Machining Hinge Mortises in Veneered Doors with CNC
We are primarily a cabinet shop, with occasional runs of around 50 doors. How do I keep the veneer from blowing out, short of stopping the program, manually clamping a backer board to the top of the door or scoring the hinge points?
Perhaps a more suitable solution for the machining of your door hinges would be the use of a right angle head (aggregate) in your spindle. In order to accommodate the bevel which you refer to, this type of aggregate head is available with a fixed angle of either 87 degrees or 93 degrees. This would allow you to reference the door either way. Another advantage to the idea of horizontal machining is that you would end up with square edges in your hinge pocket. Using the vertical tool you are currently working with would result in rounded edges which would then need to be machined by hand.
With only a right hand bit in an aggregate you can still get some chip out on one edge of the mortise, which can be minimized with a very sharp bit and slow entry speed. We machine many doors using a double outlet aggregate with right hand bit in one end and left hand bit in the other end. This works very well with absolutely no chip out.
If you can cut the door to size afterwards, it helps.
I work for a joinery company in the UK. We process a fair number of veneered panel and frame style doors. I wrote a parametric door program about two years ago designed to machine doors on pods on Reichenbacher 5-axis/Siemens 840D controllers. To get round the lack of backup on the faces of the doors, we use the following procedure. Our trick is in the first machining pass. I presume as you talk about the door blowing out on the top face you are already using a horizontal spindle of one form or another.
Looking at your standard 'U' shaped hinge cutout (opening upwards through a veneered face)… Use a standard rh cutter (we use a 12mm single flute RT cutter for consistent size), start your cut 2 or 3 mm above the LH side of hinge cutout, machine down into the door at hinge depth at a 45 degree angle, cutting across the LH hinge edge by about 0.5mm in both X and Z (cutting on the LH side of the cut, back cutting). Continue down into the door till you are around 5mm (cutter not past its centre point) in, then cut along the x-axis towards the RH edge of the hinge, continuing the backcut (avoids break out) until the edge of the cutter overcuts full hinge length by about 0.25mm. You should get a chip-free top edge using this method. Now use a standard hinge relief program making 'U' shaped cuts starting at top left of the pass at the mid point of the hinge, work outwards to clear the hinge.
I have applied this in doors with vertical and horizontal grained veneers with no problems. When cutting laminate face doors, the 45 degree initial angle needs to be knocked back to about 30 degrees.
Note you will get a slight overcut with this method. This is hidden under the hinge knuckle, however, and has never caused us a problem.
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