Squaring Up Inside Corners of Panel Routs

Advice on machining square inside corners for cut-outs in pre-laminated panels. November 13, 2009

We are running a lot of sheets of MDF pre-laminated with HPL and need to square up the inside corners. A lot of the panels are faces with doors or drawers cut out of them. We use both 1/2" and 1/4" compression spirals, but need an easy way to get rid of that inside radius. Anyone have a better idea than a square file and a lot of time?

Forum Responses
(CNC Forum)
From contributor D:
If you use a smaller bit, you'll have less to clean out. Going down to 1/16" bit, leaving a 1/32" radius, you may not want to file out any further.

From contributor M:
A corner coping aggregate could possibly work.

From contributor H:
Can you explain how that works?

From contributor C:
You need to leave a radius on the inside or the laminate will crack at the point of the square inside corner.

From contributor G:
Well, and sorry if this is obvious, you can fit and circle 45 degrees off the corner, then trim your lines meeting the corner to the remaining arc segment. It ends up about .040 off the line and max length off the line of .18" when using a 1/4" bit. Attached is a picture... Not exact - I used coreldraw.

From contributor J:
If using a CNC, all you need do is program the machine to do a Z axis movement along a 45 degree angle from the inside corner. As the V bit raises, it will create the square corner you are looking for and also relieve the stress on the HPL in the corner, as the tip usually has a .25mm radius on it.

From contributor B:
Please clarify. Are you explaining the use of a V-bit mounted horizontally or vertically? Also, it seems to me that both the V-bit or the corner coping aggregate will have chipping issues.

From the original questioner:
A clean, square, chip-free inside corner is what we are looking for. The sheets we are cutting are pre-laminated, cut with either a 1/2" or 1/4" compression. However, we have to get in there and laminate the inside edges afterward, so we've got to have square corners. Seems like the 45 conic would work nicely if we were running a 5-axis and you could control the laminate tearing out. I would think this is a common problem for anyone running a 3-axis like us.

From contributor D:
The move contributor J talked about isn't 5 axis. You simply take a 45 degree v bit and ramp up 45 degree in z at same rate as moving toward the corner. Probably leaves the same point as hitting each corner with a 1/16" bit on the surface, but with a 1/16" bit you can penetrate the whole thickness of your stock and leave maybe a swipe or two with a file for a perfectly sharp corner.

From contributor J:
It sounds like what you are looking for is a mortising attachment for the router to plunge the corner and square it up at the same time. This can be done, but still requires some hand work on the bottom to make sure it turns out. Also, yes, the previous comment I made was for an angled corner if you were to paint of vacuum form the door face.

From contributor Q:
Read contributor C's post. I just got done with a very expensive redo because of leaving square corners on already laid up p-lam. It cost me thousands to fix. Do not take to square - you will regret it one day when you get a phone call.