After a couple of years in the reprap world, I've decided I need a CNC router. After looking around at kits, I came to the conclusion I could do better. I've been reading all the stuff on the forums, and come up with this:
It's a moving bed design, and has a volume of about 600x500x100 and uses SBR20 linear bearings throughout. All the structural plates are 10mm aluminium. Driven by NEMA23 steppers with RM1605 ballscrews.
If you can see an issue, please let me know before I spend any more cash!!
What do you plan to cut on this?
Gantry looks weak in Y & Z directions, you're relying on Y-rail & supports for rigidity with no diagonal bracing. 10mm side panels might not be rigid enough. Also hard to tell what Y sbr20 are attached to so resistance to vertical deflection in Z may be an issue.
Probably want 1610 screws on X & Y.
Morning Tony, welcome to the forum.
I’ve noticed 1 or 2 things I would look to change and have concern about...
Gantry Side Plates (10mil Ali)
I would always advise that in the world of cnc, anything 10mm and below in thickness should be considered as “sheet” material and so not “plate”. If it was me I would look at a thickness of 15mm minimum for those plates. They will work don’t get me wrong but at this stage I think 15-20mil would serve you better, I can’t see any flex in the plate at those thickness’s, what are the dimensions for those plates?
Gantry Cross Section
What are size are those ali profiles connecting your side plates to form the X Axis, I’m going to guess and say they look like 45x45?, it’s hard to tell from the pic. How about beefing these up to 80x80, it will cost more yes but you could then also go with TBR20, this would then allow you to fix the rails better as TBR20 has its mounting holes on 40mil centres and that would match on the 80x80 profile slots.
Obviously there are other, added advantages of beefing those profiles up. I would then consider adding vertical ali profiles connecting the two cross sections (think ladder frame, H frame), or you could add another cross section across the full width, this would then give you an overall cross section height of 240mil this would give you a nice distance between the rails for that axis as well which always helps and should really be kept to a minimum distance of 200mil when designing and for the bearing unit centres.
What you could then do is fix a plate on the front and back of those cross sections and so connecting all together as one solid unit, at the moment the only additional support your Ali profile cross sections are getting is from the SBR rail and base, again it will work but not the best practise, though I would think just a plate on the back would be enough. At the moment your rails sit on the front of the cross section, I would move these and mount them on the top and bottom of the cross section, it will require some design changes as you would need to add some plates to the Z Axis connection plate but would serve you and the machine well for the additional cost of the plates.
I’ve attached an image for you to look at, pay attention to the assembly configuration of the X Axis, this is the configuration you should look to achieve and is considered to be the ideal way of mounting rails.
Another note on rails, the best choice you have is to use Profiled Linear Rails and other members would suggest you should use nothing but these rails, if you was to use that type of rail, you could actually make the changes I suggest and lose the 80x80 profile and keep the 45x45 again going with this type of rail would be the better choice but like everything, better normally means more cost, so I could understand why SBR/TBR would be your continued choice. I would suggest digging deeper into the details and working out what the cost difference would be in the rail types as profiled rails would be the better choice.
Z Axis Design
What I’ve said above applies here as well regarding rails, though ideally you would want the rails to be mounted to the Spindle Connection Plate, again I really don’t like the sound of 10mil ali for the plates on this.
Y Axis Lead screw
I would consider either a bearing mount for the floating end or extending the screw to the end of the bed, you could then use a simple plate with bearing and fix to the Ali profile. An unsecure floating end on a short lead screw isn’t that bad but for the longer screws you will get wobble on the screw and this really isn’t good, so ideally I would always suggest a fixing on the floating end of a lead screw. I’ve attached a second image of the z axis I designed for what will be “machine 2“, if you look the floating end of the screw is secured and isn’t a complicated thing to do.
Have a good look around the build log section of the forum Tony if you haven’t already, there is much more that has already been said and from the pictures alone you will get ideas and better understanding of the ways things could be done. I would revisit your motor mounts, there is a better design for mounting them, see Pic 3 for an idea on what you could do, if you do continue with couplings, Oldham is the type to go with rather than the Lovejoy type, in Pic 3 you can see the motor face fix's to the block, on the other side of the block are the angular contact bearings, and behind them is where the coupling lives, so the couplings live inside the block and inbetween the motor and the bearings.
Last edited by Lee Roberts; 03-01-2014 at 08:29 AM. Reason: Clarification.Me
Wow, thanks to irving2008 and Lee for the suggestions. I'm hoping to cut aluminium with this thing when its complete.
I am using 45mm cross section as I blagged a load of it from work, but I prefer the idea of bolstering the Y axis up with a 15mm plate between the two risers.
I'm committed to the sbr20 bearings and 20mm lead screws as these were donated as well.
I really appreciate the comments, firing up the CAD system as I type.
I've taken Lee's suggestion and moved the SBR20s to the top and bottom of the Y gantry and placed them on a 45mm ali bar which is just the right width for them. This has needed a redesign of the Y stage but it is now of the 'box' type with the ballnut on one side, and the router on the other.
I've also upgraded all plate ali to 15mm. Thanks for all the input!
Here is a cut away of it, does it look better:
What height is the new 45mm ali plate?
Cool that should be ok, if you can squeeze any more height into it I would try to.
I've moved this thread out of the build logs section as it's a machine design review thread at the moment, when your ready to actully build the machine please feel free to start a build log .
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