Multiple Passes introduction

Can someone explain to me the process of doing multiple passes (ie, Roughing Pass + Finishing Pass)?

I am in the midst of my first project, and I am inlaying an acrylic cover onto a plastic piece that has a recess underneath to house LED lights. I want to use a really small bit to ensure the inlay nests nicely, and carve out the pocket + recess area with a larger bit.

How do I go about doing this?

I’m using Estlcam, screenshots appreciated, etc.

The way I’ve used a finishing pass is, You set up your job to chew through the big material, which flexes the bit and machine a little while milling. Then, using the same bit, you do a full length finishing pass, which basically moves the bit where it was supposed to go, but without any stress, it leaves a nice clean, precise finish. This type of thing is enabled on the estlcam settings for a path, by choosing a finishing offset.

In my mind what you’re describing is to use a smaller bit for the hole, then use a larger bit for the pocket. This is also a setting on the toolpath.

Also, hug pockets are kind of a pain, because they just take so long. A quick shortcut might be to get two pieces of thinner material and cut hole in one and then glue them together to leave a pocket. That doesn’t always work, but I thought I would mention it so you don’t end up with a 10hr job.

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Small or large bit might not make that much of a difference in surface finish and precision but will in milling time. Depending on the accuracy of your setup, a larger bit will be just as precise if you get the feed correct. It’s the radius that might make the difference for corners. What size bits would you consider using?

I can imagine if it is a large pocket, a bigger bit will make a difference in time. I’ve been using a 3mm bit to do a 60mm pocket 15mm deep. I also have other pockets of different sizes as part of a slip fit box. I’m going to write up the whole process tonight if I can get to it.

Here is how I have it set up for the deepest operation. That is shown in red.

I"m getting a caliper reading of 69.96mm for the inside measurement. The top of the box fits inside the bottom, but it is a bit tight. I am using 3mm for the tool diameter, but perhaps that is a little small for the bit I have. I want to do some tests. That measurement might be too small for the fit I want. Still trying to figure all this out.

Interestingly enough, I had done the same operation using clamps and holding tabs for work holding and the top and bottom fit perfectly with just enough friction to hold but came apart easily. When I switched to hot glue and masking tape, the fit and tolerance got much closer, enough so that I instead of using the same line and do a part on the top and a hole on the bottom, I’m going to have to make an adjustment.

I’ve shied away from changing bit sizes because I am not sure if I’ll mess the setup and alignment. Do you have some pink foam? You can do a test in that to get a sense of what works. You can up the feed rate and not take so long and still see how the fit works. A bigger pocket and inlay will be a real test of the accuracy of your machine.

How big a pocket?

In your case, you would just select the finishing tool you want. I use a pretty small % of the tool diameter, but I just haven’t done tests to see if I could up that allowance. I’m doing cherry and walnut and the lighter pass has made a better surface finish. In the V1 documentation the range is 5-25%.

The Yellow columns in the tool setup is where you set your feed and plunge settings for the finishing pass. I think I have it set correctly. Once again, I am still learning and every part is a test.

I had a tool change routine all set up in EstlCAM and somehow it got saved over so I haven’t reworked it. Using two separate gcode files is fine and allows for a less stressful tool change. It seems to me you would want to set up a tool that is just has finishing pass settings for the second operation.

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I cut the acrylic cover with a 0.8mm, and it was perfect. The pocket that the cover will inlay into, ideally I’d like to be cut with the same drill bit. The rest can be cut with a 1/8" for all intents and purposes - the rest of the parts, particularly the dept of the pocket (~12-14mm) will not matter so much.

The part is a very irregular shape, which is why I’m using such a small drill bit. There’s odd nooks and crannies that the cover will have to fit into - I’d imagine I’m going to have to finess it into the slot regardless, with a little hand sanding here or there.

Also, the project is a Christmas ornament - it’s roughly 4"w x 6"h x 3/4"d in size.

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The acrylic is really smooth edged, no burrs whatsoever.
And I decided to do an “island” cut between the middle and inner-most dielines pictured above with the 0.8mm bit. I’ll be coming back in with an 1/8" bit and doing a pocket on the inner-most dieline at a depth of 13-15mm, and then come around the outermost die and cut the entire depth of the object.
The 0.8mm bit essentially carved out the inset that the acrylic cover will rest in, and there’s a lip of about 1/8" all the way around to prevent the shield from recessing any further into the base. This being my very first project with this CNC, I cannot express how impressed I am so far with it’s accuracy.


