Cut quality question

[jcizek]

Hi everyone,
So after months of messing around with my setup, I give in, I must ask for some assistance.
Setup: Homemade table (R&P on all 3 axes)
Hypertherm Powermax380 with brand new set of fine cut consumables
250 millisecond pierce delay
95 volts on the THC
Cutting at 110 IPM
Material is hot rolled flat sheet 0.03
Cutting at 27amps (max for the machine)

I am just not sure what to adjust when I see "X" happen for example... i have read on the net about adjusting speed, amperage, and cut height based on dross, but I can't seem to find anything that doesn't make awful looking cuts.

Any suggestions? Is it my air quality? Pressure? Speed? Height? One thing *all* my cuts have in common is an exaggerated undercut (ie, if you look at the plate from the edge, the cut path is nearly 45 degrees instead of 90 degrees) The torch is cutting an inverted "V", removing much more metal from the bottom of the cut than the top of the cut.

Thanks for any suggestions, I finally have the table, cam, and everything else working well, if I could just get the quality up on the cut I could make useful things!!

[jcizek]

Well, I tried a few more things this weekend. I found several postings suggesting that a set of parallel lines, each with a different cut speed would be a good way to check your settings. So I set (7) 3" lines. Cut at 110ipm through 170ipm (each one 10ipm more than the last). Then, I tried the same
test at 80 volts at the tip, then 85, 90, 95, 100, 105, 110. I tried the same test with 27 amps, 24 amps, 22 amps and 20 amps. The speed and the tip volts didn't seem to make hardly any difference, the cuts are all a bit on the poor side. Lowering the current made the most difference, but non of it good. It cut ok down to 24 amps, but below 24, it started to splatter all over the top. I did this with a brand new set of fine cut consumables (yep, hypertherm branded, not some third party consumables)

I am running out of ideas, I really wonder about my air quality (I live in Colorado so it's already very dry to start with). I am using all the settings that the manual for the cutter recommends to start with, and all the cuts are rough edged and poor. I have made sure to get a good ground (scratched down to bare metal with wire wheel and attached ground clamp to clean shiny metal).

I have never really seen anyone elses' plasma cuts... is plasma just not capable of making a smooth clean cut? I realize it won't cut like a laser, but i thought it would look better than a rat chewing through the metal...

I have included a top and bottom shot of the best settings I could find from the line tests. There isn't much dross, warpage no problem, but you can see the lines are not very smooth, and the corners are *very* rounded (the machine is not doing this, if you put a pen in it instead of torch, it draws beautiful sharp 90 degree corners). I also added a lead in and lead out to stop the corners from blowing out (like you see in the first post).

Anyone have any suggestions for me? Thanks.

[gurew]

put a marker where your torch is and see if you can draw a circle...

[jimcolt]

jcizek,

From the looks of your cuts:

1. The large shadow (heat) indicates that you are simply not moving fast enough. Most likely....your machine is not achieving the programmed speeds on the holes and slots. How does it do on long straight cuts?

2. Worry more about physical cut height as opposed to arc voltage. If using an arc voltage control....adjust the voltage so that the height is as recommended in the Hypertherm manual.....which I believe is .08". Any higher than this and you should expect dross and warpage.

3. On your cuts...when you see the shadow get wide and narrow around the perimeter...that wil indicate that the velocity of the torch is changing....when the movement slows, the shadow gets wider. Plasma torches produce the best cut quality at speeds that are near the point of not cutting through the plate, and consistent cut quality is produced by constant speed and proper height.

The Powermax380 was not designed for mechanized cutting....so I cannot provide accurate data....however this is probably a good starting point on steel that is .03" thick with the 380 using FineCut consumables:

Amperage (Maximum) 27

Pierce height......12"

Pierce delay.....0 seconds

Cut height.....08"

Voltage....(adjust during steady state cut so physical height matches cut height)

Cut speed....150 inches per minute.

If small features and holes are ugly....but long straight sections loook good....then your machine is slowing dramatically during cornering...or for some reason the height is changing. I don't know what your drives and motion configuration are....but you may want to experiment with icreasing acceleration and or corner speed settings.


Jim Colt Hypertherm

[Benderart]

Hey Jim, don't you mean pierce height 0.12".
12" seems a bit high......

Sorry, couldn't resist.

[jimcolt]

I have really high voltage in my shop!

[jcizek]

OK Jim, thanks for the insights, they help a lot! I know that this machine was never intended for running mechanized but it's what i have at the moment so trying to make the best of it!!

