Rockcliff Router Build, MDF

[Neale]

I built a JGRO design from MDF a few years ago. I won't talk about strength issues here, but using around 900mm M10 threaded rod as a leadscrew (with Delrin anti-backlash nuts) I find that I'm limited to around 900mm/min top speed on the X axis. This is down to leadscrew whip. The JGRO only has a single X leadscrew but that won't make any difference to leadscrew critical speed. If you want it to go faster, use a bigger leadscrew. Bigger diameter means higher rotational speed possible as well as coarser pitch for more speed at same revs. 3nm Nema 23 seems more than adequate to drive it.

[paul-l]

Thanks for the info, a bit dumb of me not to realise the bigger the dia. the faster it goes. I am currently looking at extending the X and Y screws and to fit another bearing support about 100-150mm further forward with a rubber doughnut damper. From what I read this can have a good effect on speed and reduces whip considerably. Any thoughts?

[Neale]

As a very rough guide, going from M10 to M12 with a 1000mm distance between fixed and simple bearings (typical setup) gives a critical speed increase from about 950 to 1200RPM (according to one online critical speed calculator). Given the increase in pitch as well, that's a useful increase in max speed. I don't know what a well-built MDF machine can take by way of cutting forces and hence max cutting speed, although having a decent "rapids" speed is a good thing to have even if you can't cut at that speed.

I seem to remember someone showing pictures of a damper assembly that claimed to increase max speed recently on the forum; it used some kind of fork that straddled the leadscrew to stop or reduce whip with a lever arrangement to lift it out of the way as the nut went by. Whether or not it would be easier to just fit a larger diameter leadscrew in the first place is moot, although reducing rotational inertia by reducing leadscrew diameter is a good thing in itself. Engineering is very much the art of compromise.

[JAZZCNC]

Or better still not bother and buy some cheap Acme lead screw with a more fitting Dia and pitch. You'll nail several birds with one screw.!! So to speak.

Threaded rod is a multi edge gotcha sword in that the pitch is small which requires you to spin it fast and still get low speeds. The Thread is poor and abrasive which at high speed wears nuts away very quickly. The thread pitch varies wildly along it's length so isn't accurate. It's highly inefficient so zapping motor torque which you don't have spare because the motors have to spin fast just move at what is a relatively low feed rate.

If you can't afford ballscrews then Lead screw is the next logical choice. Threaded rod isn't and shouldn't even be considered other than for bolting the frame together.!!

[mekanik]

http://www.igus.co.uk/wpck/9443/prod...zgewindemutter
Regards
Mike

[paul-l]

I seem to have inadvertently dropped into that deep black hole that is called threaded rod ,to use or not to use. My object in starting this cheap build was to have some fun and put my 72 yr old brain in gear again. I already had a design in my head using toothed belts when I stumbled across the part built Rockcliff on ebay, so I decided to stay with the Rockcliff design and see if I could finish it fairly quickly. The next 3-4 week should see it done then onto version 2 with Acme screws, Dumpster anti backlash nuts, 1200mm x 900mm bed, Kress spindle and Artcam Express if I can afford it. I read as much as I can on this site and its American cousin so with the positive help that always seems available here version 2 should be the one

[Neale]

Given where you are coming from and where you are going, then I would have thought that threaded rod for a short-term, cheap, solution would be fine. Can it handle high speeds? No. Can it handle heavy or 5-days-a-week commercial-type use? No. Will it wear out nuts fairly quickly? Probably - but see below. Will it work and give you something to play with, as well as do something useful, while working on Mark 2? Yes. Use stainless rather than BZP as it's much smoother, but if it's any reassurance, the M10 leadscrews on my JGRO router together with the Delrin nuts have been in use, intermittently, for the last 3 years and have done many useful jobs in that time. They are far from being the weakest part of the design. And I've never needed to adjust the anti-backlash nuts in that time.

It's always possible that this machine will encourage you to be even more ambitious for the Mark 2!

[paul-l]

Posted at the end of September about finishing off a part built Rockcliff moving gantry design that I bought on ebay. As usual thing got in the way and only the last couple of weeks have seen it in running order, not finished you understand but working. I had no trouble with the mechanics and the electrics were fairly easy(arduino uno, protoneer cnc shield, 4 A4988 drivers and 4 Nema 23 motors). My problems start with my lack of knowledge about g code, everything I try to cut is back to front and a mirror image of what is on screen in UGS or Grbl Controller also I don't seem to be able to know where the job is going to start despite setting all axis to zero and positioning the router to the bottom left of the bed (1000mm x 500mm). I generate the g code file in either Easel or JSCut. Any help would be greatly appreciated