Router problems following a pattern in cedar...

[Doug Hobkirk]

I am making an Adirondack chair out of redwood and cedar and having problems with the pattern routing.


I am using a Veritas plan that I glued to MDF and then cut and sanded.
My wood is straight-grained redwood and cedar with virtually no knots. It is all slightly under 3/4" thick (I am only using 2 pieces of cedar).
I used 2-sided stickies to create a "sandwich" consisting of the pattern and 2 pieces of redwood.
I cut the sandwich on the band saw a little over sized.
I run the sandwich against my "pattern following" (?) router bit -

I'm using a router table with a Bosch router.
My bit has 2 flutes just over 1.5" long plus a flush bearing.
I am moving the wood right to left, avoiding "climb cutting" (?) except on occasions when I think the L-to-R direction will cause a corner to chip off because of the direction of the grain.



My problem is that despite my caution I am still having problems with chunks of wood splitting along the grain and breaking off. I've had to glue 3 pieces back, including two that were about 6" long and about 1" wide (these were on the cedar). I also have some problems with chatter going cross-grain, especially on small (4x5) pieces.

QUESTIONS


Is the problem the particular wood species?
Is the problem that the router bit only has 2 flutes?
Does routing 2 pieces at once aggravate the problem (it seemed like it would be much of a strain since the wood is so soft)?
Or is this normal?

THANKS

[Walt Caza]

Hi Doug,
Sorry for your tearout troubles...
Did you know that LV sells router bits with bearings both above and below the flutes?
Intended to solve the exact problem you are wrestling with...
a combination of a flush trim bit, and a template bit.

By changing the bit height, you can alternate which bearing is making contact.
Flipping the pattern and blank, allows you to keep cutting, without changing bits.

I have had one of these on my LV wish list for a while. (link to bit)
http://www.leevalley.com/wood/page.aspx?c=1&p=51851&cat=1,46168,46171&ap=1

Good luck with it,
Walt

[Bill White]

Can you lower/raise the bit to take smaller bites as you follow the pattern? I guess that you would lower it if you're using a router table.
Bill

[Rod Sheridan]

Hi Doug, the issue can be grain direction.

On a shaper you can reverse the spindle rotation and flip the cutter over so that you are always going in the correct direction for the grain.

On your router can you flip the workpiece over to accomplish the same thing?

Regards, Rod.

[Jason Beam]

I've had similar brittle issues with Cedar, too. I've found that cutting VERY close to the pattern also helped reduce the chunks. Sometimes I would have to get pretty near 1/32" or so. it was tricky to do on the bandsaw, but it really helped in a lot of cases - especially those where you're cutting an arc and the grain runs parallel to your arc rather than perpendicular - the crest of the arc kept wanting to split off. By getting REALLY close to my pattern, moving very slow, and sometimes even climb cutting into the danger zones from the other direction BEFORE removing the supporting wood on either side would help a great deal.

[glenn bradley]

Flipping the pattern and blank, allows you to keep cutting, without changing bits.

Not trying to speak for Walt here but I think his point is that the double bearing and flipping capability allow you to be pretty granular about direction of cut vs. grain. The exploding chunks are generally grain direction issues and the bit Walt describes helps you be more accommodating to the material's "wishes".

I had a painstaking, bridle-jointed frame for a cabinet that I wanted to profile the inside edge on, after assembly, for reasons I can't recall (inexperience maybe?). This was a bearing guided profile but the blowout was as you describe and very disappointing.

[Lee Schierer]

I was routing some cherry just last weekend and noted that it had a tendancy to come off in chunks. I immediately reduced the depth of cut to 1/2" of what I was taking off and had only slight chip out problems, which were completely erased by the second pass. Most tearout is caused by grain lifting due toeh cut. In weak grained areas the wood will splinter instead of shaving off nicely. Lighter cuts will generally eliminate the lifting problem. You do need to watch grain reversal and climb cut those areas where the grain is at too severe an angle to the cutting edge to cut efficiently.

[Chris Padilla]

Cedar is very soft and will ding and dent if you look at it funny. I just built a 4' x 7' gate out of it and while I did route it, I only did it "just a hair" (chamfer bit on some edges), I had no issues. Still, the softer woods like pine, cedar, and redwood can be tougher to work. Trying to take a plane to pine endgrain requires an extremely sharp blade...more so than taking that plane to cherry endgrain. The softer fibers have a tendency to bunch up and get pushed together as you attempt to cut them. You may find using an spindle sander with a pattern bushing on it to work better on softer wood.

Chattering can happen with bits that are small. The larger the bit, the more mass it has, the less chatter it will impart to your wood and the cleaner cut you will get.

Also, if the flutes on your bit are straight, getting a bit that is cutting at more of a shear angle will improve the cut. CMT makes a 3/4" bit with a shearing profile that may (or may not) help your situation. (http://www.cmtutensili.com/show_items.asp?pars=RB~7/8/906.191~2)

[Tom Veatch]

...My problem is that despite my caution I am still having problems with chunks of wood splitting along the grain and breaking off...

As others have said, take very light cuts. Don't try to route it to final shape with a single pass. I don't know how to describe it except as almost like planing or sanding to final shape. Be sure the bit is sharp and clean. A shearing cut bit rather than straight flutes will help some as will climb cutting in those areas where the grain is nearly parallel to the edge. But very light "shaving" cuts with a sharp, clean bit are necessary to avoid splitting along the grain.

[Doug Hobkirk]

Very useful info - thanks!

Walt - thanks for the link - you started me searching - I've found a dual-bearing shear flush bit - BOO - I don't want to buy more bits!
Chris - note the impact of your "shearing profile" implicit in my comment above - great explanation of the action of the wood fibers.
Jason - right on point - Thank You!
Rod, Lee, Tom - I tried to focus on grain, but I needed to focus more!

[Bill Huber]

Doug, I do the same thing as you and I am using a Bosch router on a table.

The keys are 2.

1. The distance from the edge of the template to the edge of the wood on my cuts its 1/8 at most and normally 1/16, lot less wood to remove.

2. When you start the cute make sure you are staring with the grain running with the cut not against it.

Now when cutting go slow and let the bit work, don't force it.

I am using a 2 edge bit and the flutes are slanted, makes a lot of difference in the way it cuts. They call them Shear Angle Flush Trim Bits they really cut much better.
I am just about to start my 6th chair.

This is what I use to cut the wood around the template, I center in on the blade and it makes a very small edge to cut.

http://www.rockler.com/product.cfm?page=18055&filter=bandsaw%20pin