EMC2 Plasma Table

[Big John T]

Ok I finally got EMC up and running on a computer with the Unbuntu/EMC2 package. And played with the demos a bit. My background is I'm completely new to Linux but a seasoned dos/win user and programmer. I've blundered around a bit and figured out the basics about Linux. I've still not installed a binary yet on my Linux box...I own a CNC mill and am quite familiar with that end of the equation. I've installed and programmed servo controller/drives in machines... Now on to the question...

I want to set up a small plasma torch table about 24" x 24" as a teaching tool for me before I graduate up to a larger project. As I understand the parallel port can be used for controlling small projects. I want to get this project up on the cheap and fast.

Would steppers be easier or servos?

What hardware do I need?

Has anyone done a plasma table that can chat about it?

John

[massajamesb]

If you want cheap, easy and fast, look at www.xylotex.com for a 3 axis drive kit with steppers. Every thing you need is there.

For table movement on a machine that small, you might very well get away with 1/2-10 acme, if you could shield it against the plasma dust, but my recommendation is to just use some timing pulleys and belts for gear reduction, and then use some 20 degree PA 20 pitch rack and spur gear.

Here are some inexpensive pulleys and belts. Always had good luck with this guy.http://stores.ebay.com/HUBBARD-CNC-I...QQftidZ2QQtZkm

[Torchhead]

You will get (tops) about 35 IPM with the Xylotex setup (same as our 4 Axis Dual port Combo) and 1/2 10 screws. WAY too slow for plasma. You can hit 70 to 80 IPM (still too slow for most plasma) by using a 2 start 10 or 2 start 8 acme screw and matching nut.

Direct coupled Rack and Pinion will give much faster speeds but make the table low on resolution and torque. Not a probelm with plasma but bad for routing.


tomCAUDLE
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[massajamesb]

Originally Posted by Torchhead You will get (tops) about 35 IPM with the Xylotex setup (same as our 4 Axis Dual port Combo) and 1/2 10 screws. WAY too slow for plasma. You can hit 70 to 80 IPM (still too slow for most plasma) by using a 2 start 10 or 2 start 8 acme screw and matching nut.

True. My mistake. I suppose a screw with less TPI or 2 start screws would be better.
I suppose that the speed needed would be dictated by the thickness of material you want to cut, and the size of the plasma unit.

[Big John T]

Originally Posted by massajamesb If you want cheap, easy and fast, look at www.xylotex.com for a 3 axis drive kit with steppers. Every thing you need is there. For table movement on a machine that small, you might very well get away with 1/2-10 acme, if you could shield it against the plasma dust, but my recommendation is to just use some timing pulleys and belts for gear reduction, and then use some 20 degree PA 20 pitch rack and spur gear. Here are some inexpensive pulleys and belts. Always had good luck with this guy.http://stores.ebay.com/HUBBARD-CNC-I...QQftidZ2QQtZkm

Thanks for the links... I'll have to study them for a while.

John

[Big John T]

Originally Posted by Torchhead You will get (tops) about 35 IPM with the Xylotex setup (same as our 4 Axis Dual port Combo) and 1/2 10 screws. WAY too slow for plasma. You can hit 70 to 80 IPM (still too slow for most plasma) by using a 2 start 10 or 2 start 8 acme screw and matching nut. Direct coupled Rack and Pinion will give much faster speeds but make the table low on resolution and torque. Not a problem with plasma but bad for routing.

Thanks for the reply. I'm looking at my plasma manual and I need from 10 to 400 IPM for optimum travel speeds. I have a Hypertherm PowerMax 1250 plasma torch. All my cutting has been by hand so far. I think that I cut about 50 IPM by hand.

What is the drawback for cutting slower than optimum speeds?

To hit 400 IPM on a diagonal is really moving out. Heck my CNC mill only rapids at 200 IPM...

I assume that it is a straightforward calculation to determine the speed that a machine could do. The torque needed to get there and stop and change directions is at this point something that I can't calculate... But I know that it is important.

John

[Big John T]

Bump

[Torchhead]

400 IPM on a diagonal is not the same as 400 on a straight cut. The speed is the sum of the velocities (with the angle as part of the equation). That being said you can cut most metal at 200 IPM (straight line) or less just depending on the tips and thickness of the metal. The recommended feedrates for both are going to be in your plasma unit manual. It varies from one machine to another so I can give you a "rule of thumb" value.

The feedrate is one of the more critical aspects to getting a good cut. By cutting below the optimum feedrate you will get:

Wider kerfs
low speed slag (lots)
More warping and heating of the metal

conversely you will get:

sharper corners
Rounder small holes
Slag that is pretty easy to remove with a chisel.


If you are using a THC you will probably need to adjust the target volts up slightly for slower than normal cuts or you will get head dive on direction changes.

There is really nothing you can do to completely cancel out the effects of too slow a feedrate.


To cut thin material at slower feedrates use the HT Fine Cut tips and lower the current to 30A and cut height to about .040. Then you can cut back down at 100 to 200 IPM. I don't fell comfortable around a table that can move faster than I can. 500 IPM is somewhat scary and 1000 makes me want to find a protective barrier!

[Big John T]

Thanks for the info. I got the feed rates from my manual and on the lightest metal it is 400 IPM.

On direction changes why would you get head dives? Is it because of speed changes?

I have the HT fine cut tips and they do work well by hand so far...

Thanks
John

[Big John T]

It dawned on me that the speed needed to go on the diagonal is actually less than a straight X or Y direction. If I go 400 IPM in X and then in Y for one inch then when I go on a 45 I get there just as fast but travel a longer distance...

John

[ADucci]

steppers would be easier...

you need to buy 2 stepper motors, 2 drives ( see Gecko or cheaper)
then you need to connect the motors to the drives (not that simple, you will need a power source as well and some resistors before the drives...
then you need to have a table that has your rails... and rack and pinions or screws..

once you have that.. you set up (in an ini file) within EMC.. the pin configuration.
the parallell port consists of 21 pins.. you will choose at least 6 of those pins (motor 1, motor 2, plasma torch. for the motors you need 2 pins each (step and direction) and the plasma you need 2 as well...

then in EMC you simply configure your pins and there you go...

then you need some software to actually draw the designs and you are off...

Steppers are cheaper as well... and are a lot more accurate for the small user.

[mrpeja]

Hi, Torchead,
Is there constant velocity mode when running plasma table with EMC2?
Is there, also, screen for plasma parameters adjusting: piercing, delays, etc. (like Sheetcam)?
Can it run with THC300?
Best regards,
Pejic