Thanks.
Unfortunately I didn’t get to doing the test in the maxmetal that I had planned this weekend but will try later this week.
My test in coroplast with the stingray didn’t go so well. I don’t think that I can get the blade protruding enough to cut the 4.5mm material. I will try that with an O flute or a compression bit and see how it goes.
The Stingray is way too short for 4.5mm thick material.
You’ll want a full-sized dragknife for that, and will then need a CAM tool which will do corner overcuts.
Yeah, that is what I discovered last night. I will try the compression bit that @wmoy mentioned in his video and the O flute.
But now I am wondering if the SharkTooth I bought for pen work would hold an Xacto knife and do double duty as a drag knife.
As far as the software, I have Vectric Desktop now, but unfortunately the drag knife gadget that is available only works in the Pro version. Maybe I can demo that to see if it would actually work.
That is a great video. You can see how workholding options are tricky, Coro tends to move around in every direction. I don’t know much about the SharkTooth pen holder, but it would be very convenient if something like that would work for Coro. I wouldn’t even care about perpendicularity or runout, it’s such a cheap material… no matter how you cut it, it never looks great.
Ha yeah, for my first try yesterday I made sure the coroplast was “glued” down to my cutting mat but didn’t have the cutting mat secured to the wasteboard. It wasn’t an issue with vinyl, but on the coroplast it just started dragging the mat around 
The second test was better but I manually held it in place. Obviously I need a better solution. I have the mat sitting on an 1/8 piece of MDF so that the T Track slots are not an issue. Maybe I will fasten that down and the use the tack it glue on the back side of the cutting mat to have it held to the MDF. Or just use tape. On the back side as the coroplast I am cutting is slightly bigger than my mat.
If I can get the shark tooth to behave with the xacto I will report back.
Conceptually it seems like it would work although I have no experience with drag knives other than the stingray to know if there are any gotchas.
However it looks like the length of the knife handle will be an issue if the Z router is not near the bottom of the Z travel. As it spins around the handle may hit the Z plate. Seems like that could in theory be an issue with a sharpie as well but I haven’t tried.
Clamping the knife in there is a little effort to keep it straight and may be a little dicey. If I could find an xacto more like me (short and fat) it may be better.
So, I bought a cheap xacto clone from HF which had a slightly thicker barrel. Then I cut the handle shorter so it wouldn’t interfere with the Z plate.
It looked like it would do ok as a drag knife.
I was worried about how it would orient correctly but looking at other drag knives it seemed like it was basically shaped the same so I decided to try it on a thin piece of cardboard.
As I feared it didn’t orient properly or turn as freely. I even tried a second cut after manually orienting it properly for the first cut.
I spent a good bit of time trying to get the point of the blade directly centered under the shaft of the tool.
I don’t know if the blade geometry isn’t good or if the angle to the surface is bad or if if doesn’t swivel freely enough or maybe I just didn’t plunge enough for the knife to grab properly.
Maybe the spring loaded nature of the pen tool is a bad idea.
Video of my failed attempt: https://youtube.com/shorts/Ehbguazc9gc?si=Yz6U3ZDLwdk5h7AQ
You don’t want it centered, you want it past center a little bit. That’s how it works. it “drags” behind the center point. 
Ha, that makes sense and would make setting the knife much easier. When staring at the pictures of others out there like the SST and Donek I didn’t perceive that it was offset but I do remember one of the videos talking about that.
I can make an adjustment and try again. What about the spring load? In my test cases I have a cutting mat underneath so maybe I should increase my DOC as it really shouldn’t matter if I go deeper.
I guess that the zero point should still be set wherever the knife tip is rather than the zero of the spindle.
No, because the knife rotates & the tip will move. When I set mine, I rotate the knife tip forward to set X, and rotate it right or left to set Y, so the zero point is in the center of the spindle.
I think real drag knife CAM software let’s you enter the offset of the tip & it compensates for the offset.
I just use a ramp in CC to align the tip at the start, and force a little overlap at the end of the path.
So you have the tip pointing toward you and “touching” a target spot on the piece and set X then rotate and jog so the tip is on the target again and set Y?
I am trying this with Vcarve Desktop which doesn’t support the drag knife gadget which supports the offset. But I can try the ramping and add in some lead in/out vectors.
Yes. To compensate for the offset, cutting a sharp corner should look something like this…
Where the radius move is leaving the tip in the same spot but rotating the center of the drag knife in the direction of the next move. So far the stuff I’ve done with it just isn’t that critical. I’m either cutting a mask for painting, or pre-cutting the mask for a V-carve so the mask edges don’t rip out.
Thanks. I will give it another go.
I manually coded similar overcuts in the hand-written G-code for:
Um, yeah, I don’t really see that happening.
I did look at dxf2gcode which is supposed to put the corner loops in but I haven’t really tried it yet.
Yeah, it’s kind of tedious, hence my working on other ways to make G-code.
After the distraction of my failed attempt to convert the SharkTooth to a drag knife I finally had a chance to do the test cuts in maxmetal.
I used a 1/8 compression bit at a DOC of .126” cutting through in one pass except for the one with the word.
The sign place I am working with makes road signs for neighborhoods and puts vinyl iver the maxmetal substrate. So she wanted to see if I could cut some profiles. I did a small sign test and then a more fancy one and then a large one about 18” wide.
It al went really well. Very little cleanup when done. There was no real need for deburring. I just ran an xacto blade across the cut edge and wiped with an old rag.
I did have an issue with Vectric Vcarve I will post about in another thread but switched to CC and all was well.
And for the coroplast I have ordered the SST drag knife. I have to leave town for a week but hope to try that when I get back.
That’s great! Looks like I’ll be buying some compression endmills (save myself some clean-up time). Sorry the Shark Tooth/xacto setup didn’t cut properly, but your test results have a lot of valuable info (of course). Thanks!
This is the one I used for this.
https://www.amazon.com/gp/aw/d/B0B6VJQP3X?psc=1&ref=ppx_pop_mob_b_asin_title
I bought it specifically for a plywood project some time back but it may get dedicated to metal and I’ll get a different one for wood.
Both Cadence and IDC have 1/8 and 1/4 compression bits that look nice, but their 1/8 bits are on a 1/8 shaft rather than 1/4 like that Spe bit.
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