Thread: Negative template from Positive template help

In the absence of any special tools the easiest way to do this is with your top bearing 'pattern bit'. Make a frame that fits tightly around the glass out of material of a suitable thickness remembering that you will probably need to make several passes in the bench top to get to the right depth. Remember also that you will need to provide sufficient support for the router base to remain upright and well balanced. You can then attach this frame to the bench top using double-sided tape or clamps and simply rout out the recess. You probably won't find a 1/2" shank pattern bit with a cutting length less than 25mm, but there are some larger diameter 1/4" shanked pattern bits available with cutting lengths down to 12.7mm.

You can make a template using a router template guide set, but the above method works just as well.

Cheers,
David

Produce a template 20mm greater than the insert then rout with the 30mm Guide and 10mm cutter

If you used a pattern bit that had a bearing the same diameter as the cutting diameter, then running it around the outside of the glass would produce a female template to use with the same bit (I think).

Originally Posted by chrisb691

If you used a pattern bit that had a bearing the same diameter as the cutting diameter, then running it around the outside of the glass would produce a female template to use with the same bit (I think).

I don't think so, a template created that way would be wider than the glass, and using the same bit directly with the template will just copy the template and the hole in the countertop would be too large.

This technique would work if you swap to an oversize bearing when cutting the countertop so the router bit is offset from the template by its own cutting width.

You could use the same technique that I used when I cut the hole for my router insert plate. I made a template from 6mm MDF by using double-stick tape to attach the template material to a backing board, then taping the insert plate to the template. I then routed around the outside of the insert plate using a 1/4" straight bit with a 3/8" OD template guide - this produces a negative which is wider by 5/16" on each side of the original positive. Then switch to a 3/8" straight bit and a 1" template guide when using the template to cut a hole which will exactly match the original positive. Using this method, I cut a hole in my router table top which perfectly fits the insert. To use this technique you need to make sure that your router spindle is perfectly aligned to the centre of the template guide (e.g. by using a centering pin)

I believe a router inlay kit will do the job, and you can then follow the instructions here:
http://www.routerworkshop.com/revinlay.html

Basically it uses your existing object (piece of glass), to make a bigger negative template (due to the guide).
When you then attach the bushing to the guide, it then moves the bit back "in" by the width of the bit (in the case on that website, both the bushing and the bit were 1/4"), and hence the recess is the same size as your original object.

The only "missing" step in this particular tutorial is that the thickness of the original (glass) must be more than the height of the guide. I suspect 6mm is plenty. If the original object is thin (such as the one I hope to work with), then you need to make a thicker "positive" to start with.

(Warning: I have no experience with this at all - I am just about to do the same thing (and will be asking a question in another thread about where I might find a sub-base that can accept the inlay kit)....)

For the original poster, you say you are getting a piece of tempered glass. Is this something with straight sides or something with curved sides?

If it is straight sides, you can position the glass exactly where you want it on the top surface and totally surround it with strips of material attached with good double sided tape. You can then remove the glass from the centre and the surround forms the template to route around for the cavity, provided that you use a pattern bit for the job. (A pattern bit is a bit with a same size bearing at the shank, a flush trim bit is similar except the bearing is at the tip.)

If the glass is similar in size to the base of the router base you may get away with only the surround, for larger glass, you might want to place a piece the same thickness as the surround inside leaving about 50mm exposed. This ensures that the router has a level surface and will not tilt and risk damaging the cut. The internal part can be removed once you have cut the border, to allow you to route the centre to complete the job.

One issue you will have with the task is getting straight corners if you need them, as a router bit will leave the corners with the bit radius for internal corners. If you need to have tight corners, you invariably have to hand finish the last bit with a chisel. With this regard, a large bit will shift material faster but leave a larger corner radius to clean up manually. One solution is to do the job with a medium/large then rework the corners with a smaller pattern bit while the template is still in place. This will reduce the corner radius from say 10mm to 3mm quite quickly, leaving only a small amount of manual work to sharpen the corners.

Of course, if the insert is curved, a straight strip template won't work.