Thanks for all the numbers. That all makes good sense to me. But it leads me to ask another question... My table is stepper motor driven (rack and pinion, using CNCrouterParts drive mechanism which is a 3:1 reduction belt drive between the motor and the rack) X&Y use these drives while Z is a direct drive Rack and pinion. The table is all steel construction (thin walled tubing, some 1x1 and some 2x1) and is 5' x 5'. It seems to be farily common size compared to a lot of tables I read about on this forum. Clearly, there is no way for a gantry table of this mass to accelerate and decelerate to 150 IPM in a 1/2", so how do you handle cutting small holes? Seems like a bigger machine (I'm considering getting a Hypertherm 45 amp machine) would make this problem even worse?
Is the answer to cut at a lower amperage?

One thing I am suffering from I believe is a lack of a proper table to support the work. The table is still kind of an experiment and i think the work may be sagging more than it should from lack of a proper slatted table. That's next on the list to finish!

Also, can you explain why cutting at 100 ipm and cutting at 180 ipm produce the exact same looking cuts? I am watching the velocity on the EMC screen and I am definitely accelerating to the targetted velocity ( The cuts are about a foot long each) but when i look at them, I can't really see ANY difference between them (top nor bottom)

And to answer your question, no, my long cuts are pretty rough as well. Maybe just a speed thing there? I'll try running maybe up to 200ipm and try that. It doesn't seem to make a bit of difference on the long cuts either running anywhere between 100ipm and 170 ipm.

Thanks Jim and everyone else!! -james

[jimcolt]

It is certainly possible on thin material to have a very wide speed range where the cuts all look similar. On .03" steel....you can probably cut at 27 amps between 100 and 250 or so ipm without much difference in appearance. That speed range will get narrower on thicker materials.

I have two machines...one with stepper motors, one with servos. Both are pretty lightweight construction. The machine with servos does a better job on small holes and features at up to 250 ipm. The machine with steppers does a good job on small holes and features up to about 125 ipm. Acceleration is important for plasma...the servo machine definitely has better accel.

If I use the stepper machine on thin material, I first develop cut specs that cut well at speeds below about 125"....usually it requires reducing the amperage. I have never cut with a Powermax380 on a cnc machine...so I would be guessing a bit if I offered advice. I will tel you that torch height is the most critical thing...your 380 will cut best at a consistent height....and it is designed to run close to the plate.


Jim Colt

Originally Posted by jcizek OK Jim, thanks for the insights, they help a lot! I know that this machine was never intended for running mechanized but it's what i have at the moment so trying to make the best of it!! Thanks for all the numbers. That all makes good sense to me. But it leads me to ask another question... My table is stepper motor driven (rack and pinion, using CNCrouterParts drive mechanism which is a 3:1 reduction belt drive between the motor and the rack) X&Y use these drives while Z is a direct drive Rack and pinion. The table is all steel construction (thin walled tubing, some 1x1 and some 2x1) and is 5' x 5'. It seems to be farily common size compared to a lot of tables I read about on this forum. Clearly, there is no way for a gantry table of this mass to accelerate and decelerate to 150 IPM in a 1/2", so how do you handle cutting small holes? Seems like a bigger machine (I'm considering getting a Hypertherm 45 amp machine) would make this problem even worse? Is the answer to cut at a lower amperage? One thing I am suffering from I believe is a lack of a proper table to support the work. The table is still kind of an experiment and i think the work may be sagging more than it should from lack of a proper slatted table. That's next on the list to finish! Also, can you explain why cutting at 100 ipm and cutting at 180 ipm produce the exact same looking cuts? I am watching the velocity on the EMC screen and I am definitely accelerating to the targetted velocity ( The cuts are about a foot long each) but when i look at them, I can't really see ANY difference between them (top nor bottom) And to answer your question, no, my long cuts are pretty rough as well. Maybe just a speed thing there? I'll try running maybe up to 200ipm and try that. It doesn't seem to make a bit of difference on the long cuts either running anywhere between 100ipm and 170 ipm. Thanks Jim and everyone else!! -james

[jcizek]

OK thanks again Jim, this info is very useful. My acceleration settings are pretty conservative right now, i'll check to see what I can get away with and see if that helps. Also, i'll try reducing my amperage a bit on the thin stuff. Finally, I'll do some testing to see what voltages correspond to what torch heights on this material and shoot for the numbers you gave me.

I really appreciate all the insight, I had kind of given up on the table after a lot of frustration but I have SO many projects that would benefit from it's use (getting kind of tired of replacing nibbler tools that wear out from all the use, not to mention the cramped hands!!)

Thanks again, i'll report my progress after I've had a chance to try all this!
-James