Questions/answers/discussion about initial setup of your CNC Shark

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Re: Why?

Postby tonydude » Thu Feb 22, 2018 9:13 pm

Using vcarve is better than using quick engraving when using v bits. The depth of carve is based on the width of the vectors that the bit fits into. A 60 v bit will carve deeper than a 90 v bit etc.. The further apart the vectors the deeper the cut. Watch and do the tutorials, it helps a lot. I've been doing this for 6 yrs now and still watch the tutorials, there is always something new.


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Re: Why?

Postby Rando » Fri Feb 23, 2018 7:28 pm

No matter how you choose the toolpath, a v-bit in wood has two problems, and it looks like maybe you're fighting both of them.

The underlying cause is that the human eye is VERY good at seeing variation in line thickness. Don't ask me why, something to do with yummy possums on tree branches or something, I dunno. My point is that two parallel lines that have variation is VERY obvious to humans. Just like an out-of-round circle will seem just that little bit off, even if you can't "see" it clearly.

Well, when you carve with a V-bit as your weapon of choice, the positional change for each unit of vertical positioning error, the error in the horizontal plane is 1:1 for a 90-degree bit, and 1:1.7 for a 60-degree bit. (And don't get me started with the "0.004" tip" 4-degree engraving bits where the first 0.010" is actually 120-degrees!) So, compared to even a tapered ball end mill (typically 6-15 max side angle), the error in the visible line at the top is huge.

So, what's causing your vertical positional issues? Wood. That's your big problem. But really, in essense, it's that the top surface that the v-bit is cutting into is not where the v-bit thinks it is.

Okay, first, forget that stupid virtual zero. If your wood is deformed like a pyramid, then maybe you can make it work. But wood warps in a cylindrical section around the shape of the tree. And you can't model the inside, much less outside of a cylinder with a pyramid. It doesn't work. The "ends" of the cylinder, wherever they are, are given ZERO of the calculated offset, while in the middle of the design it's given 100%. It doesn't work. :ugeek: They need at least 9 points, and they said they didn't want to make people do the extra four points to make it work properly. Plus having that many points would require "actual math".

If you really insist on getting an accurate v-carve on the deformed surface of a piece of wood, there IS a pretty easy way. What you do is use their scan probe, or a real 3D scanner, or even your own measuring goodness, to create an STL solid model of the deformed wood. Then, you have Vectric map your toolpath onto that surface. Works WAY better than that lame virtual zero, and you don't run the risk of breaking a bit, or getting hit, or just plain confusing the entire system. I love it. And remember, no matter HOW bad your halfway-decent STL model is of the board, if it's closer than a flat piece of wood, then it is technically "better" and will therefore most likely provide at least incrementally "better" results.

But, in the end, no carving can survive without the material it's carved into. No, I don't mean that in the mystical sense. I mean that our ability to perceive it for the design it is has to do with the precise placement of that design onto the surface of the material. And since you've designed your design as a flat design, it needs to be put on something flat, or it won't look right.

So, if you're going to use a v-bit to carve you have two choices: either you flatten the board, or you flatten the "field". Either way (here's the important part): If you want accurate cuts with the v-bit, what those cuts are cut INTO is just as important as defining where to put the bit.

Does that make sense? Either flatten the whole board, or flatten a circular/heart-shaped/etc. **field** into which you carve your design. That way, even if the board wasn't flat, the place where your design is located IS flat.

Okay, one more then I'll stop. If it's some relative thin stock and you WANT it to have the design on the bowed surface, sometimes you can screw down the board to the spoilboard so it goes flat during carving, and then springs back when it's done. Don't cut the parts out, though, and don't cut the screws ;-).

Hope that helps (and makes sense...I'm not convinced it did ;) ),

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Re: Why?

Postby Jacduck » Tue Feb 27, 2018 2:01 pm

Thom, thanks for the insight it gives me much to ponder. I am going through tutorials incessantly, even loosing track of email etc. I think I will get it someday but I suspect there are more issues in the future but I still don't understand a o,2+ beginning and then back down to the assigned 0.1 plan on mdf.
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