Spiral Bits

[Halfathumb]

I've never used one but looking at the post on French Feet on Lumberjocks he used one to carve the feet. What is the benefit of using a spiral bit instead of a straight trim bit?

[Hank Knight]

Spiral bits cut with a shearing action and generally yield cleaner cuts than straight bits. They can still tear out chunks of the workpiece when used with reversing grain direction. The bit I used on my desk apron is a hybrid spiral bit called a "compression" bit. It has more cutting edges than a standard spiral but and they are arranged so they compress the wood from the top and the bottom toward the middle as they cut. This dramatically reduces the chance of tear-out. The downside is that they are expensive. The apron and feet on my desk had grain running in every direction. I had one chance to get the cut right and I couldn't afford to risk tear-out. For me, this was justification enough for the purchase of the bit. It was well worth the cost; it did a beautiful job with absolutely no tear-out. Here's a photo of my Whiteside compression bit.

9D6169EE-9183-4F46-88FF-EF661A751FF8 by Hank Knight, on Flickr

NOTE: The bit wasn't much help carving the feet. Like any edge trim bit, it only works on flat 90 degree edges. I used it only for flush trimming the apron and as much of the feet as I could. When the router base encountered the up-curve of the feet, I stopped the cut and finished the job with a block plane and scrapers.

[Bill Holt]

I don't have that bit but I believe the advantage is smoother cut.

Once again I was late on the response.

[Halfathumb]

Spiral bits cut with a shearing action and generally yield cleaner cuts than straight bits. They can still tear out chunks of the workpiece when used with reversing grain direction. The bit I used on my desk apron is a hybrid spiral bit called a "compression" bit. It has more cutting edges than a standard spiral but and they are arranged so they compress the wood from the top and the bottom toward the middle as they cut. This dramatically reduces the chance of tear-out. The downside is that they are expensive. The apron and feet on my desk had grain running in every direction. I had one chance to get the cut right and I couldn't afford to risk tear-out. For me, this was justification enough for the purchase of the bit. It was well worth the cost; it did a beautiful job with absolutely no tear-out.  Here's a photo of my Whiteside compression bit.

9D6169EE-9183-4F46-88FF-EF661A751FF8 by Hank Knight, on Flickr

NOTE: The bit wasn't much help carving the feet. Like any edge trim bit, it only works on flat 90 degree edges. I used it only for flush trimming the apron and as much of the feet as I could. When the router base encountered the up-curve of the feet, I stopped the cut and finished the job with a block plane and scrapers.

Hank:
Thanks for the info.
Jim

[handi]

The difference is that a spiral cutting edge slices like a knife, cutting from top to bottom or bottom to top through the stock. A straight bit chops through the entire thickness at once more like a chisel.

The physics that make it cut cleaner are VERY similar to angling a plane in use.

One thing to keep in mind, spiral bits do cut plywood faster, but leave a fuzzy edge compared to straight bits. Solid woods, no problem, but plywood does tend to fuzz.