Tear-out cutting a long mortise - Router Forums
 
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post #1 of 8 (permalink) Old 08-02-2013, 03:15 PM Thread Starter
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Default Tear-out cutting a long mortise

I am making a crib. I cut a long mortise into the cross pieces into which I will put the slats with tenons and spacers. There is tear-out along one side of the mortise. I am using a Bosch 1617 attached to a table that I made and a 1/4" straight bit . What could cause this? What should I be doing differently?
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post #2 of 8 (permalink) Old 08-02-2013, 06:50 PM
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Pics would help. I would guess from what you are saying you need some sort of backer board, a piece of wood behind the piece being cut.
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post #3 of 8 (permalink) Old 08-02-2013, 06:55 PM
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It depends on the type of wood being used. Some softwoods are prone to tear out, especially if the cutter is dull.

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post #4 of 8 (permalink) Old 08-02-2013, 08:43 PM
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Originally Posted by hankh View Post
I am making a crib. I cut a long mortise into the cross pieces into which I will put the slats with tenons and spacers. There is tear-out along one side of the mortise. I am using a Bosch 1617 attached to a table that I made and a 1/4" straight bit . What could cause this? What should I be doing differently?
Hi Hank - If I read this right you are basically just running a slot. The slats fit into the slot and then spacers placed between them.

I'm kinda with James, bit may be dull, cut depth may be to deep. With certain woods and veneers you sometimes need to do a light scoring cut, ~1/32" or so just to break the surface and then do the full depth. Also, with a 1/4" bit you probably need to do the slot in two or three passes, depending on the depth. Slot cutters seem to be more forgiving on tearout than straight bits IMO. Another thing that may help is switching from a straight bit to a spiral.
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post #5 of 8 (permalink) Old 08-03-2013, 01:08 AM
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Semantics, regionally meaning the same thing... Groove, mortise, spline, dado... We all seem to understand descriptions.

Is it a full groove the length of the stock or partial, like a blind dado?

*** here is what is going on and the why of it:
You are using a 1/4" bit in one pass to create a 1/4" groove. Most likely to break out in some woods if trying to cut that in in one pass... Reason for that, is one side of the bit is traveling the feed direction of the cut and the other side of the bit is moving in reverse direction of how the cut should be fed... That off-side is likely to break out and have scorching. See below to get around this by other methods.

- Agreed that a slot cutter is less likely to break out than using a straight bit.

- Dado's are less likely to break out it you route with a slightly smaller bit and do in two passes, one pass each direction.The first pass with have slight break out on one side. The second pass goes in the opposite direction / the correct feed direction for that second side, cleaning up that side.

- Past experience shows me that a spiral bit in one pass- if an up-cut spiral is going to be less tearout than a straight bit if only in one pass. And if you go with a slightly smaller bit in two passes... BUT- Depending on the wood and how sharp, a down-cut spiral is less likely to tearout than an up-cut spiral, but you have to watch that the sawdust doesn't built up too much. Down-cut spirals are also good for doing dado's where there is a veneer, as it has a tendency to push the veneer to the other wood as is cuts, instead of an up-cut that will have a tendency to pull on that same veneer. Be cautious on using a down-cut bit as (depending on the wood) it may have a tendency to try to float or lift a router (if hand held routing) or your work-piece (if table mounted routing... Because the cut is pushing against the work-piece. It's a different feel than most people are used to, but you just need to be aware of how it works and adjust to it.

- Then again, it could also be cut with a dado stack, but don't know how popular they are in Israel... I think grooving blades are easier to get on that side of the pond, but are pricey.
- About 10 other ways to do it...

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post #6 of 8 (permalink) Old 08-03-2013, 05:13 AM Thread Starter
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Thanks. Very helpful. I'm using pine, so the softwood comment makes sense. I made a groove the length of the piece, leaving about 2" at each end. The groove is about 12 mm deep, I cut it in 3 or 4 passes, so that should not be the problem. I'm using a Ryobi bit from a starter set. I didn't see the tear-out in earlier work, so I am guessing the main problem is the bit - got dull after not much use. As I'm new to routing, I'm learning in practice what I've read. A good spiral up and probably down are on the horizon.
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post #7 of 8 (permalink) Old 08-03-2013, 05:48 AM
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Quote:
Originally Posted by hankh View Post
Thanks. Very helpful. I'm using pine, so the softwood comment makes sense. I made a groove the length of the piece, leaving about 2" at each end. The groove is about 12 mm deep, I cut it in 3 or 4 passes, so that should not be the problem. I'm using a Ryobi bit from a starter set. I didn't see the tear-out in earlier work, so I am guessing the main problem is the bit - got dull after not much use. As I'm new to routing, I'm learning in practice what I've read. A good spiral up and probably down are on the horizon.
Hank I agree with your conclusion, if the bit is from a set and not top quality then as soon as the edge goes off a small amount then it can tear soft wood, I am not a fan of cutter sets as they never seem the same quality to my normal bits, if you are going to router all the time then buy better bits, you will not be sorry. NGM
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post #8 of 8 (permalink) Old 08-03-2013, 08:46 AM
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I also made 2 cribs like that. Make sure you do multiple passes and use as straight a grain piece of wood that you have. Of course do not use cheap or dull bits.
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