# How does a worktop jig work?

I got myself a worktop jig like this one:

It is normally used to make 90 degrees joint for kitchen worktops, with what is called "hockey stick" cut:

The jig instructions call for a 30 mm template bushing, and a 12.7 mm x 50 mm, or 1/2" x 2", router straight bit.

Now, I understand that the 30 mm template bushing will keep the bit center along the center of the groove - what I fail to understand is why should a 12.7 mm bit work, while any other bit doesn't work.

For what I understand, when I use this jig to make cuts, when doing the "female" cut you will be discarding what is "south" of the groove, in the picture above; when doing the "male" cut you will be discarding what is "north" of the groove.

The two edges will then necessarily have two different radii where the groove bends, so just like you cannot fit the two edges of the jig groove together, you will never be able to fit the two edges of your joint together; unless you have a 0 mm diameter bit, of course.

Now, my understanding is clearly flawed since lots of people use a jig just like the above everyday. What is it that makes this 12.7 mm so special for this jig? How is it possible that the two edges will fit together, since they are offset in opposite direction with respect to the groove center?

• Too early in the morning for me to do maths, but I'd suggest that your life would be easier if you just took this on faith. (It really does work!) Plus, long router bits in anything other than 12.7mm are hard to come by. May 29, 2021 at 17:33
• That depends on where you live :) this side of the pond, I can get many metric bits, such as a 6x50 as an example. I have found a 12.7x50 but I need a router for that because mine is metric, so while I surely trust it will work, before doing the leap I want to understand HOW :) May 29, 2021 at 17:37
• Oh, and if your router is tiny (ie, doesn't go bigger than an 8mm collet), then it's almost certainly underpowered for worktops. Here, it would be a 1.5hp minimum; in the UK, I'd aim for about 2000 watts; don't know about your locale. May 29, 2021 at 21:49
• Hi Vladimir, welcome to Woodworking. Bear in mind the expected end user for these, basically it's intended to be pro-only and virtually all pros them will have at least one 1/2" router. Now if you don't have a 1/2" router then you don't have one, but as long as your router has enough grunt and you can get a suitable bit — it's the cutting end that's most important here, not the shaft! — you should be able to make this work. May 30, 2021 at 5:47
• In terms of the geometry it's really very simple. You know when you route a 90° corner you get a different inside and outside radius, right? That's all this type of jig is compensating for, so you end up with the same radius on the male and female parts (which are cut in two separate operations). Anyway that aside, since you can get 1/2" / 12.7mm straight-cutting bits on smaller shanks a router with a 1/2" collet is not a requirement for occasional or one-off use. And in case you don't know, even with a 1/2" router these cuts aren't done in a single pass! May 30, 2021 at 15:32