A Curious mind wants to know

[acourtjester]

Using a Hypertherm 85 on a CNC plasma table cutting 3/4" steel what is your plunge rate set at??

[djreiswig]

I use a ramp pierce with the plunge the same as the travel speed. Tom @ CandCNC says it messes with the DTHC, but it works fine here.

[acourtjester]

What I was looking for is the speed the Z moves for the pierce.

[beefy]

All I have ever done for plunge rate is set is at the fastest speed my Z can move (Z rapid speed).

In a nutshell, I have my torch come down to pierce height, whatever that may be for the nozzle used and material thickness. I then have the pierce delay set long enough so that the pierce is actually completed, i.e. the torch has pierced all the way through the sheet.

From that point, all I want is for the torch to RAPID down (plunge rate) to cut height and start cutting. I used to also get confused and worry about plunge rate until I thought what the heck am I worrying about. Just pierce, then immediately get down to cut height, and start cutting. Then enable THC once the torch volts has stabalised at cut height.

Many systems have "THC DELAY" which is a setting in seconds for which the CNC controller completely ignores anything to do with torch height control. I'll be very surprised if Tom has said a ramp pierce messes with THC because you can set the pierce delay to however long you want so that there is no DTHC during the ramp pierce period + any plunge to cut height (which may not even exist with a ramp pierce) + a period of stabalisation at cut height.

It's always worked for me - blast a hole through at pierce height, then get down to cut height ASAP (plunge at rapid speed), the allow the torch volts to stabalise, then enable THC.

I'd love to know if there's any reason to do it differently.

[djreiswig]

See this post and his reply in the next post.
http://candcnc.com/supportforum/viewtop ... 77&#p13540

[acourtjester]

Thanks for the reply I was working with another guy remotely (via email) (he is using the 11 J post processor for CancCNC setup) I have always just left the original rate of 60, but the thickest I have cut is 1/2". It seemed his reload of the tools in SheetCam it had 120 and that seemed to cause a problem (he did not state what). Not being the "All Knowing Oz" I posed the question here for the commercial type that have cut 3/4" for their answer.
As usual a questions asked, an answer given

[jimcolt]

If the z axis speed (some call it "plunge rate" though that is more of a machining / routing term than a plasma cutting term!) is too high expect rapid oscillation in the z axis (you can feel it while cutting if you place your hand on the torch....rapid vibration). This will cause striations or roughness in the cut edge. Set it as high as you can without causing oscillation for best performance on most machines. Jim Colt Hypertherm.

[railmaker]

I have a Hypertherm 1100 and have had it for a few years now. I have cut 3/4" and 1" very successfully by preheating the material to 150 deg or more with a rosebud. I realize not all may have this, but even a weed burner would work. Preheating is never a bad thing when it comes to working with steel/iron/metal. Anyway, it's always worked for me.

[beefy]

If the z axis speed (some call it "plunge rate" though that is more of a machining / routing term than a plasma cutting term!) is too high expect rapid oscillation in the z axis (you can feel it while cutting if you place your hand on the torch....rapid vibration). This will cause striations or roughness in the cut edge. Set it as high as you can without causing oscillation for best performance on most machines. Jim Colt Hypertherm.

The plunge rate I was talking about was based on Sheetcam plasma settings and is the speed at which the Z "plunges" from pierce height to cut height.
I have never heard of "plunge rate" being applied anywhere else in plasma cutting. In any case "plunge" generally refers to a downward diving movement.

On the 2 systems I have used the Z speed you mention above has a specific name, and it is a completely different setting from the general Z axis rapid speed setting.

On the Candcnc system I had, it was called "THC Rate" and was a percentage of Z rapid speed if I remember correctly.
On my current UCCNC system it is called "THC Feedrate" and is set in units/min.

So if most other systems are similar, you can set your Z axis rapid speed to whatever the drive/motor combo is capable of doing with its load.
It is the Z THC speed setting that causes the oscillation when set too high.
I have no idea if less developed cnc software just uses the basic Z rapid speed setting for THC control.

[hsolve]

I think I'm with beefy on this one. My Z just moves down at the 'rapid speed' down to cut height from the pierce height and have never changed it. I have cut 20mm steel and 25mm alli, I have a PM125 and that may be making the difference. It does seem to take longer to pierce through alli than steel