Replies

1. 
   RalphBarker | Sep 25, 2010 07:05pm | #1

   rabbet size

   Actual. Otherwise, the fit is sloppy.

1. 
   jyang949 | Sep 25, 2010 08:20pm | #2

   Thanks. I thought maybe the smaller size plywood was to allow for seasonal movement. Time to order a new router bit.

   Janet

1. 
   RalphBarker | Sep 25, 2010 08:59pm | #4

   Look for bits made for plywood sizes: nominal (actual): 1/4" (7/32" or 5.5mm), 1/2" (31/64") and 3/4" (23/32").  You may still experience some variation in thickness along the edge, particularly with construction grades, so some trimming may be necessary. Or, as Hammer suggests, make two passes with a smaller bit.

2. 
   jyang949 | Sep 26, 2010 11:40am | #5

   But plywood is more of a

   Darn. I am trying to fit new plywood to an old rabbet that is exactly 1/4". I suppose that is why the existing plywood didn't hold up well. The previous owner relied on glue to fill the gap. 

   Plywood is more of a "made" product than plain lumber. Can't they manufacture sheets that are a better fit for popular bit sizes? 

   And speaking of sizes: Wasn't the U.S. supposed to have gone metric by the year 2000? In Europe, is their plywood a good fit for metric bits?

   Janet

1. 
   RalphBarker | Sep 26, 2010 11:47am | #6

   plots

   I think the reduction in plywood sizes was due to a conspiracy between plywood manufacturers and router bit makers. ;-)

2. 
   DonStephan | Sep 26, 2010 12:13pm | #7

   Good 1/4" baltic birch

   Good 1/4" baltic birch plywood may be close enough to a true 1/4" for a good fit.  Another option would be to glue strip of veneer to edge of undersized ply, sand to fit well.

1. 
   jyang949 | Sep 26, 2010 01:41pm | #8

   My plywood is ordinary birch, 7/32" thick. The veneer will work, though. Thanks!

   Janet

2. 
   hammer1 | Sep 25, 2010 08:25pm | #3

   If you are doing a full dado, you want it to fit, not so you have to hammer it in but tight enough that you have to press firmly and it will stay there solidly, or at least that's what we strive for. Plywood varies. Some router bit manufacturers make bits that are supposed to fit. I have some of those as well as some 3/4" bits that have been sharpened several times. Out of six bits that I have that vary slightly, I can usually find one that is a perfect fit, but not always.

   Sometimes you have to use a smaller diameter bit and make two passes. Another way is to make a smaller dado and shoulder the plywood to fit. I prefer to use an undersize bit and make two passes. I replace my round router base plate with a square wood one. Then I size the edges to be slightly less than each other in relation to the bit. With this method, I can clamp a single guide and make two passes by just turning the router and using different edges of the plate. There are many other ways to do the same thing.

   Before I do any dadoes, I do some samples on scrap. Then I know if I have the correct size bit or if I have to use another method and what that method is. It does take some practice when you want a great fit and you have to be able to repeat it several times. I wouldn't use stack dadoes on a table saw for a crosscut dado on plywood but I would if cutting with the face grain or using solid stock. You use shims between the cutters to fine tune whatever size you want.

1. 
   SeaMonkey | Nov 12, 2010 02:22pm | #9

   why not the shoulder route ?

   Hi - I am doing the same thing, in thicker (expensive) plywood.  i have seen several methods for fitting dados to the exact size of the plywood, and all seem successful, but cumbersome.  I am wondering why it is not faster to just rout the dado slightly narrower than the narrowest plywood (say 1/16th) and then rabbett the plywood ends to that exact width ?

   Relatedly, has anyone used the festool router with guide for routing dados in long sheets ?  thanks !