Firstly a big thank you to everyone who has shared their experience on this site. This project would still be a vague pipe dream without your generosity.
As mentioned elsewhere, I have a trip to China coming up in just over a month and I'm hoping to use that trip to bring back some of the biggest bargains.
The research continues at a pace although not as fast as the time is counting down! I've had a chat with the guy who has access to the heavy engineering part... His Abene mill looks like a beast, but it's bound to be really useful along the way. It does of course mean that the whole design process has to be done in a rather tight timeframe which is less than ideal given my lack of understanding of the problems. Targets are design finished mid October and running before xmas! ROFL!
After trawling what feels like a majority of the internet, the design considerations so far (in no specific order as this thinking as still changing too rapidly!);
Small A3+ CNC capable of working wood, plastics and some light alloy (slowly) budget around 1000GBP
Working area of not less than 60cm x 60cm x 15cm. Overall I guess somewhere around 80cm x 80cm(?)
Design to incorporate adjustable height bed, 0-15cm, 15 to 30cm.
Box section steel frame at least 25mm x 25mm - filled with sand (thanks JAZZ!) a bit like this one, (Is there a name for the high side design?)
In the above pic, along with some other changes I'm thinking that I'd add a third vertical under the top rails and possibly add a cross bar at the back roughly where the steppers are at the far end for better stiffness and also triangulate the bottom of "open" end. I have seen another similar design that uses a substantial C section for the top rail too…
20mm supported round rail on X,Y,Z haven't found a price for profiled, I expect it will be outside my budget.
This is the area giving me the most problems at the moment, currently from what I've read, I think I need;
Steppers - 23(?) 3nm? (still trying to play with Irving's excellent spread sheet)
Ball screws - X,Y; 20, 10mm pitch (approx. 800mm long) FFZD ball nuts
Ball screws - Z; 20, 5mm pitch (approx. 300mm long)
The top rails that mount the supported rail for the X to run on would seem to be the first construction problem and perhaps one of the reasons this is not a very popular design(?).
My plan is to Mig weld the end verticals (35 - 40cm ish long) to the top rails then clamp them to the big Abene mill and skim 10 thou or so off to level them up. Then invert them, clamp the machined faces to the table and weld the bottom part of the structure together. That should (I'm thinking) give me to flat and level and parallel surface on which to mount the supported rails. The "bottom" of the structure is sure to be less accurate and more uneven, so use car body filler to level it on the stand/ table with a laser level. Then finally, mill the spoil board level to the Z axis.
Setting up the X supported rails; build the Y axis, then set one of the X rails by eye and using the other side of the Y as a guide, fettle, shim and file for best fit of the Y, bolting down gradually.
steel gantry, design to be decided
Z Axis alloy, design to be decided
2.2kw water cooled spindle - (noise!) PC cooler mounted on Z.
Gecko G540, although the idea of a handful of the M542's / 752's and breakout board are growing on me.
70v power supply
vortex dust removal
Z zero switch
temp sensors via Arduino
Still a thousand questions without answers, not least of which are;
Is the gecko g540 really worth the extra money for the fancy magic inside it??
If the Y axis supports ends up about 1m apart, whether to go with 2 steppers for the X or use just 1 and belts?
Any thoughts, suggestions or eggs most welcome! :-)
To me that machine looks like an excellent basis and the changes you suggest sound worthwhile, particularly the triangulation. I would be inclined to leave the sides open as it may be convenient for long pieces. By all means make the steel box section that supports the rails bigger to compensate.
Milling the surfaces for the linear bearings flat is a good idea, but you want to do it after all of the welding as any subsequent welding may throw it off again. If the milling machine is big enough then it's easy enough to clamp the whole frame to the bed - the difficult bit is clamping it to the bed without distorting. If it twists when you clamp it to the bed, then reverts when you take it off then it's a waste of time...
An alternative to milling is using epoxy resin. Put barriers round the top of the box section where the rails round with a channel / tube between. Pour the resin in (couple of mm thick I reckon) and gravity will self level it extremely accurately.
