Making small router jigs

[Giff]

Can anyone tell me an accurate method for making a small router jig for this keeper. It is 80mm x 15mm and I was hoping to use a 13.8 insert ring with a 10mm cutter. I have done this in the past with oblong / share sections by first routing a template + the difference of the ring but don't know how to do this properly with the radius edges...has anyone got a method for doing this ? Geoff

 

[sunnybob]

trace it. cut it out of 18mm mdf with a bandsaw or hand tools. Hand smooth it to the exact shape you want. Then fix the template to work piece with double sided tape and rout round it with a bearing guided cutter.

 

[Giff]

Wouldn't it be then + the thickness of the bearing cutter ?

 

[nev]

Wouldn't it be then + the thickness of the bearing cutter ?

Nope. The 'hole' in the template would be + thickness, but when you use that to guide cutter you are back to original size

 

[Giff]

Still not got it ? sorry for being a bit dim......so I cut an 18mm s/s to the part.Then cut round that with say a 12.7mm flush bearing cutter ? Is then this the right size to use as a template ?
Sorry if it"s a long query to a simple problem !

 

[nev]

Sorry, senior moment here #-o

You copy your shape by tracing around it onto 12mm mdf. cut it out with jigsaw or whatever and sand to finish. stick this to your new stock and use a bearing guided cutter to trim to size.

Have a look at this video from about 7.50 minutes on.
https://www.youtube.com/watch?v=C45Fff_F3f4

 

[Giff]

I don't think my request was very clear....I am trying to make a female template as the drawings. Not a flush trimmed piece from a template. It would need to be sized up from the drawing ( exact size of piece) to fit the guide bush 13.8mm with a 10mm cutter. How do I enlarge the template hole by the exact amount ?

 

[MattRoberts]

You can find a washer with the same ring diameter as your guide bush, or make one from wood. You then place a pencil tip inside the washer, and trace around the piece. That will draw a perimeter around the piece the exact size of the guide bush. Cut it out, and you have your template

 

[sunnybob]

I would do it on a table mounted router.
That way the template is the exact replica of the original.
If doing it with a hand held, then it would be best to make an internal jig as per the drawings above, but then you would need the width of the original plus the diameter of the cutter to get the correct sized hole.

 

[RobinBHM]

I would think you hole needs to be 83.3 x 18.3

You need to add 1.9mm to the radius of the ends

If the radius ends of the keeps is 15mm then the radius will be 16.9mm on the guide -Ive assumed the strike point for the radius is the base line of the keep.

 

[nev]

I'm pants at maths so what I would do is Place bush and bit against an edge and measure from edge of work to edge of cutter? take that dimension and enlarge your sketchup shape by same amount using the offset tool. print it out at 100% stick it to mdf and cut it out with a jigsaw etc.

 

[Giff]

Ditto on the Maths Nev. I know that's the way to do it but I can't even see .3mm never mind cut to it! Using the bush and cutter as a guide for drawing sounds a good idea. Thanks. Geoff

 

[Eric The Viking]

As I understand it, you want to make a "negative" of your original keeper - a hole exactly the same size as its shape.

It needs a pair of guide bushes and a pair of cutters of different sizes. I remember Ron Fox talking about this in years past (you might still find his videos on the Wealden site). There are limitations, which I'll get to below, but anyway, roughly, it's this:

To calculate the combinations you need, there is a very simple formula. Use either the diameters or the radii of the cutters, but be consistent (always radii or always diameters).

Bush2 = Bush1 + Cutter1 + Cutter2

The first pass, running round the outside of your original piece, uses Bush1 and Cutter1. The second pass, round the inside of your template, uses Bush2 and Cutter2. Obviously, the bushes will always be bigger than the cutters (as the cutters fit inside them).

The trick is to make that work with cutters and bushes that you actually have, and you'll probably need a complete set of bushes for that reason!

There is another issue with this though: accuracy.

Imagine you're cutting a circular hole, using a hollow circular template. You run the guide bush round the circle, making a smaller hole inside. Any mistakes can only happen into the waste material in the middle. If you twitch (and I do!), you make a bump in the circumference of your wanted hole, which you can remove by making a cleanup pass with the router.

If you're cutting round the outside of a shape, the same thing applies - mistakes go into the waste material.

But making a template this way is VERY difficult: mistakes will go into the final, wanted surface. You might minimise this by using a slightly smaller Cutter1 first (followed by the correct Cutter1) so that you are taking off a small amount of material to get to the finished shape, but any mistake will still translate finally into a bump in your wanted negative shape.

The other issue is the radii of your curved keeper: in order to follow that curve properly, Cutter2 must be smaller than the curves you want to follow. If it's bigger you will get the radius of Cutter2, not the curve. And in any case, you'll get the Cutter2 radius where you have square corners (so you'll have to square those off by hand).

Hope that helps.

E.

By the way, what is a "keeper" in this context? I don't recognise the thing from the picture

 

[Giff]

Thanks Eric
I must say that in the original post I was hoping for a solution just like that, which avoids measuring and using the "keeper" in this instance, as an initial template.
I will search Wealden's site as well, for Ron's video....he always made things look so simple !
The "keeper" is the back of a mushroom holder (keeper?) for a casement espagnolette shoot bolt.
Geoff