Automated height control

I experience the touch getting stuck in some areas after cutting especially when the metal expands when it is cut. By any chance is there an addition component we can buy that can help with automatically controlling the touch height when cutting?

THC is not available for the Crossfire. However, a couple of people were modding theirs to have THC. They posted build stories, photos, etc.

Without THC you have to be more careful to get your height, speed and power settings correct. Also, if you’re seeing a lot of warp or tip-ups that the torch encounters you can set the toolpaths to avoid running over previous cuts as well as change the order of cuts so they’re not all together in one area which will allow for cooling and less warp.

If you’re sticking at the ends of cuts, try increasing your lead-out, reduce the angle and increase the distance so it moves away from the cut piece before shutting down and running it’s head cooling cycle.

do these thc controls continuously adjust the height as they cut?

Depends. There are THCs that just react to commands in the machine control files. Those take care of what other CNC systems define as “retract height” where as the head moves from cut to cut it raises to avoid hitting material. It also allows for a different pierce vs running cut height. You often want your pierce height to be higher than when cutting to avoid splash & interference as the arc initially hits material and hasn’t established the vaporizing cut that allows of the plasma stream to make it through the bottom to exit.

The next step up is when your torch and the plasma unit support a feedback loop that monitors arc voltage and adjusts the height to keep it in the correct range for cutting - the head will make vertical adjustments as the cut proceeds and the material height changes. The torches that support this will have additional wiring vs the standard 2 wire on/off trigger wiring we use for the Crossfire.

There are 3rd party THCs that you can buy and potentially fit to the Crossfire (the Crossfire control board has connectors for THC management) but you’ll be pioneering in the community :slightly_smiling_face:

That might be where I want to go with this for mine as my cutter has connections for ok to move and 1/16, 1/50 divided and raw arc voltage. I assume the voltage changes with the distance from the sheet and that how it varies the distance. I’d rather have the best cut quality possible and less aggravation especially when I eventually expand the machine and have a bigger chance of uneven surfaces. there’s also the idea that people might bring me random material to cut that might not be the best quality.

Yes, that’s how it works.

Does that mean things like paint and rust can affect the cutting distance?

yeah, but it adjusts. That’s why they’re great. But that’s also why they’re a PITA. Sometimes they’re finicky and getting them to work can be challenging too. Mixed schools of thought out there although the consensus is that they’re worth more than whatever bother you experience.

No, Paint and rust to not alter the voltage/ height relationship with a plasma cutter. It can affect the sensing of the surface, which is critical to achieve accurate pierce height. Jim Colt

The best height controls for plasma are not standalone systems, separate from the CNC electronics. The best are integrated into the cnc as the z axis, with arc voltage feedback Those with similar features that have been in industrial plasma’s for well over 20 years (Ohmic sensing, voltage sampling, anti dive, and more) are not at all finicky. Unfortunately many choose their plasma height control based on cost, and the low cost ones are better than no height control but not nearly as good as the well developed integrated height controls used on many cnc / plasma machines. On my home shop machine I can nest a few hundred parts and walk away, returning when the job is complete. Jim Colt

which control are you using? there is Z and A axis available on the langmuir controller as far as I understand so this should work like your saying provided the plasma has the right outputs

I’m a little unclear on this and its definatly something Im going to want to do. There is a very good write up already but it doesn’t look like that system is actively adjusting height as it goes, it seems more point to point. My tables not here yet so I’m still trying to predict what I’ll need as I get set up.

The z axis (torch height) is fixed on the Langmuir. Plasma (for best overall operation in terms of cut quality and consumable parts life) needs to set an accurate pierce height, pierce delay time, then before x and y movement needs to index down to the ideal cut height. With the Langmuir you either compromise on one or both heights, or you do all of the pierces, stop then do the cuts at cut height. Fortunately on materials thinner than about 3/16" you can pierce and cut at the same height without too much impact on consumable life and cut quality. Thicker materials require higher pierce height and pierce time delay. Jim Colt (41 years at Hypertherm, cnc plasma cutting for that same number of years!)