That is a really great kidney bean you have there :).

Seriously, nice work. It looks sharp.


Thanks for the help everyone - posted the final piece in the “things you’ve made” section. Very happy with the results, and looking forward to my next project already.

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I’m back with more concerns about multi-passes, and seeking some advice.

I’m making a sign out of wood, and ripping it all with an 1/8" bit. How do I go about getting crisp corners on the areas circled in the below photo?

Is that a multi-pass thing?

(I work off of an SD card, if that matters)

The bit won’t fit in there. You need a smaller diameter bit. Or, if you use a V bit, it can get very small by making a shallower cut. Just beware that any error in Z will make an error in width. Sharper bits are less prone to width errors.

Here is a video (from an old version) that shows the concept:

If you just want the corner cut out to provide clearance for mating parts, you can choose to overcut that corner. I don’t know what it will do for the A, but it might look ok for the E.

I have been thinking a lot about how to carve out letters in outline and also how to do carve out hte insides of letters. It is a definite are to make something look good. Font choice and size definitely requires a matching bit. The key word is radius. Radius of the cutter and radius of the inside hole. The radius of an inner hole or inside corner needs to be equal or greater than the radius of the bit you are using. Text just isn’t easy to make without a very roundish looking font. or going gigantic. Hence the vbits.

You could sharpen the inside corners uing a 1mm bit as a finish pass. Not sure who practical that would be with tool changes and not breaking a bit.

So, when I select a different bit as a “Finishing tool”, does the machine pause at the end and return to the origin to allow me to switch to a different bit? How would I continue the gcode if that’s the case? Mind you, I’m not directly hooked up to the CNC with my PC - I run everything off the SD card.

I’m ok with running a “finishing pass” with a finer bit, but does that mean just creating a separate gcode?

Also, when I try and setup a path on Estlcam for the V-bit, no matter the depth, it shows the entire width of the tool on the preview. Is this normal? (see below image)

This is it’s own ball of wax. There is a configuration for what gcode to use during a tool change. The magic is the M0 command, which will pause the machine until you click on the LCD. So the gcode ends up being something like:

Lift the Z
somehow adjust the Z to the top plane
Set Z=0
go back to clearance plane

I haven’t done that myself.

You need to use the carve operation, not the part or tool operation.

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As Jeffe mentioned, you would put a tool change script in your EstlCAM setup to assist in pausing, making the router move to a location easier to manage, change the tool, then go back, using the LCD knob.

I had a good routine all written up, but it got wiped. As soon as I recreate it, I’ll share it. There are some other examples on the forum of tool change routines. You’ll find the startup, finish, and tool change code in the Setup > CNC Programs > Texts tab. There is where you can put in code for startup, which I have to get my vac and router started up with the IOT relay I use.

Until you get that routine figured out, a separate gcode file can be used, as long as you keep your original work offsets and your mount hasn’t wandered any in the first operation.

V-bits are a good study in geometry. Depth of carve and and width of text are related to degree of angle and width of tool. I have found that a 3mm 45 degree vbit does well with text that are an inch and a half high or less width no part of the letter getting thicker than 3mm.

I have yet to try and carve a pocket with a flat botton and have the sides finished with a vbit. Still figuring that one out.

I returned to ask a question, and I think this is more or less what my option is.

I carved around all the lettering and details with the V-bit carve function, but it didn’t carve any of the negative space as a pocket, like the video above suggests. Am I missing a setting or something? Otherwise, everything looks crisp, including the tight corners.

I think I’ll tell it to go through and finish the negative pocket space with a 1/8" on a separate gcode, but in the little time I played with it, it was returning an error stating that the 1/8" couldn’t fit in some areas between the lettering.

Leaving out of town tomorrow for a week, so I have some things to think about while I’m away.

Certainly appreciate the help and advice thus far.

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IIRC, there is a pocketing tool setting in the carve settings. If you use that, it should cut out the inside.

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There is a setting in the carve tool that will help you with the negative space problem.

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I think that is the issue, not being confident and set up to do tool changes yet easily in one set of gcode. That’s my conundrum. Had it ready to go and did one test and then I lost the code in the tool change text that I felt comfortable with and understood. Also get a display message and then getting the knob clicks right on the LCD. It does take some clear steps.

BTW, If you’re having trouble telling what it will do, it would be interesting to try grblgru to simulate the results.

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