New build - fixed gantry mill using extruded profile

Hi all,

I have been lurking for a while following other people's builds and absorbing the excellent advice so freely given on this forum and I thought it about time I stuck my head above the parapet!

I am planning on building a machine that will be used initially for 3D machining of aluminium and PU tooling board with a cutting envelope of approximately 620 x 330 x 130mm. The frame is mostly 80 x 40 and 40 x 40 profiled extrusions and I hope to use it to make parts that replicate sand casting tolerances which are around +/-0.37mm (10mm feature size) up to +/-0.9mm (600mm feature size). I have schemed Nema 23 motors driving 1605 ballscrews through belt drives - these are currently at 1 : 2 but the design allows for 1 : 1 drive for more precision and higher torque if required. All axes use profiled rails - 15mm on the Z and 20mm on the X and Y. The spindle in the design is a DIY as well - it is an ER20 collet which I am looking to drive with a 2kW RC brushless motor (not shown), again belt driven to give some flexibility on operating speed / torque.

Given the size, I may also consider the 'going vertical' option and have the table upright as I can see good benefits for chip clearance (we have a 2 axis CNC at work which sounds more like a grinder when you don't clear the chips well enough)

The design is about as far as I can take it without more guidance. I am looking to start procuring the hardware shortly so I thought it a good time to get some feedback on what I have so far. All comments gratefully received.

Many thanks,

Paul.

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Hi Paul,

Looks good thou got a Few things you may want to consider.?

I wouldn't have the ballscrew and motor moving with the table your just moving weight around unnecessarily which will limit the acceleration and seen has you want to mainly do 3D machining then acceleration is important. Given the bed will be heavy anyway this saving will become welcome.
This brings me to the next point.? The bed.!! It's a bit weak with no supports in the middle so I'd beef it up a bit, has you have it even if you used Ali plate for the Bed it will flex when plunging in the middle. A few cross braces will sort it.

Next the belt drive.? I wouldn't use those Idlers like that and just make the motor brackets slotted so the tension can be taken up by sliding the motors, KISS approach it works and it's easy.

Next Rails..!! If you haven't already got them I would use 20mm rails all round.? Reason being 15mm are very low with tiny bearings and on the Z axis they make it so you have to mount on quite high spacers to create the clearance for the ballscrew. Even with 20mm you have to machine in rebates for the screw blocks to clear.

Hope this helps.!

In my humble opinion it won't cut aluminium very well because it is not designed to cut aluminium.

Overhangs are your enemy. A good solid base is your friend if you want to get the best out of a fixed gantry.

The tool is unlikely to stick out more than 2" from the chuck, and that's being generous. Try dropping the gantry down to 2" clear of the table and cut the Z axis travel down to 2". See how much more rigid it looks.

If you go vertical and have a good brick wall, consider dumping the frame altogether and Rawbolt everything to the wall. Maybe add extra diagonal braces to stop the gantry flexing sideways.

If you really think you will want to engrave the top of something 130mm thick at some later date, add provision to move the gantry rail up another 80mm. All it takes is a few extra bolt holes. You don't need to worry about extending the Z travel because the tool isn't going to be 130mm long.

@Jazz - thanks for the input. I can see the benefit of what you have suggested and I will look to incorporate the changes. I haven't purchased any hardware yet (thought it would be wise to get some feedback first!) so the move to 20mm rails all around is an easy one.

@ Robin - I undertsand the benefit of low overhangs, but I am looking to 3D machine parts up to about 70mm deep (a shallow trough form so I don't need a 70mm long cutter as long as the spindle body clears!). That combined with the 50mm tool height (and a little margin) gave me the 130mm height. I will still look at the deflection of the gantry at worst-case conditions to see if there are stiffness improvements to be had.