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Why did my routed recess come out 1/8 in bigger than my pattern?

1292 Views 3 Replies 3 Participants Last post by  NiteWalker
I'm sorry for the long preamble with my question, but I figure it's a good way to get a few more tips and pointers …

I'm fairly new to woodworking and I decided that I wanted to build my own router table insert for my table saw. (It's a Ridgid R4512). Without boring you with details, I laminated together a 3/4 in piece of melamine board with a 3/4 in piece of MDF. I edge banded this with 3/4 in Oak. I got all of this sized to fit the opening in my table saw. No sweat.

I ordered a Rockler router table insert plate and their pattern (being new to the game, I decided it was worth the $10 for a ready made pattern). And, I used my Hitachi router with a 3/4 in top-bearing pattern bit from Diablo. I used the fixed base on my router (probably a mistake) and routed the pattern.

The first thing I noticed was that the collet chewed up the top of the pattern. Obviously, I should have used the plunge base for the necessary depth on this rout. But, I also managed to gouge the pattern with the router and hence my recess reflected the same gouge. Not a big deal. It fit in nicely. But, being an idiot, I went to cut out the hole with my jigsaw and forgot to leave a rabbet for the plate! Yes, I could have glued on some rails, but I wanted to try and not to gouge the side this time. (Better is the enemy of good!)

I built another, and this time, in addition to sloppy routing, I cut the corners for the screws too tight, and I couldn't get the leveling and attachment hardware in. Another one for the scrap pile.

(Anyone need two slightly imperfect router table inserts???)

OK, try #3. Built another table insert. (I'm getting good at it now.)

And, I built a new pattern from 1/4 in MDF in 3 in wide strips. I used double stick carpet tape to hold the pattern tight against the plate. Then I used my plunge base this time (major improvement!) and I made a center piece so the base was evenly supported throughout. Not necessary, but I found it a lot easier.

I routed everything, and the corners might have gone a bit more round than the plate, but I was happy enough with that. I sawed everything out (properly this time!), and I went to fit my plate. Now I found that my recess was 1/8 in wider around than the plate.

How is this possible???? Did I have the bearing riding too low on the pattern? The rocker plate locks down, so I don't feel it is critical to redo this. It won't shift around …

But, I'd really like to learn from my mistake.
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The pattern may be designed for a 5/8" copy ring using a 1/2"
bit. This is a pretty common way to cut out patterns.

I would glue narrow strips of something like oak in there,
squaring corners, and re-route using a copy ring.
The pattern instructions were for a pattern bit and no ring. Then when I made own replacement for the pattern, I butted the pieces of MDF up flush to the plate. So, there shouldn't have been any offset there. This was the one that came out larger than intended.
You'll want to take very light passes.
Also, it looks like your last attempt was routed in the wrong direction, which is why the corners are a bit gouged.

Were the pattern sides square to the faces?

Also, FWIW, I never felt the need for a router plate. It eliminates and possibility of installation problems.
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