When you're feeling bored...

[Robin Hewitt]

If you've got a spare minute or ten, help me out, tell me where I'm going wrong, maybe fill in a few blanks.

I've been web searching and think I'm starting to understand the problems associated with this plasma cutting malarky. You don't need to type it all in, I've already done that, all I want you to do is correct it...

Machine torches cut thinner plate than hand torches because they are limited to what they can pierce. The only difference is the switch.

It is better to have a lot more amps but 30 is okayish for up to quarter inch plate. The machine runs on top setting, the cut speed is adjusted to match the material being cut. Faster is better because the heat is more localised.

The tool tip is a consumable but I only need one size unless I start messing with the amps.

There is something called finecut but it costs a lot and I probably don't need it.

There are two ways of initiating the arc, one of which is electrically noisy, hard on the tool tip and best avoided. If I buy a new Thermal Dynamics Cutmaster 38 it has the better system and I won't have this problem.

The plate I am cutting will make an electrical contact to the table slats so I fix the big crocodile clip to the table rather than the plate.

I move to the cut start on the table then physically measure how far the torch is above the material to be cut at that point.

If I was laser cutting I would start away from the cut line and then move in because it blows a wide hole at pierce. Nobody mentions doing that for plasma. Perhaps it's obvious, perhaps it's unnecessary

I start the cutter by shorting out whatever the little switch on the handle shorts out. It is okay to use a relay to do this. Whether I dab it to initiate the circuit or hold it down is vague.

I watch the current flowing in to the plasma cutter box from the wall socket, using a one turn transformer shorted out by a resistor. When it ramps up, past a tide mark of my choosing, I assume the thing is ready to cut.

I then wait for a preset pierce delay before descending to cut height and starting to move across the plate.

I now measure the voltage between the metal I am cutting and something inside the torch.

Before long I remember that voltage and adjust the torch height to try and maintain it. Whether it increases or decreases when I move up and down is a mystery.

When I reach cut end I do something to stop it cutting which is probably very obvious to anyone who already has a plasma cutter.

At some time I have to stop cutting because the thing has a duty cycle and I don't want it to stop halfway through a cut or burst in to flames.

I can run it off the shop air line but will need to dry the air if I don't want problems.

Some of that has to be right because I ain't stoopit, but the devil is in the detail

[massajamesb]

I would put in corrections, but it would involve writing all new sentences, which you said you do not want.
If you want quick and simple answers,
1. No real difference. Both use same consumables, machine is rated for thinner pierces because it can't angle in, like a hand torch
2. Every plasma is different. you can vary amperage and speed to get best cut.
3. yes, mostly
4. Finecut is made by Hypertherm for use with Hypertherm only. Has specific uses for thin material
5. You are referrring to high frequency start or touch start. TD does not use these. = good
6. Always ground to plate
7. move torch tip to plate and set that as "Z" zero
8. You are referring to "leadin/leadout". Yes, this is done with plasma. Look at www.sheetcam.com for a better idea
9.yes, and the relay will stay on the entire cut time
10.no
11.yes.
12. for a digital Torch Height Control, yes. No THC means no voltage measuring
13. better left to Torchhead(www.Candcnc.com)to explain
14. M code will tell torch to go on/off
15.yes
16.yes
17. exactly. Devil x2
Sorry for not correcting yours, but this was the easiest and most direct. Need more in-depth? just ask.

[Robin Hewitt]

I appreciate that, just give me a while to digest it

[Torchhead]

Originally Posted by Robin Hewitt If you've got a spare minute or ten, help me out, tell me where I'm going wrong, maybe fill in a few blanks. It is better to have a lot more amps but 30 is okayish for up to quarter inch plate. The machine runs on top setting, the cut speed is adjusted to match the material being cut. Faster is better because the heat is more localised.

30 amps is not enough to do smooth cuts in 1/4". I use 40 amps on anything above 14ga and 60A on 3/16ths 1/4 and 3/8. The more current the straighter the sides and the less cleanup. Using low amps means you have to move slower and that means more dross and more heat Perfect cuts are a dance of the right tip, the right current setting, the right feedrate and the proper tip gap (height).

The tool tip is a consumable but I only need one size unless I start messing with the amps.

There are circumstances where "messing" with the amps is required. On smaller machines only one size tip is normally available. On larger machines several sizes are avialable and needed for different thicknesses of material.

There is something called finecut but it costs a lot and I probably don't need it.

Finecut is just a tip with a smaller oriface designed to be used at lower amps and gives a smaller kerf and allows lower feedrates on given thin material. They are not more expensive than regular tips unless you have to change out the electrode and/or swirl ring. Hypertherm finecuts for the 1000 series use the same electrode and swirl ring as the 40 and 60 a tips. They cost about 5 bucks.

There are two ways of initiating the arc, one of which is electrically noisy, hard on the tool tip and best avoided. If I buy a new Thermal Dynamics Cutmaster 38 it has the better system and I won't have this problem.

