Help with Making a Custom Router Template

[JAW911]

I have a number of flush mounted wall-mounting brackets to install on wooden boards. I want to create a template for my plunge router so I can accurately inlay them flush as you would a hinge for example. I understand that the MDF router template hole needs to be larger than the bracket to allow for the offset between the collar and the 3.2mm spiral bit (the brackets are only 44mm x 16mm x 4mm and rounded at each end). What I want to know is how do I draw the bracket shape exactly, say, 5mm larger than the bracket all round so I can cut it out of MDF with a jeweller's saw (it will be a complete hole so cannot use my bandsaw!) - freehand will not be accurate enough. I expect it is a really simple exercise to do but I can't see how to do it accurately!

 

[dzj]

Take a strip of mdf. Cut it into 4 pieces, so that when glued up, they form your required shape.
Gluing this to a thin ply substrate (with a larger hole than your jig) helps to keep all the bits together.

 

[JAW911]

I don’t see how this explains how to make the shape to the required size.

 

[Sgian Dubh]

Let's say for discussion's sake you have a guide bush (collar? in your description) with an outside diameter of 18 mm, and a 3.2 mm diameter cutter.
Calculate (18-3.2)/2 = 7.4 mm. 7.4 mm represents the required offset. In this case to enable routing 44 X 16 mm recesses, make a rectangular hole in a piece of MDF or similar that is 58.8 mm long X 30.8 mm wide, or 59 X 31 mm (as near as damn it) to make a template.

You could also manufacture the required template hole out of four pieces of MDF, or similar, by cutting two bits ~59 mm long, spacing them ~31 mm apart, and enclose the open ends with another couple of bits of MDF all four bits held in the right orientation with masking tape: for this latter method you'd glue the four bits to a (thin) base board. Once this rig is made, set up your router with the guide bush and your 3.2 mm cutter, and plunge through the thin base to reveal the cutout dimension. You now have a usable template. Slainte.

 

[Nelsun]

I think the idea is to make the template the exact size of your bracket and then use a top mounted bearing bit to ride against the edge. That's rather than making a template for a guide bush and working out required offsets.

Taking 4 lengths of MDF (each longer than the edge of the bracket they'd go against, wide enough to give your router base enough to ride on and allow you a means to keep it in place) and glueing them around your bracket would give you your template in lightening time. But you'll be left with a fragileish template.

I'd go for drawing your target hole on a piece of mdf, cutting it out with whatever you have (e.g. jigsaw) leaving a hair inside your lines and then finesse it to fit. You'll then have a one piece template with no joins to snag the router base.

If you do go with a guided bit you need to make sure your template is thick enough to give the bearing something to ride against without making the recess too deep if that makes sense. A 4mm recess and a 6mm thick template would mean you'd need, allowing 4mm for the bearing to ride against, a bit with a max cutter height of 6mm... not that easy to find. A 12mm or 18mm template would let you use a taller cutter.

Or you can go with a guide bush and template with offsets to allow for the distance between outside edges of the bushing and cutter. It's a more complicated method but would let you use just about any straight bit you have to hand.

 

[JAW911]

Thank you very much - that all makes sense now.

 

[xy mosian]

To find the offset of guide collar to cutter. Un plug the router, if it was plugged in, mount the cutter and guide in the router and measure the distance, radially, from the outside of the guide to the cutter. Make sure the cutter is rotated so that the measurement is to a cutting edge.

Draw around the mounting plate of the bracket on the material of the template. Cut out this shape, as a hole with fair degree of accurracy. Again with unpowered router, place the cutter in the hole with the guide resting on the surface of the template. Move the router around the hole, with the cutter touching the hole edge, and use the outside of the guide, and a pencil, to draw the required template size. Cut out the template. This method does require the offset to be known.

xy.