Thread: CNC blind inside corner solution

The difficulty I've found is lining things up perfectly when going to the CNC bed after another cutting operation. I usually end up +/- 0.5mm since there is error in placement of the original holes on a drill press (or doing another operation) and also in zeroing the CNC. You may have a higher quality CNC that is super accurate, however there's always a risk of misalignment.

Why not just clean up the corners with a sharp chisel? Are you making a thousand of these? If so I'm curious about the application that requires such complex joinery!

Edited - misunderstood original description.

As above, post processing seems necessary for your desired outcome. Inside corners are a known issue with cnc bits. Laser cutting works for thin materials as bit effective diameter approaches zero.

Originally Posted by PFH

The difficulty I've found is lining things up perfectly when going to the CNC bed after another cutting operation. I usually end up +/- 0.5mm since there is error in placement of the original holes on a drill press (or doing another operation) and also in zeroing the CNC. You may have a higher quality CNC that is super accurate, however there's always a risk of misalignment.

^ This

Why not just clean up the corners with a sharp chisel? Are you making a thousand of these? If so I'm curious about the application that requires such complex joinery!

^ Also this

It's gonna be a hell of a lot faster just chiselling the corners square than stuffing around trying to align things exactly where they need to be.

The sharpest corner that a CNC can cut is defined by the radius of the cutter.

One neat solution is to fillet the edges of the tenons to the same radius.

[Remember, the edges of a Domino floating tenon are not square.]

Originally Posted by PFH

The difficulty I've found is lining things up perfectly when going to the CNC bed after another cutting operation. I usually end up +/- 0.5mm since there is error in placement of the original holes on a drill press (or doing another operation) and also in zeroing the CNC. You may have a higher quality CNC that is super accurate, however there's always a risk of misalignment.

Why not just clean up the corners with a sharp chisel? Are you making a thousand of these? If so I'm curious about the application that requires such complex joinery!

Edited - misunderstood original description.

Thanks for the good advise, I don't have a production line so I agree a chisel would be quicker. I am considering building a Kronos krmx02 cnc machine and was just trying to figure out the limitations so my expectations aren't too high. I was trying trying to figure out if it is possible to cnc something complicated like crazy Japanese joinery.

You could use one of these corner chisels.

Could you turn job sideways so the corners are the bottom of the bit not the edge? I suppose that just moves the problem

Originally Posted by lyricnz

Could you turn job sideways so the corners are the bottom of the bit not the edge? I suppose that just moves the problem

Usually on a CNC Table you are cutting sheet material.

A very small percentage of CNC's have a facility for holding stock vertical, but these are quite rare.

If you have a mortise machine clean the corners up after being in the CNC, just clamp a sacrificial piece of scrap timber in the corner to give the chisel something to bit into.

Have just had a series of phone calls with a friend who was having difficulties conceptualising the CNC issues, so I drew some diagrams to assist.

CNC's are very good at cutting joints with rounded corners such as jig saw joints, they cut exterior square corners very accurately, but with interior corners the corner cannot be sharper than the radius of the cutter. You can make them sharper by going to a smaller radius bit but then it wears faster and is much more fragile. Repeated breakages are a PIA.

I just used a simple mortice and tenon joint for illustrative purposes, and will divide this post into three parts:

1. The problem,
2. The traditional solution, and
3. My hybrid solution.

Hand Cut Mortice & Tenon

This favoured joint has been around since Egyptian times and still works brilliantly:

M&T 1.jpg


CNC Non-Solution

If we simply try to replicate these shapes on a CNC we strike problems:

• Exterior corners are square and therefore fine,
• Interior corners are rounded to radius of cutter.

M&T 2.jpg

This "joint" cannot be assembled because the square corners of the tenon overlap the rounded corners of the mortice, and
the rounded "shoulders" at the base of the tenon.

The traditional solution to this problem is to cut the radius of the cutter past each interior corner. This is a quick and dirty solution, a cheap solution, at the expense of a little strength and a lot of aesthetics.

M&T 3.jpg

In the drawing I have deliberately made the corner excesses large for illustrative purposes. The four corners as numbered:

1. Rounded internal corner cannot be assembled with square tenon,
2. Tool path extended vertically by half a diameter,
3. Tool path extended laterally by half a diameter,
4. Tool path extended diagonally.

You also have to similarly extend the tool path on the tenon to remove the shoulder at its base.

In the assembled joint, the tool path extensions are clearly visible. Tool path 4 is probably the least offensive.

Besides the aesthetic issues, there is some weakening of the joint because of the material removed and the reduction in gluing surface area.

This is the solution that I use. Note I have access to a large commercial CNC that reliably cuts to and accuracy of less than 0.1mm; this is high precision work.

First: I cut the basic shapes on the CNC:

M&T 4.jpg Square tenons and rounded mortice!

Second: I round over the edges of the tenon. This can be done with a chisel, gouge, file or router. Best results are with a trim router and a round over bit with bearing of the same radius as the CNC cutter used to cut the mortice.
Either, the shoulder at the base of the tenon must also be squared or the short edges of the mortice rounded over. The latter is easiest using the trim router.

M&T 5.jpg M&T 7.jpg

This joint assembles with very tight jointing.


Another drawing of the assembled joint.

M&T 6.jpg

Nice idea, rather jealous of your access to a big machine.

Thanks Graeme,

I think M&T 5.jpg looks like a good solution.