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correct way to offset peg holes for open m&t rafter joints?

Started by hayton1960, October 04, 2005, 03:19:36 PM

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hayton1960

I am reasonably conversant with the concept of using offset holes to drawbore when making m&t joints-except that is when doing an open m&t on a rafter couple peak, I am not sure in which direction to offset; at what angle or direction from the shoulders? :-\ :P
Also how would you peg a simple half lap rafter couple peak together? Doesnt seem possible to offset, do you just squeeze them up with a strap come a long and "pin" them with a wooden peg?

Could anyone give me some advice on this please?
Many thanks, Jonathan :) ;)

Jim_Rogers

When we locate a hole location on a tenon we do it by measuring the location as an offset from the shoulder of the tenon. Usually 1 1/2" off the shoulder. Then we locate the hole at a distance away from the bearing surface, sometimes 2" sometimes 1 1/2" depending on the joint and layout.
We do this easily by placing the framing square tongue against the shoulder and draw a line which will automatically be 1 1/2" off the shoulder surface. Then we rotate the square to the other surface and draw the second line.
How we locate this hole spot is we create two lines that cross on the tenon. This should look like a plus sign (+) often called "cross hairs" like the cross hairs in the telescope of a high powered rifle.
We also locate the hole for the mortise piece in a similar way, with cross hairs. Then we bore the hole on the cross of these lines in the mortise timber, dead on the center of these two lines that create the cross hairs. Next insert the tenon into the mortise and site into the hole previously bored. You should see the cross hairs on the tenon in the center of the hole, if you're layouts were correct.
Then insert the bit into the hole and turn in reverse or counterclockwise to prick the hole location on the tenon. This mark will insure you have the correct starting location for your hole, if you don't intend to "draw bore".
But we now use this location as the true "working point" for our offset for draw bore.
To visualize which direction your hole should move to, in-order to correctly draw bore, place your hand over the tenon. Hold your hand out as if you were about to shake hands with someone, vertically that is.
Now imagine your hand is the mortise piece of timber. Move your hand in the direction you want your mortise timber to move to tighten the joint against the shoulder of the tenon. This is the direction you want the new 'draw bore' location to move towards. Move your new location in this direction and make a small mark on your tenon about 1/16th to 1/8th of an inch depending on what type of wood it is. A smaller move in hardwoods.
Now if you're joining two pieces an angle then you have to draw bore in two directions. So now rotate your hand so that it moves in the second direction that you want your mortise timber to move in and in this direction you should offset your new location another 1/16th to 1/8th.
So you'll be offsetting in two directions or into one quartrant of the cross hairs corners.
I don't have my camera here today to take some photos of my hand to show you the offset, but I will have it here tomorrow and I can do it for you then if you need to see it. Just let me know.
This method can be used for draw boring a brace to a post/beam or an open mortise and tenon joint at the peak of two rafters. I can also be used with two rafters meeting as a half lap joint. Just consider one rafter the tenon and the other the mortise and it should work, as you still want to offset the hole in two directions to pull the joint together at the shoulders and down.
I hope I've explained it so you can visualize the move and understand the procedure for determining the offset location.
If not ask more questions.
Jim Rogers
Jim Rogers Timber Designs
Jim Rogers Sawmill
117-R Jewett Street
Georgetown, MA 01833-1833
800-422-6250
www.jrsawmill.com
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

hayton1960

Hi Jim,
Thankyou for replying to my question :)
I had a look earlier today in Mr Sobons book TF Construction, page 123, theres a small diagram at the top of the page. Is that the same as what your describing?
Cheers, Jonathan ;)

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