Design Help Pt2 Required for CNC design/Build

[MikeyC38]

Hi Mike
Good to hear someone getting use out of my spreadsheet. Nice to put something back into this forum.
Just a comment on the x axis bearings. When mounted on their side it does make for a neat installation but be aware that because the blocks are open (sort of C shaped) that they will open up slightly due to the gantry mass and to a less extent the cutting forces. I've not done the calcs but suspect the effect is small so don't panic. Interested to hear if anyone has worked out how much they deflect for a given side load (or vertical in this case) or has experience of this orientation.

As for twin screw - I have a gantry about 600mm wide with a single ball screw and I can rock the ends of the gantry back and forth by hand up to about 1mm if I push hard. Having said that I can happily machine balsa, liteply and ply and they come out at the right sizes. But I would still say twin screw is better and would like to upgrade when funds allow because I could then up the feedrate.

Thanks routercnc for the spreadsheet and the comment re the X axis bearing placement. I've seen them placed like this on another design, but somehow felt that vertical placement would be better for carring the vertical weight of the gantry and resisting racking. For the gantry I will be using 20mm plate sides with 120x40mm heavy extrusion (8.8 kg/m) for the y-axis on the basis that the box section resists twisting from the cutting loads better than plate and is lighter, reducing the forces required to accelerate and stop the gantry.

BTW I too am a model flyer (Gliders and power) and have in mind to build a 1:3.5 scale (4.25m) "Reheinland" glider with all the bits CNC cut - once I have done my brother's guitars!

Kind Regards
Mike

[Jonathan]

I've not done the calcs but suspect the effect is small so don't panic. Interested to hear if anyone has worked out how much they deflect for a given side load (or vertical in this case) or has experience of this orientation

It's going to be tricky to calculate as it relies on the strength of the bolted joint between the round rail and the support. The aluminium surface at the joint is a V-shape, so the rail only makes two line contacts. There's not many bolts (and if you don't threadlock them they can work loose) which overall means the round bar can, and do, bend in that direction upon the rail. Check it with a dial indicator and you should be able to see the effect.

For the gantry I will be using 20mm plate sides with 120x40mm heavy extrusion (8.8 kg/m) for the y-axis on the basis that the box section resists twisting from the cutting loads better than plate and is lighter, reducing the forces required to accelerate and stop the gantry.

See you mention making the frame very strong, which is great, but with only one ballscrew you're kind of defeating the object of doing that. Not saying don't make the frame strong, as you can't change that later (without wasting material/money) unlike adding an additional ballscrew which is obviously easy enough.

BTW I too am a model flyer (Gliders and power) and have in mind to build a 1:3.5 scale (4.25m) "Reheinland" glider with all the bits CNC cut

I bought a 500 size helicopter recently, so I'm also almost a model flyer. Not dared actually fly it yet! Couple of bits to cut from carbon on my router first. So far I have just stuck to model cars.

[routercnc]

Hi Mike,
Good to see a fellow flyer on the forum. I'm sure you know this already but Chris Williams writes a good 'big glider' column in QEFI magazine. Once you cnc cut parts from CAD designs there's no going back to carbon paper and all those hand methods!

Hi Jonathan,
Good point you make about the rail itself deflecting - I was think only of the block itself which is of course open to allow the supported rail underneath. That makes things more complicated and either needs FE, or someone to do a quick test!
If you've never flown a model heli before then join a club and get trained because otherwise you will crash it in seconds! Lots of PC simulators out there which are worth a go because you can often connect your own Transmitter (using the buddy lead).

[MikeyC38]

Thanks everyone for the very kind and useful advice. Here are the revised plans with most of the stuff hopefully taken into account:

1. Revised Z axis with bearing blocks fixed to the Z plate against the gantry
2. Gantry sides beefed up to 200mm width to reduce racking . 20mm thickness
3. More detail added in drawings

The endplates will be extended to allow for a future upgrade to twin ballscrews as everyone is suggesting! and belt driven to avoid synchronisation issues as discussed earlier.

Thanks for the guidance on sources of aluminium plate.

For the eagle-eyed viewers you may notice that the y-axis bearings are floating... this is not deliberate but something I realised that a 750mm ballscrew is the total length, not the threadlength and as I want to bolt the BF/BK blocks to the gantry sides then I will have to reduce the length of the y-axis round rail and support. Question here - is it best to dismantle the supported round rail then cut the rail and support seperately and re-assemble? or try to cut it all in one go?

I'm really getting into the 3D solid CAD in a big way - it really does help you work out how things need to fit together and how on a 300mm ball screw, the maximum travel you can get on the Z axis using supported round rails would be no more than 150mm at best!

Thanks again everyone!

[JAZZCNC]

Question here - is it best to dismantle the supported round rail then cut the rail and support seperately and re-assemble? or try to cut it all in one go?

Cut it in one go. . . . I do it by first cutting the round rail using 1mm thin disc in the angle grinder, cuts like hot knife thru butter then finish Alu base using mechanical hacksaw but hand held will work just same.

[Jonathan]

The distance measured parallel to the Y-axis between the spindle and the Y-axis ballnut/screw is currently quite large. This will cause the cutter to deflect parallel to Y as the rails offer little support in that direction. Between the Y-axis rails is a good place, hence why a lot of people mount them on separate pieces of extrusion.

[routercnc]

The vertical plate on the back of the Z axis (to which the Y axis ballscrew is attached) is trapped between the small top and bottom plates. This means it needs to be machined accurately to ensure the Y axis bearings sit correctly. It would be better to make this plate full height and have narrower top and bottom plates so that everything can be set before tightening the bolts.