bearing-guided router bits

[milanuk]

Aren't they normally supposed to cut *flush* with the bearing edge, so you can run the bearing along the template and get an exact match?

I chucked my Bosch 1/2" dia. bearing guided bit (1/4" shaft) in my Bosch Colt trim router, put it inside a template I had made (had to cut a clearance hole to get the bit down far enough to where the bearing would engage the template), and went at it.

I was a little surprised when all was said and done and I removed the screws holding the template onto the piece of scrap I was practicing on... the template had been made by drilling out waste and then working with a couple files for a press-fit. Now it had a very small, almost imperceptible line just below where the bearing had touched the template the first pass... mostly noticeable because above that line, you could still see the slight imperfections/variations from my file work. Below that, everything (including the rest of the template!) was machined smooth. As such, it was just a skosh bigger than the original, and now it is no longer a press-fit in the work piece. Not exactly 'sloppy', but definitely no longer a tight fit.

Did I do something wrong, or am I using the wrong tool for the job?

Thanks,

Monte

[pelligrini]

The bearing could be off a little.

There are also Pattern Bits that have a bearing that is larger than the cutting diameter. Check the image mid page for some of the MCLS bits: http://www.mlcswoodworking.com/shops.../bt_flush.html but your problem is in the different direction.

[milanuk]

I got to thinking about whether maybe the sides of my template weren't parallel and the pattern bit just trimmed them up a bit? If the bit were really bigger than the bearing, it seems like I would have had a series of ridges/lines, one for every time I made a new cut (took me several passes to get down to where I wanted to go), but there wasn't.

I measured the bearing, and it measures 1/2"; the bit measures 31/64" so it shouldn't be cutting anything outside where the bearing contacts. I'm guessing I get to re-do my template - either that or line it with a couple passes of masking tape to snug things up again and give it another go - and use it to make a thinner template this time around using some 1/4" MDF I picked up @ the Borg instead of the 3/4" stuff I have been using.

[phi1l]

Has the bit had much use? With use, the bearing surfaces can wear a little allowing the cutting surface extend farther into the work.

[phi1l]

you might want to try a guide bushing instead of a bit with a bearing.

[pelligrini]

I don't think the Colt's are set up for guide bushings. Mine's just a couple weeks old and still in the box though.

[master53yoda]

I use 1/4" tempered hardboard for my patterns. it doesn't wear and i don't have to worry about it breaking if it is real narrow.

[Habe]

I have a bosch colt and there is an accessory base made to use the Porter Cable guide bushings. It is a round style base instead of the square type base that comes on the colt as standard. I got mine at Rockler, but it is a colt accessory so any dealer should have them.

[cabinetman]

Most of the flush cutting bits with a bearing that I use, (and brand doesn't matter) don't cut flush to the bearing. The cutting diameter is a smidgen smaller than the bearing. What can happen is when the bearing is run against the template, it could be pressing in to the template, causing the bit to cut that depth proportionally.
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