Cutting really thick/long mitres accurately

[mrbmcg]

Hi Folks

I'm about to cut longish (150mm) 45 degree mitre joints in 2" thick (expensive) American Cherry for the base of a large bookcase I am making.

I guess it's just my technique, but I'm never quite satisfied with the mitre joints I seem to produce. I've tried the hand mitre saw, the power mitre saw, the table saw and even the radial arm saw.

No matter how I endeavour to make the cut I invariably end up out with the wax fill-in sticks on at least one corner of a frame.

My question then is, given that I don't want to spoil a 1700 x 150 x 50mm piece of lovely cherry (or take 2-3mm off the length of the piece ), how would you guys deal with this type of cut, and mitres in general?

I'm a bit of a power tool woodworker (although I think I have a plane or two somewhere ). I've read about shooting boards and the like but no real explanation on how to construct or use them.

Any advice? Jigs? Help?

Cheers
Bob

 

[Alf]

Bob,

A Versatile Shooting board
A Ramped Shooting Board for End Grain
A Donkey's Ear Shooting Board (I'm guessing this is the sort you need for this job)
Three shooting board designs
Yet another version
Mitre jack, a rather more complicated alternative, and not really necessary, but what the heck. :wink:

Hopefuly Jeff Gorman's page in the first link will answer your questions, but if not come back and yell.

Cheers, Alf

 

[Aragorn]

Is the board completely flat to start with? If it's cupped at all, then the mitre won't work out. In fact it will double the obviousness* of the cupping.
For a mitre like the one you describe, I just plane the boards flat (with a machine :roll: ) and cut it on my Makita chopsaw, clamped to the saw using the clamp provided.
I usually make an initial cut about 1mm off the line and then trim to length by taking off the extra 1mm in one nice smooth quick cut.
Biscuits and glue to hold it together.
I would also make test cuts in similarly sized pine to double check the exactness** of the 45º and fine tune the angle as necessary before turning to the cherry.

*obviousness - is this a word? Can you say this? :?
** exactness - is this a word? :?

 

[Chris Knight]

I reckon you are making things hard for yourself in using such thick wood. The more surface of mitre you create, the more possibilities there are for imperfections to prevent the two mitred pieces joining properly. I never use more than 3/4 in thickness for cabinet plinths.

A mitre jack or a shooting board to Bob Wearing's design - like this one I made can do the job of tidying up a poorly cut mitre but I would generally cut them first on my tablesaw and that is usually a very good result. You can always shim the work to the fence with post it notes or similar if fine tuning on the saw is needed

 

[johnelliott]

I agree with Chris, the wood you are using is VERY thick for the purpose. Even if you get the mitres right, there is every chance that they will open if the wood shrinks or expands in the future (which it probably will).
Personally I hate mitres and will design them out at every opportunity. Good luck whatever you choose to do
John

 

[mrbmcg]

Thanks for the info guys.

The mitres aren't 50mm wide all the way down, just at the widest point. If you imagine the profile it is 150mm high, and 50 wide. There is a 25mm by 100mm rebate taken out of the back then a full width of 50 mm. This is to support a 1" thick chery veneered MDF base which is 1650mm wide. I guess I could glue a support rail underneath instead of moulding it like this, but the 100mm x 25 piece I am ripping out of it is perfect for part of the face moulding so I figured I could skip a short glueing up stage. Maybe I should reconsider :wink:

This is the profile here (in very rough ASCII) then the top edge will be moulded. The widest part is 50mm.

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Is it generally true that wide mitres will always open up? If so, is there a width that you shouldn't go over?

 

[Aragorn]

I reckon go for it as you've planned.
Applying the moulding in glued up pieces just leaves the possibility of more mitred pieces to match perfectly and opening up in time.
If the wood is flat and well seasoned, biscuited and glued well it shouldn't open up any time soon!
To be honest, what you are talking about isn't that big a mitre. It's only a 6" wide moulding. When I do Victorian skirting restorations, they might be 8"-10" or more, and might be 40mm thick at their widest.
Knock it up in pine and do test cuts and fits. This way you'll get a good idea of what you can handle and how good a fit you are going to get with the cherry.

 

[mrbmcg]

Yep, I think so. The table saw has usually yielded best results for me too. I think I just need to take a little more care probably.

I will mess with it tomorrow night and let you know how I get on. :wink:

Cheers
Bob