noob trying to get some of the CNC pie

[jestah]

Hi

after years of thinking about it, reading about it and dreaming about it i have enough time to start designing a cnc router to cut out my carbon skateboard moulds and decks.

I would rather slower rapids and higher repeatability and resolution as it is very important that molds be as balanced as this can to help reduce speed wobbles in my speedboards (at 90+km unbalanced boards start to feel VERY scary!). I would be happy with 100-150ipm rapids. My cutting area should end up around 1500x1200x250-300 to allow me to slide a full sheet of ply/mdf down and cut the other side as i cant quite afford to build a full sheet at the moment. I also need a bit more z as some of my other projects are a bit taller. I plan to cut mostly MDF or wood but would like to be able to cut thin carbon/glass fibre laminates, small Al parts and maybe resin infused mdf.

I plan to built it from steel box section (either 200x100x6, 250x6 or a mixture of the two) that is welded together. I will be using slides and ball screws and maybe a 2.2kw spindle from :

http://cgi.ebay.com/linear-rail-SBR1...742.m153.l1262

what size slides should i use? I was guessing that 20mm would be good for the x and y with 16 for the z. I plan on using 20mm 10mm pitch ballscrews with for the 2000mm x and 1600mm y and 16mm 5mm for the 500 z. are these stiff enough for the rapids im planning? I could preload them if they start whipping but would also like to know if its just easyer to go to the 25mm screws.

I have attacheds some very basic thinking for how i would like to construct my table and gantry but am not sure im going about the z in the same way as most (often i see the carrages mounted to the z tool plate)

Electronics wise i would like to use the G540 for the fact that i am not so good with complex wire bundles and feel that i should be able to find motors that will give me the grunt to move this gantry. I under stand that the drive is limited to 50v and 3.5A but is that 50v total across all motors and 3.5A per motor?

thanks for helping me iron out the finer points of my desings as i feel im still stabbing in the dark.

jestah

[finishlinewrx]

+1 for the g540 driver

Here are nice recomended packages with motors and psu.
http://www.kelinginc.net/CNCNEMA23G540Package.html

100-150ipm is going to be slow on a 5ft table. but if your happy with 30-40 sec or so travel it doesnt matter.

I like the frame design myself.

[jestah]

Originally Posted by finishlinewrx +1 for the g540 driver Here are nice recomended packages with motors and psu. http://www.kelinginc.net/CNCNEMA23G540Package.html 100-150ipm is going to be slow on a 5ft table. but if your happy with 30-40 sec or so travel it doesnt matter. I like the frame design myself.

it will be slow but im still struggling to optimize the specs of all the different parts to get the best middle ground. Once i have it all drawn up in solidwork ill be able to guess how much the gantry is going to weigh then work from there.

[finishlinewrx]

Are you planing on using just one screw on your X axis?

You might look in to doing one down each side of the table. With a belt drive or even two motors to sync them.

[jestah]

Originally Posted by finishlinewrx Are you planing on using just one screw on your X axis? You might look in to doing one down each side of the table. With a belt drive or even two motors to sync them.

I am going to use to screws down the x, much like the 4x4 hybrid. I will add a timing belt if i need to but im going to slave them in mach first.

[finishlinewrx]

The Z your talking about.
I dont see why it wouldnt work and im sure other have done it both ways.

The only reason i ran the carrages on the moving side was because i felt the rails were heavier since you have to lift them up and down. But i could also see why if other felt it added rigidity to the Z.

[jestah]

The weight they add is a good point i did not think about. i had added them as i felt that would cut down on having a two long plates/boxes (this has just one long z box section that the spindle is mounted inside.

I was planning on having the motor mounted to this as well but then this just keeps stacking weight up on the z. I had planned on using a 5mm pitch to help with vertical resolution as well as giving some extra ease to the motor.

[jestah]

So after a lot of thinking, many LATE night and a bit too much cursing at solidworks i have come up with something that you may find very interesting.

Here is my y and z that i am saying will weigh some where around the 50kg mark with my spindle attached. they will both be made using carbon prepreg that will be formed in a female mold that will be cooked off at 120c and 2.5-3 bar. We are also thinking of making the table from carbon as this would also allow us to recess the ballscrew and apply more preload with less defection than if we mounted it with bearing blocks.

I have gone with 25mm 10mm pitch ball screw for the 2.8m long x and 1.8m long y and 16mm 5mm pitch for the z and would like some help selecting motors. sadly i dont think the 540 is going to cope with this so i have decided to get 4 203v and a pmdx-132 bob. I am aiming at rapids around 200ipm but will drop this down as more low end torque and repeatability is better for what i want to do.

thanks in advance
jestah

[BobF]

My 305 oz/in motors with 1/2-8 2-start screws calculate out at 200+ lbs of force. If I changed my drive to a G540, the same motors would produce close to 425 oz/in and the force would be proportionally higher.
2.8m is a very long screw. I would look at multistart screws with higher pitch. 25mm if possible. The multistart screw is more efficient and the force produced is higher than you think.
Look at Nook industries for the formulas to calculate this. Choose a screw that is close to what you plan to use and see what you get.

[jestah]

im locked into a ballscrew from:

http://stores.ebay.com/linearmotionbearings

where on the nook can i find the ball screw force calcs or do i just use the acme details?

thanks
jestah

[Jason3]

Hi Jestah,

Great to see your build is progressing! Really looking forward to seeing the Carbon gantry - saw some pictures of your boards, and they look seriously good.

I think your ball screw choice is good. Let me know if you want some help with the numbers for motor choice

Best regards,

Jason

[jestah]

i went with these screws because the price is right for us in new zealand as acme rod and ball screws are BOTH 10 times the price of what we an get here.

so using the 25mm ballscrew with what im assuming will be a 20mm root D i should be able to spin my longest screw at 470rpm. Drop this back to 400 rpm to be safe is still 4m per min that gets my spindle from one corner to the other in 40 seconds or so... well fast enough for me.

Where i stumble is working out how big of a motor i need. Using the Cnc_calc spredsheet i come up with needing 1.2Nm, 175oz, at 400 rpm to move a 100N cut (im assuming 10kg is enough to do light pass in mdf, foam, and slow cutting of fibre glass and al.) with a Additional weight of 60kg for the gantry and spindle. This seem VERY low to me but i get a feeling i have made a mistake

I am still unsure as to how multi start screws they move the same distance but reduce the force needed?