Age old problem - Tearout with router

[LCHIEN]

The above is a picture using a 1/4" solid carbide upcut spriral bit. 25,000 RPM, 1/16th inch deep cut. Router table mounted router and a fence. Birch Plywood.
This is why I don't like doing dadoes on a router. Table saw dadoes are always perfectly clean (of course a blade set costs $100).
I always get tearout doing grooving on a router. It cleans up (mostly) running a sanding block over the grooves but sometimes you have to get the block around the edge to clean it up.

Just to review so we don't get into a discussion, a upcut bit has the spiral so it pulls upwards (towards the router) when using router. In this case it was a table mounted router so the real world direction was down (towards the camera in the pic). So I get similar results with a straight flute bit.

Anyway, will a spiral downcut bit really fix this and make a clean edge? I don't have one, I guess they are about 16 bucks at MLCS.

Or do I have a technique problem. Router is working pretty hard even with shallow feed, I usually won't take more than 1/8th in this situation. Here I only took 1/16th inch. Moving moderately fast to avoid burning. A deeper cut risks breaking the bit and burning from too slow a feed.

[jussi]

I bought several whiteside compression bits (up and down spiral) on eBay before and they work great. No tearout at all. It's not as fast at ejecting chips as a pure up it so you do have to move a little slower to avoid burning.

[leehljp]

I have one down cut spiral, and it has been a while since I used it. It cut great and smooth as a dado. I later used it with a router guide to smooth up a side on plywood and the top was smooth like a saw blade cut. But the bottom was frayed somewhat like your picts.

I looked at the up-down compression bits like Jussi mentioned but never bought one. The type of cutter edge does make a big difference, IMO.

[phrog]

I have read that you can run a piece of masking tape along the surface to be cut so the blade is cutting within the margins of the tape to avoid this problem. Never had reason to try it since reading that - so I don't know if it works. But a special bit would be a much better choice if you have one.

[jussi]

You could also run a marking knife first but depending on how many you have to do it could be very time consuming.

[LCHIEN]

Oh, yeah, forgot about the compression spiral bits. Would be good for through-slots.
But expensive! $30 for a 1/4" compression spiral vs $17 for the down spiral.

[TimTucker]

If the table saw works well, why not use that and just make successive passes with a standard blade rather than a Dado set.

Seems like the only difference would be incrementing in multiple passes horizontally rather than vertically. Depending on your router setup, adjusting the fence on the saw seems like it would be easier than adjusting the router height as well.

[poolhound]

Or do I have a technique problem. Router is working pretty hard even with shallow feed, I usually won't take more than 1/8th in this situation. Here I only took 1/16th inch. Moving moderately fast to avoid burning. A deeper cut risks breaking the bit and burning from too slow a feed.

Loring, Upcut bits are probably not the right choice for this. They are intended to clear waste efficiently from a deeper cut like a mortise. As others have said a down cut is intended to get a crisper edge. given your described scenario where the board is upside down on a router table and I assume you have DC connected there should be no issue clearing the cut. I regularly do cuts like this with a straight bit and get pretty clean results.

Given that you said this is a 1/4 inch bit and you are taking only a 1/16 and that the router is working hard I am thinking something else may be going on as that doesnt sound right. You should be able to take way deeper cuts than a 1/16 and have the router eat plywood like butter. this is very odd is there any other clues you can give us? how does your setup work with solid stock?

[JimD]

I have a jig that requires a top bearing bit that I used to use for dados with a router. The bearing and the bit are 1/2 inch so the idea is you set the distance between the guides to your material thickness plus a little and route away. It never gave me results like you show. I don't know if your bit is a bit dull or if the small diameter is a contributor. The tips of a 1/2 wide bit are moving a lot faster than the tips of a 1/4 wide bit. The bit I use with this jig is just a straight bit. If I make a dado with the router, that is normally what I would use.

[dbhost]

Interesting. I have never done dadoes with a router, always done them on the table saw... I have done plenty of rabbets though, and never get tearout... I DO get tear out trying to do dovetails from time to time, I am pretty sure it is a technique problem.

[cwsmith]

I don't recall ever having cut a rabbet/dado in plywood, though I have done so in pine, cross-grain. I use a straight shear bit. (I think "shear" is the proper term; basically the cutting edge is at a slight angle, instead of straight up the the side of the bit.)

For something like ply where the veneer is sort of brittle, I think I'd score the edges with a knife or tape them. I have gotten that kind of tear-out when cutting ply with a circular saw. The best remedy for that was to simply tape the blade path and then take care in removing the tape afterwards. While it didn't seem to bother the blade at all, I don't know if it might gum up a router bit though.

CWS

[dbhost]

I would think the tape would gum up a router bit. Might be worth it to test that idea though...

[poolhound]

I would think the tape would gum up a router bit. Might be worth it to test that idea though...

I have used painters tape to protect against tear out with all types of saws and the router. not had a problem gumming things up.

Now duct tape that would be a whole other problem :-)

[Black wallnut]

I also have mostly used straight two edged bits to do dadoes in plywood without this chipout. Some times I use a single edge bit for narrow slots. It has been so long since I have used my 1/4" spiral bit to dado plywood or solid wood for that matter that I can't remember what my results were.

[jussi]

Yet another option is to use a pattern bit with an exact fit jig that has the slot cut by the bit itself. That way ithe jig acts like a splinter guard when used.