Pilot arc stability

[Tallguy64]

Hey all... So i have a question, why else would i be posting here right? And be for tech support flames me i know i know the pp256 is not designed for cnc use, but that's what i currently have set up . now that is out of the way. I have been mainly cutting signs made of 16gauge steel. I have done thicker and not had any problems but again i know its not recommended. I am looking to replace my pp256 on the plasma table and have a dedicated unit for it and looking heavily at the 80s looks like it should work for the stuff i want to get into cutting and then some and i can stay at 100% duty for most what i would be cutting.

So i will start out with this, i cut this sign the other day and i noticed a problem. And trust me i know this is long winded but it will all make sense. So this sign i was cutting out had several trees in it and the are VERY jagged lots of movement on my table not smooth at all. What i was noticing was lot of jumping between pilot arc and main cutting, i can try and correct this by changing some setting on my table but during this switching i noticed once and a while i am loosing arc all together, at which point it will relight the torch and it keeps on trucking away as if nothing happened (by the way when i mean "it" relights the torch i mean the plasma unit itself, right now my machine is not setup to detect whether the plasma is on or not). When it does this it must send out some nasty emf because one of my drivers picks it up and i lose a step. which isnt a huge deal if it is only once or twice but these trees wrecked havoc on my machine because i at the end of cutting a 24" sign i was off by about a 1/8". So ignore the issues with the cnc table they are more for story purposes as this table is emf proof minus these nasty spikes.

So on to the questions.

1. When switching between main and pilot arc, there really shouldn't be any delay or cut out of the torch should there?

2. I have noticed random cut out of arc before, could moister in air be a cause of this? i know that it is bad on consumables and mine seem to be doing good. but i have no good way to tell. i am running three air driers like the one that was originaly provided with the unit, the first see moisture but the next to are always dry.

3. Is the hf actually supposed to relight the torch automatically with out "letting up on the trigger" if you will?

4. with blow back start if there is loss of arc will the torch automatically relight?/ what is the length of time loss of arc that it can relight with in?


I know i can change some setting to try and minimize the amount of jumping between pilot and main but with some of the designs i have it is impossible to eliminate it completely and i need to know what is causing my cutting out before i get a new unit. Well that and prove to the wife that this can make some side money before i spend anymore on it.... i already tired all my angles, and she said what i am making now looks great why do i need to spend more... "because tech support said so" didn't work sorry boys.

[Rambozo]

1. It depends, there is no delay when switching between pilot arc and main, but there can be issues if the cut forces the reverse. I suspect that your CNC path has the torch moving over places where there is a previous cut, and that can always be trouble. A lot depends on the software that creates the torch path with proper kerf compensation and corner rounding for continuous motion. Often it is also a sign of the actual feedrate not keeping up with the programmed rate due to poor acceleration. This can happen with rapid direction changes like what you mentioned. More of an issue with steppers than servos.

2.Moisture will exacerbate any problems as well as create some. You can use a desiccant drier that will change color to confirm moisture content levels.

3. No idea on that model.

4.No. The blowback mechanism is started by the flow of air, so the postflow needs to finish before there can be a restart. This requires that your CNC get the arc good signal and be programmed for the proper restart sequence. However, if things are working as they should, you shouldn't be losing the arc in the first place. A restart will always leave a mark in the workpiece. For a lot of things that would scrap the part and is unacceptable.

There are a number of things you can do to help make your torch path work better. Try to make sure your CAM software is optimized for plasma cutting. Make all your corners as small arcs so that the direction change can be split over both your X and Y motors. Also you can rotate your program to an angle to avoid straight lines being only X or only Y moves. Tune your steppers to get the best performance out of them that you can. Shield your cables and electronics to keep the EMI from messing you up. HF can be a pain to deal with, but it sounds like it might not take much to fix that if you are only losing steps. Often HF will cause some machines to crash or reboot, so it could be a lot worse. Have you experimented with different power , air, and tip sizes? You might be able to find a combination along with a feedrate that will keep your arc going. Another option is to break your path into sections and have two cuts meet in the sharp corners, instead of trying to get the torch to make those kind of sudden changes in direction. Keeping everything smooth is difficult, but it really pays off in better parts and less machine wear and breakage. Not to mention higher speeds.

The PP60C has an HV start that is tailored for CNC. I would like to see an 80 or 100 amp version of that machine.

[performance]

If it is a standard 256 it is HF all the way. There could be issues with the HF gap. And it would interfere grossly with the CNC. Otherwise, there should be little issue with interference.

[Tallguy64]

thanks rambozo, you gave me a couple things to think about. Software wise i am using sheet cam to generate g-code and my machine is running using planet cnc controller. So far i love the setup. as for shielding i really cant do much more, all of my electronics minus the laptop running the system are in a steel grounded box, everything is in the a star ground pattern running double ground rod. All of my cables are bundled smartly inside of plastic/ copper woven shielding. Like i said i am not to worried about the emf that i am suffering from right now, if i correct the cut out problem then i will have no problems at all . I didn't think about kerf width but you are right if i have it set to narrow compared to what i am actually burning the the machine will be trying to hard to cut stuff that has all ready been burnt. I did turn up my acceleration and will try that sign again, originally i had it set at an insane speed and i was causing issues with vibration in the whole table and also would even lose steps do to stupidity acceleration of 75 , so i kinda went to the opposite extreme and haven't touched it since i first got it working.

As for experimenting i have to a certain degree, the one question i had is with air pressure. What i was aiming for is good detail so that means minimizing my kerf as much as possible, so i dropped my amperage to 25amps upped my feed rate to 55 ipm and am running a 35? amp nozzle i believe. i currently have my air pressure set at 65... what ever the manual said it should be set at. How much can i actually play with the air pressure with out causing machine problems? or will it only effect my cut quality?

Also does that sound about right for cutting settings for 16 gauge? 25amps 55 ipm. I am getting good results on my smooth stuff. I thought it sounded slow but any higher and it dosent cut though. Any thoughts?


I was guessing that was the case with the blow back, i am not to concerned about that i will just hook up the arc-ok signal when i up grade to the unit.

I was dreading someone was going to say spark gap but i think you might be right, ill check that.