Mitre Madness

[Chems]

I'm currently converting a very long church pew into a corner seat. Its going well. All I've got left to do is mitre the back of the seat parts together.

The back of the seat has a slight recline, do I measure this angle, then set my saw to 45 degrees and instead of cutting straight up the seat back cut at the angle the seat is reclining?

I hope that makes sense.

 

[Steve Maskery]

Hi Chems
No! Don't do it!

A compound mitre (which is what we are talking about) means you have to set a bevel and a mitre,

I used to have a little calculator on my website which did it, Or rather it should have done it, but didn't. Despite trying to get a BSc (I) and a PhD (Mathematics) to have a look at it, I still didn't get it sorted.

These days, Sketchup is the easiest way to determine the angles. If you don't do Sketcuhp, let me know the angle of incline of the backs and I'll do it for you.

Cheers
Steve

 

[Chems]

A bevel and a mitre sound like what I was thinking.

Like a mitre upwards with the saw set to 45 degree cut. I have a program a bit more advanced than Sketchup I'll try it with that.

 

[chris_d]

I'd love to see a photo of the finished article since I fancy doing something similar....

 

[Chems]

Its a job for a friend, she got the bench which was 14feet of ebay for £60. Its been an easy job to convert it. I think its red oak, I planed down one seat to see if it would look nice bare but it doesn't so it will be getting a painted finish. I have some pictures, I'll post them tomorrow when I've done this Compound Mitre.

Steve, after saying about the calculator a quick google search threw up this site which seems to have the right calcualtor right at the top.

http://jansson.us/jcompound.html

 

[Steve Maskery]

The point is, Chems, that neither angle will be 45 deg.
S

 

[Chems]

No that seems to be what the calculator is saying. I'll have to measure the angle tomorrow an give it ago. I don't like doing 90 degree mitres let alone tilting 90's.

 

[Escudo]

My brother Dave has no common sense and generally is pretty useless at most things, he is also lazy (rarely gets out of bed before 11).

However, he is a doctor of mathematics and a chartered mathematician which has its uses when it comes to angles.

Just thought I would throw this resource on the table.

Tony.

 

[WellsWood]

You might find something here to help.

Always do a test cut first though, obviously.

 

[matt]

If I'm interpreting correctly, then I suspect you're using a mitre so the top edge has a nice neat join? If that's the case, perhaps it's worth just doing a mitre for the very top and a square cut the rest? Sorry, not easy to explain.

 

[FogggyTown]

Good luck! I was optimistic enough to try to take on a piece with nothing BUT compound mitre joints.

Drove . . . me . . . NUTS! Couldn't get a formula going so it was days of trim & offer, trim & offer, trim & offer. I sort of made it in the end but never again.

 

[Chems]

Ok so I need some help.

If I draw this up in my 3d program I get a 45 degree bevel cut with a 10degree mitre.

All the calculators I've used say other things but there parts don't mate up.

Anyone got any experience that can help?

 

[Chems]

I think I got it figured, standard 45 degree bevel on the saw and 100 degree mitre on the vertical. I found a very easy way to work it out in the end. I'll test it tomorrow an see how it goes.

Edit, when I say 100 degree on the vertical thats cutting from the bottom, if it was on a mitre saw the angle would be 10 degrees.

 

[Steve Maskery]

I'll be interested in this Chems! I really do think you are wrong on the 45 bevel.

Try 45.438. Or rather,that's what needs to be left, so tilt 44.something. OK I admit it's not a huge difference, but the more you slope the sides the bigger that deviation from 45 becomes.

Cheers
Steve

 

[Chems]

Ill run it again steve.

Basically what I did was to intersect to planes with a 10 degree lean then work out the angles. I'm going to test on a few bits of scrap with the CMS later on.

 

[Steve Maskery]

Yes, that's what I did, too.

Remember, though, that the bevel angle is to be measured using the mitred edge as the main axis, not the original 90 edge. That's where the difference originates.
Cheers
Steve

 

[Niki]

Have a look at this post
http://www.woodworkforums.com/showthread.php?t=51565

On post #6, there is a PDF file with Excel....you can change the inputs and get the angles...

Also, on post #15 there is a table of compound miters....

You can see the formula on the link on post #17
http://www.woodcentral.com/bparticles/m ... mula.shtml

Another one here
http://www.woodcentral.com/bparticles/miterangle.shtml

And another chart here
http://www.woodcraftplans.com/compound_miter.htm

Hope it helps
niki

 

[Chems]

Niki thats perfect. They are discussing the exact same calculation as I am.

Steve, I think your right a slight deviation in the bevel is needed. I tested on some scrap and did seem to get it nearly right.

 

[Chems]

Thanks for all the help. I got around to doing it today. Didn't work an I was very confused. I had measured the slope off the back of the seat from the seat base, can you guess whats coming? The seat itself had a 5 degree slope to make it more comfortable so that was throwing out the calculator that Niki linked to. Once measuring properly which was 75 I was away.

So for anyone reading this in the future, all the links you need for compound mitres are in this thread!

Chris, started a WIP thread here

 

[Lord Nibbo]

Or you could use this calculator

http://www.csgnetwork.com/sawmitercalc.html