High-Speed 3 (4?) axis cnc router

[rherman]

I am embarking on building a high-speed cnc router table. I only need a work envelope of 24" x 24" x 6" in the xyz. Actually the z only needs to be 1" (.5" cut depth and .5" clearance). I have two machines, since they are general use, they are not designed for high-speed. I am cutting wood only to a max. z of .5". I am looking to cut at 600ipm and up with 1/8" and 1/4" shanked bits. Right now, 180ipm is as fast as my 4' x 8' x 6" router table with steppers can go. Since my loads will not be great, I am thinking a rigid, but light gantry and high speed spindle with servos will get me where I need to go. Anyone have info. on cheap servos and drives? Also, accuracy requirements are not as tight as my metal cnc machine. I can probably get away with +-.010" (.020" total delta). Great forum.

Thanks,

Rob

[rherman]

I dislike answerning myself, but has anyone built anything that can cut in light passes at 600 ipm? Thanks.

Rob

[ger21]

I doub't you can get to 600ipm on a 24"x24" machine. That's 10 inches/sec, barely enough room to accel and decel.On a small machine like that, even 400ipm might be more than you need. But you can probably get 400ipm with rack and pinion and steppers. Shopbots use closed loop steppers and can cut at 600ipm, rapid at 1800ipm. You shouldn't really need closed loop for light wood cutting. A good source for servos is http://www.homecnc.info. And Gecko drives to run them.

[wisp]

You also really need to think about a spindle that can run high enough rpm for that feedrate. At 600ipm, with a 2 flute cutting bit, to get a .006" chipload per tooth, you need 50,000 RPM.

Something to think about.

[rherman]

Thanks for the info. Wouldn't a 3 flute bit at 24000 rpm (hitachi router with speed feedback) give me approx. .008 at 600 ipm? Maybe 300 ipm is more realistic. My bought machine, a Practical CNC machine, runs open loop steppers on rack and pinion. I can rapid at 200 to 250 ipm, but when I am cutting complex 3d surfaces it actually cuts at around 90 ipm as opposed to the 135 ipm I command it to run. I am looking to significantly cut down on my machining times. How about servos and 36" x 36" table? Thanks.

Rob

[ger21]

Originally Posted by wisp You also really need to think about a spindle that can run high enough rpm for that feedrate. At 600ipm, with a 2 flute cutting bit, to get a .006" chipload per tooth, you need 50,000 RPM. Something to think about.

You're not using the right tooling then. I looked in an Onsrud catalog, and the recommended chipload for a 3 flute spiral chipbreaker (1/2"D) is .02, which is 10,000 rpm at 600ipm. THis is for hardwoods, plywoods and mdf. Our router at work only goes 400ipm, and I use similar tooling @ 14,000. http://www.vortextool.com

[Chris D]

The slowdown you see could be the result of the computer, not the machine. When machining complex 3D shapes, you are often cutting lot's of little X,Y,Z moves, the computer has to process a lot more data for that than if cutting in a single axis as the same speed. This problem exists even on industrial machine tools.

Originally Posted by rherman Thanks for the info. Wouldn't a 3 flute bit at 24000 rpm (hitachi router with speed feedback) give me approx. .008 at 600 ipm? Maybe 300 ipm is more realistic. My bought machine, a Practical CNC machine, runs open loop steppers on rack and pinion. I can rapid at 200 to 250 ipm, but when I am cutting complex 3d surfaces it actually cuts at around 90 ipm as opposed to the 135 ipm I command it to run. I am looking to significantly cut down on my machining times. How about servos and 36" x 36" table? Thanks. Rob

[rherman]

ger21: That's because you're using a 1/2" D tool. I am using 1/8" and 1/32" Dia. tooling. Angular velocity at that diameter is a lot slower than your 1/2" bit at the same spindle speed.

Chris D: I don't think it's the computer. I have a WinCNC controller with high-speed card. I think the software may be limiting it due to the accelerations. I generated the same toolpath in different Cam package, and it spaced out the line segments at .020 on average as opposed to .009 or .0012 on average. This is not my stepover, but the CAM packages interpolation to fit the surface data. I am cutting with a stepover of .015 max.

Maybe I'm just asking too much given the work envelope is so small and tight; however, I was motivated by seeing those high-speed machining centers cutting metal almost as fast as I cut wood. Thanks.

Rob

[Halfnutz]

Please excuse my ignorance, but I am new to this and dont understand what the need or desire to go that fast is based on. It is obvious that this is a major concern for many people, but I dont understand why. Why are people so concerned with the speed of thier machines, especially while cutting? Is this production based, ie to produce more parts faster, or performance, ie do the finishes turn out better, or both, please clue me in. Thank you,

ps also to note the quick change R8 collet set shown is a nice tool, (I have a set) but TIR is much greater with these than with a standard collet. I get MUCH better finishes with standard collet's than with the QC set.

[rherman]

In the manufacturing business speed is key, if finish is within spec. But, I am machining 3d surfaces over an area from 4"x5" to 24"x24". It takes me 4 hours to machine a 5"x6" surface. I am using v-bits or 1/32" ballnose bits with a < .015" stepover. Take a look at my user gallery here. If I could cut twice as fast with the same quality, that's twice the money made that day.

Rob

[Halfnutz]

Where is your user gallery? I would like to see it. Thanks,

[ger21]

http://www.cnczone.com/gallery/showg...0&ppuser=12847