Flush trim bits question

[James Jayko]

After selling my house, I've been using a public workshop here in Philadelphia. Most of my tools (including bits) are in storage at the moment, so I've tried to use some of the orphan bits left at the shop instead of buying them again if possible.

I am trying to take a template from 3/4" BB ply and glue/brad up to about 7 layers of 3/4" to make a form for some bent laminations. When I added the layer and tried to trim it flush, the bit left it a few thou proud. I tried several bits in the box and got the same result over and over again.

So my question is...are these just all worn, cheap bits where the carbide has worn away ever so slightly leaving the cut slightly proud? I cannot imagine a scenario where a flush trim bit was intended to leave the cut just slightly proud intentionally. Just thought I'd get ya'lls feedback before I head to the local Woodcraft for a shiny new, $100 Whiteside bit.

[Prashun Patel]

Yes, some bits are designed to do that intentionally.

It could also be the case that the bearing doesn't line up perfectly. That can happen on cheaper bits.

It could also be the case that when template routing on a table, that the face of the template is not perfectly perpendicular to the bit cutters.

I've never had the carbide wearing away to cause this; when mine get worn, they just dull and burn, but still route flush...

[Brian Tymchak]

Maybe they have been resharpened a few times leaving the cutting edge slightly undersized relative to the bearing.

[James Jayko]

I can't imagine they've been resharpened; none of these appear to be particularly high quality bits. Again, these are just the bits abandoned by other woodworkers at this public shop.

Maybe I'll bring some calipers to Woodcraft to see if bearing vs. cutter head varies by manufacturer.

[roger wiegand]

Probably a reason they were abandoned. One of my first bits was from Sears, it did that. None of the current Whiteside and Freud bits do.

[Bill Dufour]

If it was mine I might hit the bearing outer race on the grinder while it was spinning.
Bil lD

[Mike Kees]

If it was mine I might hit the bearing outer race on the grinder while it was spinning.
Bil lD
Bet that would end up precise.

[Richard Coers]

If it was mine I might hit the bearing outer race on the grinder while it was spinning.
Bil lD
While what was spinning? Hit it on the grinder while the bit is spinning at 22,000 rpm????

[Lee Schierer]

If it was mine I might hit the bearing outer race on the grinder while it was spinning.
Bil lD

If the bearing is working, wouldn't it just rotate as fast as the grinding wheel that is touching it?

[Jim Becker]

My recommendation is to "just say no" to old, dubious tooling and buy something fresh. There are so many choices available for reasonable money, both from the "names" and on Amazon, such as Yonico brand. Dull and/or damaged tooling not only cuts terrible, it's also less safe to use.

[Eric Arnsdorff]

I agree with Jim!
When I got back into working in my shop I tried an old flush trim bit I had (2 straight flutes) and it did ok but burns and not completely flush results. I bought the Infinity compression mega bits and I was blown away at the quality of the cut and the ease of cut.
It always amazes me when I finally convince myself to stop being cheap how much better things end up.

[johnny means]

The correct answer is to make all your parts from the same template with the same bit. They won't match the template, but will match each other. Worn bits do shrink ever so slightly, that's what wear is. Really, though, if you're doing bent laminations that slight discrepancy will be the least of your problems.

[Bill Dufour]

If the bearing is working, wouldn't it just rotate as fast as the grinding wheel that is touching it?


Yes, in theory,then you slowly angle it so it running 20-45 degrees off of parallel. The bigger the angle the faster the grinding action. I would spin it in a drill for lower rpm difference to reduce grinding action.
Or you set them parallel and rest a piece of wood on the bearing to slow it down and cause some grinding to occur. The fastest grinding effect will occur if they are rotating against each other's spin direction

[Mike Wilkins]

Just recently used a flush trim bit in a set of pattern bits that came from a popular tool retailer; straight bit with a bearing on top. No only was the cut surface full of chip-outs, but the bearing came off the shank. Removed it and replaced it with a name brand spiral pattern bit and got smooth surfaces for my effort.
I suggest getting a decent quality bit and stop your frustration.

[andy bessette]

The correct answer is to make all your parts from the same template with the same bit. They won't match the template, but will match each other. Worn bits do shrink ever so slightly, that's what wear is. Really, though, if you're doing bent laminations that slight discrepancy will be the least of your problems.

This. A few thousandths of an inch difference between the pattern and the workpiece is laughably insignificant.