Wobblycogs CNC Mk2

[njhussey]

Graham, liking the new design! When you want some 10mm and 20mm Ali plate for your Z & Y axis give me a shout as I've got loads stashed away (a 2200 x 950mm piece of 10mm 6082 (albeit cut up into smaller pieces now...) and a 1065 x 850 piece of 20mm) and won't be using anywhere near all of it. I'm sure we can come to some arrangement...

[Wobblycogs]

Thanks Neil, this CNC building lark is expensive but every little saving helps. I saw a massive piece of 25mm aluminium plate go on eBay over the weekend. I was wondering if you'd grabbed it. I took a punt but I wasn't trying too hard as it's thicker than I need.

[njhussey]

Well when you have settled on the sizes you want give me a shout...free Ali in exchange for machining services could be good?

[Wobblycogs]

Sounds like a plan but I think you might be over estimating how quickly I can get this machine built

I'm going hell for leather at it presently but I'd guess it's still going to be a two or three months at least before it's built.

[njhussey]

Quicker than mine at the rate I'm going!! I'll probably look at it again when I get back off holiday and blinking re-design it again!!

[Wobblycogs]

Well I've broken rule number one and bought the first pieces of the machine before the design is properly finished. I decided to take the plunge and get some second hand THK profiled rails. They are a bit longer than the supported rails shown in the design above so I'm beefing up the gantry. The X and Y rails are SHS15R@1040mm and the Z is SSR15XW@410mm. I've since read that SSR are better suited to horizontal alignment because of the bearing arrangement but to be honest they are working so far below their rated maximum load I'm not worried.

I have a quick question about mountings the rails on extrusion though.

For the Y-axis I was thinking of doing this:



Does this look like a good idea? There's 4mm of aluminium and the bolt is an M5 so that would mean 5 threads. I'm concerned that if there's the slightest bit of warp in the extrusion it'll throw the rail out because it's fairly snug in that central channel.

For the X-axis I've got a couple of options:



Neither seems to be a great option but the one with the rail over the T-slot looks precarious to me.

I'm seriously considering ditching extrusion and going with rectangular section. Any one know where I can get some with decent wall thickness?

EDIT: Scratch that last question, I must be going blind, Aluminium Warehouse do 2500mm 100x50x6 box section for £52. That makes it significantly cheaper than the 60x90 extrusion I was going to use.

[Wobblycogs]

I couldn't get off to sleep last night for thinking about the CNC so I decided to get up an do a bit to the design. I wanted to address a couple of issues:

1) the rails didn't mount very cleanly onto the aluminium extrusion
2) it felt like too many parts were becoming sufficiently complicated that I would need access to a milling machine. I know I could get some / all of parts made for me but part of the fun of this project is coming up with a design I can make myself.

With those issues in mind I switched over to using aluminium rectangular section 100x50x6. The other major change was to mount the rails on the front of the gantry rather than top and bottom. I'm not sure if this is the greatest of ideas but I think it'll make the build quite a bit easier.

The shots below show the Y and Z axis together. There's a bit of tidying up to do moving bolt holes but the basic structure is in place.



If vibration is an issue with the box section on the gantry I'll pack one or both beams with fine gravel / sand. Anything shown in red will be tooling plate.