torch misfiring randomly

[04dmax04]

hey guys, im new to the forum and also a newbie to cnc. i have found this forum super helpful well worth the price to join! i purchased a used table a week ago from my old employer. the machine was running well for him in his shop. get it to my shop and have had nothing but problems. him and i cannot figure it out. its a 10 or 12 year old 4x8 ldr table, TD cutmaster 82, rev 20 pwm box, autocad 2004, sheetcam, mach 3. running off the same dell desktop. all new TD consumables. put in an earth ground was only able to go 5 feet before hitting ledge. anyways, i can run the same g code over and over and it always misfires. it will pierce shut the torch off move an inch pierce again. other times it will pierce and almost completly cut shape out. keep in mind im running the same cut settings from the last owner that had no issues. any help would be much appreciated!

[WyoGreen]

Sounds like emf noise issues. Make sure your plasma return cable is not coiled up somewhere, and make sure it is clamped directly to the work. Is everything more or less located in the same places as they were in your bosses shop?

[04dmax04]

when you say plasma return cable would that be the yellow cable that comes from the table computer to the plasma cutter? and i always clamp directly to the steel im cutting. i have even been grinding where i clamp. and yes it is almost identical exept he had refridgerated air dryer. im running off cheaper dual in line filter system for now as a cheaper alternitive to start up.

[djreiswig]

The return cable is also called the work lead. It's the big cable that comes from the plasma cutter along with the torch lead. Some refer to it as the ground, but it's actually a positive voltage, so not truly a ground.
It should always be attached directly to the material being cut.

[adbuch]

Sounds like emf noise issues. Make sure your plasma return cable is not coiled up somewhere, and make sure it is clamped directly to the work. Is everything more or less located in the same places as they were in your bosses shop?

Having the work lead cable coiled up can act as a large inductor in the circuit and produce fluctuations in the divided arc voltage used for torch height control. This results in the "sewing machine effect" where the torch "bounces" up and down as the divided arc voltage fluctuates. Here is an example of a Hypertherm 85 with a 25 ft. work lead coiled up.

https://youtu.be/AJIrqI_bv98

Torch Bouncing.jpg

Here is an example of the same part being cut on the same cnc as above, but with a Hypertherm 45xp being substituted in place of the Hypertherm 85. The work lead cable is still coiled up, but the part is cutting fine with no "sewing machine" action. The 45xp produces much less switching noise than the 85 and hence does not affect the divided arc voltage.

David

https://youtu.be/TCUags6rfe4

torch not bouncing.jpg