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Cut Rules and DTHC Settings for Detailed Cut-outs

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Cut Rules and DTHC Settings for Detailed Cut-outs

Postby Econdron » Tue Jun 13, 2017 3:00 pm

I'm using a CandCNC Full Automation bundle with CommandCNC for Linux. Honestly, most of the parts I'm going to be cutting out for business are dead simple, very little detail. But I do like the artwork type things for personal projects and such, and I want to be able to cut out more detailed work upon request. I've refrained from coming and asking questions at the first sight of trouble, and I've gone back through the manual and found settings that were off, etc. I've gotten pretty decent results, now I'd like some tips on cutting detailed projects. For one, can anyone share a list of cut-rules you use in sheet cam for detailed artwork? I've got one that turns DTHC off on small shapes, and one that slightly slows down the torch and turns off the DTHC at the end of a cut. Any other big ones I'm missing?

Also, I've been playing around with the VAD (Velocity Anti-Dive) settings, as well as the auto-sets and sensitivity of the DTHC. I THINK I've got a grip on what the settings do, just having trouble finding good settings that work. I just cut out some letters, and the problem is the voltage spikes when back tracing the other side of the letter where it's cutting partially over the kerf on the other side. It all happens so fast, so I'm not positive exactly what's going on in the computer, but I'm guessing the voltage starts to increase, so the torch starts lowering, until it crashes. Apparently the voltage spike is low enough that it's still within the range to activate the DTHC. Anyone willing to post a screenshot of the DTHC Settings in CommandCNC as well? Would be much appreciated!

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Re: Cut Rules and DTHC Settings for Detailed Cut-outs

Postby motoguy » Tue Jun 13, 2017 4:04 pm

I too think it would be helpful if there were a way to deactivate THC when crossing over previously cut (or partially cut) kerfs. I'm not sure if that would be on the Sheetcam side or CommandCNC side. I asked the question in the CandCNC forum, and Tom mentioned that crossing a previously cut kerf is a drawing error. I see the point to that, but I've run across scenarios where I need to cut across a previous kerf, or (more often) cut a detail so small (inside detail on a letter, or something) that it'll backtrack across a cut (or partially cut) kerf. If I KNOW this is going to happen on small parts, I put those parts in their own layer and disable THC for that layer (by putting some huge number in the "dist before DTHC" setting in Sheetcam).

Sometimes, if it's a large cut, you can't do that, because you need THC active. I'll sometimes babysit, and when we get to the questionable location on the cut, disable the THC with the pendant/at the keyboard.

I'm new to CommandCNC as well (about a year and a half with Mach). I really appreciate the VAD feature, but I'm learning it's not a panacea for all situations.

1) I've created a rule that deactivates THC and slows cut speed in tight corners, in order to maintain a nice, square edge. This is mostly for thicker (non gauge) materials. I ran into a situation on some 3/16" fire pits where my tabs would not fit into my slots. Though VAD kept the torch from crashing on the cuts, it maintained (full) cut speed. This didn't let the arc "dwell" in the corners long enough to make them nice and square. By slowing the cut speed .15-.2 before and after the corner, the arc dwells longer, blows more material out of the corner (makes it more square), and fixes my fitment issue. I'm happy when I can assemble right off the table, no grinding/filing necessary.

2) I've created a rule that can disable THC on corners and arcs. This can let me use VAD for most of the drawing, but have a rule to "tweak" THC in certain areas of corner/curves. I also set the feedrate to 99% in the rule, so I can see where the rule takes effect in Sheetcam. Without a feedrate change, the cut line won't change color (which is what I use to see where rules kick in/out). I'll adjust the "dist before" and "dist after" to suit my needs in the particular drawing/cut file.

3) the default EOC (end of cut) anti-dive is set to run all the time.

4) (Small) holes go in their own layer, with reduced feedrate and (possibly) no THC. The smaller the circle, the greater the chance I've disabled THC, and slowed the feed rate WAY down. I cut some gauge panels for a customer out of 14ga. The holes need to be very square, and as close to perfect as possible. I'll typically slow them down and disable THC to get a nice, round, square cut. The extra dross is an acceptable tradeoff.

I really like the VAD. I view it as more of a "safety net". I also use it to calibrate cut height in areas where I couldn't use THC with Mach. Cutting the stars in a tattered flag file, for instance. In Mach, I had to use a corners rule to keep from crashing out, or disable THC for the layer; they typically had the same THC on the stars. This meant I touched off quote often, to make sure I was at a suitable cut height. With VAD, I don't need to use ANY rules on the stars. I just bump the VAD limit to 98% or so (high because of the rapid direction change and fast feedrate on thinner materials), to make sure there is no hiccup/bump/drop of the torch while cutting. This usually still lets the THC sample the cuts in several places on the star, ensuring a correct cut height. This sampling per cut means I can spread out my ref distance number, and touch off less often.
Bulltear 6x12 w/ Proton Z axis & watertable
CommandCNC/Linux w/ Ohmic & HyT options
Hypertherm Powermax 85 w/ machine torch
Solidworks, Coreldraw X7, Inkscape, Sheetcam

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Re: Cut Rules and DTHC Settings for Detailed Cut-outs

Postby tcaudle » Tue Jun 13, 2017 4:42 pm

Cut rules can get yoou in trouble if you don't know exactly what you are doing. An example is where you are trying to keep the DTHC from turning on at the beginning of a cut. The command to turn it off in the code will happen BEFORE the suto DTHC On code happens based on the Z position.
The over usage of cut rules will cause more problems than it solves. The VAD and DTHC are designed to do cutting that does not violate the normal plasma cutting process. Plasma is melting and vaporizing metal and the flame is not rigid. Get two cuts too close and you get a condition where its like cutting over a kerf and causes the voltage to rise. The VAD won't help unless the velocity has been reduced to be able to follow the toolpath. SheetCAM will not let you overlap cuts and refuses to go into an area to narrow for two full kerf widths. Even then you are going to get diving problems where the distance between cuts is less than about 4X the kerf width. Part of this is being able to look at a design and understand where the problem s areas are going to be. Sometimes its easier to alter a design than to try and force it to do something its not designed to do. A really important setting is in the Operation called Min Cut Length for DTHC. The reason to use that on smaller cuts is to keep the DTHC from ever turning on at the beginning of the cut. The way the system works is it turns the DTHC off at the end of a cut and then turns it back on AFTER the pierce and AFTER the Z has gone down to Cut Height as long as it does not violate the Min Cut length. So DTHC is always off by default until certain conditions are met

We have cut some extremely detailed artwork on 14 ga at 45A at 225 IPM and watched the blue DTHC enable LED on the box flicker like a 2year old playing with a light switch. it never dove on the corners. Is it perfect and will it work if you violate the clearance rule between cuts? No, but its good enough that we have turned off all cut rules. Recently we cut some 18ga. not real detail but some square corners and it did well.

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Re: Cut Rules and DTHC Settings for Detailed Cut-outs

Postby Econdron » Tue Jun 13, 2017 11:04 pm

I understand what youre talking about and it makes a lot of sense. So what is the recommended method of cutting when sections of grass are to be cut from a picture for example? This is in several of the files included when I became a contributing member here. Lots of sharp angles where the kerf overlaps at the return of the spike for each blade of grass. Or letters I see overlap quite a bit on smaller letters. Sounds like if you're cutting within 1/4" of another kerf line, the DTHC should be turned off? Then the job needs to be programmed in a way that turns off the DTHC in trouble spots?

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