Help with rebate router cutter on plywood

[anynick]

A beginner with this technique, and struggling. I want to put a rebate in 18mm birch plywood bookcase uprights, to accept a back panel. I have a brand new Trend rebate router cutter. I have done one experiment and the tear-out is terrible - 1" or more of the veneer layer torn off and thrown around the workshop.

The rebate cutter produces a 12.7mm rebate which should leave 5.3mm remaining, though so much of the ply was torn away that in practice the bearing was running on the next layer down, so there was actually mostly only 4.5mm-ish remaining. I took a 5mm depth pass in one go. The router was running along the edge of the plywood but supported by a surface level with the plywood edge - I don't believe it was a tipping problem. The rebate was across the grain of the veneer layer.

I can think of a few things to try, but was hoping someone could short-cut this and tell me the right way to do it. Things I could try:
1. Only put rebates parallel to the grain of the veneer layer - I suspect this would at least limit the length of the tear out.
2. Take shallower passes, though from the tear-out I got I'm not sure how this would help
3. Use a parallel guide to limit the initial width of the rebate, take several passes, and finally let the bearing set the finish width - sounds lengthy and it's not obvious to me that it would help.
4. Use a straight router bit and parallel guide to a take a couple of mm out, down to full depth, then switch to the rebate cutter.
5. You can't rebate plywood with a router - I hope this isn 't true!

Help!!

 

[Sgian Dubh]

It's possible the surface veneer is not as well attached to the core veneer as it could - the vagaries of production can be a cause of that. Another might be that you're using a low quality product - not all birch plywood is good stuff. You most definitely can rebate plywood with a router. I've been doing exactly that for more decades than I care to remember, ha, ha, and seldom have I needed to employ the first suggestion below, but it's possible you're taking a bigger bite than your router and cutter can handle.

Anyway, in this case:
*1. try using a cutting gauge to score a line 12.7 mm in from the edge across the grain of the surface veneer. It wouldn't hurt to do the same going with the grain too.
*2. take bites only about 3 mm deep working down to your required depth. Slainte.

 

[Random Orbital Bob]

Just one thought. Has your router got a speed control dial and if so is it wound up as fast as it will go or at least to the recommended limit of the cutter?

 

[anynick]

I'm wondering if I am doing this all wrong. Here's a picture, I hope you can make out what's going on, the ply is clamped to a support piece, the router bit has created the 12.7mm rebate, leaving 5.3mm of the 18mm, and I have gone 5mm deep. You can see how bad it is.

Yes, the router has a speed control, and it's at max - in fact, I did wonder if I should slow it down rather than speed it up, because the cutter is 35mm dia overall. In general the birch ply seems to be very high quality, but I suppose there might be an issue - it saws well enough though, with virtually no breakout.

 

[DonJohnson]

I may be wrong, but is the breakout because you are cutting 'on the edge' and the cutter bottom is naturally pulling the surface way from the other plies as there is nothing supporting it.

I would have expected the cut to be made with the router on the face of the plywood, so that the side of the cutter is trimming the surface layer - if you understand what I mean.

 

[deserter]

I may be wrong, but is the breakout because you are cutting 'on the edge' and the cutter bottom is naturally pulling the surface way from the other plies as there is nothing supporting it.

I would have expected the cut to be made with the router on the face of the plywood, so that the side of the cutter is trimming the surface layer - if you understand what I mean.

Agreed

 

[nev]

You could also clamp a sacrificial piece of ply (in red) to the face that is being torn out. That way the remains of the sacrificial will hold down the leaves of the ply that would otherwise be torn off.

 

[RobinBHM]

Are different size bearings available for your cutter, it looks like you need a 5mm rebate.

It is tight but you should be able to run the bearing on the 5.3mm on the final cut to depth.

 

[Hemsby]

I may be wrong, but is the breakout because you are cutting 'on the edge' and the cutter bottom is naturally pulling the surface way from the other plies as there is nothing supporting it.

I would have expected the cut to be made with the router on the face of the plywood, so that the side of the cutter is trimming the surface layer - if you understand what I mean.

+1 for this.

I also would have thought that the router base would have been on the ply face, then use a standard cutter. Cutting as you are your cutter is picking at each layer and lifting it towards the surface :?:

Also with the router base on the ply surface you would have more control.

 

[Sgian Dubh]

I'm wondering if I am doing this all wrong.

Now I see your problem. You're methodology is wrong. Rotate your router through 90º so that its base rests on the wide face of the board, as others have said. The problems will go away as long as you use the router pushing in the correct direction. The right direction is a normal cut, which is, with the edge of the board facing you that you want to rebate, move the router from left to right. Slainte.

 

[anynick]

Thank you everybody. Obviously (now) it makes much more sense to do it the other way round. I had got profiling and edge trimming in my head, stupid of me. So now I need to buy another bearing - the spending never stops does it.

 

[Sgian Dubh]

So now I need to buy another bearing - the spending never stops does it.

Not necessarily. Your router I'm assuming came with a side fence. You will probably find you can use that to set the width of cut, and use the depth stop settings to set the depth. You may even be able to use your existing cutter, although you might need to remove the bearing.

However, you do come across as so very inexperienced in basic router use that I really wonder if you ought to sign up for some instruction, or at least get someone familiar with routers to set you off in the right direction. Router cutters operating at full speed really don't care if they're cutting wood, flesh, bone, etc, and if you don't know what you're doing with the tool you could end up cutting the latter two rather than the intended target of wood. Slainte.