All start systems are either HF (kinda like firing a spark plug) or contact start. The modern torches up to about 100A use a clever variation of the contact start. The electrode starts out in contact with the tip and the air when you start the cut pushes the electrode back from the tip and creates the arc.

The plate I am cutting will make an electrical contact to the table slats so I fix the big crocodile clip to the table rather than the plate.

You need excellent contact with the material being cut. Just clipping to the table will cause problems. Clip to the work (is called a "Workclamp")

I watch the current flowing in to the plasma cutter box from the wall socket, using a one turn transformer shorted out by a resistor. When it ramps up, past a tide mark of my choosing, I assume the thing is ready to cut. I then wait for a preset pierce delay before descending to cut height and starting to move across the plate.

Probably won't work well for a small inverter unit. Current waveform is not easily sensed with convetional 50/60 hz magnetics on small inverter.

If you setup the arc sensing (current sensing) right you don't need a set pierce delay as long as your controller software has provisions to hold motion with a signal.

I now measure the voltage between the metal I am cutting and something inside the torch. Before long I remember that voltage and adjust the torch height to try and maintain it. Whether it increases or decreases when I move up and down is a mystery.

You measure the voltage across the electrode and the workclamp which is the actual arc gap voltage. The higher the tip the higher the voltage. If you attempt to cut air the voltage climbs.

The circuit is simple you just have to move the head in the opposite direction the real voltage is away from a target voltage (usually about 140VDC on Hyperthemr and lower on TD). So if the voltage starts to climb above the target you move the head down to lower the voltage and if it starts to fall you have to lift the head.

Only problem is that there is a lot of noise in the signal, it changes rapidly and you need to hold the voltage to within about +- 1.5VDC (out of 140) or the head bounces down the metal like a basketball.

You also have to ignore the huge spike from the pierce move or the tip plops down on the metal like a dead Cod

When I reach cut end I do something to stop it cutting which is probably very obvious to anyone who already has a plasma cutter.

The G-code generated from your handy CAM program (like SheetCAM) puts in the torch on, torch off and head raises automatically. Also lead-ins and pierce height and initial cut height. "Pierce high, cut low"

At some time I have to stop cutting because the thing has a duty cycle and I don't want it to stop halfway through a cut or burst in to flames.

Duty cycle is hard to predict. It depends on the surrounding temp, the duration of the cut the thickness of the material and other variables. One thing is for sure. Making extended cuts with a small machine on a warm day in 10ga or thicker will let you find your machines duty cycle!


Tom Caudle
www.CandCNC.com

[Robin Hewitt]

At risk of outstaying my welcome by reponding to this quality information with more dumb questions, is 2" lift on the torch more than enough? I've just about finished AutoCadding a 1 x 2 metre table and it's the last complicaton. After allowing for some sizeable losses I calculate my 160 oz.in steppers will drive the x axis gantry at around 870 inches/s/s, I could theoretically start stepping at 60 ipm before I even have to start worrying about acceleration ramps

I am kind of restricted to 30A if I go with a Thermal Dynamics box. The only thing that comes out of my wall socket is 240VAC. The Cutmaster 38 is on sale over here in 240VAC but all their larger models seem to be 3 Phase, I think our 50Hz might be a problem.

I can get a 40 Amp cutter nil problemo, such as the R-TECH 40HF http://www.r-techwelding.co.uk/ . It has a better duty cycle and a machine head option for an extra 50 notes... but it's an unknown quantity and puts me outside the comfort zone. Grey importing a Cutmaster 51 from the States in single phase 60Hz was a nice idea but leaves me with little or nothing in the way of warranty.

According to the glossy, "With 30 amps of available cutting power, CUTMASTER 38 has plenty of power to quickly and precisely cut 3/8 inch (9.5mm) steel. Genuine Cut means 10 inches per minute cutting speed, an excellent smooth cut surface, and little or no dross with no need for grinding or rework".

Is that only on sunny days when there's an R in the month or what? Limited by the 6mm pierce it sounds like I wouldn't be over taxing it, or is the 20 ipm at 1/4" too slow?

The torch tip has to touch down to strike the arc? Putting it right in there with the burning iron?

[massajamesb]

2" lift should work, as long as you have a area to jog the torch to, where the plate cannot strike it while being loaded, and you can also change your tips out.

The higher the current, the cleaner the cut edge. There are other factors, yes, but you will get a cleaner edge on thicker metals with more amperage.

