Comments sought on new build: A CNC Router for RC Gliders and Planes

[njhussey]

Hmmm seem to have been away for ages again...still now bought a small welder so will start cracking on with the fabrication.



Finally decided to do 2 machines, one for balsa and one for ally...the one for ally will be a small one I can also use for labels etc...

[njhussey]

I really should stop playing about with the design and get on and build the damn thing.....but I keep reading posts on here and changing things. Was going to go with a framed gantry but then everyone says to keep the sides down to a small height to increase the stiffness.

Here's my latest design, can you guys comment on it please? It's going to be predominantly for cutting balsa and ply, the odd bit of plastic, carbon fibre and possibly aluminium components for RC planes, helicopters etc. There's only 100mm of height, but as the thickest balsa I'll be cutting is probably 1/2" it should be fine.

The box frame is 40x40x3 mild steel, the flat plate is 12mm 5083 Ali, the rails are SBR 16, most probably from linearmotionbearings2008. There are a few things which need tweaking to make them fit (Z axis bearing plate needs to be made the same width as the bearings etc.) so I'm aware of them, it's just the basic concept/design.

[Jonathan]

I would make the mounts for the X and Y stepper motors/ballscrew fixed end bearings stronger since they have to support the axial force on the ballscrews, so any deflection in those mounts directly causes inaccuracy in the tool position.
On my machine I have SBR20 and SBR25 rails and it is clear that the 25mm rail system is substantially stronger, presumably (apart from the obvious reasons) because the bearing blocks have an extra row of balls. I urge you to consider at least using 20mm rails as it shouldn't cost that much more, although it does mean increasing the box section size to 50mm.
Have you considered using pulleys and timing belts to drive each axis? This can simplify mounting the motors, reduce resonance, improve feedrates or resolution, among other things.

[njhussey]

Thanks for the comments Jonathan, I'll beef up the mounts and possibly look at pulleys. This router will be used for mainly balsa and ply so I cant see the need for the 20mm or 25mm rails, surely the 16 will be strong enough? The only Ali I'll be cutting on it will be thin stuff for horns, brackets etc. out of 1mm or 2mm thick.

If you look at the below router it's made out of 30x30 Ali and skate bearings and cuts balsa fine for making models! Yes it's not fast or particularly accurate but was fit for the purpose it was built for...



I will be making a smaller stronger router/mill for doing Ali blocks/brackets etc at some stage in the future and this will be made with linear rail and all the other goodies!!

[njhussey]

Just got myself some 10mm Ali plate from fleabay for a (what I think is) bargain price.....aluminium plate 10mm thick 2200 x 950 mm approx 87" x 37 1/2" Pickup floor | eBay Now going to look at the design and get drawing some parts to get made :D Need to invest in a pillar drill now to get all the holes drilled nice and square!

Jonathan, I've been looking at the 20mm rails and you're right they're not that much more so will get them...

[njhussey]

Chai quoted $246 for the rails for my router so I think I'll get them when I've got the money soon. Picking up the Ali plate in a bit and have someone with a CNC router going to cut the bits for me :D

Just one thing to ask. would it be better to have the y axis rails spaced apart and the ball screw (well trapezoidal rod for now) through the middle and have some plate on the back to stiffen it up? This would bring it nearer the spindle and reduce the twisting when cutting?

Very quickly moved things around as per the below (yes I know things need sorting but just wanted to show what I was on about...picture paints a thousand words and all that!!)

[njhussey]

Picked up a 2200 x 950 x 10 sheet of second hand (holes in the edges) Aluminium from flebay for the princely sum of £52 the other week.



Finally got round to taking it to a local sheet metalwork shop to get guillotined into smaller peices.



I've now got 5 pieces:

2 off 1000 x 5000 (going to be bolted together and then bolted to the steel base to make the router base, holes don't matter as they'll be out of the cutting area)

2 off 1000 x 450 and 1 off 950 x 205, pieces to get the gantry sides, brackets etc. out of.

Now going to do 2 routers. One is purely for cutting 1000 x 100 balsa sheet and will be a combination of MDF and aluminium. All the items I have so far purchased will be used on this. I will then be able to cut my teeth on this to get used to using the CNC and whilst the machine is hopefully generating some money cutting balsa plane kits I'll be building my other router.

Balsa Router.



The "proper" router will have 3Nm steppers, 80V drivers, 20mm supported rails, 1610 ballscrews belt driven etc. etc. Rough picture below, the 12mm Trapezoidal screws and smaller steppers are still shown here as I've not re-drawn it yet.



If anyone feels so inclined to do so I'd appreciate any feedback, comments etc...

[Jonathan]

Picked up a 2200 x 950 x 10 sheet of second hand (holes in the edges) Aluminium from flebay for the princely sum of £52 the other week.

Not fair! I hope it doesn't turn out to be a difficult grade to machine.

Be careful with the amount of Z-travel on the MDF/aluminium machine. Clearly you want to have as small a Z-travel as possible to reduce the deflection from having a relatively weak Z-axis and gantry, but there still needs to be room to change tools. This could be a problem if you need to use drills, unless you buy short drills and/or have a hole in one corner of the bed which you jog the machine to for performing the toolchange.

[njhussey]

Not fair! I hope it doesn't turn out to be a difficult grade to machine.

It was far less than I was prepared to pay so was well happy.....I'll find out if it is difficult when I send some pieces to Haydn to machine for me. I've still got to do the calcs and design the bearing housings etc. Where would the best place be to have the ball screws on the X axis, they seem to be sticking out quite a long way. Would it be better to have them under the 50x50 box that the rails are sitting on?

Be careful with the amount of Z-travel on the MDF/aluminium machine. Clearly you want to have as small a Z-travel as possible to reduce the deflection from having a relatively weak Z-axis and gantry, but there still needs to be room to change tools. This could be a problem if you need to use drills, unless you buy short drills and/or have a hole in one corner of the bed which you jog the machine to for performing the toolchange.

As the tallest piece of balsa is likely to be 1" I'll look at lowering the gantry and reducing the travel. Want to reduce the gantry size anyway as the sum of all the lengths is just over 5m in length so just more than 1 length :)

Below is the sort of thing I'm looking to cut on the MDF router. No drilling, I'll be using mostly 2mm cutters (somethinig like this I guess 10x Carbide Drill PCB End Mill Engraving Bit CNC 1.2mm | eBay )in the E50 Proxxon I have but yes will look at height for tool changes.

[njhussey]

Well after work I stopped by our local steel and Ali merchant (not the one we use at work) and got 3 x 1.5m lengths of 25mm x 25mm 10g Ali then went to B&Q and got an 8 x 4 sheet of 18mm MDF which they cut down into manageable lengths. Liberated a couple of 1.2m lengths of 3/4" medium red band pipe from work today (we haven't used it for 7 years) so this weekend unless I get collared to do DIY I'll be marking out and cutting out bits for the cheap crappy router I'm doing ;)