Bowed miter cut

[Lee roy]

Hi all,

Need some help with some unexpected mitre cut results.
Im making some basic wall storage boxes out of 32x200 hoop pine, using a incra mitre 1000. To start, blade was checked dead on 45, mitre gauge parallel to the blade, everything cut fairly straightforward, mitres are probably the best I've cut front on BUT when I dry fitted everything there is a distinct bow in the middle of the cut. Appears slightly worse inside than out, but it's on both faces.



This had me scratching my head, Stock is all flat and true, I didn't find the stock 'pulling' under load, and it was dead flat to the miter bar and end stop all the way through. I pulled the saw apart and checked the mitre slot and the blade with a dial indicator and didn't find any unusual problems. Admittedly, my throat plate won't fit with the blade tilted at 45 and I didn't have another so I made the cuts without the throat plate. I figured 1" stock max unsupported wouldn't be a big issue with 30mm stock (I will rectify this!)
I made several other miter cuts in the same config with 15mm stock and they are all fine. Does my problem lie simply from the lack of throat plate? I don't have access to a planer at the minute so I can try on the other side of the blade where there would be 5mm unsupported, but I am reluctant to muck around on the left side of the left tilt arbour!

Anyone shed any light ?

[elanjacobs]

On large mitres like that I generally cut about 1mm over size, then re-cut it. That ensures that there's zero blade flex and zero chance of the timber being pulled off the fence during the cut.

Bit of extra time to do it, but I've never had problems.

[Lee roy]

When you say cut 1mm oversize, you're talking the thickness? Or recutting the mitre -1mm?

I've considered re-jointing each face, I can't thickness as they're too short and worried they'll snipe, but something tells me the mitres won't line up as well if I re-joint them.

[elanjacobs]

When you say cut 1mm oversize, you're talking the thickness? Or recutting the mitre -1mm?

I've considered re-jointing each face, I can't thickness as they're too short and worried they'll snipe, but something tells me the mitres won't line up as well if I re-joint them.

I mean length.