First design - Few questions

[9designs]

Thanks for the encouragement, I did read and appreciate the little rant. I guess you could say I’m into engineering, apprenticeship trained and qualified, working as a mechanical engineer for the last 25+ years :)
So my gut instinct was telling pretty much want your posts have been leading to.

Anyway to try and put some fact & data to it and to defend some use of wood haha.
I compared two beams, one steel 50x50x4mm 1400 long. Constrained both ends and applied a 750N load in the middle. Deflection = 0.225mm

The same applied to a Marine Plywood 25x200mm beam. Deflection = 0.0345mm







OK the steel is the same in Z and Y.... The Ply only in Z. So perpendicular bracing required. (Ply also has lower thermal expansion and once sealed should be OK for humidity).

What would be a suitable spindle motor, single phase, from where? I can then look to see if I can adapt the mounting to take it.

OK some rendered shots of my hybrid Aluminum/Wooden wonder.... :)









Gantry assembly complete comes in at around 77Kg. The center of mass is between the spindle and the Section8 cross beam. Cutter is inboard of the gantry end plates which are 275mm wide with two HGW25HC per side.

Width between gantry is 900mm, give a cutting width little over 700mm.
Bed length is just under 1400mm. Z clearance 130mm.

I'm sure it will raise a few eyebows

[mitchejc]

Looking very nice! Wood and aluminium together is just sooo easy on the eye :-) Interesting comparison of the wood and steel, just keep in mind that the force in the direction you simulated is actually very small in gantry routers. Because the cutter is located relatively far from the gantry the force on the gantry is more rotational rather than straight in Z and Y, due to the leverage created by that offset. Would be interesting to see the same comparison with that in mind.

I think the experienced guys on here might recommend two ball screws for your long axis to prevent racking.

I've often wondered about the way you have your rails on the gantry because it does sort of make sense to me. Not sure if someone has tried that yet but it might be a bit tricky to do the alignment. I guess the same applies to the way your Z rails are oriented, those Z parts will have to be perfect to get those rails parallel. For Z its also generally recommended that your carriages are fixed to Y and the rail to the moving Z plate.

I do agree with your comment that a lot of frames are way over designed but don't underestimate the importance of a good solid foundation for your machine. Some will likely disagree with me here but the stationary base of a machine can never be too solid. For starters, you don't want the whole thing walking around when its briskly moving that 77kg gantry during cutting:-) What you have there looks relatively sturdy to me but I'll leave it up to the pros for comment.

[JAZZCNC]

Ok got to be careful here don't want to sound 2 faced after the ranting but I agree with Mitchejc that the base needs to be strong and sound just like any well engineered structure the foundations are important.

Now this doesn't mean going bonkers but I'm afraid it does mean much more than you have drawn there. I'm not getting into the Steel/aluminium is better than wood argument because you clearly have your sights set on using wood so go for it.! . . But I'm afraid you'll need much more structrue than what you have there.!

The other things I see are classic under estimations of forces acting on parts of a CNC machine. Like the gantry sides are too flimsy for there height and when in use they will flex and resonate when short fast back n forth moves are happening while cutting. Just try shaking 20Kg back n forth really really fast to see how it feels and the effect it has on your arms.?

Single or twin screws depends on what your cutting but your border line on the width for using 2 screws. If your planning on cutting harder material with any intent of decent depths then you'll want twin screws.
If you stick with single then make the lower spanning brace much wider to help combat racking.

Z axis design is bad news and will be a proper bitch to setup. Why re-invent the wheel just use the proven concept of rails on front plate bearings on rear it works perfectly and is easy.
Y axis screw is way too high for my liking and you want to try getting it lower if possible.


Other things with design to think about is how your going to build it and allow for adjustment etc for setting up.! I don't see much.?
Also don't forget cable runs etc.

[9designs]

Hi Jazzcnc, thanks for the feedback.... I hadn't done much calculations of the side supports, so I've now gone back and tried to calculate some forces and deflections. The absolute values are perhaps hard to quantify but the level of improvement is at least relative.

Initial layout I applied a 500N lateral load to one side, ignoring the opposite side for sake of easy calculations. Load just applied to the top fixing hole of the Section8.
The analysis show stress concentrations as such.





Adding some ribs and thickening the bottom plate that holds the carriages, also added the Section 8 to better simulate the load area.



Stress plot looks much better etc.




I hadn't correctly put the weight of the spindle and stepper motors in, so even with the extra material thickness I'm at 66Kg. Hopefully keeps with one X axis ballscrew.

The base, though not detailed is intended to be anchored to a side wall of my garage that it will stand next to. So it won't go for a walk and will further supported. That said I can always beef it up after testing etc.

For spindle is this the sort of item recommended?
http://www.cnc4you.co.uk/Spindle/Spi...VFD-plus-Mount
Is it worth it for a DIY machine?
If so I'll need to look at the Z axis anyway so will have another go at that. I was wanting to keep the offset from the Y axis down. Many designs seem to have a large distance, increasing the moment and forces from the cutter. I was looking reduce the offset.
I'll have another think.... and revisit the layout.

What sort of DOC is normal from Gantry type machines? Tying to get a feel for the forces to expect.

I can see the costs running away a bit on this project