30 amps will cut 1/4 plate, but it will not be as clean or as fast as 40 or 50 amps, and you will be running at the edge of the duty cycle of that TD plasma, if cutting a large piece of 1/4" plate. You have three levels of cut quality with a plasma,
1.quality AKA genuine
2. good AKA rated
3.sever AKA snaggletoothed beaver teething device. Almost like a torch cut. (this is a bit of an exaggeration, but so is their advertisement )
With that TD plasma,
1/4 is quality, 5/16 is rated, and 3/8 is sever.
It will cut 3/8, but it will do it rather slowly, and the edge won't be as nice as you would expect.
The torch tip should not touch the plate surface, but hover between .06-.10 off the surface, depending on plate thickness, amperage used, cut speed, etc.

[Robin Hewitt]

Hadn't thought about clearance to change cut heads, so much to learn, have to bodge in a small rack extension

Here's another one... Many cutters have a ring on a stick around the hot part. Is that for dip measuring the flying height, or to detect tilted off cuts before there is an expensive collision?

I'm leaning more and more towards the R-TECH plasma cutter. It has 40amp cutting, 240V 50Hz 28A input, 60% duty at full power, costs around $1390 with the machine cutting head option and 2 year warranty.

Their ad says, "HF pilot arc enables easy starting of cut - even on painted surfaces, with auto re-ignite ideal for cutting mesh etc". Does that sound like the good arc initiation circuit or the bad?

I have a 'phone number, are there any pertinant questions I should ask before parting with my hard earned? 28 Amps at 240 Volts sounds like a lot when you are used to working with a 45 Watt soldering iron

[millman52]

Originally Posted by Robin Hewitt Hadn't thought about clearance to change cut heads, so much to learn, have to bodge in a small rack extension Here's another one... Many cutters have a ring on a stick around the hot part. Is that for dip measuring the flying height, or to detect tilted off cuts before there is an expensive collision? I'm leaning more and more towards the R-TECH plasma cutter. It has 40amp cutting, 240V 50Hz 28A input, 60% duty at full power, costs around $1390 with the machine cutting head option and 2 year warranty. Their ad says, "HF pilot arc enables easy starting of cut - even on painted surfaces, with auto re-ignite ideal for cutting mesh etc". Does that sound like the good arc initiation circuit or the bad? I have a 'phone number, are there any pertinant questions I should ask before parting with my hard earned? 28 Amps at 240 Volts sounds like a lot when you are used to working with a 45 Watt soldering iron

Someone else correct me if wrong but I think the HF pilot arc means "High Frequency" = computer glitches or total shutdown.

[thkoutsidthebox]

You can us a Phase converter to get 3 Phase power on your single phase mains. This is what I'm intending to do if/when I ever get around to buying and building a plasma cnc. Here's one link from google, but a search for 'Phase Converter' throws up lots of links. They can be quite expensive though.

http://www.phase-a-matic.com/?gclid=...FRHgXgodgg4kdw

Torchead, the mechanism you referred to about the air pushing the tip away from the sheet, whats this called and is it standard on most new machines now (Thermal Dynamics or Hypertherm) ? i.e:Should I ask for it when buying?

[millman52]

Originally Posted by thkoutsidthebox You can us a Phase converter to get 3 Phase power on your single phase mains. This is what I'm intending to do if/when I ever get around to buying and building a plasma cnc. Here's one link from google, but a search for 'Phase Converter' throws up lots of links. They can be quite expensive though. http://www.phase-a-matic.com/?gclid=...FRHgXgodgg4kdw

May I ask why you need 3 phase power for a plasma cuttling machine?

If you actually do need 3 phase I have plans for balanced, build your own converters. Not all that hard to do & much much cheaper than purchasing one. I have 7 RPC in use in my shop now. I do run welding machines on a couple of them. For resistance type loads, building them is not quite an exact science. So I'm not sure how well they will work on a 3 phase plasma machine.

[thkoutsidthebox]

My understanding is that 3ph is more efficient for any large machine which draws a lot of power, or needs more torque. You cant get 415V on 1ph power. I'm considering using phase converters to let me get a bigger welding machine later (Better duty cycle), and possibly for a larger table saw also.
Although I'm not sure if I need a phase converter, plus an inverter to up the voltage aswell, or will the phase converter do both?

[millman52]

Originally Posted by thkoutsidthebox My understanding is that 3ph is more efficient for any large machine which draws a lot of power, or needs more torque. You cant get 415V on 1ph power. I'm considering using phase converters to let me get a bigger welding machine later (Better duty cycle), and possibly for a larger table saw also. Although I'm not sure if I need a phase converter, plus an inverter to up the voltage aswell, or will the phase converter do both?

Building a RPC to run other 3 phase motors is simple enough. For a welding machine however some of my home brew RPC work better than others for that. Unfortunately there seems to be some difference on the motor you choose to build one for a welder. My rule of thumb for a welder RPC is to use the oldest, heaviest, ugly, bulky motor you can find. You definately don't want anything that is labled energy efficient.

I am running an entire machine / fabrication shop with 5-6 employees on 220V single phase with the use of RPC's For the amount of work we do my power bill is very reasonable. For this level of power usage though I do have (2) 400A 220V service entrance drops.