THC Setting Issue?

Hi all

after doing some nice work on my little CNC plasma table, I have now got a job which uses all of its travel capacity.

The steel is a bit bowed, probably max 4-5mm variation so not too bad across 625mm.

What happened on the first cut was that it started out perfectly then the cut started losing penetration intermittently, my guess was that the THC was not tracking properly the bow in the sheet and i ended up with too high a cut.

I am stuck between it being the cheapo Proma THC i bought, or a setting issue.

In Mach i have the THC anti-dive at 90% and THC Z rate at 7%.

The rate was low because before it seemed to be overshooting, i.e. the THC loop could not keep up with torch travel and was adjusting too much, too late.

I think the Z axis screw is 2mm pitch and direct drive from a small Nema23 motor.

Maybe the rate is too low, the THC is crap, or something else is not right.

As this is my first table, I have no idea how much travel a THC is supposed to compensate for so I'm not sure where to pin the blame.

Any tips would be much appreciated.

I have no objection to getting a new THC but must know before if it is the right thing.

I have got it working on slow cuts now, I lowered the HYS setting to 1 and increased Mach3 RATE to 10% now it cuts good at slow speed

On a faster cut i see it follows upwards very nicely but does not follow downwards fast enough, leaving me with a very big cut height.

If the metal is shaped 0mm - 10mm - 0mm across the X axis, it will follow 0mm - 10mm but then it will finish on about 5mm not 0mm.

Any ideas??

Well it looks like the THC is just no good, if i tune it for slow cuts it works well, but could never manage a cut at 3300mm/min, pretty certain if up the tuning to go faster it will bunny-hop on slow cuts due to overcompensating. :(

Anyone recommend a THC ?

I have tried contacting CandCNC but had no response yet.

I had this reply from Proma....

"THC 150 has about 150ms (0,1 - 0,15s ) processing time, o don't sure what many time need mach for reaction.
total time isn't very well for it. I think that better is use model SD (step dir) or full system MD+DV. both systems works alone (without software) and total delay time is about 70-80ms - for high speed still it's not very fast but better that compact THC."

Sounds like speed is not their thing?

I think your problem is those Americans. They all make huge plasma cutters driven by enormous stepper motors and when it comes to THC the Z axis already has a huge stepper motor on it.
Maybe a second lightweight DC motor driven Z axis mounted on the MEGA stepper driven Z axis could be the solution. You do not actually need 2 tons of lifting force to move a plasma head.

I see where you are coming from but I designed and built this table myself ;)

Its driven by tw medium size Nema23 motors for X & Y plus a small Nema23 motor on the Z axis, it has a floating head etc.

I think the main issue is poor loop response time of the Proma THC unit, its fine on slower cuts but get up to 2300mm/min and higher and it cant cope.

Cant find a different unit that will work with my little Hypertherm 30xp though, they all want built in outputs for the THC and I don't have that.

try cncdirect.co.za. his thc works very well for me

Thanks, just emailed
them

I have 4 of Proma THC and been using them for over 2 months so here is where i am at:
3mm mild steel light bent at 2-3 cm per feet i can cut at 5500mm/min at 60A all day long, mostly use it at 3000-3600mm/min at 30A so i do not bother to be near the machine at all
2mm mild steel cut mostly @4200mm/min @30A
Back to your problem: you did not mention the delay--> lower it on the THC so you get faster response from it, and if this does not help, you might need to change the computer or do a fresh install.
B.T.W i use linux exlusively but i had a winXP/Mach3 setup for a client that does production work with it on a 3.5mX2m work envelope with a Hypertherm MAX100.
Regards,
Tom

This issue was resolved by fitting an MP3000-DTHCII from CandCNC, this unit can control the z axis very fast which is just what was needed for my finecut consumables that like to run at only 0.75mm cut height.

It's working very well now.