A 'woodie' Christmas puzzle

[RogerS]

Imagine you want to have a top made from two types of wood (say, ash and oak). The ash is a semi-circle and the oak will go round that semi-circle as a border. You don't have any pieces of wood large enough to make each part out of one piece and so decide you need to make some templates from ply and then glue together enough pieces of your ash and oak and use a bearing guided trimmer and your templates.

So you rout out the template for the border thus...


and that will give you two good edges (inner and outer) as a guide for your bearing guided trimmer to make the oak border.

But what about making the template for the ash semi-circle ? Take that other piece..the semi-circle left in the ply after making the border template ?

Well, sounds like a plan...we have a nice snug fit...


....only....as we move round the circumference we start to have a problem

and when we get right round to each side....oh dear ! The other side is the same.


How would you make the template for the ash semi-circle ?

Answers in a PM please....

And a Merry Christmas

 

[GarF]

Bearing guided rebate cutter to waste out the middle bit? I had a similar issue making this... Creating matching internal and external cuts gave me a headache. I think I ended up using an intermediate scrap piece. HTH

 

[RogerS]

Answers or suggestions in a PM....please !

 

[Trainee neophyte]

Is the problem the missing kerf? If you replaced that, would things fit as expected? (I may not have understood entirely what is happening here, but I then spend quite a lot of my life puzzled).

 

[SammyQ]

Kerfs, not single kerf surely?

Sam

 

[Yojevol]

You need a Guide Bush Collar like this.
This will give you an EXACT fit between the male and female parts. you will need to make a pattern which is smaller than your final shape. The amount of undersize is calculated from the bush and cutter diameters. The collar wall thickness needs to be the same as the cutter diameter. With just the bush in place use the pattern to cut the female part. Add the collar to the bush and use the same pattern to cut the male part.
I think I've got that right, it's a long time since I last used mine
Brian

 

[RogerS]

You need a Guide Bush Collar like this.
This will give you an EXACT fit between the male and female parts. you will need to make a pattern which is smaller than your final shape. The amount of undersize is calculated from the bush and cutter diameters. The collar wall thickness needs to be the same as the cutter diameter. With just the bush in place use the pattern to cut the female part. Add the collar to the bush and use the same pattern to cut the male part.
I think I've got that right, it's a long time since I last used mine
Brian

Way too complicated ! There's a much simpler solution.

 

[Terry - Somerset]

Not sure whether I fully understand but:

1. I assume to cut the semicircle, the router is is pivoted around a centre point on the ply sheet

2. After cutting the outer semicircle (oak?) increase the radius of the cut by the width/diameter of the router cutter.

3. You will need to move the pivot point on the ply so that the increased radius will all be into fresh ply. If you keep the same pivot point with an increased radius you will simply make a lot of noise in fresh air!

You should then find that the semi-circle cut by the router cutter will match the inside radius of the outer already cut.

 

[RogerS]

Not sure whether I fully understand but:

1. I assume to cut the semicircle, the router is is pivoted around a centre point on the ply sheet

2. After cutting the outer semicircle (oak?) increase the radius of the cut by the width/diameter of the router cutter.

3. You will need to move the pivot point on the ply so that the increased radius will all be into fresh ply. If you keep the same pivot point with an increased radius you will simply make a lot of noise in fresh air!

You should then find that the semi-circle cut by the router cutter will match the inside radius of the outer already cut.

True. But I've already made one cut and so why make another ? Besides which the pivot point has long gone as it's a BIG circle.

 

[Yojevol]

True. But I've already made one cut and so why make another ? Besides which the pivot point has long gone as it's a BIG circle.

Just follow Terry's instruction and all will be clear.
The point is that the whole cutter must be on the wastewood side of your joint arc. Therefore for the outer (female) part the Router/cutter C/L must be outside the joint line by the cutter radius and for the inner (male) part the C/L must be inside the joint line by the same amount.
This means that to make the ash pattern semi-circle you need to increase the trammel length by the diameter of the cutter. This means in turn that the trammel pivot point must be moved to get clear of your border pattern.
Have a good Xmas day
Brian

 

[RogerS]

Thanks go to everyone who participated in this little puzzle.

As many have pointed out, the gap is due to the kerf of the router cutter. It was counter-intuitive to what I had been basing my approach to this project ..after all a table saw blade has a kerf but once you've made the cut, the two pieces are still effectively "Parallel". So I beavered on oblivious to my impending 'problem'.

So one of the solutions as has been suggested was to simply adjust the trammel and make a cut of the correct radius (to match the other piece). Perfectly feasible but not in my case because I'd already dismembered the rest of the board to give me my component pieces and, as that radius is very large, had nowhere left to mount the pivot point for the trammel. Bit of a gotcha.

Neither did I want to go out and buy another sheet of ply. So late one night I had a flash of inspiration (and also as others have suggested) simply replaced the kerf. Bit of an offcut, sneak a bit off in the thicknesser, glue and pin. Job done.

 

[woodbloke66]

You need a Guide Bush Collar like this.
This will give you an EXACT fit between the male and female parts. you will need to make a pattern which is smaller than your final shape. The amount of undersize is calculated from the bush and cutter diameters. The collar wall thickness needs to be the same as the cutter diameter. With just the bush in place use the pattern to cut the female part. Add the collar to the bush and use the same pattern to cut the male part.
I think I've got that right, it's a long time since I last used mine
Brian

Way too complicated ! There's a much simpler solution.

It may be complicated Rog, but it's the right solution and will give you an exact 1:1 fit. Two different guide bushes and two different cutters...but it ain't simple :lol: - Rob

 

[RogerS]

.....
It may be complicated Rog, but it's the right solution and will give you an exact 1:1 fit. ...

So does mine

 

[woodbloke66]

.....
It may be complicated Rog, but it's the right solution and will give you an exact 1:1 fit. ...

So does mine

It's called a bodge, Rog :lol: - Rob

 

[RogerS]

.....
It may be complicated Rog, but it's the right solution and will give you an exact 1:1 fit. ...

So does mine

It's called a bodge, Rog :lol: - Rob

Cheeky sod ! :lol:

 

[woodbloke66]

Cheeky sod ! :lol:

:lol: :lol: - Rob

 

[SammyQ]

Tssk, tssk, tssk.... :roll:

Sam

 

[woodbloke66]

Tssk, tssk, tssk.... :roll:

Sam

Rog and I have known each other for years Sam; no worries about 'joshing' one another - Rob

 

[SammyQ]

No worries Rob. I was indulging my love of 'pulling your leg' by mock criticism. I knew you would be up for it and, although I've never met Roger in person, I have nattered to him by PM and I reckoned he could take 'a bit of bant'. I hasten to add that I expect to be 'slagged off' in exactly the same manner! I grew around, up and worked with, fellow Ulstermen, for whom such sham pontificating hypocrisy were routine conversational rituals - and led to hysterically funny situations betimes. Bad habits die hard. :?

Sam