How do I improve mitre accuracy?

[MusicMan]

Yes, they are reasonably hard steel,. especially the two riding points, but likely to be much better on a Starrett than on a cheapo. That's why they cost more.

Yes, uneven wear of the points will send the angle out of true. But as I mentioned, you can true this up yourself.

If you have three squares and they all match (as in Custard's picture but also with a match of in this case the big and small), when on a flat surface such as a saw or planer table, then that is a sufficient test for accuracy. Technically, a single pair match doesn't quite do it, as each could be off by an equal but opposite amount. But then they cannot each match with a third square. So buy squares in threes!

 

[Racers]

The small points are for adjustment, its easer to adjust a small amount of metal than a long flat grove.

Pete

 

[worn thumbs]

All of the previous replies are correct as far as achieving accurate mitres is concerned.Should we be asking whether the original rectangle is equally accurate?To minimise waste,and mistakes,I would suggest adapting custard's approach to fitting individual pieces.Instead of making all the angles identical and hoping the cumulative error will be very close to zero,make each piece fit it's location and use an angle that connects the intersection points of the outside edges with the intersection points of the inside edges.I find that a preliminary cut on a bandsaw followed by cleaning up with a disc sander works well.The same result can be achieved with a tenon saw and a sharp chisel,if thats what you have.

 

[johnnyb]

I find it best to feel identical lengths with fingertips. Try it it can reveal tiny length issues. Often I'll touch the table saw blade gently pull back a recut...job done. On false cut from 8 can result in an issue. As often as not the cut is false due to twist or sawdust or not being straight or not perfectly square that custards fitting is essential.

 

[devonwoody]

I got into the swing of mitres in box at corners (those tissue boxes) over the period of 7 years there were hardly any failures.
They were cut on a tablesaw horizontally and blade 45%. The major points were wood block stops and the sliding table that relied on the mitre guide plus the table edge which was a fixed side that was always static and never changed the blade bearing point under the table top which fortunately came supplied accurately set.
Never had the same success with picture framing at long lengths.

 

[John Brown]

This is probably a stupid question, but has anyone ever made a picture frame using "cope and stick" instead of mitres? Is there some obvious reason why this would not work or be a bad idea? Having a couiple of router cutter sets for frame and panel doors, I've often wondered about this.

 

[AndyT]

This is probably a stupid question, but has anyone ever made a picture frame using "cope and stick" instead of mitres? Is there some obvious reason why this would not work or be a bad idea? Having a couiple of router cutter sets for frame and panel doors, I've often wondered about this.

I think this could be the explanation.

What you describe would work, of course, but only for the very simple ovolo type mouldings found on window sashes or cupboard doors.
Many picture frame mouldings are (or were) very complex, with re-entrant mouldings and surface carving to add to the complexity.

The one universal method that works on all of them is the mitre. So set yourself up to cut mitres and you can make frames using whatever the customer chooses.
Use of the mitre becomes so widespread that we learn to expect picture frames to have them and would possibly be distracted if they didn't.

(Of course this is all a generalisation and there will also be historical differences - for example if you do an image search for "altarpiece" where the framing is more structural and complicated than a simple flat frame to hang on a wall, you will find plenty of other construction details, including mortise and tenon and mason's mitres. And in Victorian homes, the "Oxford" frame was very popular, with half-lap joints and stopped chamfers.)

 

[John Brown]

Thanks for the answer. Makes sense.

So there is nothing stopping me making a simple frame by such a method, if I choose to. It would certainly seem to be easier than cutting mitres, from what I've seen in this forum!