How do we set Z home and plunge depth when using a drag knife and flat probe?

Mostly Printed CNC - MPCNC Troubleshooting
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I’ve got gcode generated from EstlCAM to cut a sample vinyl heat transfer sticker. EstlCAM wants to make two or three passes for each cut, plunging a little bit more each time. This leads to the cutout getting done just fine and dandy for some parts but scraping the mirror under the decal for the final pass.

I followed the setup to get the drag knife at just the right depth but since it’s plunging, and thus pushing up the drag knife itself, I’m guessing I need to do something better to set the Z home. I have the flat probe sold in the store, but since the drag knife itself is not ferrous, it doesn’t register when probing Z and I have to use the steel tubing as the probed point. Thus, the drag knife gets pushed up due to the slight difference between the tip of the drag knife and the tip of the tubes.

I’ve tried using Z babysteps while cutting to dial it in but it seems like it’s just not working - if it’s babystepped high enough to just barely cut over the course of three passes, it does not fully cut the vinyl. If it’s babystepped low enough to cut on successive passes, it will drag and fail to cut completely. This is with heat transfer vinyl that I got cheap at a craft store to practice on, so it’s still a learning process, but is there a guide or steps I can follow to dial this in a little better? It feels like I’m flailing around trying different experiments and not getting any successful cuts.

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@MohnJadden

Retract the blade completely by loosening the locking nut and unscrewing the barrel until the knife is not protruding. Mount the drag knife and zero the Z axis by clipping the lead to the barrel and putting the plate under the whole drag knife assembly. Use the same gcode as any other z-zeroing process (adjusting for the plate thickness). Save the Z zero value.

Move the Z axis up and then screw the blade back down, but just about 1.5 times the masking material thickness. Set the depth of cut to about 1X the mask thickness, and only one pass. The mask will get cut, but the mirror wont have enough pressure to get a scribe mark.

This is the procedure with many vinyl cutters and you can find a bunch of videos on Youtube about setting the cutter position vs material thickness.

Mike

2 Likes
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Starting out in hard mode, I see…

So the machine that I’m using my drag knife on, I set the drag knife depth so that when the face of the knife body is against the material, the cut depth is exactly what I need. Test by clunking the drag knife down against the material and cutting a small circle.

Setting the machine depth is a bit trickier.

My vinyl cutter uses a servo for the Z axis, and is configured so that the maximim down position would lightly drag the face of the knife body against the bottom board. The knife holder has some spring from the pushrod, so it will apply pressure but not too much. This results in a cut through the vinyl that is comsistent.

DTF vinyl is thicker and slightly grippy on the cut surface which makes it harder to deal with. Too much pressure and the knife body eants to stick and the holder flexes. Not enough and it wants to ride up and not cut through.

It’s a tricky balance, but a few things that Ive found that work better.

A cool room, about 18°C seems to be the balance between the adhesive on the cut side being less sticky, and the vinyl getting stiff and resisting being cut. Warmer and the knife wants to stick, cooler and the blade doesn’t like to cut all the way through. (Hence my machine is in a cool corner of the basement.)

I used a Cricut sticky mat to stick down the vinyl. I get away with a bit of painter’s tape with the regular adhesive 3M vinyl, but something firmer seems to be needed with the DTF.

In the CAM I have the cut set to -1mm with the servo range defined as -1mm to 4mm (5mm throw.) So setting the zero has nothing to do with the material, since that is the physical range of the servo.

When I had the drag knife on the Primo, and later on the LR2, and LR3, I would back off the point of the knife into the body and set zero by the knife body. The mounts for the machines would then flex a little when the knife went to -1mm (actually about 0.5mm below the material surface, because I set it as a hard zero on top of the touch plate.)

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I just grabbed clearance-rack vinyl without regard for type, lol - if there’s an easy mode material to start with I’d love to get a rec on what to buy. I’m new to CNC and while I have some experience with the hellscape that is 3D printing, I still have a lot to learn before I strap on the router.

@mbamberg Thank you so much for the detailed walkthrough - I didn’t even really have the terminology to think in terms of masking material thickness, etc., so it’s been a lot of trial and error. I haven’t been zeroing in gcode - just using the sensor before each run.

I haven’t had any luck getting the sensor to register a touch when the clip is on the copper barrel of the drag knife - it’s been doing better when I clip to the push-down tip of the knife and use the sensor to detect the very tip of the blade itself. Probably not the best course of action but I bought plenty of spare blades with my drag knife to account for beginner’s error.

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I have the one from the V1 store and it works there.

For my servo activated one though, I adjust the servo pushrod attachment, so no probe, just visual, and that works for me.

You shouldn’t really need to worry about super exact zeroing of the Z with the drag knife. The knife blade stick-out handles the cut depth, and a bit of flex is designed into how the holders work. I use the touch plate because I can and it’s a bit less work. (Only a tiny bit less.)

Well, the adhesive vinyl is a bit easier, the material is thin, so even a scratch is pretty close to enough, it doesnt take as much pressure to cut and the surface is slippery, so doesn’t put as much force on the knife. All in all, most stick on vinyl will be easier than the DTF stuff.