Is this a router bearing issue or poor technique?

[ironhat]

I'm routing a 3/8" radius on 1.5" thick cherry and I'm getting a depression which corresponds with the top edge of the bearing. This occurs along almost the entire length of all of the boards and the only way to preent it is to back off on the pressure applied against the bearing. Of course, the downside of this seems to be the occassional bobble in the routed edge. f I re-do it with more pressure I get a good edge but the groove reappears. I have cleaned and re-oiled the bearing but it just might be bad. I believe this is one of the large sets that I got from Brian but I wouldn't swear to that (just by way of identifying the bits - no slight intended to Brian's good name and products). So, what have I got? Bad bearing or bad technique?

[Uncle Cracker]

I'm betting on a bearing problem. It shouldn't gouge or scrape against the work, even under load. A replacement should be easy enough to get. Just change it and see if the problem goes away.

[LCHIEN]

The bearing of course should spin freely to minimize any rubbing marks, The bearing rotates at 23,000 RPM relative to the bit so that keepeing them clean and lubricated is important.


But I'm not 100% sure you have a bearing problems although that could cause your problem. More likely if the bearing were not turning you would get burn marks along where the bearing was contacting the wood.

Remember that the bearing surface is only about 3/16th wide in one dimension and a line contact in the other direction so if you push hard against the bearing it will leave a mark even when its rolling correctly because of the small contact surface.

My technique when using bearing guided bits is to not force the bearing against the workpiece real hard in any case, but to push very lightly. Yes, you get an ocassional bobble or incomplete cut but the beauty of the bearing is, it limits the travel perfectly every time so you can make as many passes as you want and stop only when you hear no more wood being cut ("ZZZZZTTT"). On each pass, press very lightly so as not to make a bearing impression. In fact, I normally don't try to cut all the way to the bearing on the first pass. Taking light cuts also allows you to go pretty fast and thereby avoid burning which is easy to do with cherry, IIRC.

It's not uncommon to have bearing marks even then but if you don't push too hard, they can be removed with very light sanding.

Additional note: are you using a table or handheld?
If handheld, and you are getting a impression from only one side of the bearing you may not be holding the router square to the work, thus putting all the pressure on one edge of the bearing. I usually do roundovers on the table with a safety guide pin - ensuring perfectly square routing and the bearing impression, when I get it, is as wide as the bearing itself, not just one edge.

[ragswl4]

When I route a straight piece with a bit that has a bearing I set the router fence even with the bearing then back off the fence .005". I have an Incra Ultra so its fairly easy to do. Otherwise do the same and then back off your fence about the thickness of a piece of paper. It might take a couple of adjustments of the fence to get it right. Make test cuts in scrap first. That should keep the workpiece from pressing on the bearing too hard.

[Tom Hintz]

What LCHIEN said.
I went through this with bearing tipped router bits and the only answer I came up with was using the bearing to follow the wood, not press it against it. It takes some getting used to but once you do it a few times is simple enough.
In hardwoods now, I hardly get any line at all. In soft stuff like pine that little depression is all but unavoidable but does sand out easy. Whenever possible, I use the rougter table so I can isolate the bearing.

[LarryG]

Adding to what's been said:

First choice: router table and fence, with the bearing isolated or removed.

Second choice: edge guide on handheld router.

Last-resort choice: bearing-guided bit.

A table fence or edge guide (which is really just a hand-held fence) will average out any imperfections in the wood or pressure on the router over its entire length. A bearing guided-bit is the last resort because it applies all the pressure on one tiny spot and will amplify every nick or imperfection in the edge it's following.

[ironhat]

Well, I cleaned the bearing and re-oiled it. I got the same result so I moved the fence into position (sorry I didn't mention that, Loring) so that a straight edge was just even with the bearing ="s better result. Thanks for the input.

[LCHIEN]

Well, I cleaned the bearing and re-oiled it. I got the same result so I moved the fence into position (sorry I didn't mention that, Loring) so that a straight edge was just even with the bearing ="s better result. Thanks for the input.

Yes, that'll work.