MY 4`x 4` table

**Steve-m** · 08-09-2008

HI All

I will start my build log with a repeate of my introduce your self post, I'm sorry to those who read it but those who didn't this is self explanatory.

my project is a 4`x 4` plasma table, at the moment I have the Bridge 75% complete with a home made Z axis with 150mm of travel with a Trapezoidal lead screw, and a 3Nm stepper. A 1.4mtr, 25mm linear rail Y axis with a 1mod gear rack and pinion, also with a 3Nm stepper. the X will be 10 cam follower bearings running on and guided by 75x12 cold rolled steel, driven by 2 off 3Nm steppers, one each side. all designed in Auto CAD. the plasma unit is a Thermal Dynamics 75xl with a capacity of 20mm. I dont have any electronics yet, other than a PC with Mach3 on it, I am hoping to get a Candcnc torch head control from America, these are some of the earlier pictures, tomorrow I will take some fresh ones, unfortunately things have come to a temporary stop, I burnt the motor on my miller, The fan stopped working and I cooked it drilling the gear racks for the "X"

I hope this is of interest to some

Looking forward to hearing your comments and advise

Steve

**Steve-m** · 08-09-2008

Here are a few more pictures

**Robin Hewitt** · 09-09-2008

Interesting, I think the design philosophy is that you know it's going to bend like a banana with the welding but everything adjusts on slots so you can compensate. It may weigh a ton but you have overpowered it to overcome that? :D

I think it will work well. Not sure you can pierce thick plate without angling in but you can always start from an edge or a pre-drilled hole.

I'm just doing the circuit boards for mine. I went for a solid iron bed with a lightweight aluminium gantry, but that's just me.

Problem is you really need a CNC plasma cutter to make the plates that hold the bed rails. I may just fudge the bed then cut them.

I think THC is fairly simple but hyped up as difficult so they can sell you the kit. If you buy kit you are stuck with whatever output they choose to give you and you rely on them to sort out your problems remotely. I like to feel in control even when I'm not :D

I want to get an oscilloscope on the torch volts and move it up and down a bit, see what there is to work with. I don't have an 'arc good' output so I need to detect the HV igniter sparks when the main arc fails.

The bit that still concerns me is setting to the pierce height on thin plate. I drop down on to the sheet until it takes the weight of the torch the back up. BUT, suppose it bent under the weight of the torch and then followed it up? You may think you are lifting when you actually aren't. I was thinking maybe fire a 40v spark down through the tip and see if it goes, but maybe that's over-egging the pudding.

**m_c** · 09-09-2008

THC does sound quite simple in theory, but I'm guessing it's more complicated than what it first seems.

Basically you need to know when the plasma is active, so you can then measure the torch voltage (doesn't campbell's use a suitable placed inductor, rather than direct monitoring?), smooth that voltage/signal to remove spikes/dips, then compare that to a reference voltage. Then depending on voltage, either signal the torch to be moved up (increase voltage) or down (decrease voltage).

Sounds quite simple, and I'm sure you could design a crude set-up to move the torch up/down, but getting something on the level of one of the commercial/pro-hobby boards would take quite a bit of work.
Would certainly be an interesting project for someone with the electronics know-how...

**Robin Hewitt** · 09-09-2008

As I understand it...

1: Drop the torch till it touches metal.

2: Back off to pierce height

3: Establish an arc, wait for the pierce

4: Drop down to cut height and start moving X Y

5: After a short movement, measure the volts and save that as the benchmark.

For torch height I used 4mm studding with a 2:1 reduction pulley off a zippy DC motor. There is a chopper disk on the motor shaft for initial position and a digital voltage regulator to control the speed. I plan to measure the torch voltage many times a second then switch the motor speed/direction/brake accordingly. Not much of a plan but it's all I've got :D

**Steve-m** · 09-09-2008

Hi

Well Robin the idea originally was bolt together aluminium 60 x 60 for the bridge, that's what all the holes are for in the plates, I can change at a later date, every thing is only bolted. I rang some one in Yorkshire abut some they said they had it in stock, I drove 150 miles there one way, only to be told sorry they made a mistake, I said ****** ***** **** off! and made it in steel.

I plan on making the table then setting the whole thing up with a dumpy laser level and shims, the bridge bowed when I welded it but only by .02mm, I set it up on the mill put the center bolt in the linear rail and a DTI in the chuck and clocked it parallel then bolted it, I then assembled the Z upright, put the DTI on that and clocked the gear rack 2 ways so that's parallel to the rail. all the slots are for setting up gear racks and reduction belt adjustment, I may pivot the motor and belt plate on springs to "track" the gear rack if I have problems

As for cutting thick material, I mostly want 5-6mm occasionally 10mm, so I have plenty of spare capacity, its not for work only another tool for my "Play pen"

My plasma has all the CNC connections on a plug already, "okay to move" "torch on" "power" and dose not have the HF start, so no damaging spikes, Its real strange it will fire in the air, and with out the earth it will gouge 10mm plate 4mm deep ( I could not work out why it would not cut, DOH!!) I can also touch cut at full power, 60A, running the shroud tip directly on metal, so it is possible to simply rest it on the metal and drag

Unfortunately electronics and I dont get on, Im colour blind and resistors are a nightmare, so I dont do any circuit boards at all, unless its plug and play. I played with a osiloscope, when I did my degree, no matter how hard I tried I could not find ITV and the picture was rubbish all spikes and wavy lines, and it was so small LOL

Steve

**Steve-m** · 10-09-2008

Hi all
here are a feww pictures I took today,

Your coments and views will be apreciated

Steve

**m_c** · 10-09-2008

Looking good.

I've got a copper/boxwood hammer that looks very similar to yours, although mine's a little bit more (ab)used looking than yours :-)

**Steve-m** · 10-09-2008

AHH Mine is a copper - leather one, You want to buy a not as abused as yours mallet???

I also seem to have a collection of ball pane hammers, Im sure the buggers are breeding when Im not looking today, there was 9 of them, and they were all out of the draw at the same time

LOL

Steve