Wood splits with flush trim bit on router table
bburgboy
| Posted in Jigs, Fixtures, Tips & Tricks on
Can anyone explain what’s happening here? On several different occasions, using this template to trim mahogany with a flush trim bit, the wood has splintered as I progressed down the cut. I had trimmed the stock to within a 1/16″ to the line before routing, so only a small amount of wood was being routed. I suspect that I was moving into the grain – I must admit that I had hard time reading the grain on this wood.
In the other photo, you can see the piece shattered when the stock bounced as I tried taking small bites from the end grain. Never seen wood do this before, but I don’t often work with mahogany.
Bit was an after market item, but not very old. I had router speed on my Bosch at about 3….maybe 15-18K rpm.
Replies
You were using a top-mounted bearing bit, right? The Bosch is a good router, although I think you were under-speed and too deep on the cut. And yeah, the grain could've caused climbing. Did you try flipping the piece over?
All I can say is if you have a piece to practice on, I'd try that. Cut 2 pieces; one end grain, one a slice with the grain. Lay them side-to-side. That might give you an inkling on the grain.
Trim bits are between 1/4 and 1/2 so should run at 20,000 rpm to have enough cutter speed due to the small diameter. There should be no bouncing, your blade either dull or defective.
I think you have hit the nail on the head with grain direction. End grain is particularly tricky at times. In you first pic you would want to go fro the top-center out to each edge. Ideally using a top/bottom bearing bit versus climb cutting although climb cutting is fine on non-problematic material.
If you hold your hand up with your fingers together, palm facing you . . . you want to route from your pinky to your longer fingers, not the other way around.
Spiral bit is needed. The slamming of a flat edge flush trimmer will peel mahogany's grain. The wider the bit, the better your result will be. A compression bit would be even better. I would go 1/2" at least.
As others have said, this is due to grain direction, which is difficult if hard to read. I find the tone of the router often changes with the grain though, so you can get fair warning.
You can minimise this with as many of the following as you can or dare do:
1. Climb Cut - always a bit of a risk, but with larger pieces is safe provided you consider what will happen if the wood gets grabbed. A guide pin will make this much safer, or at least less unsafe. Climb cuts with the grain produce much finer results than cuts against the grain.
2. Stepped cuts. If you are not really close to the line then either change out the bearing on the bit for a slightly larger one or consider a few wraps of blue tape round the bearing for the first cut, then remove it for subseqent cuts.
3. Use a half-inch shank. These vibrate much less.
4. Use a larger diameter bit (same reason as above) @_MJ_ is as usual correct - at least a half inch diameter for most work.
5. Use a spiral or raked bit. Compression bits won't help your problem any more than these, but do produce better finish on both sides of the work.
6. Use the correct speed. Usually maximum.
It is generally not possible to take shorter bites, but it can be done if you have a short bearing guided bit. This can be advanced up (or down) the work with the bearing running on the recently cut surface. This will of course copy any errors but errors are less likely to occur.
In addition to what others have added, that mahogany has very interlocked grain. It makes it especially susceptible to doing what you've shown.
Get that router speed up. A bit that size should be on the highest speed.
Thanks to all who commented. Next time, I will: a) increase speed, b) look to avoid biting into the grain where possible and c) get some other bits - either larger diameter or spiral.
I like the latter option - always looking for an excuse to buy some new stuff.
This is why I own an oscillating spindle sander....
But, I think _MJ_ has hit it on the head with a spiral bit. Rob_SS has some excellent advice there as well.
But, after building a few Telecasters, I was convinced into buying one of those Ridgid oscillating spindle sanders and have found it excellent for cleaning up work like this.
Thanks FIveBirdsCustom. I too have a Rigid oscillating sander but being stubborn, I wanted to learn how to rout with templates on my router table. Next time, this project will be on the sander.
Bit speed issue i.e. not fast enough, and/or dull/dirty carbide. I clean my bits quite frequently.
Ahhhh, cleaning bits. That reminds.....
LOL
BburgBoy, I use templates all the time. But there are just some jobs they don't work well for... end grain and climbing cuts on a lot of woods, like mahogany, curly woods, and even some softwoods like pine, can be a real PITA to get nice clean results, unless you are super diligent about grain direction, sharp and clean bits and taking little tiny nibbles.
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