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Discussion in 'CNC Mills/Routers' started by Bradles, Sep 5, 2017.
basic start to design of cnc machine
Bradles published a new build:
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The only suggestions I can offer would be in regards to the gantry end plates. While it appears that you have cut them back (L shaped them) to get both ends out of a slightly larger piece of material I would recommend against doing so. Flexure can happen in many areas on a system and one of the more overlooked locations is the gantry end plates. Stiffness in the end plates is relevant to controlling lateral sway of the gantry and controlling lateral sway is relevant to keeping the side of the bit in contact with what it is cutting rather than skipping off the surface. Carrying them out full width will double the stiffness resulting in a stiffer machine which leads to a better cut.
The other item in regards to end plates is the height. While I understand the urge to make the gantry high enough to cover all the "what ifs" doing so only reduces the stiffness of the machine. And the loss of stiffness in this case is not a simple linear loss, the losses are exponential. Doubling the distance between the X and Y rails does not merely double the flexibility, it increases it 8-fold. This is why it is imperative for those seeking the greatest possible stiffness in a machine to reduce the vertical rise to the absolute minimum necessary to do the work the machine is designed for. If you are needing to do thicker materials on occasion, it is better to put a false bottom in on your spoil board and drop the bottom out to allow for thickness than it is to jack the gantry up.
1) mount your end mounts in a movable spot, like with Tnuts to the rail. You do not want perfect center locations as they are not made concentric and require ~1mm of adjustment range.
2) make your end plates a complete square. Your L will flex. Use at least 3/8 aluminum considering how high you are making it. Check my build thread for more advice
You need wider/stronger horizontal support. 2020 isnt enough, espicially without a table to sit it on.
Honestly ball screws are overkill with V-wheels and V-rail. I would use linear bearings like MGN9 or 12 size.
Can be found for not much more than the cost of a set of full price V-wheels these days.
aliexpress linear rail
OR just go V-wheels and dual-belt like a standard OX heavy duty (the HD uses 2080 or C-beam for all)
the dual-belt is not the limiting factor in the OX
Also you really won't need that much torque for steppers.. any crashes and you will bend and break stuff with the screws for sure.
Partially agree. I needed my 16mm because it was a 1000*1000 machine. 500mm is fine with the 8mm acme rod.
You cannot break the screws with the motors..... Nothing breaks either. Your plate just hits the end cap and just loses steps.
Not worried about breaking the screws, but you are probably right, just have strong end stops! 397oz-in steppers at 48v (high speed)
I worry with mine that i will strip the belt when i hit an end stop, even with 269oz-in it's amazing how strong they are.. but with 1605 screws they make a whole lot of force.. It's easy to get carried away Jogging!
thanks for all the replies guys,
The way the dimensions sit in the poorly taken picture is 1mx1m so therefore would prefer to still use ball screws.
and not worry about changing them later down the line.
I was tempted to use hiwin linear rail but didnt quite know how i would mount it, but may have a new idea now that i have actually started to design on CAD.
Horizontal support was just used as guide, most likely will have a actually table and wooden frame made up as using the 40x40 extrusion as legs is a waste of money at the price it costs.
As for end plate yes i will look at slotting them to account for non concentric parts
Will look at getting some better pictures up over the next few days.
Wondering for those with experience is there any rules of thumb as per offsetting the z spindle centreline? im assuming the closer to the physical centre of moving gantry plates would be best due to having more central weight balance.
Will have to update images to current revision3 as display pics always seem to be taken from phone which are terrible quality
Centered is always ideal. No reason not to....
you do NOT need two sets of linear rail on your Y axis! One pair is plenty for each side.
12mm screw is too small for 800mm. You will get whipping!
I hope your side gantry plates are 3/8 aluminum. No need to go over 1/2"
Your screws need a place for the bearing mount, and that must be adjustable. Chinese ballscrews never run perfectly straight and true. Keep atleast 1mm of adjustment in every direction.
Your Z would be better with the linear rails on the front face and only having one plate on the top for the ballscrew nut, or just design it to bolt onto the back of your Z plate for easier manufacturing
Use all four motor mounting holes!