Some design questions

[cropwell]

Hi Chaps,

There are (at least) two problems with selling my current machine. a. Overcoming the shame in admitting I bought it and b. I wouldn't wish it on anyone else ! The wee beastie is really only a collection of bits of ali profile and some trapezoidal screws. When I mean rebuild, I only intend to use the steppers, spindle mount and T slot plate. Some of the remainder of the machine might furnish profile and motor mounts, but the rest would go into my non-ferrous collection.


Chai has quoted me on supported rails and bearings, leadscrews, ballnuts and end bearings. I have requested they are only machined on the driven end, so I can turn them to the final fit. For the small cost of the rails and leadscrews, I have been generous in length allowance. The machine can only be 600 x 500 x 200 MAX. I have a MIG and 4 saws capable of slicing steel, as well as 4 angle grinders (there is a short but very boring story as to why 4 of each !).

On a small machine, the usual practice seems to be to drive the long axis with one leadscrew centrally under the machine. How problematic would it be to put it to one side ? Could any skewing be sorted with a parallel motion mechanism of wires and pulleys like draughtsmens' tables ?

My main thought at this early stage is 'why put a leadscrew where it gets shit thrown at it ?' Are there any major mechanical disadvantages to starting the design as a gantry with the long axis screw at the top ? I think that I probably ought to get some drawings done to clarify my thoughts.

Cheers,

Rob

[routercnc]

Rob,

There's no shame in buying an MD machine, or any starter machine. At that stage you need to be sure this hobby is for you and that is one of the ways to do it. You will also learn a lot about what is important and what factors are required for a really good machine.

Many, myself included, have considered pulleys and wires to slave the other side but in the end I've not seen many builds like that. In the end with wire tensioning, stretch and setting up concerns etc. most opt for just buying another screw and belt slaving or double motors etc. I would suggest you do the same. At 500-600mm wide it be well worth it to go to double screw.

Going back to your thoughts - if you put a single leadscrew at the top in the centre, it will have to be very high to clear the Z axis and stepper. The drop to the gantry will be so long that you will loose considerable stiffness with the mount.
If you put a pair to each side, but still high up then you will have long supports up from the bed to mount the steppers to (assuming raised gantry design), again loosing stiffness.
If you put a pair to each side, just above the bed then you have a fairly optimal position in that the driving force is similar in height to the cutter. The stepper mounts are short and simple. I think if you use side shields down each side of the bed (e.g. perspex), or dust extractor for wood you will be OK.
You can also run the screws even lower down, outside the gantry sides and run the screw level / lower than the bed, and this also works. I think it is not quite as good as the previous for cutter force offset, but this might easily be outweighed if the mounting is more convenient and can be made stiffer.