CNC Plasma Table

In December of 2005 we started building a CNC cutting table to use with our plasma cutter, we underestimated the total effort required (as usually) so it took us almost three years to complete with an estimated 3500 hours of effort. The total cost for the table materials is now close to 1.000.000 ISK ($5.000 USD) which is a bit more than we hoped but very reasonable keeping in mind the large size of 6.5x2.5m (20x8.5ft). The table is built specially to cut the hull plating for our boat out of 6x2m sections (18x7ft). When building a table of this size for a reasonable price the main problem is in obtaining the accuracy, when the beams are more than 6m (18ft) long they must either have good backing or be substantially larger and heavier than we can afford.

We bought a combo-board and a THC 300 from Bob Campbell www.campbelldesigns.com , we later replaced this with a superior unit called MP1000.Both units measure the voltage on the plasma cutting circuit and use the voltage drop to determine optimal cutting height. The THC300 and MP1000 are designed by Tom Caudle but manifactured/sold by Bob Campbell (THC300) on one hand and Tom directly via his company CandCNC.

Although the THC 300 is designed to use Mach2/Mach3 we bought it and have now finished implementing a linux system running EMC2 which surpasses Mach2/3 both in features and reliability (Thanks to alex_joni, cradek, jmkasunich, Fenn and chinamill).

For more information on how to get your Torch Height Control running with EMC2 see the EMC2 Wiki article, the plasma-thc and plasma-thc-sim configs ship with EMC2 as config samples and can easily be modified to suit a specific machine.

The table includes 4 motors on 3 axis, initially we had not planned on implementing torch height control but it soon became obvious that even with adjustable backing the material would always deflect which would affect the cut quality, speed and accuracy.

The Y axis (longitudinal) has two motors which are slaved in software, they control the gantry on which the X axis are operated. The X axis cart is driven by a single motor of the same type used for the X axis. The Z axis uses a direct drive (no gearing) unlike the X and Y Axis.

The motors are of the stepper kind and use matched drivers, each of them produces 460lb/oz or torque at a maximum of 2000RPM. Unlike regular AC or DC motors the stepper motors generate most of the torque at low RPM which reduces the need to gear them down.

We bought our motors with the matched drivers and power supply from Ebay for $610 USD which is really quite good for this kind of power and a good brand.

Our brand is Pacific Scientific which since then has been merged with Danaher Motion, among the other thing they produce motors for is the Segway :D.

We would have preferred to use proper reduction gears but the cost of these is roughly $1000 per unit, we therefor opted to achieve the reduction using timing pulleys and belts with 15/45 teeth per wheel. Using pulleys had the additional advantage that we were able to eliminate play in the pinion/rack by using two pinions per side and adjusting the pulleys for slight tension between the two pinions. The best sources of gears/pinions/pulleys/racks seem to be McMaster-Carr and SDP/SI.

Initially we only intended to use the table for plasma cutting work but now we have added a router to it for joinery applications so we can do all the woodworking for the boat with CNC.

For more pictures of the CNC Table see Plasma Table Gallery

For videos see Google Video and search for CNC + Rugludallur

Tips for dealing with High Frequency noise and grounding

00. HF/Plasma Grounding is all about inductance, not resistance. Make sure you understand the difference.

01. The best conductors/wires for interference grounding are those with the most surface area spread furthest apart(low inductance @ Radio Frequencies), flat copper strips, braided copper strips and large diameter copper/aluminum pipes are typically the best.

02. Don't use aluminum conductors underground, above ground they are almost equal to copper in inductance to higher frequencies and are therefor very suitable.

03. Shielded cabling should only be tied to ground at one point, either side or center. If possible try terminating all shielding at the common ground point.

04. You should not be able to measure any significant conductivity between the PC Case (Ground) and your Cutting Table/Plasma cutter Chassis, resistance should be near infinite (a very high ohm value if any)

05. A cheap way to test your ground is to hook up a light bulb to the ground and one phase (115v/230v), make sure you hook up to the phase before a residual current circuit breaker, otherwise you will trip it since this test will create an unbalance between phase/neutral.

06. Copper tubing and cladding is typically much cheaper than wiring and is often much better at conducting high frequencies.

07. Don't put ferrite beads on grounding wires except if you don't want them to conduct High Frequencies, make sure to check if your power cables have molded beads on them and remember that they include the ground wires.

08. I recommend adding "ferrite beads" to all leads from the plasma to the sensor card except the ground (chassis), wrap the wires a couple of times around the ferrite depending on size of ferrites and wires.

09. I recommend adding "ferrite beads" on all signal/power leads into a computer/THC, this includes all home/limit switches, parallel, serial, network and AC but don't put one on your ground connection except if you are absolutely forced to use the same ground path for both the plasma cutter and computer (try to avoid sharing ground for them at all cost!)

10. A relatively cheap but superior ground can be made by using 1/2" copper piping as a conductor hooked up to 10 square feet or more of a buried corrugated hot galvanized plate, try putting it in damp mud/clay (organic) layers, avoid rocks and sand!