1st plasma table ?'s for my first post

[johnnydmetal]

Hello,
I've been a fabricator since the mid 80's and just quit a job working in computer networking to return to fabrication working from my home shop 4 days a week. my shop is a large 2 car 32x22 with a full loft area for storage. I enjoy building usable equipment from scratch. Besides the satisfaction I get when its finished and works right, it allows me have the equipment I need without spending the big bucks. The same goes for restoring old shop equipment like my 1933 Brunner compressor, Southbend H10, 30's era Craftsman Drill Press, 6x8 electrolysis tank, and 15hp 3 phase RPC to name a few.
I have a Mytec Plasma cutter that will cut 3/4 inch MS and sever 1 inch. It's a Canadian brand but is likely of Asian decent. Though I can cut well with plasma and Oxy/Aceteline I would love to have a computerized cutting table for greater accuracy. With it I will be able to take my work to the next level. Plus it will free me up to do other stuff while the machine runs the cuts off.
I realize some around here feel plasma cam is a hobby grade machine for an industrial machine price from what I have read here. I need a machine that can work for 8 hr's, so I want to do this the right way.
5 yrs ago I bought plans for $5 a cutting table with a self drain water system that can be built for the cost of the steel and about $75 in plumbing including the pump on Ebay. The same guy is selling them here http://www.andyinmo.com/ for 19.99 now.

So I have kept searching around and reading and I think I know what my project outline is:
1. Build 4 1/2 ft x 4 1/2 ft cutting/water table.
2. Set guide rails for x for 4 ft x 4 ft work area.
3. Build gantry for x & y.
Rack and pinion McMastercarr
4 p# 6325k83 Spur Gear 20 Pitch, 20 Teeth, 1" Pitch Diameter, 3/8" Bore $12.67 Each
4 p# 5170t3 Angle Gear Rack 16 Pitch, 1/2" Face Width, 1/2" O'all Height $55.67 Each
4ft long w/12 set holes
4. z axis for torch. ?
5. Standard windows compatible PC.
6. CNC control card in the PC.
7. Mach 3 software.
8. 3 controllers.
9. 3 motors.
10. Build exhaust fan/hood.

There seem to be all kinds of drive setups available from cable or chain to rack and pinion and I've seen screw types too.
I was thinking for accuracy a rack and pinion would be my choice.
I felt though I should ask here what if any negatives there are to doing it that way?
What other setup would you suggest instead.
Also as for the control card and controllers I found this sight http://www.ohmikron.com/
Thier Ohmikron Unipolar 3-Motor Stepper Control Kit sells for $117.95.
The motors they sell are 180 oz-in of torque in 200 steps per revolution and only cost $39.95 each. Never having purchased or assembled this stuff this seems like a cheap price and it plugs into the PC's parallel port leaving room to locate the PC where I want. The maximum cable length is about 3 m or 9.75 feet.
Is Ohmikron any good?
Are the controller or motors worth the price?
What NEMA motor size should I be using for a steel gantry?
Will the 180 oz-inch work for my build?
Would I be better off shopping elsewhere?
What is the z axis of choice?
Ok, I know I'm asking a lot but the more I read and look for parts the more I realize I need guidance.
Thanks
John

[Torchhead]

Do some more homework. Plasma needs high speeds compared to routers. Steppers lose torque at higher speeds. Rack and pinion ususally results in a gearing increase (eg a 1" pinion moves along the rack 3.14" per revolution). With high gearing you get plenty of speed but really low torque. Torque is what allows you to accelerate. It's like running a road race with lots of turns and corners in a car with one high gear and a 25hp motor. If it's geared for the top speeds of the straightaway it will take corners like a dump truck

You will get pretty poor speeds and performance with small motors (180oz-in are tiny by most standards for a bigger table) and the low voltage supply. That setup might be okay for a small desktop mill but not for a plasma cutter that needs to run a full shift.

You stated that you want to do it right but then selected the cheapest solutions you could find (?). There are dozens of questions about plasma cutting that you need to ask if you expect to be cutting 8 hrs a day. With the wrong setup you will spend more time and money replacing tips, recutting scrapped parts than actually cutting and making money.

Look at the commercial tables and realize that if they could get by with cheap components and stay in business they surely would.

You need to worry about things like: a constant cut gap, plasma friendly CAM software, pierce cycles, different pierce and cut heights, surface touch/detection, optimum feedrates, artwork generation (no, you can't load a picture (bitmap) and cut something no matter what the marketing DVD says!).

I see people buying 1200 oz-in motors (arg-arg) that are WAY overkill and others trying to use small under-sized ones. I think a 296 oz-in geared 3:1 down driving a rack and pinion is on the lower end of the target range for effective speed and torque. You can't just choose to cut at lower speeds if you can't get to the ones recommended since slower cutting vastly effects the cut quality.

Read the table build threads and look at the free table plans. I don't think you will find many using unipolar 180oz-in motor drives.

Also think noise avoidance (EMI, RFI) both radiated and conducted. The cards you listed do no appear to have any provision for input signals back from the table with any form of isolation. If your Plasma uses HF start then you have a level of noise to deal with that will leave you frustrated when the wrong interface components.

There is no "Z axis of choice". It's just another axis. If you want control of it for height you have to design and build it just like you would the X and Y (but maybe with a leadscrew instead of R & P). If you want control of the height while cutting you need to read about Torch Height Control (THC).

