Flush trim and straight bits grabbing the wood

[Matthew Schneider]

I was trying to using a flush trim on maybe 1.5" of eastern red cedar on a router table, and it kept trying to grab/throw the wood and tear chunks out of it. I've noticed the same thing trying to take light but full length passes with a straight bit.

I didn't think this was just bad practice, but maybe I'm wrong? What could I be doing wrong? I'm definitely going the right direction. The bits should be sharp, especially the flush trim bit which is fairly new. I'm thinking maybe the speed is too high or low? But I'm not sure, and scared to experiment.

The flush trim bit is 3/4" diameter, and was running at around 19,000 rpm.

[Rod Sheridan]

Hi Matthew, I'm no router expert, however I use a shaper for pattern copying a lot so here goes;

- make sure you're feeding against the bit direction of rotation. (If the bit rotates counter-clockwise you feed from right to left

- your speed sounds OK to me, low speed results in grabbing/kickback

- do you have a starting pin or lead in ramp on your jig so that you can start feeding the piece in a controlled manner? Otherwise it can grab when starting.

- is the grain of the wood in the correct direction? You may need to flip the wood over for some grain directions (Shapers can reverse spindle rotation to take care of that).

- don't try to take off too much in one pass, perhaps you have to much material to trim off?

Regards, Rod.

[glenn bradley]

I'm definitely going the right direction.

Using a fence or no? Whether there is a fence or not you should be feeding the material from right to left with the material between you and the bit. Just making sure the material isn't between the bit and the fence.

I do a lot of pattern routing which is fundamentally the same process; a reference material ride the bearing and a slightly oversized piece of material is cut down to meet that reference surface. While it is certainly possible to route uphill while flush trimming this can be avoided with a top/bottom bearing bit.

In template routing there are curves and one frequently crosses a direction change in the grain. I route to just before the direction change, then flip the piece and route downhill to and just past that location.

[Mel Fulks]

Sometimes just using a coarse piece of sandpaper to slightly round the saw torn corners will make for less grab tear.

[John Lankers]

Cedar can be so nasty sometimes that it is better to sand and not bother using the router, the fibers are just to soft to back up the cut.

[John TenEyck]

When I have or anticipate that problem, or when I want to guarantee no tear out, I use a handheld router and use a climb cut. If you clamp the work down, and set the bit to take a light pass, it is not problematic. I would take at least 2 and more likely 3 passes to create a 3/4" round over.

John

[Prashun Patel]

"kept trying to grab/throw the wood"

That doesn't normally happen feeding in the right direction. Where are you starting the cut on the piece? At the end? Is it straight or curved? If the curve is convex or straight, I would set the fence almost flush with the bearing so you can use the fence as the 'starting pin' and feed completely from the right to the left - into the bit, instead of having to pivot the piece onto the bit close to - but not AT the corner.

[Matthew Schneider]

It didn't occur to me to even think about grain direction, so I could have been doing that wrong.

I was using a fence set just barely behind the front of the bearing, as one would a starting pin.

Again, definitely sure I was feeding in the correct direction, it's something I always think about before cutting.

As far as too much material goes, I was using a lot of the cutter, but trying to take the lightest passes I could. I do this with a handheld router not infrequently without issue, but with a normal straight bit, it quickly tried to throw the piece away (although that was a while ago, and I don't remember the details of what I was doing).

[Lee Schierer]

When routing the edge of boards, it is a good idea to take a close look at the grain along that edge. Look for areas where the tip of the cutting edge will be hitting the ends of grain. These areas are likely to splinter. A light pass first can also help prevent serious tear out and splitting. I've had some pieces that I made where the only way to prevent destructive tear out was to climb cut those areas. You have to take light cuts when climb cutting and the router and work piece have to be handled with care because the router will tend to self feed. Climb cutting will eliminate the reverse grain tear out. What usually works best is to read the edge, mark transition points with a pencil or chalk and then use a combination of normal cutting and climb cutting to get a perfect edge.

[Wally Alexander]

I had the same problem as you trying to pattern cut pine with a 3/4 flush trim bit. It got to where I was afraid to use it. I switched to a smaller diameter bit, I believe 1/2 inch, and that took care of it. I can't explain why. I will say though you still need to be wary of taking light passes and be aware of grain.

[glenn bradley]

It didn't occur to me to even think about grain direction, so I could have been doing that wrong.

This combined with too big a bite is my best guess.


I was using a fence set just barely behind the front of the bearing, as one would a starting pin.

This concerned me as to the amount of material per pass but, then I kept reading ;-)


Again, definitely sure I was feeding in the correct direction, it's something I always think about before cutting.

Ok, we're good there.


As far as too much material goes, I was using a lot of the cutter, but trying to take the lightest passes I could. I do this with a handheld router not infrequently without issue,

I do the same at the router table with or without the fence present (starting pin for curved material). I just take 1/16" or so per pass and end up with the template riding the bearing. The fact that you are getting "grab" makes me lean toward grain direction, type of material, etc. I can get away with things on beech that I would never do with something fibrous like shedua for example. Setting the fence for a shallow cut and then stepping it back could help if grain direction turns out to be correct. In that case I would suspect the material and using the fence as a "restrictor" could solve the problem.