Preparing mitre joints in oak

Ok - you know the approx angle between the central tread and the bays to either side, 135 degrees, so you want 67.5 approx on the end of each board.

The mitre saw should cut cleanly on the upper face, and may cause some splintering on the lower face. another sacrificial board underneath may minimise this. If you do have unacceptable splintering, you can use a straight router bit and run the router against a guide batten to cut a new clean edge. I'd think the boards are likely to be too large for the router table.

As far as getting the angles right, I'd want to cut both ends of the central board first, so it will fit tight into the recess, with angles of approximately 67.5 - the exact angle doesn't matter - that it is exactly the right length does. Then I'd slip both end boards under it in exactly the position you want them, and then mark off the edges of the central board onto them. Depending on your mitre it may be possible to set accurately enough to follow this line, or you may nead to cut slightly overlength and use the router and guide batten to exactly follow your line.

Can you explain how good the supporting surface is underneath (absolutely smooth and flat?)? Glued and screwed will give you the strongest joints between the boards, but at only 20mm thick the boards will flex under bodyweight if there's the slightest uneveness under them - and the joints will eventually fail.

How will you fix the treads in position? Through holes for screws from the top, countersunk and plugged?

A stronger joint would be to cut a tongue and groove on the mitred ends (easy if you have a bearing guided router). This gives more glue area, and if the joint ever does work loose, only part of the tongue is exposed - you can't see through. Stronger still would be thicker timber bearers (say 40mm) attached to the underside of the treads (with continuity across the underside of the joints in the treads), again glued and screwed through the treads and into the bearers.

As always, practice any techniques new to you on scrap offcuts.

Mark the joints with a sharp 'marking' knife ( or even a Stanley knife at a pinch) all round the mitre, cut the joint on the waste side (just taking out the mark) and keep the sharp, fine toothed saw 'flat' (as you will with a mitre jig anyway) as you cut the joint - and all should be ok with a nice clean joint.

IF you want to clean the joint up after cutting, just leave the mark in and finish off with a very sharp, 'fine' iron on a hand plane, preferably using a 'shooting' board for accuracy (not a router).

The real secret of getting a good finish on this sort of job is to have the confidence in your abilities to do it.

Of course, if you have access to a radial arm saw using a sharp TCT blade - then that would be perfection.

Brian G

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