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  1. Quote Originally Posted by No1_sonuk View Post
    What sort of price is 18mm 3/4inch MDF at the moment?
    607mm x 1220mmx18mm in B&Q is 10.79 plus 4 free cuts.

    2440mm x 1220mmx18mm is 14.99 plus 4 free cuts.

    2440mm x 1220mmx25mm is 21.68 plus 4 free cuts (in you can find one that stocks it).

  2. #32
    Having looked at the price of ground steel and oilite bushes at 60+, together with a 20mm router bit, I am beginning to think that I might modify the Rockcliff type C using 20mm rails, support pillars and pillow blocks from Zap, they aren't much dearer, and would probably be easier to fix in place and align.

  3. #33
    Hi Lee,

    I've read through this interesting (old) thread and wondered if you managed to finish the machine and if you did how useful you found the machine to be.

    Does it do what you wanted and is it fairly accurate?

    I'm planning my first build but don't know whether to jump in with both feet and build a big (expensive) aluminium job straight away or do a cheaper MDF one to get a grip on it. There's so much to learn.

    I want my machine to work wood and make a lot of items which I hope to sell to earn much-needed money so the machine will have to do some production work.

    Many thanks.

  4. #34
    Just bought a part finished Rockliff D with a working size of approx. 900mm x 600mm not sure of the Z. The guy who I bought it from had to give up the workshop that he had. I have taken it apart and propose to strengthen it in various ways in the hope of getting a bit of speed from it. I'm using 4no. 270oz Nema 23s driving M8 threaded stainless steel rod for screws. I am making some anti backlash nuts although I'm not looking for super accuracy. My reason for contacting was to ask if Lee Roberts ever finished his Rockliff as I can't find any trace of it.

  5. #35
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 6 Hours Ago Has been a member for 5-6 years. Has a total post count of 1,167. Received thanks 212 times, giving thanks to others 5 times.
    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.

  6. #36
    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?

  7. #37
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 6 Hours Ago Has been a member for 5-6 years. Has a total post count of 1,167. Received thanks 212 times, giving thanks to others 5 times.
    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.

  8. #38
    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.!!

  9. #39

  10. #40
    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

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