Re: [CAD_CAM_EDM_DRO] Big CNC router

Doug: I hope what I plan to do here will be legible!

> We want to build a router table to handle 4'X8' sheets of .125 aluminum with
> +/- .005 in./ft. accuracy in drilling and slotting. Speed is not a major
> consideration here, as most of the work is prototype. On the other hand,
> the machine will later be used for cutting patterns in plywood and particle
> board, so we will eventually need high feedrates. >> Been there, done
> that. Shipped to England, flew over, assembled in January '86. Run by an
> old PET!
>
> 1. Should I use preloaded nuts on rolled ballscrews or are the nuts tight
> enough without preloading? >> I used BSA rolled ballscrews with dual
> preloaded ball-nuts. The "slop" seemed to be at least twice the per-step
> move, which was 0.005"! So, the preloaded nuts were necessary, in my
> opinion.
>
> 2. Most of the low end or home built machines use cast aluminum toolplate
> for the gantry. Is this preferable to a steel weldment? >> Depends on
> what is preferable. If not having steel-cutting and welding gear is
> preferable to tolerating SOME shimmy, then the Aluminum might do. If you
> can heli-weld the alum., it will almost-certainly do.
>
> 3. Larken and a few others are using dual screws for the X (moving the
> gantry) axis. >> WOW! Then I wasn't all-wet on that '85 design! (Cost a
> bundle for the two huge screws!) This seems like a sensible approach. >>I
> thought so, too... Should I use a chain or belt to couple them or would
> dual motors be advisable? >> I used #35 chain, and this machine got used 8
> hours per day, and the chain had to be replaced every 20 days, average.
> NOT good! So, the ol' boys removed ONE screw, and the stiffness of my
> "gantry" was sufficient for them, so, they removed the HUGE (MO-172, was
> it?) "X-motor" and put it on the END of the remaining X-screw, with one of
> those flexible shaft-couplings, and they are happy. I was embarrased when
> I saw it, but they slapped me on the back, "No problem, Yank! This bugger
> has run for tens of thousands of hours since you left us with it! It paid
> for itself before the first chain broke! Since, it has made us millions!"
> But, dual motors might be a good idea. I think I have heard they did
> that, too, later. I am assuming that dual servo motors driven with
> problems one might encounter with
> steppers. I am using CNCPro.
>
> 4. Is cast aluminum toolplate the best option for the table? How thick?
> MDF would not work with coolant. >> Hmmm... I used Melamine on MDF for
> the top of the "English X-Y", but that was a TOTALLY dry operation. for
> metalworking, Alum. would be OK, but the thickness you select would depend
> entirely upon the spacing of the frame-members "joists"?) under it. If
> they were, say, 12" apart, 1/2" plate might be OK for milling flat stock.
> Maybe even 3/8". But I think I'd hafta "see" the operation before I'd
> guess less than 1/2" plate. Won't it get "routed up" all the time, and
> need constant replacement???
>
> We have about $6,000 in the budget for this machine. >> Lotsa luck! I
> quoted $25,000 U.S., and SPENT $35,000, by the time I was done, and my
> lodging and board was paid by the customer in England! Oh, and the freight
> over was paid by them on delivery. That is, the actual $ spent for
> steppers, drivers, power-supply, circuits (I built my own interfaces,
> etc.), steel, some outside job-shop machining, case-hardened shaft-ways,
> linear ball-bearings (Thomson stuff), hi-F 3-phase router-motor and
> phase-converter-generator-motor thing all totalled $35,000. Had a
> working-surface of about 60" x 120". OH, the "frame" was hardwood, built
> BY the customer! I had steel "bolt-ons" for the mechanism, so all was
> fitted, run, tested here, before knocking-down and shipping. I saw the
> wood frame first upon arrival there, for installation. Never again!
> Questions? Jan Rowland, Troll
>
> Doug
>
>
>

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