I see. I thought the langmuir controller had 2 unassigned axis we could use for something else like a stepper for Z.

Oh… do you mean as it comes standard it as no Z axis?

I’m confused then. If paint & rust don’t alter the height relationship but then how can the sensing be critical to get accurate height?

I thought the raw feedback was interpreted as a change in the height so if it’s running along on a non-painted or rust-free surface and then encounters a patch of rust or paint it will sense the change and adjust appropriately.

What am I missing?

I think, maybe he means, if the electrical connection is made through the plasma to the metal being cut then it can do its job. If it can’t get an arc then it can’t adjust the distance. But in that case I think your not cutting either. So the plasma arc, if its established, would blow through paint and rust and it wouldn’t matter.

It’s interesting about peirce height and cutting height being diferent. Is this something you can program in mach3?

Heh if my table was here this is the kind of thing I’d love to try through experimentation. You could hook a scope up and see the voltage drop with diferent surface problems. I just heard ups may have lost my water table. Hopefully the crossfire makes it tomorrow like its supposed to

Mach 3 can support that - I know in Sheetcam it’s a separate setting. I think (but don’t remember where) you can do the same thing in Fusion. I just don’t know if it’s at the toolpath line level or at the setup.

When piercing on a cnc machine the torch is perpendicular. Piercing is like drilling, the chips (molten metal) blows out the top of the hole until the hole penetrates the material. Hypertherm plasma systems all have a factory pierce rating and factory specs that include a pierce height and a pierce delay time. These are developed in order to allow efficient piercing that does not damage the shield and nozzle, and possibly the retaining cap and torch. The pierce specs are in the many pages of mechanized cut charts in the Hypertherm operators manuals for 45 amp and larger air plasma systems. Molten steel that enters between the nozzle and shield will affect cut quality and cause double arcing that slows down the process and immediately damages the nozzle orifice…the orifice is what shapes the arc and provides consistent edge angularity and cut speed and edge smoothness. One bad pierce can damage the nozzle. This is why pierce height is different than cut height. If you pierce at the correct pierce height then stay at that height for cutting, expect more edge angularity and perhaps top dross (always indicates cut is too high). Proper cut height is critical. Pierce delay? This is the time (very well tested by Hypertherm) from the moment electrical energy flows through the work cable, until the pierce penetrates each thickness of metal…that it takes to pierce. On systems with properly engineered height controls the torch locates the surface, retracts to pierce height, fires, stays at pierce height until the pierce delay times out, then indexes down to cut height and starts on the x-y path. Once the machines exceeds 85% of book cut speed the z axis is then controlled by arc voltage feedback. Be happy to answer any questions on height control functionality, it is a very important part of good quality cnc plasma cutting. Jim Colt

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Arc voltage feedback height control is a reading of the voltage between the (negative) torch electrode and the positive work cable (some incorrectly call it a ground lead). Think of the plasma arc as a resistor, when the material is thicker or the torch is further from the material the resistance changes and the arc voltage rises. So an electrical / software controlled comparator circuit monitors this arc voltage and adjusts the z axis to maintain the cut height that provides best cut quality. This is tricky as there are many parameters that affect the voltage / height relationship. A few variables are: Electrode wear (as the electrode wears the voltage increases and simple height controls will correct by moving the torch closer. Better height controls auto calibrate and compensate for electrode wear). Cut speed, slow down the speed mid cut and you will see the torch get closer to the material. Better communication between a good height control and the cnc control allows for height freeze and anti-dive when deceleration for intricate details naturally occurs. Low cut air pressure changes the resistance of the arc, and the height of the torch. Moisture in the air changes the arc characteristics and affects height, not to mention consumable life. Rust and painted surface have virtually no effect on arc voltage based height, but may have an effect on initial surface sensing, depending on the method of sensing. Arc voltage height works fine submerged. (Note that I worked for Hypertherm for 41 years, recently retired). Hypertherm designed its first arc voltage based height control in around 1975, so it is not new technology! Jim Colt

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This is all good information Thanks jIm.

Submerged in water you mean?