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Flapjac
08-30-2004, 09:56 AM
I presently have a small cabinet shop and have the opportinunity to expand, this will require a CNC router able to handle large sheets of HDPE (5' X 18' X 3/8" thick). I would appreciate any advice on the pros and cons of attempting a project like this.

Rack & Pinon or belt- 2 drives on the X ?
Any Ideas/advice would be greatly appreciated.
Thanks

JRoque
08-30-2004, 03:40 PM
Hi. I'm no expert but you might be better off building (3) 5'x6' machines than one 5'x18'. I can imagine the error alone at 18' will make the machine impractical. Three machines in line will be manageable and way faster.

Regards,
Julio

DDM
08-30-2004, 07:21 PM
I'd say go for the 5'X18' router. Take a look at the Shop bot (correct me if I'm wrong), it shouldn't take anything to make it 3 times the length. For the drive, I would go rack and pinion on one side and on that side make it so the gantry can't skew. Our school has a plasma cutter that's got a 12 foot bed on there that can get within 10 thou easy. When building it a lazer level would probably be nice because chunks of iron 20 feet long can flex quite a bit. Umm can't think of much more right now other than I like big pieces of equipment. :cheers:

DSL PWR
08-30-2004, 07:32 PM
Just look at how plasma tables are built.

ynneb
08-30-2004, 07:45 PM
Am I reading wrongly? Does 18' mean 18 feet? 6 Metres ?
Does raw material actually come that long?
It must weigh a ton. How long is the longest piece you plan on cutting?
Would it be better to cut the sheet down each time to suit a smaller table?

DSL PWR
08-30-2004, 09:10 PM
You can get steel plate in 8'x20' and it is set on plasma tables. I've seen machines that could handle 2 full sheets. Once you have the width in a gantry with rack and pinion, you can build the machine as long as you want!

Flapjac
08-30-2004, 11:20 PM
ynneb,
Yes, a 54"X17' sheet of 3/8" HDPE will weigh aprox 150 lbs. I've been looking into a 1"-1" lead ball screw with +-.004/ft. This brings into play sag/whip issues. I will have to spin the nut in order to get any kind of speed. I'm concerned with using only one screw...racking. Anyone have any ideas I would truly appreciate any ideas.

Thanks

ynneb
08-30-2004, 11:35 PM
That would be a mighty expensive lead screw.
Personally I would be looking at either rack and pinion or belt driven for that length.
Definately try and drive the gantry from both sides, or make your gantry very wide so as to avoid twist. You may then need to make your machine a little longer so as to accomodate for the wider gantry. Still, drive it from both sides as a preference.
You can use the one big servo to drive it from both sides with drive shaft going across the gantry. It is possible to have 2 servos, one for each side, but there are other negative issues to consider there.

Flapjac
08-30-2004, 11:44 PM
ynneb,
What problems would I have running two steppers on each side of the x axis? Would I need a separate (4th axis) or could I simply wire the motors in parallel? Would a stepper with a encoder solve any of these problems??

Thanks

ynneb
08-31-2004, 12:05 AM
This is an area that I have never worked with, but from other peoples previous posts i have noted these issues. If one stepper lost steps you would get twists. If one stepper stopped working for some reason you would get twists. I know you would need a seperate drive for each stepper and you run the second drive as a slave.
If however you had one bigger stepper and a drive shaft you would avoid all these issues. I guess it wont be long b4 someone with first hand experience will come and explain better than me.

DDM
09-06-2004, 11:09 PM
On a rack and Pinion system what kind of considerable backlash is there? I know I was the one to bring it up but I'm still in the dark on the gearing and possible backlash. Would a person have to have some kind of preload to bring the pinion gear into the rack gear or is there two seperate gears to prevent backlash.

Is it worth trying to mess with a rack and pinion system on some of the smaller machines with y axis being 4' or is this type of setup more centered toward the larger machines where ballscrews/acme rod would whip so badly?

Also if you were to put a rack and pinion system on a smaller machine what kind of skew would the gantry experance seeing how the drive is on one side and not centered across the gantry?

Carl

Carver
09-08-2004, 08:05 AM
Just a few quick thoughts.

Rack and pinion is the only practical way to go. You have no error of accumulation on the long travel.

Regardless of the construction method, do a stress relief after final welding, but before component assemble.

Regularly check it ( the bed) for flatness even if you think the concrete slab it is sitting on is stable. Slabs flex seasonally without cracking and you can be machining a parabola without knowing it.

Servos will keep you from having to enslave the opposing drives on the long axis to eliminate skewing.

I have a 8' x 12' router, and have no accuracy problems with it at all. In fact it came from a cnc manufacturer that used it for milling ways on other cnc machines.
I am exposed to some 44' long cnc used in the boat building industry and they have the same accuracy at 44' as the small machines do at 4'.

Best of luck, Phill

turmite
09-08-2004, 01:17 PM
I have a 5'X9' Digital Tool machine I bought way back in 1991. It has steppers, rack and pinion and uses a drive shaft between the two y ways. This machine also uses Bishop Wisecarver vee ways and wheels. I have never had a day of problems with the rack and pinion nor the Bishop stuff. I routinely machine pistol grip blanks that wrap the frame of semi auto target pistols and some of those grips hold functiong parts to the frame of the gun. Once the program is correct I have never had to worry about it. So with all that said my suggestion is:

Rack and pinion
Bishop/Wisecarver
Steppers or servos
Mach2 for the control
Gecko's or Rutex for drives depending on selection of motors

Mike
ps If you haven't seen Phil's site you really need to go take a look. I am hoping he might adopt me some day! :p

DJB282000
07-25-2005, 02:29 PM
I am also interested in a larger table and it seems like the more I read the more i am pushed to the rack and pinion idea.

Does anyone have any recomendations on the size and pitch of the rack.