Hi All
I’ll first start by apologizing for being vague and confusing here but for simplicities sake, I’ll use the example of a donut. Let’s say you want to route out a donut shape, .5" high and 4" in diameter. No big deal, it’s a simple shape, but how does the CAM software know that you want to cut the perimeter and then the hole in the middle rather than cut out the entire donut, leaving the hole as a “column” in the middle?
I’ve not got a handle on this yet and I made a rooky mistake with this today and ruined my stock. I should have looked closer at the tool path but I didn’t. Anyways, any and all help is greatly appreciated.
The cam software you use to make the shape creates a bunch of lines to make the shape you want. The software will create nodes at the ends of these lines that it took to make the shape. In a circle its usually a lot of small arcs that make up a circle so the ends of the arcs are connected by nodes. These nodes are what the program uses to trigger changes in the cutting parameters including the entry and exit points. Most cam software has a node editor where you can change these in detail.
What software are you using?
In your example, you’d have 2 circles drawn - a 4" one for the outer ring of the donut and then a smaller circle centered in the first, say maybe a 1" circle for the hole in the donut.
The outer circle would be a profile cut and the inner circle would typically be a pocket cut. The profile cut would be defined as an “outside” cut so it cuts outside the line and you get the full 4" circle.
The other two types of profile cuts are “inside” and “line” where the bit is entirely inside the circle resulting in a 3.5" circle if you use a qtr inch end mill. Cutting on the line would result in a 3.75" circle with the same end mill because only half the diameter of the bit is on the inside of the line.
The pocket cut mills out all the material inside the inner circle regardless of the size taking multiple passes to clear out the inside of the circle.
You could also do a profile cut on the inner circle (using an “inside” cut) as long as your end mill is less than 1/2 the size of the circle (hole) you’re cutting. You’d end up with a little column of wood left in the center but it would be faster than a pocket cut.
Now if your material bis 3/4" thick but you want a 1/2" thick donut, you’d make a pocket cut 1/4" deep for the outer circle (& the inner one too depending on the software you use - some will define the pocket as the space between vectors, others use the whole shape). That will leave you with a recessed 1/2" circle 4" in diameter. Then your profile cuts will leave you a thinner donut/ring
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I’m new here and barely understand the software but I think that it’s you that’s telling the machine through the software what to cut and where. Cutting out the entire donut shape free of the stock would be a profile cut and it should always be last or you wouldn’t be able to cut the center. It’s the tool paths that the software calculates based on what you designed and the order that you put them in that makes the desired cuts….I think.
But I have wondered why a tool path starts where it does and maybe moves to another spot and makes the same cut and then later connects them sometimes appearing to be running all over the place seemly randomly.
Hope I didn’t make a foolish reply.
Much of the answer depends on the software you are using. I use carbide create and fusion 360.
In cc, I pretty much have to think thru the order of all the cuts and which ones to pick. Sometimes adding detail to the design to constrain a cut. The CAM choices are super basic and I occasionally need to find ways to try several different approaches, looking closely at the sim, to get what I want. For CC, it would be up to me to pick the right cuts to either clear the donut hole or to leave it as a free standing cylinder in the middle.
In f360, the CAM part is wicked smart. Sometimes I need to try a few different things (because I am newish to the CAD part and super noob on the CAM side), but more often than not, I pick adaptive 3D, select a bit, and leave the settings alone. When I view the sim, it’s better than what I had been thinking. Using the donut example, adaptive clearings would clear the hole. Contour cutting would leave it as a cylinder. Also, I can dive into the settings in adaptive and make it leave it as a cylinder if I wanted to. It mostly does what I wanted without me having to be explicit and provides the fine control to change it if it is not exactly what I wanted. F360 excels in the Computer Aided part of Computer Aided Manufacturing.
As for you last question, it’s a total mystery how the software decides to cut a little here, cut a little there, then cut between the first two to join them together.
Mitch, are you from the Boston area? “wicked smart”
I’m using MeshCAM and Carbide Create. Mesh doesn’t easily do pocket cuts so I’m guessing the easy way for me to do the donut example is firstly a pocket cut using CC and then the profile cut using Mesh. So that means, (I’m assuming), that I need to create two separate g-code files. I was hoping to avoid that but no big deal as the controller can bring my spindle back to the probed start.
Nope. Got it pretty much right.
The only thing is that the outside profile cut is usually made with something called “tabs” added in the CAM. These are small little bits left in the final passes of the profile cut that hold the piece in place. Otherwise the piece can break free during the final pass and then it gets trashed because it’s not where it should be.
I generally use 4 tabs spaced around the piece that are 0.2" long and 0.1" tall. Since I use VCarve, I define them as 3D tabs which come out as little hump shapes vs just a rectangular block. The tool can speed through those faster than square edge tabs.
The router will rise a bit when it comes to the edge of where you told the software to place the tab and then lowers back to the cutout depth. Once the machining is done, I use a multi-tool or utility knife to cut the tab and remove the piece. A little sanding takes care of the stub.
This is what they look like on a project I just did. You can see them at the top and then the left & right where the waist of the guitar narrows in.
If you use clamps to hold your material down, these are critical. If you use the glue & tape method they may not matter if you make sure you’ve got the area under the cutout part well covered with the tape & glue because the tape will hold it in place. But I’m conservative and add the tabs anyway as a little insurance.
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@Maplehead, I am originally from Minnesota, glad to know some phrases transcend region 
I haven’t used mesh cam. With CC, it’s their pocketing I like least since there is no control and no ramping into the material. It doesn’t mean I don’t use it that way…it’s so easy to go from zero to gcode that I use it even when better tools would be …umm …better !
Once your machine coordinate system (homing) and work coordinate system (WCS) (zeroing xyz) are set, no need to probe anything unless you change a bit. You can run as many gcode files back to back on the same work piece (or another for that matter**) as long as they share the same WCS. You can jog the machine around. Let it sit overnight. When you press play, it goes to where it needs to be and starts cutting.
** I’ll use a fixture, typical an L bracket attached to the spoil board, so I can clamp another piece of material and batch out another part. The aligns my lower left x and y so the new piece starts in the same spot. As long as my material is the same height or my design accounts for (small) variations, hit play and cut a new piece.
This is embarassing. In Meshcam there is a toolpath option called Cutout. Guess what it does? Yup, it cuts the piece out on the outlines, easy peasy. Oy