Fret slotting runout

  Good afternoon everyone,

In May I received my Shapeoko 3 XXL, and I absolutely love the thing. It is everything that I’ve ever wanted in a CNC. Not too big, but big enough.

I have built a lot of things so far. But I’m in the middle of a Les Paul build and I wanted to do the fretboard on the CNC and I’m having a horrible time with the fret slots. I have a bit that is .023 and when I go to run the code for the slots it is always to big, so big that the frets are loose in the cut slots. I’m using Fusion360 for the CAD/CAM. Carbide 3D has already send me another router and I have the precision collets.

My question is, has anyone had this issue when cutting fret slots. And if so, how did they fix it?

I’ve been dealing with this for months, and I would really like to finish this project.

Thanks in advance.


What is the nominal fret slot width? If you target is 0.023, you will not get that with a 0.023 cutter. If your target is 0.028, you might.

The smallest endmill I have is 0.015" (0.3mm, actually), and it will cut to a depth of about 0.75mm (1/32"). It is one I do not use unless I must. Look at it wrong, it will implode. But when needed, it is the bee’s knees (and several other body parts of assorted pollinating insects). But slow. I have used it for 0.5mm (0.020") slots within 0.02mm


Maybe try negative stock to leave and sneak up on perfection?


Thanks for the replies. To answer enl_public the width of the slot needs to be .023. So you’re saying go a couple sizes smaller with the end mill? It will offset the runout of the machine. Ok, got you. That makes sense.

And griff, I’m relatively new to fusion when using cam, can you explain negative stock?

Thanks again for the help.

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The slot is ALWAYS wider then the tool. How much wider depends on a lot of things, including machine rigidity, spindle runout, tool runout, tool deflection, and, if multiple passes are used to get to depth, you may get a mild taper to the bottom, as the tool will take a little more at the top during successive passes. As Griff said, leave a little stock then tune to size, but the tool must be smaller than the desired width. It will be slow with this small of a tool.

My go to for his kind of thing is use the adaptive strategy if the slot is enough wider then the tool, which controls tool engagement, and select “stock to leave”. If your desired slot is 0.023, and you are using a 0.015 tool, and enter 0.002 for the radial stock to leave. In this case, you may not be able to use the adaptive, but you should still be able to select stock to leave.

Another option is to use a pointed tool (engraving tool) or a small tip vee tool to give a starting/locating groove, and finish by hand using the appropriate fretting saw.

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There is a cam option in fusion to have it follow the center of a slot with your bit. I’m on mobile and can’t think of what it is now. I used it for fret slots with a .024 bit without issue.

As mentioned above, your endmill needs to be smaller then the width of your fret slot. In the “Passes” tab click on the “Stock to leave” box and select a value for axial and radial, say 0.1mm. This will result in an undercut slot.


Thanks everyone. Much needed help. I have ordered in smaller bits, so we will see.

Now the next question is what order do I do the cuts. Radius, fret slots, inlay cutout, and then contour cutout?
One of my problems through this process is how to maintain the correct zeros into the next steps. I have run a lot of practice cuts and sometimes the fret slot job will not even cut the slot the first couple passes.

Thanks again for the help.

Could you elaborate? How are you setting zero? What do you mean will not cut the slot the first couple cuts? How many passes? What step down cut-to-cut?

As to order, the fret slots should be last, as they are the highest precision requirement, and use the smallest tool. You want to remove the smallest amount of material you can.

Sorry for the late reply. But I decided to go with a fret slotting jig from LMI, to take care of the fret slots.

But my process is that I secure the material, put the Carbide3D probe on the same reference point of my model in Fusion and run XYZ zero. I have heard some people us the bottom of the material to reference, but i use the top of the material not the model as mine. I do not know if that is correct or not.

But in some instances the first couple of cuts, not only dealing with cutting these fret slots but anything, do not touch the material.

Thanks again for all of the help. 


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On the “Stock” tab, check that there is no additional stock.

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Ok i checked the and there isnt any additonal stock. I do have everything else figured out for the slots, i ran a test piece tonight and everything was beautiful. And then the last three fret markers had these weird machine marks. Any ideas?

Can you share the Fusion file?

Yep, not a problem. Thanks for the help. And if you see anything else wrong with it, please let me know.

FretBoard 0625 v10.dxf (1.9 MB)

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