Plasma table, motors, drives, controllers

Hello,
About three weeks ago i got bit with the bug and have been looking over these forums, searching google, ebay, and just trying to find out anything i can about building a plasma table. Finnally the other night, as a proof of theory i put together a square with 2x3 tubing and some roller blade (608) bearings i had and made a rolling gantry. After slotting some bolt holes i was rather suprised with the amount of slop i was able to remove and how little deflection i had. So here it is. I have decided to go full plunge. My table is a little under 5x9 so it can accomodate a full 4 x 8 sheet. My goal is to be able to cut brackets for use on offroad 4x4 suspension out of 1/4 inch steel. So my questions are as follows.
1) Cnc setupWould this be a reasonable setup, good power supply? stepper motor size? good drivers?
2) If those stepper motors are reasonable would acme thread be ok for the table. I understand backlash can be a problem with acme thread but for the purpose of this table just cutting 4x4 brackets would acme thread be ok?
3)If acme thread is ok, what would be a good size. I have found quite a few postings on this but mostly for smaller tables. Would 2 start be good for a 4x8 table. I need some level of accuracy but still be able to get to the other end of the table in under 2 minutes.
Any help would be greatly appreciated. Links, threads, blogs, anything that would answer these questions for me.
-Aaron

Acme thread (or leadscrews) are not the best choice for plasma cutting. Three main problems:

1. Plasma requires cuttings speeds of 100 to 300 IPM depending on material and thickness. Feedrate is a major component for decent cuts. At 48 volts you will get maybe 600 to 650 RPM from the motors you list. With a leadscrew you get speed reduction. Lets say you pick a 2 start 10 acme (5 tpi). Your top speed will be about 130 IPM. That will limit you on speeds. It is also really slow for rapids on a table that big. Also remember that the faster you spin a stepper the lower is torque. When the RPM curve crosses the required torque axis the motor stalls (losses steps).

2. Leadscrews exhibit a condition called "lead whip". It's cause by centrifical force over the length of the ballscrew at RPM. In short, the longer the leadscrew, the slower you have to spin it to prevent lead whip. Larger diameter screws have less tendency to whip but precision nuts for acme screws above 3/4" are hard to find and more expensive. Even if you get a course thread leadscrew you may still face the RPM limitiation.

3. Plasma is one of the dirtiest cutting processes you can do. It's vaporizing metal with a 30,000 plasma arc and blowing it out with 70 psi air. The result is smoke (vaporized metal) and dust (nuggets from microscopic to the size of BB's). The smoke drifts all over the shop and settles like black volcanic ash on all exposed surfaces. It's conductive and abrasive. It's rough on moving parts and especially exposed leadscrews and tight fitting nuts.

A 5 X 9 table will take about than a minute to go from one end to another with the setup I used for illustration. Not bad but sit down and time a minute. You will long for the day you can do 300 IPM or more. Loading and unloading the table sometimes takes more time than making the cuts!. A table that big will eat up a big chunk of shop floor. Then you have to have clearance either in the sides or the ends for loading material. A 4 X 8 sheet of 10ga is a not something one man can handle without a forklift or overhead lift.

If your goal is hobby cutting then time does not mean as much. But then if you want to do hobby cutting you can get by with a 4 X 4 cutting area for 95% of your cuts. If your goal is decorative cutting then you never work in areas 4 X 8. Even bigger "scenes" are cut from smaller pieces. Never buy a table for the sized material; buy it for the size jobs you will be cutting.

If your goal is commercial then you should look to more robust and complete systems that will help you keep your cost of consummables and scrap from bad cuts to a minimum.

A mixed use table presents other challenges but we won't go there in this thread (:-)

BTW the above is not based on hearsay of from reading other posts. My first table in 2000 was ballscrews and servo's with an old Miller 500 HF start plasma and no THC. I cut commercially for 5 years (over 2 of it with that rig!).

TOM CAUDLE
www.CandCNC.com
Totally Modular CNC Electronics
Low Cost Digital Torch Height Controls

Hello Tom and thank you for your reply.
Due to plasma dust getting in the threads, suffering whip over the 8 foot span, and not being able to achieve the proper ipm, i think i wont go with acme threads.
Loading the table shouldnt be a problem, and table space is available in a room i have off the back of my shop. I work for a forklift company in charlotte and came across a forklift with a blown head gasket and in need of a brake overhaul. Two weekends later and 300 bucks, i have a working forklift. Kinda lucky.
So,
1) Would gear and rack be better with a 3 to 1 belt reduction?
2) Should i buy components seperately, better drivers? better motors? or is the kelinginc set good for me?
3) If gear and rack is a good setup, does anyone have a thread, or post that can describe pitch, teeth and all that i need to know about gear and rack.
Thanks
Aaron

It's pretty easy to stick three motors, three drives, a power supply and a cheap breakout on Ebay and call it a kit. It's like putting a raw motor, transmission and 4 tires together and calling it a car "kir". Unless you have some experience in building CNC electronics from parts and pieces you need to factor in a lot of your time for learning.

I think at this point you need to focus in on the mechanical part of the build. About all you need is to know the physical size of the stepper motors you will be using. All 23 frame and all 34 frame motors have the same mounting flange and basic body diameter. They vary mostly in length and dynamic specs.

For ready made CNC mechanical components visit

www.CNCPartsKit.com
www.K2CNC.com


On a table that size (5X9) you HAVE to drive both sides of the ganty. Either do it through an axle and via R & P on both sides or with "electronic" gearing (Dual stepper motors and independant R & P on each side. Either approach works but the latter gives you double the torque to move the heaviest part (the gantry) and allows independant suspension so the motor/transmission can be pulled into the rack to reduce gear backlash.

Do some research on the table builds in this forum and there are some free plans as well. Also spend some time and study as best you can table designs from the guys that build them to sell. The designs are not always the best but when you see the same type of drive show up repeatably then it's one to consider.

The R & P gives you a speed increase (to speeds you can't use) and a proportional decrease in torque and resolution you could use. A belt reduction of at least 3:1 gets you back some or all of the torque and resolution. The ratio in R & P is determined by the Pitch diameter (DP) of the pinion gear. It's PI (3.1416) times the DP). The bigger the gear the greater the ratio.

With a 1" DP pinon and 3:1 belt reduction transmission you are back to parity on speed and torque/resolution. At closer to 5:1 you still can have 350 IPM with the 48V based motors and increase resoltuion and torque.

I read in amusement where one table vendor uses a direct drive stepper to R & P and uses it as a "feature". (No belts to break, lower maintenance, etc). The gantry has to be pretty light to get the acceleration you need for sharp corners. You still have 1/3 the resolution you would have (which effects accuracy) but plasma is not inherently that accurate so why worry? (rhetorical question).

TOM CAUDLE
www.CandCNC.com
Totally Modular CNC Electronics