Moving table VS moving gantry

[fer_mayrl]

Hey Guys,
Ima firm beleiver of definite purpose designing when it comes to building CNC machines. What I mean is, the last machine we built is a big router, suitable for well... routing we have done some 3d releif work and v carving, but the size and weight of the machine make it a little unsuitable for that aplication, mainly because of vibrations, nothing that cant be  fixed by adding supports to reduce the shaking, or using quantum in the near future. The machine is big at 12' by 12'.

Lately we were approached to make a thermoforming tooling out of aluminum. Although we were able to do it, i didnt like the machines performance that much, I know I could have taken deeper depths of cut (cutting at 0.010") and at faster feeds (30 ipm right now). but the machine wouldnt allow it. The mold was about 20" by 26".

Now I was thinking, if I were to build a machine just for that kind of jobs, small print (maybe 48" by 48" or something) designed specifically for milling aluminum at decent rates. What would the best aproach be?
I was thinking in using AC Servos at about .85kW 2000rpm, max rapid rates at about 200ipm, with ballscrews al around, 0.2" lead, with a 2:1 reduction, that turns out at rated force of almost 2000lbs, and a max force of 6400lbs. Using HiWin rails maybe 25mm.

And on top of all this, stiffen the hell out of the frame and all components. That is why i was thinking of going with a fixed gantry and moving bed. that way I can stiffen the gantry a lot without concerns for weight.

So basically I am looking for opinions on that aproach, components and the using of a fixed gantry vs going with the moving gantry.

Thanks in advance
Fernando

[ardor]

Hi Fernando.

It is a matter of preference .. or the final application that determines if it is a fixed or moving gantry design.
Fixed gantry CNCs need a lot more real estate to house the machine, while the moving gantry can be made just as strong and vibration free as the the fixed one!

I have been designing and building CNC machines (Laser cutters, Glue applicators, Routers, Engravers etc..) for a few years and I had no problems with the vibration of the gantry or the router head!

For a smaller, up to 400W laser cutter, I would use the moving gantry with the laser generator right on the gantry. Over 400W I prefer the fixed gantry idea with the generator right on the gantry to reduce the beam delivery distance.
Routers and glue applicators, I prefer mostly moving gantry design, to reduce the area required for the machine.
 
My smallest machine was a 2' x3' engraver, while the largest was a 12' x 56' dual gantry CNC for foam cutting, everything else was in between, either a 4' x 8' or 5' x 10' or a 2m x 3m effective work area table size.  The Z axis on my machines varied from 8" to 36".  (Routers up to 10 HP.)

It is only a matter of recognizing and applying the basic engineering principles and incorporating them in your design if you want to create a sturdy machine.
For light gantry, use aluminum, for extreme strength, steel tubing. Deal with deflection and you will handle vibration as well.
For deflection look into the modulus of elasticity and the section modulus... that is where the secret for a good design is hidden. (In the shape of the gantry and the material used in the extreme fibre!

It is not just the way you design the gantry that will determine the strength and vibration. The interaction between the table's structure, the linear guide rails and the ganty sides, they all play a role in the creation or the dampening of the vibration.   
Don't use rotating (live) ball screws, use the live nut design.
Accumulative play means larger vibration.
The price of decent linear guide rails and ball screws is so low today, that a good quality machine is only a few thousands of dollars away. 

Sorry, ... I do not wish to preach.

Actually, I would like to get some information from you and from the other members on this forum.

I want to come out with a new line of very low cost but very capable CNC routers for people who want to start up a new business on a low budget.
I need the input from people who used the Mach3 and the EMC2 operating systems. I need input from people who have a direct comparison between these and the Fanuck, Fagor, A B, Siemens etc. G code systems in a professional production environment.

I will appreciate any input by helping with your design and manufacturing ideas.

 Regards,

Stephen I. Molnar

[fer_mayrl]

Thanks Stephen,
I agree on all the points you said. We have not gotten into building the small machine for aluminum cutting, If we ever need one, i think i might go with chinese machines, cheap, look well built, and it they lack in some departments, theres room for improvement.

I wont be able to give you much advise on what you asked since I have only used Mach3, an amazing software by the way.
Regards
Fernando

[zoltan]

Hi Stephen,

I am looking for building a 100-400 W laser cutter. Could you give me some advices, regarding laser unit, optics, etc, as design solution and suppliers of parts? Also, as you said that you designed CNC since few yaers, is it possible to share with us some of your designs?

Thank you,

Zoltan