inside.
Thanks for fast reply. Have been studying your design really like it. Looks like I may going to build something like it.
Cool. If all goes well, I should have some video of it in motion soon. I have gone a little crazy and have recently attached a 8inch LCD (to view GRBL Controller), and later may look at connecting a Raspberry Pi. I am also building my 1500mm wide one at the same time (I have posted some photos of it earlier). but I have gone through the trouble of making more detailed photos of construction that I will post up at a later date. Just note that when buying skateboard wheels, there are different grades based on wheel hardness (Durometer). I purchased "95A" wheels only because they were readily available (I think 100A is the hardest) and 56mm Diameter.
I believe the wheels I ordered are rated 99A, should be good. Think inking about adding a piece of 20x40 rail on top of the 20x80 y rail for extra height for Z. If you recall how long the Y rail is, I would appreciate that info. Hard to,tell from pictures how you have X andY, and stepper motor attached.
On the smaller CNC The X rails are 2 x 1000mm 60x20 and the Y are 250mm 80x20 (a 500mm piece cut in half) The Skateboard Wheel centres are drilled at 30mm in from either end on the Y beam Another option is instead of 60x20 profile for the X-Gantry, is the C-Beam. The motors are Nema 17 Bipolar 84oz.in(59Ncm) with the Openbuild Nema17 motor plates holding them in position. The larger machine is using the standard 2 x 1500mm 80x20 for the X-Gantry, and the Y-Plates are 350mm 80x20, with Nema 23 motors. Maybe instead of trying to attach pieces together for the Y, is trying to source some wider t-slot. Will connect and work the same way if you want to add height.
Perhaps you could use a rack and pinion system where the lower wheels drive the machine. The large chip issue might not be an issue then. Portability of using any old 4x8 sheet would be an issue, solved by solid racks that clamp to the 2x4.