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I got myself a worktop jig like this one:

Worktop jig

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

Types of cuts

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?

Vladimir Cravero
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  • 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. – Aloysius Defenestrate May 29 '21 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 :) – Vladimir Cravero May 29 '21 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. – Aloysius Defenestrate May 29 '21 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. – Graphus May 30 '21 at 05:47
  • Regardless of what you have now in terms of straight bits, why not try some test cuts on scrap (scrap anything, not worktop material) and see how the pieces fit together? Seeing the results firsthand will give you direct insight into how the angles work out in practice with the different radii of cutter. – Graphus May 30 '21 at 05:51
  • Hello @Graphus, thanks for your insights. I am more than willing to buy a half inch router, I have found a cheap model that fits 6, 8, 12 mm and quarter and half inch bits. I am definitely going to make tests with the half inch cutter, but I still would like to understand how it is possible that the two edges will fit. I understand that from ww perspective the answer might just be “get the proper tool for the job”… Perhaps I should try a different stack like math, for example. But I will try with different cutters, that is good advice. Thanks! – Vladimir Cravero May 30 '21 at 07:02
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    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! – Graphus May 30 '21 at 15:32
  • How about using a copy bit aka flush trim bit (a straight bit with a bearing) for this template? I also don't have a 1/2 in bit or a bushing, but I'm thinking to use a flush trim bit with such a template to get the job done. Has anyone tried that before? – Tudor Jan 29 '24 at 06:15
  • @Tudor it is better to ask for a new question, this website is not a forum. nonetheless, you cannot use a flush trim bit, the parts will not match – Vladimir Cravero Jan 30 '24 at 09:17
  • Here is a quick walk through : https://www.axminstertools.com/eu/ideas-advice/ujk-technology-worktop-jig/ – Zahid Asif Feb 26 '24 at 05:38

2 Answers2

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Here is a diagram I made to show how the jig works. The top part shows the offsets between edge of bushing and edge of cutting bit as the centerline. The bottom diagram shows how those two sections are then separated to account for the thickness of the cutter and then shifted over with the dimensions.

Obviously the dimensions of the bend radius have just been selected at random but these can be anything you want. The main focus is that in the bottom portion the radii of the green and orange sections have different centerpoints.

Made a couple edits to the image as I had some things slightly wrong but it is correct now.

Jig diagram

Marmiketin
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Mt attempt hope this helps someone! Maybe get a cnc out of some laminate or composite panel!

enter image description here

Neil A
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