Compound angles- compound mitre joint.

[marcros]

On a splayed tray, If I put the side flat on the deck, i understand that the compound angle is 10 degrees off square for cross cutting and 44 degrees for the saw blade tilt. Give or take. Not greatly explained, but there are plenty of calculators online for it. http://www.blocklayer.com/compoundmiter.aspx

if the pieces are upright- as they would nearly be on the tray then the blade is 7 degrees, and the cross cut nearly 45. These I can picture.

However, the RAS hasn't got a table set up yet and i don't have a SCMS, so the compound angles are really a bit irrelevant.

Am i correct in thinking that if i tilt the side to the required angle (ie chamfer the bottom to 10 degrees, so that when it stands up, the sides are splayed), then i can just cut them at 45 degrees?

 

[Sgian Dubh]

Am i correct in thinking that if i tilt the side to the required angle (ie chamfer the bottom to 10 degrees, so that when it stands up, the sides are splayed), then i can just cut them at 45 degrees?

Yes, assuming the corner is 90º. What you're describing is what's usually known as the 'spring' of the moulding, most commonly heard mentioned when dealing with wooden cornice mouldings. The spring is the angle the moulding describes to the wall. The Americans are hot on this stuff, although they call it crown moulding, and most of the online sources giving guidance for setting bevel and mitre angles on chop saws come out of North America.

The two common moulding types in North America have flats on the back side at the outer limits of the moulding's width cut at either 45º/45º or 38º/52º. If the mouldings are small you can simply set them up on the saw table at their spring angle and cut the appropriate mitre, e.g., 45º for a square corner, or 38º, usually with the top edge towards the bottom. It's when the mouldings are too wide to stand up and still allow the saw to simply chop downwards that life gets relatively complicated. This is when you have to lay the moulding down flat on its back face on the saw table, and then set both the mitre cut and the bevel angle to get the appropriate compound cut. It's also useful if you know how to calculate the angles you need for anything that doesn't fit the standard profiles, springs, and the like you'll see in cornice (crown) mouldings, e.g., for making various polyhedra such as a tall four sided pyramidal like structures that might be used as, for example, a display stand.

Anyway, that's all probably more b*llox than you really wanted to read, but assuming you want to make a tray with sides simply mitred at the corners then your plan should work. If you want to execute some joinery, e.g., through dovetails, it won't. In that case you'd need to work out the dihedral angle and from that work out what two angles you'd need to cut the end of each piece to fit into the corner, and then be able to execute the joinery with similarly marked angles. Slainte.

 

[Steve Maskery]

If you do it that way, you might find that a matching support block behind your workpiece is a good idea. Just bevelling the bottom edge will mean that the workpiece is not properly supported against the fence. A 10 deg wedge will fix that. I wouldn't just hold it by hand, you are asking for a snatch.
S

 

[marcros]

Am i correct in thinking that if i tilt the side to the required angle (ie chamfer the bottom to 10 degrees, so that when it stands up, the sides are splayed), then i can just cut them at 45 degrees?

Yes, assuming the corner is 90º. What you're describing is what's usually known as the 'spring' of the moulding, most commonly heard mentioned when dealing with wooden cornice mouldings. The spring is the angle the moulding describes to the wall. The Americans are hot on this stuff, although they call it crown moulding, and most of the online sources giving guidance for setting bevel and mitre angles on chop saws come out of North America.

The two common moulding types in the North America have flats on the back side at the outer limits of the moulding's width cut at either 45º/45º or 38º/52º. If the mouldings are small you can simply set them up on the saw table at their spring angle and cut the appropriate mitre, e.g., 45º for a square corner, or 38º, usually with the top edge towards the bottom. It's when the mouldings are too wide to stand up and still allow the saw to simply chop downwards that life gets relatively complicated. This is when you have to lay the moulding down flat on its back face on the saw table, and then set both the mitre cut and the bevel angle to get the appropriate compound cut. It's also useful if you know how to calculate the angles you need for anything that doesn't fit the standard profiles, springs, and the like you'll see in cornice (crown) mouldings, e.g., for making various polyhedra such as a tall four sided pyramidal like structures that might used as, for example, a display stand.

Anyway, that's all probably more b*llox than you really wanted to read, but assuming you want to make a tray with sides simply mitred at the corners then your plan should work. If you want to execute some joinery, e.g., through dovetails, it won't. In that case you'd need to work out the dihedral angle and from that work out what two angles you'd need to cut the end of each piece to fit into the corner, and then be able to execute the joinery with similarly marked angles. Slainte.

No, just the b*llox that I was after, Richard. Thanks! It is only 3 inches tall, so i will give it a go. My normal dovetails need some practice- adding some dihedral angles isnt going to help matters!

I do have a morso machine, so to make a 90, i can just use that.

 

[marcros]

If you do it that way, you might find that a matching support block behind your workpiece is a good idea. Just bevelling the bottom edge will mean that the workpiece is not properly supported against the fence. A 10 deg wedge will fix that. I wouldn't just hold it by hand, you are asking for a snatch.
S

Thanks Steve. I was thinking of a wedge and a stop, but that may have further complicated my already verbose post! Good reminder though.

 

[marcros]

i cut the angles tonight. Used the morso that i got a little while ago, and it made a great job of them.