DIY CNC 8x4 mdf build

[JAZZCNC]

And i am first on the list usually . Hence nobody commented till now.

Err you sure about that Boyan.? . . . . On this occasion I didn't post because he's obviously not done much research otherwise wouldn't be using MDF/V bearings also know when man's gone to this much effort and trouble he's not going to listen to anything we say So why waste the effort.!!

[Robert Millard]

Hey peeps why all the negative vibes???
I'm just playing at this anyway... We all can't afford the expensive linear rails
plus this isn't something I'm building to last I just wanna cut a bit of ply

I've seen plenty in mdf that work perfectly well. If I wanted to spend 3k plus I'd be building from steel but I don't. I have a wood work shop and all of this was stuff I had kicking about damaged boards etc so it's free so far. If it doesn't work I won't loose much sleep about it. If at the end I fancy a posher job I'll design something I bit better. So please don't shoot me down this is supposed to be a friendly forum not cocks are us!
Hi boyan I'm going to use 25mm ball screws for the main gantry 30mm would of been a bit better speed wise but as its a mdf build I won't be running a very high speeds I've yet to spec the steppers which I'll do when I've done the calcs. I agree I may need to stiffen things up a tad and Vee bearings were my second choice but cost wise I couldn't knock them as I found a job lot for £45 quid
Ive got a job I thought it would be handy for so as long as it does a decent enough job cutting some shapes from 18mm ply it's saved me having to sub the job out. If it doesn't it'll keep the boiler burning for a month or so!

[Neale]

Hey peeps why all the negative vibes???

Ah! You used the M word! Forum members around the country will be hanging up sprigs of garlic over their keyboards to keep out the wicked spirits of MDF!
Now, I'm probably the only forum member (or, at least, the only forum member prepared to admit it) to have built and still use an MDF CNC router. Personally, the experience of using this close relative of cold-rolled cow dung has somewhat influenced my views, but the machine works. In fact, I've just popped into the warm to do a quick CAM update of a design I'm cutting on the machine this evening. For reference, my machine has a cutting area of something like 600x400mm. Over the three years I've been using it, it has gradually been sagging and warping its way into oblivion, and while it wasn't that great to start with, it's worse now! I've had to bolt square steel tubes along the sides of the bed to try to keep it somewhat flat but still end up machining spoil boards or spacers for any job that needs halfways decent accuracy. The Z platform is sagging and is now a few degrees off vertical, and I've fudged it by packing out the spindle supports so that the spindle is vertical, but I can't make significant depth cuts in multiple passes as the cuts don't line up accurately enough. At the moment I'm cutting 3.6mm ply with a 3mm cutter, and running at about 500mm/min. I can hear the laughing from here - that should be around 10x faster but the structure of the machine won't take it.

More to the point, though, and assuming that you can build a torsion box structure, maybe judiciously reinforced with steel, that is strong enough to do the job, you still need to think about the subtleties of the design. For example, using ballscrews or whatever. I'm using cheap threaded rod as leadscrews on my machine, and that limits me to 900mm/min max. That's because of the leadscrew critical speed, which is the rotational speed at which the screw starts whipping and shaking things to death. I've just looked up an online critical speed calculator which suggests that the critical speed of a 25mm ballscrew, 3m between bearings (which is about what you are looking at) is just under 300RPM. If you use 2510, that means about 3m/min max speed. That might be OK, but most new builders are surprised to find that things like ballscrew inertia are critical to machine performance as this limits acceleration, and that's a big ballscrew you're planning. There's a reason why big machines often use rack and pinion or ballscrew with rotating ballnuts. There are plenty of other non-obvious design decisions and trade-offs.

Personally, I'd be surprised if you can build a very successful machine of this size in MDF, but in any case bear in mind when you are buying the more expensive bits that you might want to use them on the Mk2 when you get dissatisfied with the Mk1! That's what I did with my own machine, and I'm halfway through building a bigger machine in steel at the moment. But I won't say don't bother with the MDF machine, but be prepared to be disappointed. However, as a hobby machine, MDF gives quite a lot of fun per pound. I've learnt a lot from mine but based on that experience, you are being pretty ambitious with yours.

[Robert Millard]

Hi thanks for that wow 3yrs!! I only need mine to last 6 months so that gives me a positive. My whole machine is 3m long but the ball screws will be 2.6m which makes a fare difference to the critical speed
I may rethink the gantry sooner rather than later and swap out the bearings for a better option in time ... Just see how it goes
i did give sagging a fare bit of consideration when I built the table which is why I made the 2 sides as separate load baring structures in there own right and joined with supports from 25mm plyI agree though the expensive pieces I may wish to reuse
thanks for your input :)

[JAZZCNC]

So please don't shoot me down this is supposed to be a friendly forum not cocks are us!

Yes it is and I'm sure you'll get lots of help with comments like this.!!!

If you just want to shapes in Ply wood buy good Jigsaw it will be quicker and do better job. .