Flush trim bit question* PLEASE SEE ADDED POST

[Mark Rios]

Am I correct in thinking that a flush trim bit should cut only to the outside plane of the bearing? In other words, shouldn't I be able to, in theory, make a pass on the router table with a flush trim bit and and a square edged piece of wood and have no material taken off?

My question comes from using the flush trim bit in the Woodline set of router bits that I got from Woodcraft a while back when they had them on half off sale. Remember that 10 piece set for $39.95? I realize that these aren't necessarily the cream-of-the-crop bits and the these ten cost almost as much as one and a half toop-rated bits but I tought that they should at least have accurate machining. Maybe the bearings don't last very long or that they don't stay sharp very long but shouldn't they cut accurately?

I edgebanded some plywood with 1/8" oak. I made a pass through the router and sometimes the bit would take off the edge material AND some of the plywood. It would cut inside the plane of the bearing. Am I doing something wrong or should I just not use the bit anymore for finish stuff?

[Laurie Brown]

A flush trim bit should cut flush to the bearing and no more. If your bit is cutting deeper than the bearing, I wouldn't use it.

Most of my bits I got on sale at Sears or Woodcraft, so they're not the top quality either, but the flush trim bits I have all cut to the bearing.

[Steve Cox]

Since the problem is not consistent the issue is probably more with technique than the bit.:( When trimming like you described it is very easy to tilt the board you're trimming a little too far and cause the problem you describe. A high fence on the router table plus some feather boards to keep things moving without tilting help me. I know this from personal experience with a much more expensive flush trim bit.:rolleyes:

[Laurie Brown]

Ah, I didn't read the original post correctly. I thought the problem was happening all the time. If it only happens sometimes, then it's not the bit.

[Mark Rios]

Sorry, I wasn't clear above. By sometimes I meant when making a final pass. Some of the pieces have almost 1/8" of material to take off so I would make a light pass and then the final pass. Somepieces have 1/16" or less and I would only make only one pass. In either case, on the finish pass the plywood had some material taken off. Also, I did take the time to make/set-up a tall fence so that I could be sure and keep the material square.

I have better bits but I thought that I would use these up. I guess these will be used up faster than I thought.:D

Thanks very much for the confirmation and for the info.

[Marion Rood]

If you bought them at Woodcraft, take them back. CMT is 25% off right now at woodcraft. Apply the credit to a good bit.

[Marion Rood]

Ah, I didn't read the original post correctly. I thought the problem was happening all the time. If it only happens sometimes, then it's not the bit. Laurie,
See you're new here. I'm new myself, but welcome.

[Rob Blaustein]

Mark,
Sorry if these are obvious questions: Are you sure your edge is 90 degrees to the face of the board? Other than checking it with a machinist square a hint would be if you have this problem on only one side of the ply. You might try running the bit over a scrap piece of ply without any edge banding to see the pattern of how the bit is cutting. How about bit depth--you should only need the cutting edge to go just a hair below the edge band. That would minimize any cutting of the ply.
Rob

[Dave Falkenstein]

It sounds like the bearing is undersized or the bit is oversized. If you can, measure the bit and bearing diameters to pinpoint the problem - use a caliper if you have one.

Woodline has excellent customer service. Call them, and my bet is they will replace the bit or bits that are cutting too deep, with little effort on your part. I had a problem getting the wrong bit on an order from Woodline, and they sent me the right bit which arrived the next day, and told me to keep the first bit I got.

A better way to do edge trimming on large panels is to build a jig that holds the router flat on the panel surface, and uses a flat bottomed straight bit to trim the edging. There is a plan for that jig in the book, "Router Magic" by Bill Hylton, available on amazon or in most large book stores.

[Jeffrey Makiel]

I don't think it's the quality of the bit. When you set the bit height from the base of the router, was it at the minimal distance such that the bearing just surpasses the edgebanding to ride on the board and no more?

The problem you describe often comes from setting the bit depth too far out from the router's base. When this is done, any slight tilting of the router or workpiece will lead to a scalped part. You will not even know you're tilted either because all it takes is a very small tilt.

The disadvantage of using the minimal depth setting is that sometimes the bearing gets gummed up from the edgebanding glue or contact cement. However, this is easily remedied by cleaning the goop off the bearing with your fingernail (of course, while the routers off!). In fact, this is why I use cheap flush trim bits for laminating work, and I never had seen a defective non-flush trim bit that could cause this problem...at least to your extent. The other disadvantage is that you don't use all the carbide cutting edge on the bit. In fact, I only use about 1/16" of the edge closest to the bearing...which is the second reason why I use cheap flush trim bits.

cheers, Jeff :)

[Charles McCracken]

It sounds like the bearing is undersized or the bit is oversized. If you can, measure the bit and bearing diameters to pinpoint the problem - use a caliper if you have one.
Another possibility is the bearing off center (not concentric with the bit). If carbide and bearing measure the same then this is the likely culprit.

[Mark Rios]

Okay, here's an update.

First I must state that I was wrong and that I apologize to Woodline for any negative information and/or ideas that I am responsible for. These are NOT Woodline bits. They are Woodcraft bits. I apologize again to all and to Woodline.

Second, some of you replied to my thread earlier regarding the gluing up of mdf for a fence and my mistakes in doing so. (Thanks again to those of you who responded). I thought that I had taken great pains to make sure that the second set of panels came out flat throughout so as not to cause any out-of-squareness issues with the fence. Well, I checked and my glued up panels turned out to be okay; flat, flat, flat. So I turned my attention to the triangular fence body itself. I wouldn't have guessed it but it isn't square to the table. So I took the fence off and apart and checked the 90 degree corner of the fence with my 7" Incra square and it is, in fact, out of square, thereby causing the scalping (thanks to jeff for identifying the word I was looking for).

Third, after checking the WOODCRAFT bit very carefully, the bearing does seem to have a little bit of movement to it (side to side) but I tend to think that the scalping problem was probably caused by the fence.

Thanks agan to all for your input and help.

[Michael Adelong]

Mark,

Remember to check that the little cap screw holding the bearing in place is tight.

I also had an issue using a Woodcraft 1/4" flush trim bit recently. I made a small vertical rabbit about 3/16" deep along an entire edge of my MDF torsion box assembly table. I went about 8" around the first corner before I saw what was going on. The stupid screw came all the way out and both it, and the bearing, hit the floor. The little 1/8" nub that that goes into the center of the bearing and that the screw goes into was acting as a bearing.

It was my first time using a flush trim bit. Learned a good lesson. From now on, I will be checking the bearing every time I use a bit. Good thing it was just an assembly table. :cool:

Michael