Best way to avoid tearout on the backside of a mortiser's cut?

[Dan Mitchell]

I have a very good quality General mortising machine which I have used with great success on "hidden" mortise & tenon joinery, but now I would like to use it in a through-tenon situation where both sides of the mortise will show. If anyone has any tips for avoiding tear out on the backside of the cut, I'd like to hear them. I know I can backup the underside of the workpiece with another bit of wood, which I have done int he past with varying degrees of success (and a fair amount of screwing around, making sure you have a completely non-cut portion of the backup piece under each new cut) and it also occurs to me I could attack the mortise from both sides, letting the cuts meet in the middle, but of course this latter approach introduces the possibility of not getting the cuts aligned perfectly. Chisels and bits are pretty sharp, I have one of those diamond-encrusted cone hones which I have used on them. Wood will probably be Honduran mahogany.

Perhaps pre-scoring the outline of the mortise with chisels or a utility knife?

TIA

Dan

[keith micinski]

Pre scoring sounds less accurate and more of a pain. If you don't want to back it up then just turn it over. As long as you use the same face for reference they should line up fine.

[Josiah Bartlett]

I use a small cross-slide vise mounted to my mortiser's base. It limits the maximum depth a bit but it does allow you to set it up very accurately. I use that when I want through mortises, flipping the piece end for end to attack it from both sides.

[Eiji Fuller]

Make the mortise in your prepared piece that's 1/16-1/8 wider than your final size then joint or plane to size and remove the tear out at the same time.

Cheers

[Neal Clayton]

cut the edges of the board to be mortised after the mortise. voila, problem solved .

edit: bleh, beaten by a measly minute!

[Mike Cutler]

I do what Josiah recommended. I've had great success with a both a cross slide vise and just using the fence with stops clamped to them.

Fiberglass packing tape, or blue painters tape, is your best friend when using a mortiser.

[Rod Sheridan]

When making through mortises I mortise from both sides, keeping the same face against the fence.

Regards, Rod.

[Kent A Bathurst]

Fiberglass packing tape, or blue painters tape, is your best friend when using a mortiser.

Bingo.

Blue tape, and I put a piece of whatever-is-laying-around-plywood under the workpiece for a backer. I don't have any real trouble keeping fresh scrap under the mortise target.

And - sharpen, sharpen, sharpen. The chisels are sharp to start, but there is a lot going on there - esp dense hardwoods, like the QSWO I often run - and I don't get real far before I dress the chisel up a bit.

There is also what I think of as the "modified G&G approach": when the design of the piece allows it, rather than make the joint "invisible", I will go the other way and make it more pronounced, by rounding over all the edges of the mortise and the tenon. Doesn't work, of course, when the piece is Stickley/Ellis/etc.

Never thought of the "oversize and trim back" method. Will keep that in mind.

[Frank Drew]

Dan,

There are times when there might be no choice but to mortise in from opposite sides, such as with through tenons on passage or entry door work. Careful layout and careful stock prep is the key, ensuring that your material is precisely four-square, and that your fence is truly 90º to the table, then flip end for end as suggested above.

[Richard Dragin]

Just as when through drilling, use a backer piece of wood to avoid blowout of the back side.