My 1st Hybrid Steel/Ally (1150 x 850 work area) (comments/advice please)

[mitchejc]

Ah, good to see another South African on the forum. I'm very much in the same boat as you trying to figure out what's the right direction for my build. There's much more knowledgeable people on here than me so I'll leave the comments on your design to them.


Apart from the great build threads here's some of the stuff I found extremely helpfull with a design


To calculate critical speed on ballskrews and motor sizing a great help is also. Can't find the link now but If search on here you'll find it
MotorCalcs.zip


Extremely usefull to compare stiffness of different design options.
cnc_stiffness_calculator_v8 (look at post #26)
http://www.mycncuk.com/threads/7733-...r-design/page3
Or you can just spend endless time fidling with FEA software and be none the wiser :-)


A few general comments:
Helped a friend of mine build a router and he got those Wantai steppers and motors from MicroRobotics and he's very happy with them.


Not sure what your requirements are wrt the 4th axis but be realistic in your expectations. I have a UniCam router with a 4th axis and I have not used that functionality in the 5 years I have the machine. Maybe it has changed recently but in my experience I have not found any reasonably priced software that can do true 4th axis toolpathing. Sure, if you want to engrave text around a cylinder there's plenty free stuff around that will wrap a 2d path around a cylinder.


The stuff from CNCDirect are top notch. Piles of wood dust made on my router over the years and the SKF bearings are still smooth as silk. You may also consider the Hiwin rails. I ordered Hiwin rails from Wendy's Bearing Store http://www.aliexpress.com/store/637430 Payed on the Wednesday and they were in my hands the next friday so I'm very happy with her service and the rails seems to be of very good quality and probably comparable to the SKF suff. The chinese ballskrews and nuts is not comparable to the SKF producs but considering the price I think they are great value for money. I decided to give them a go so we'll see how that works out.


Good luck with your build and if you find other useful local suppliers for the bits and pieces please let me know, I'll do the same.

[EddyCurrent]

@lebies
There are other things but for a start these jump out at me.

1. A spindle with at least an ER20 collet is a MUST I would say, also a 2.2kW spindle would suit the machine better.
2. The Z bearing blocks are too close together in the vertical direction, these will be weak regarding twisting as they are.
3. With an X of 1200 you might find 16mm ball screws will whip at higher speed but I'll let jazzcnc comment on that, he might suggest changing the pitch to slow them down.
4. You might need to think about 20mm rail instead of 15mm
5. Rather than a long Z travel, think about an adjustable bed. The Z will be very weak at full extension as shown.

I look forward to seeing your control panel design.

[routercnc]

Hi Lebies,

Welcome to the forum. Agree with Eddy, plus a couple more views on your design:

That is a large machine to also cut aluminium. It's not so much about taking lighter cuts and going slowly to compensate for lower machine stiffness (which will be likely with a large machine), but just that you don't have the stiffness at the cutter so it will vibrate, get hot, and go blunt. To compensate you will end up beefing things up but at this size it all gets heavy and the design could run away with you. Just be realistic about what it could do at this size and don't expect too much on the aluminium cutting.

You have a very large offset from the Z axis to the Y rails - I can see that you are trying to mount them on the rear part of the gantry where the gantry is locally stiffer, but I think you will be worse off overall due to the offset. I would mount them on the front of the gantry closer to the Y rails, then if the ballscrew clearance allows push the central extrusion forward as much as possible. Then close off the open back with a simple plate of moderate thickness, say 10mm. That way you are getting back towards an overall box section.

The end plates on the gantry look very thin. I would use 12mm minimum here (aluminium), and for the ones underneath to mount to the X axis bearings.

Will you add a bed to this? The spacing of the cross members of around 600mm is quite large. Another 2 member would be my minimum. Don't underestimate the bed stiffness as this is what hold the workpiece against the cutter forces. Vibration here is just as bad as vibration on the tool - difference is it is much easier to do something about by adding more support.

X axis motors - you mention running them in opposite directions to each other? Not sure why you need to do that. They should normally run the same direction (both clockwise or both anti-clockwise) otherwise the gantry will be pulled into a serious racking problem! Perhaps you are confused about running twin ballscrews on one axis, or maybe I misunderstood your intentions there.

Looking forward to see how it progresses. I'm sure you'll get plenty of other support as you go . . .