Required accuracy for a torch height control

[beefy]

Working on designing an external torch height control.

If I've done all my calculation correct then the WORST CASE error reading would be 0.8 volts.

Anyone think that is too much error for a THC.

Keith

[jimcolt]

Most height controls accurately control arc voltage to plus or minus about 2-3 volts. For every volt you will see roughly a .004" change in height (this varies a bit with thickness and power levels). Controlling the arc voltage tighter than this usually creates some oscillation in the z axis, which will show as striations (roughness) in the cut edge.

I think it is funny when THC manufacturers advertise .5 volts accuracy......because the arc voltage itself fluctuates more than this (due to the moving cathode attachment).....which means the height control will oscillate rapidly up/down to try to compensate. In all cases when the edge gets rough because of this oscillation, you need to slow down the z axis speed or deaden the THC reaction time, which ultimately loosens up the voltage deadband to at least the 2-3 volt range. In reality controlling to plus or minus 5 volts is ok on materials thicker than .125".

One of the overlooked components in a THC is the mechanical tightness as well as the lack of "overshoot" of the z axis lifter. A loose lifter requires that the voltage range (deadband) is loosened up, a lifter that does not instantly stop (braking) will do the same. The z axis lifter, properly designed will be the most costly part of any THC.

Jim Colt Hypertherm

[beefy]

Thanks Jim,

and thanks for all the extra info you threw in there, never hurts to learn a bit extra. Glad to know I don't need to go the extra mile with the design.

I know what you mean about manufacturers advertising super accuracy. I've seen that a few times on plasma tables where they advertise repeatability to 1/1000th inch. As though the floppiness in a plasma jet could take advantage of such accuracy IF it were true. A lot of sales are made on deception while perhaps not quite telling a lie, they just don't tell the whole story.

I learned all about the dead-band requirement and the correction speed a few years ago when my Z-axis was playing jack hammer. I'll be putting a user adjustable dead band & THC delay in the mix too.

Cheers,

Keith