You can mill the spoil board with the router when it's finished. Having an adjustable height bed is great but it's hard to keep it level when you move it!
I can highly recommend the PM752's. I've neither seen nor used the G540 in real life but I doubt the 'clever stuff' it does is worthwhile for a machine this size.
For that length I think you'd be fine with 16mm, 10mm pitch ballscrew, not 20mm diameter. The bigger screws have a higher inertia which will harm your acceleration and top speed to an extent, however the critical speed is greater so with big enough motors it would go faster. I managed to get 60m/min on my 16mm, 10mm pitch Y-axis which is a 910mm long screw, about 840mm unsupported ... which is clearly much more than enough! That is with PM752 and 3Nm Nema 23 motors. You don't want Nema 34 for a machine this size as the moment of inertia of the rotor is greater which, among other things, means they operate at lower rpm.
Thanks for the help Jonathan, some good ideas there! Much appreciated!
The last couple of days have actually seen some progress with the first iteration of the bill of material is now complete although it's stiill going to need several more iterations before all the bugs are ironed out.
I'll try and get an extra set of some of the elements to use initially as hot swap spares with the option of making a 4th axis after xmas...
I can't find a good reason to keep the gecko G540, so that's gone and 1610 ballscrews instead of 2005 too.
The adjustable bed might wait until I need it! the potential solution was going to be a removable second bed that could be removed.. I would put some designs up at this point but everytime I try using google's Sketchup I must be taking a stupid pill! Just can't get my head around it yet.
more to follow. :)
Are there any thoughts about using 6 bearings instead of 4 on an axis, specifically here I'm thinking of 25mm supported rail?
...Which also leads me to wonder if there is anywhere to avoid or prefer with the placement of the ballnuts? Centre of mass or axis of the spindle seem to be the most logical? Does that sound reasonable? I noticed that on the picture above, (which I should apologise for not crediting to "Nhyf" on youtube before now) there was a comment about the placement of the X axis ballnut placement...
I'm trying to have the bottom of the Y axis about 150mm above the work surface, with 2/3rds of the spindle mounting plate supported on the Y axis when the spindle is at the lowest point of travel... is this something worth the effort?
Locating ball nuts has 2 major thoughts, keeping it rigid and avoid introducing twist. Don't put them out on a limb.
The Following User Says Thank You to Robin Hewitt For This Useful Post:
This is a problem on my machine at the moment:
I think aligning the X-axis ballnuts with the center of mass in the XZ plane (or at least in Z) is a good plan as when the X-axis accelerates you've eliminated the turning moment about the ballnut. Ideally you would do the same for aligning the X-axis ballnut with the Z location of the cutting force, but that clearly moves so the best you can do is an average.
thanks guys! :-)
I'm fairly happy with the state of the design for Y but the Z still needs some more work. Looks like about 150mm from the ball nut that moves the Y to the centre line of the spindle. Using HWIN 15mm rails would drop that to around 130mm, but 20 mm of those numbers are the spindle mounting plate! The Z ball nut is 94mm from the spindle centre. But it's difficult to see with these crayon drawings :rofl:
As long as the ballnut is directly between (zero distance measured parallel to X-axis ... hard to describe) the two Y-axis rails then I think 130mm will be fine as the aluminium plate in between is very strong and should not flex much. If you have not already try setting the rails/blocks on the Z-axis into the plate - i.e. mill a slot for the rail and pocket for the blocks. That will reduce the distance further though you'll have fun fitting the Z ballnut as that will also require a cutout.
Yep, I get what you mean about putting the ball nut between the rails. I might save the pocketing for V1.1 (if it turns out to be a problem!) I'm already surprised at the amount of machining that the design is going to require, I had thought of this as just bolting it together, ha!
I wouldn't drop the Gecko's I have a G540 here and it's beautifull bit of kit, thou that said I would go with 203v's much more flexable with extra growing room. That said I'm slightly bias because thats what's on my machine. IMO they are the Dog's b@#/?ks and worth the extra.
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