Which router bit and techique for this?

[Fishnaked]

I'm building my home which has exposed 4x8 rafters throughout with tongue and groove decking over that. All exposed from inside. Where the rafters butt up against each other, I will be tying them together with Simpson 18" connector straps on top. I want to route grooves in the tops of these rafters so the Simpson straps will sit inside the grooves so the tongue and groove decking lays down nice and flat to the rafters. The grooves would only be 1/8" - 3/16" deep and 1.5" wide.
 
I plan to set all the rafters in place then come back and make the grooves. Since they will be hidden, they do not need to be pretty. I just need an easy and quick way of cutting them while they are in place.
 
I have a router with plunge base attachment...though I have never used the plunge. I don't have a straight bit which I assume is what I need but am not sure since a router is not a tool I use often.
 
Which bit and technique do you all recommend?

[BpB123]

I'm building my home which has exposed 4x8 rafters throughout with tongue and groove decking over that. All exposed from inside. Where the rafters butt up against each other, I will be tying them together with Simpson 18" connector straps on top. I want to route grooves in the tops of these rafters so the Simpson straps will sit inside the grooves so the tongue and groove decking lays down nice and flat to the rafters. The grooves would only be 1/8" - 3/16" deep and 1.5" wide.
 
I plan to set all the rafters in place then come back and make the grooves. Since they will be hidden, they do not need to be pretty. I just need an easy and quick way of cutting them while they are in place.
 
I have a router with plunge base attachment...though I have never used the plunge. I don't have a straight bit which I assume is what I need but am not sure since a router is not a tool I use often.
 
Which bit and technique do you all recommend?

I would use an edge guide, one pass from each edge would ensure the slot is centered.  If the diameter of the bit is less than 1/2 the width of the desired slot, you will have to do a clean-up pass with the edge guide re-adjusted.

I will leave the type of bit to others

Sounds like an interesting project, please post pictures.

-Brian

[jteneyck]

If looks count for nothing you could just freehand it with a router using a mortising bit or a center cutting straight bit or endmill.  If looks count then make a template and use a collet on the base of your router to follow the cutout in the template.  Screw the template to the rafters, route the slots, remove the screws and move to the next one.  

John

[MstrCarpenter]

How about mounting two fences to your router base (or make another) to keep the bit from cutting too close to the edge? The space between them would be the width of the rafter + the width of the slot - cutter dia.. I assume you'll be making the the plumb and birds mouth cuts on the rafters before setting them. I would cut the slots on the ground as well. 1-1/4" dia. mortising bits (for 5/8" rad. butt hinges) are fairly reasonable.

Another option would be a Porta-plane. Most cut 3"+ wide , so you'll have to sacrifice some blades to make the narrower cut you need.

[Bill Holt]

If looks are not important, doing the mortise on the ground would certainly be faster and safer.  If it were me, I would set the router to the desired depth and just free-hand the mortise.  And yes, a straight bit would be the right choice.

Is there a reason for doing the mortise with the 4x8 in place?

[Tapper]

Just make a jig for the router that straddles the rafters. The hole in the middle matches the size of the mortise you want, minus the margin for a PC collar. Couple of clamps to hold in place while routing - easy peasy.

Doug

[Fishnaked]

Is there a reason for doing the mortise with the 4x8 in place?

Just the sequence I had in my mind but, I do understand how it would be easier on the ground.

Thanks to everyone for the suggestions! Was pleasantly surprised how many responses there were.

Will post pics as per Brian's request...if I remember and have time. I'm working solo mostly so will have my hands and mind pretty full!

[mr_skittle]

Just make a jig for the router that straddles the rafters. The hole in the middle matches the size of the mortise you want, minus the margin for a PC collar. Couple of clamps to hold in place while routing - easy peasy.

Doug

I second the idea of a jig that fits over the rafter. I imagine a piece of plywood with a couple of cleats fastened to it that tightly capture the rafter. Cut a hole in the plywood the size of the recess you need to route, and bam. Put a countersunk hole so you can screw it to the rafter to hold it in place, or make the jig big enough to accommodate a clamp without interfering with the routing operation. I've attached a photo of a similar operation. One last idea, if you are up on a ladder or something working on rafters in place, I'd use a small trim router. Routers are heavy, especially plunge routers.

   

[Wild Turkey]

That size/depth makes me think you could snap a chalk line for the centerline and use a Forstner bit to hog off what little wood is in the way.
Two ways to do it -- 1-1/2" or 2-1/4" bit, depending on what your drill can handle.

   

   

(Sorry if images are too big. I tried resizing them but couldn't get new images to replace old ones 
 )

[rwe2156]

Check Marius Hornberger's YouTube he has an excellent example of a large base plate with a dual fence. I think it's ideal for what you're doing. It doesn't have to be that elaborate, just a plate and a couple fences will work.
And yup - do them on the ground!!