Thread: Tear out on a rebate

Originally Posted by Maxx02

I'm trying to rout a rebate on a piece of pine for a cupboard door frame. I was taking shallow cuts and routed an inside channel first before doing a second pass on the edge. I keep getting ugly tear out, what am I doing wrong? Should I have done the outer pass first before doing the final pass inside?
IMG_4728.jpg

What method are you using? (1) Table mode (2) Side Fence Attachment (3) Rebate cutter (4) or any other method?
Or is it simple going against the grain?

Tear out when rebating is a pretty common problem. It's too easy to set yourself up to have the edge fibres completed unsupported and therefore relying on the sharpness of the cutter, rpm/feed, grain direction and the timber properties in general.

Cutting with the grain will solve a great deal of the problems, but it is not always possible, or with rebating it is not always the easiest option. Ideally you select your work pieces and orient them in a way so that the later processes like rebating or edge profiling allows you to easily rout with the grain. It is something easily forgotten until you actually pick up the router and check the material only to find you have forced yourself to rout against the grain, doh!

Supporting the edge fibers, even when cutting with the grain is a very good habit to get into. I would normally do this type of work on a router table using a fence, even if the router bit is equipped with a guide bearing. I use the fence instead of the bearing because the fence aids control and therefore a more consistent feed speed, but it also acts as a chip breaker reducing the tear out. If you set the fence close enough to the cutter, any tearout will only fold back until it hits the end of the fence and then it will break. By the looks of your photo, you are having splinters being torn off indicating zero edge support. Ideally, if it is worth your time, you would add a temporary fence and use that to create a zero clearance fence. This when used properly almost always prevents the tear out that you are getting. And it actually sounds much much nicer when it is cutting, because it is actually cutting rather than splintering the timber apart.

Now to use a fence to support the fibres also means you need to have the fence in contact with the edge. So you can't really cut multiple passes, which is fine because it's only a rebate, single pass all the way baby.

The other thing, which is really nice when using veneer panels, is to use a cutting gauge to score a line where the rebate will be. This is something I almost always do with veneers because it is rare that I need to do a lot of the work which warrants setting up a zero clearance fence. I don't know about any fancy router bits (i reckon they exist for big big $$$$, bugger that!), but rebating heads in square end tenoners or on spindle moulders have turn over knives acting as knicker blades. These knives pretty much do the same thing as the cutting gauge, but it does it a millisecond before the main blade cuts through. It's not fool proof, but it does a pretty good job.

In short, I would set up my router table with the rebate bit in. Set the fence the appropriate distance to make the depth of cut. Set the fence super close to the blade and I would cut with the grain if I can.

If I was doing it hand held, I would set the router with a fence and climb cut about 1-2 deep into the face, then I would do a second cut a full depth into the face using the bearing or fence. That's two passes and handheld, bah humbug I like my router table

Here's my set up. I think grain direction might be the issue, thanks for the advice.IMG_4729_Fotor.jpg

Another thing is to use a rebating bit. You can use a standard straight router bit like you have in the photo. But check that the outer lower tip of the cutter is the lowest point on the cutter. Otherwise if any part of the inner bottom edge of the cutter is the lowest point, it won't cut through the material, but instead smash its way through. Pay very careful attention to the trailing edge of the carbide tips, sometimes these are ground flat or angled upwards.

To me it looks like you are trying to do the full depth first, DON'T. Try setting up to do the width in full (if you wish) and do the depth in small incraments.

My brother is a retired manual arts teacher and he showed me a few techniques to help with this sort of problem (that I will probably be shouted down for using) and it is intended to limit this sort of tearout in brittle timbers. The first technique is a three pronged attack, first use a small diameter bit say around 1/4 inch, second limit the depth of cut so the full depth of rebate is achieved only after around 4 or more passes. The third is the tricky and some may say dangerous part. You very carefully and only when the timber is fully controlled repeatedly push it onto the cutter from the front to create a scolloped edge. This cuts the long grain and limits the length that can splinter. Then make a pass against the fence to around 1/4 of the final depth or about half the diameter of the bit being used. Reset the depth of cut to the next increment and repeat the scolloping and routing along the fence. Keep going until you have a rebate.. Only do this technique with a small bit and if you are confident you can control the timber, use extreme care.

One of the other approaches was to route a grove and expand it until it is a rebate, cutting from inside the timber can prevent splintering. The last option is a variation of the second and is not always possible but to create the rebate using two cuts with a three or four tooth slot cutter bit. The cuts are at right angles to each other. The idea here is that both side of the cut groves are supported with both cuts..
I hope this helps. All the best.