Then lastly there is the dust and smoke problem. Often overlooked, you will quickly discover that a plasma table will deposit a volcanic type dust (abrasive and conductive) in/on ANYTHING within 100 ft. Water trays help but are only about 25% effective. They do little for the smoke and dust from the cutting process. Your electronics will suffer, your lungs will suffer, and your computer will present you with blue screens.


Tom Caudle
www.CandCNC.com

[Redline]

I built mine as cheap as possible.It will cut at 200 ipm.I used a Xylotex control board with 3 nema 23 425 oz motors.It has leadscrews with wipers at the nuts. I went with 3/4 inch linear rails and bearings also with wipers on the rails.It will run 8 hours a day but I couldn't imagine the mess it would make.I only run it 10 hours on Saturday.I have no ventillation but I do wear a resperator.Campbel Designs THC.

Total 3500$

[massajamesb]

I think the Mytech plasma is HF activated, making it a tough sell on a plasma table. Nothing like high frequency to make your motors go nuts!

If you want a smaller hobby table, look at Hobbycnc kits, or Xylotex kits.
Industrial size table? Gecko drivers hands down. There is no substitute.
You should also look at what www.kelinginc.net has in store, they have Gecko drivers, and some of their own, at reasonable prices, and I highly recommend their 425 ozin motor. I have one on my X axis on my torch table, and I can get upwards of 500 in/min when I run higher voltages.
One thing I would consider is, what are you cutting? If you are cutting really thick stuff, you can "gear down" your smaller steppers enough to get adequate torque. Then you are trading speed for torque.
This is not to say you shouldn't gear down a larger stepper, I feel you always should on a XYZ table, no matter the motor size.
I had my oxyfuel table up and running originally in the 500 dollar range, including computer and software. I have made some changes to it now, cleaned it up some, added a Z axis, etc. and it is still under 1000.00 all told, including the Hobbycnc boards I originally bought to run it!
Look around this site, take in a little info. See what others are using, find out what works. And always listen to Torchhead when it comes to plasma. There has not been a question I have had yet that he has not already answered.

[massajamesb]

" Rack and pinion McMastercarr
4 p# 6325k83 Spur Gear 20 Pitch, 20 Teeth, 1" Pitch Diameter, 3/8" Bore $12.67 Each
4 p# 5170t3 Angle Gear Rack 16 Pitch, 1/2" Face Width, 1/2" O'all Height $55.67 Each
4ft long w/12 set holes"


I don't think you will want to run 20 pa gears on 16 pa rack, unless I am looking at your list wrong.

[johnnyb]

Hi Massajamesb, I have been thinking about getting a couple of the 425oz steppers from www.kelinginc.net. I see you are getting around 500 ipm, what controller and voltage are you using?

And how are you driving your X axis (lead screw, rack and pinon) Any information would be great!

I am running the Hobby CNC chopper board around 40v I can get around 80ipm using 127oz steppers with 1/2-10 Acme leadscrew. I am looking for more power but I heard that the 425oz steppers are great for touqe but some say they are slower due to the larger size. Like I said above I need more power but I am afraid of lossing speed.

Thanks,

John

[massajamesb]

I am now running Gecko 201 drivers at 24 volts, 20 pa 20 pitch gear rack, with a 1:2.8 gear reduction. I originally had chain drive, and also had Hobbycnc drivers, but I changed all that as I went along, though they did work just fine.
I have the 425 ozin motor on X, with a jackshaft, so both sides are driven from one motor. I have a PowermaxII stepper (150 ozin) driving Y.
I borrowed a 60 volt supply from a friend, and 500 IPM is no prob. The most I get with 24 volts from my Geckos or Hobbycnc was about 300, if I lube the crap out of my bearing ways.
I am running Quickstep3 as a controller program, and my breakout board is from Brent Hubbard on Ebay. It has built in relays for controlling my torch, etc. They are about 55 bucks for the whole board.
You are on the right track to use Mach 3, don't forget Sheetcam! Great programs, both of them.

[massajamesb]

And I should mention that those speeds listed were all with chain drive being used. Got a little noisy at those speeds, but it worked. I haven't gotten a chance to see what the rack and pinion will do.
I will say that I can stand on X or Y, and it will carry me around the table, no prob. Plenty of torque, plenty of speed.

And yes, the larger motors will be slower. You will have to increase voltage to make the difference up (though you are at the limit of what that board can do), at least using leadscrews. Though the difference in torque will give you a higher cutting speed, versus a higher no-load speed you are getting now, I believe.

[johnnyb]

Thanks massajamesb!
I thought about going the chain route. Right now I am spinning the drive nut instead of spinning the leadscrew because of whip. I was afraid of going the chain route with my small steppers but I think the 425oz with reduction would give me good touqe and speed. My table is not a plasma although one day I would like to be able to adapt to plasma. Being a router table I need the cutting force but also would like to have nice rapid speeds.

Thanks again!

[massajamesb]

I believe you will have more than adequate rapids with that motor, but you will be able to cut faster than before. That motor is the largest I have ever found that the Hobbycnc board is capable of pushing to full ability. Now, since the Hobbycnc board is Unipolar, the actual motor torque will be closer to 300-350 ozin, still way more than 127 ozin motors. If you are using leadscrews, gear reduction should not be neccesary.

[massajamesb]

I had good resolution with chain and sprocket, but good resolution with a torch and good resolution with a router are two different things, in a way. I don't know if it will be the best bet for you, although setting it up that way is fairly inexpensive.

[johnnyb]

With a 1.84" chain sprocket and a 1:7.2 stepper reduction running at 1/4 step would give a resolution of .001 which is great for me.

Running the stepper at 400 rpms would give a speed of 322 ipm.