Routing rabbet for back panel in 3/4" plywood cabinet. Lots of blowout - Woodworking Talk - Woodworkers Forum
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post #1 of 8 Old 09-20-2019, 01:01 PM Thread Starter
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Routing rabbet for back panel in 3/4" plywood cabinet. Lots of blowout

Question for you guys.

I built 3/4" BB plywood cabinets for end tables and a dresser. I didn't use the table saw or a dado blade to cut the rabbet for the back, as I wanted to just rout it in, and chisel the edges square. I thought that would be the best plan.

I got LOTS of blow out of the top veneer layer though. Thought it might be the bit, but that was not it, brand new bit does it as well. I tried a with the grain cut, tried a climb cut, everything seems to screw up the top veneer layer.

Is it just a bad sheet of ply? Am I doing something wrong? I've never routed a rabbet like this in plywood before, only hardwood, with no issues.

Should I have used a dado blade and just stopped short and finished it off with a chisel?

Should I have used a dado blade on the sides and cut a groove, then slide a back panel in and butt up against the top?

Halp

Last edited by Tom Hoppe; 09-20-2019 at 01:05 PM.
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post #2 of 8 Old 09-20-2019, 02:41 PM
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You can always score through the veneer with utility knife and then make your cuts, but if the glue up was not good you would have to go through a couple of layers to be sure not to get tearout.

I got some red oak ply from Lowes and it would tearout like crazy. I suspect it was glue-starved or perhaps some new "ecololgy" rated glue did not work very well.

A test would be to score through a narrow strip with a utility knife and the attach some packing tape to the veneer, adhere it well and pull up the tape. If the adhesion is reasonable it will not lift off. If it is substandard it will.
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post #3 of 8 Old 09-20-2019, 03:38 PM
where's my table saw?
 
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Use one of these first ......

Score your line on the inside using one of these before using the router.
https://www.rockler.com/rockler-whee...BoCjfgQAvD_BwE



The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #4 of 8 Old 09-21-2019, 05:12 PM Thread Starter
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Duh. Why didn't I think of this? The first post, I was like "this is going to be difficult to get the right depth", and the second post was "ohh this is one reason marking gauges exist"

Thanks guys!!
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post #5 of 8 Old 09-21-2019, 10:35 PM
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You shouldn't try to take the full cut either. Make repeated cuts at increased depths, or increased widths. Trying to take too much in one pass, especially in plywood, can lead to all kinds of problems. 1/4" or less per pass is what I would do. If the veneer still chips, try a down spiral bit. The larger the bit diameter, the better. Use less than 1/2 the bit diameter.

Charley
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post #6 of 8 Old 09-21-2019, 11:52 PM
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Here are a few hints about those wheel marking gauges:

* They roll with a very large radius, as in "roll off your workbench."
-> Grind a flat spot on one side of the round fence so the wheel marking gauge won't roll off your workbench. The flat spot won't affect the utility of the tool.

* To use the wheel marking gauge, slide it along the edge. You do not roll or twirl it.

* The bevel on the cutting wheel will pull the fence against the edge of the wood.
-> Focus on the cutting wheel. You don't have to press the fence hard against the edge. Let the cutting wheel do its job and keep the fence engaged.

* The back end can wiggle as you slide the marking gauge along the edge.
-> Practice on scrap wood until you can get a smooth cut without wiggle. It does not take much practice.
(It just occurred to me that waxing the fence might help it slide easier, but I never tried it.)
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post #7 of 8 Old 09-22-2019, 11:08 PM
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Actually, there is no good reason for you to get blowouts on BB ply from cutting a rabbet with a router. I do it often enough without problem. The most I get is a burr that I have to sand off. This is both with the grain, and cross grain. You should NEVER need to score for that type of cut. Something is amiss. First off, what type and brand of bit were you using? What was the size of your rabbet? Mine are usually 1/4" x 3/8" for a plywood back, taken in one pass with no problem, using a 3/4" straight bit with a 1/2" shank, and an edge guide, or set up on the router table. I find these bits to cut it more cleanly than a standard rabbet bit. For brands, I use both Whiteside and Freud with equal success. If the bit is not the issue, then I would suspect the plywood. Were you cutting the rabbet on a factory edge? They are notorious for chipping and veneer splitting at the edges. And I am referring to nearly all plywood on that count. Anyway, this is all based on my experience doing a similar cut.
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post #8 of 8 Old 09-23-2019, 07:52 AM
where's my table saw?
 
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OK, you shouldn't be getting it, but ......

You said this:
I got LOTS of blow out of the top veneer layer though. Thought it might be the bit, but that was not it, brand new bit does it as well. I tried a with the grain cut, tried a climb cut, everything seems to screw up the top veneer layer.



So, even with a new bit, and changing feed directions it still happens.

I suspect the plywood has defective glue. Can you try the same process on a different piece from a difference source? A table saw with a dado set would have been a better solution, but who would have thought you couldn't use a router? Scoring seems to be the only I see, for a short term solution.

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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