Calculating compound angles

[RogerS]

I've got some coving to cut to fit on top of some kitchen cupboards. The coving slopes at an angle of 30 degrees. I found this useful spreadsheet http://www.woodcentral.com/bparticles/bpindex2.shtml#downloads (compound mitre angles) and it works for external corners.

But what mitre/bevel angle do I need for internal corners? :duno:

 

[Steve Maskery]

Hi Roger
Isn't it the same only opposite? Whether a corner is external or internal only depends upon which side of it you are standing, does it not?

I'm sure the angles must be the same only negative, or something like that.

Go on then, tell me I'm wrong
S

 

[Chems]

Sounds about right so if your angle was 143 for example on the external, take 143 from 360 and thats your internal angle.

 

[RogerS]

Hi Roger
Isn't it the same only opposite? Whether a corner is external or internal only depends upon which side of it you are standing, does it not?

I'm sure the angles must be the same only negative, or something like that.

Go on then, tell me I'm wrong
S

Ah...but if you are on the 'inside' of the inside corner (if you see what I mean) then the cornice is leaning away from you and not towards you?

The spreadsheet gives me 26.5 for the mitre angle and 37.7 for the bevel angle for a slope of 30 degrees.

Actually thinking it through a bit more, isn't it a case that the inside angle is 30 degrees and no bevel but needs to be scribed to match...ummm..maybe not

 

[jasonB]

Probably easier to just make a simple stop to hold the moulding at the 30deg angle upsidedown in the saw then just set the mitre angle to 45geg and not worry about flipping the saw over, thats how I do it unless you have a very tall moulding that won't fit in the saw.

There is a lot on the Dewalt USA site but I don't think it covers a 30deg angle, search for "compound angles"

Jason

 

[RogerS]

Thanks Jason...angles worked as a search term ...good article. Like the phrase 'pretesting on scrap is essential'

I can see the email now......

Dear Kitchen Manufacturer

Please can I order some more cornice?.....

 

[Steve Maskery]

Roger,
There is another way to do it which does not require any calculating at all.

You can take a piece of stock, say 2x2, and mitre the end, then rip a 30 degree bevel. You can then use a sliding bevel to measure directly from this test piece what the angles need to be for your bevel/mitre combination.

HTH
Steve

 

[RogerS]

Jason - as an afterthought....how do you stop breakout on the facing side if the piece is upside down?

 

[jasonB]

Its not actually upsidedown ie face of moulding on the base of the machine.

You just hold it as if upside down eg the top of the moulding rests on the base of the saw and the back of the moulding goes against the fence, in otherwords the base of the machine is the "ceiling" and the fence is the wall" if it were a cornice that goes to the ceiling.

A bit like this

Jason

 

[RogerS]

Ahah..thanks for that.

R

 

[mack9110000]

The only time you need a compound angle cut is when the moulding is too high for the mitre saw's fence, then you would have to cut on the flat using the bevel and angle functions on the saw.The jig from Rutlands is clever, but they can be made from scrap timber quite easily.
mack

 

[BradNaylor]

I've had to do countless compound mitres over the years and have stopped trying to calculate them at all. It does my head in!

I find it far quicker simply to arrive at the correct settings by trial and error on some pieces of scrap. Then, when you've got there, make a note of the angles on the saw so that you can swing the blade from side to side for the different cuts.

I made an easy 20 quid on Saturday helping out a neighbour who was struggling with the facia boards on his corner summer house. He had spent four hours trying to get the compound mitres right and was running out of facia!

It took me ten minutes with a few pieces of scrap wood and an experienced eye. He now thinks I'm some kind of genius!


:lol:

Dan

 

[bugbear]

I've got some coving to cut to fit on top of some kitchen cupboards. The coving slopes at an angle of 30 degrees. I found this useful spreadsheet http://www.woodcentral.com/bparticles/bpindex2.shtml#downloads (compound mitre angles) and it works for external corners.

But what mitre/bevel angle do I need for internal corners? :duno:

Try googling "crown molding", which is what our friends in the US call this.

This style is popular over there, and much discussed.

BugBear

 

[RogerS]

This is doing my head in.

I've worked out which bit goes which way up and on which side of the mitre blade and which 45 degree mitre angle to cut (LH or RH) and so many thanks, chaps, for that.

What is vexing me big time is that I have on the left hand side of the kitchen, wall units that effectively form a U. So the sequence that I've got to cut is external - internal - internal - external - external. All cut to micro-millimetre accuracy length it seems to me otherwise you get gaps. But even when you do that because the wall units aren't perfectly square to each other (due to walls/manufacturers tolerances etc/alignment) you still end up with gaps (albeit small but visible).

So what do the pro's do? What's the dodge?

J....a....s.....o....n......help!!!!

 

[George_N]

So what do the pro's do? What's the dodge?

J....a....s.....o....n......help!!!!

Filler and paint?

 

[jasonB]

Glue the joint first with mitre fast and then you can tweek the fit either by flexing the moulding or running it ever so slightly out of parallel to the carcase.

Wax filling sticks for natural wood and worktop joint sealants for painted will also help if the joint is not spot on or the finish has chipped out.

Jason

 

[RogerS]

Thanks guys.

I've got some Mitre Mate (although it's over a year old) but didn't have much success with it. Is that why you recommend Mitre Fast, Jason?

 

[jasonB]

I tend to use the one Isaac Lord do, the lid on the flat bottle tends to last longer before it gets gummed up.

If your bottle has been opened and stored for over a year it will probably have gone thick and not be as good as when new.

Jason

 

[MikeG.]

I think the pros would probably do all of the corners first out of short lengths, then fill in between with straight pieces............and you would be amazed how much filler they use!!

Mike

 

[hpl]

The amount of filler depends on the accuracy of the person cutting the joint. A well cut joint won't need much filler, if any, a poorly cut one may need a lot. Also a quality mitre saw makes so much difference.

I would follow with Jason, mitre mate and tweaking is the only way when everything is slightly out of square.

Johnny B