How to set a holding compass

How to install a hidden lid support hinge

One of the problems with the trunk doors opening upward, and also with my display case with the window at the top, is that the door does not stay open on its own unless it is turned beyond the vertical, toward the back of the cabinet.

The thing is that in my display case, if I turn the door beyond the vertical, the hinges will be forced, and surely the wood of the window will touch the wood of the structure and leave marks. Installing a retaining compass to hold the window open will solve the problem for me, and to make it look more elegant it will be a compass that is embedded in the wood when the display case is closed.

How to install a retaining lid support hinge:

How to measure the position of the compass hinge

The first time we install a retaining compass we usually try to position it so that it rotates the same way as the door or window. But with a compass as simple as the one I am going to install, if I want to open the door to 90 degrees, the union of the two pieces of the compass would bump into the back of the cabinet.

The correct way is to set the retaining compass so that it rotates opposite to the showcase window. And since the two pieces of the compass measure the same, the distance from the axis of rotation of the window to the position for the clamping screw in the window will be the same as the distance to the clamping screw in the display case structure. So the window will open to 90 degrees if I put those screws in the correct position. All I have to do is calculate that separation.

In the video we see that I do not take into account that, as the screws go below the surface where I measure, instead of the 13cm I should use a smaller measurement to make the calculation. Calculating with a diagonal of 10cm I get a separation of 7cm which I think would have given me a good result when milling the grooves and positioning the compass inside them. In any case, even if the slots are short, it is very easy to readjust the milling machine to make them a little longer once they have been milled.

Change milling cutter bearings

A trick to be able to mill a slightly deeper groove is to change the bearing of the grooving cutter. This is very easy to do if you have the right size Allen wrench. In this case I put two smaller bearings so that they would have more bearing surface and thus leave less marks when pressed against the wood.

Milling a groove in wood

To embed the retaining compass I mill a groove in the window and another in the structure. Using the milling machine and a slotting cutter all that is complicated is measuring so that the slots are in the position I want, and one right on top of the other. And in this case I have to take into account that the window protrudes 6mm to the sides.

Put the retaining compass in its correct position.

To hold the compass, but allow it to rotate, I use a pair of screws that pass loosely into the compass holes. I have to put the compass hinge with one end in each slot, so that both are at the same distance from the axis of rotation of the window, and mark the position where to put the first screw.

Check how the holding compass works

After marking where to drill the pilot hole for the first screw, I close the window and mark the position for the second screw making sure that the compass will be horizontal, and therefore will fit into the slots and allow the display case door to close fully. The screws I use are brass, with a more decorative head than normal screws.

Drill pilot holes for screws

I can now drill pilot holes, slightly finer than the screw threads. And then I drill 4 or 5 millimeters with a slightly thicker drill bit, to accommodate the smooth area of the screws without problems.

When I place the retaining compass it is too loose, and when I close the window it does not fit straight into the slots. But with a washer on the inside of each slot I managed to keep it aligned with the slots while closing.

Putting a screw in the mortise hinges

Although before placing the brass screws I had to fasten the hinges with a screw (they have a notch prepared for this), since with so much removing and putting and manipulating the window of the showcase, they came out too easily from the holes in which they are embedded.

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