carbide spiral upcut bits don't plunge?

[LCHIEN]

I looked at the MLCSwoodworking site today and they say their spiral bits are plungeable. this is the illustrationin the their brochure:


The end is straight across and the cutters meet in the middle - there's a cutting edge that will plane away the bottom of the hole. And that's exactly how my MLCS bit looks (but I didn't use it).

Close examination of the Holbren bit I have (it's marked Holbren) shows the end to be an inverted "V" and it has cutting edges, not straight but slightly wavy, that have a gap of about 3/32" in the middle where the end of the bit is just flat and higher than the tips. THis corresponds to the burnished but uncut pip I had in the bottom when I tried to plunge it, and only the outer tips cut in. The V is quite shallow but definate.

So that explains why this particular bit wasn't able to plunge.
I might try it with the MLCS bit, it both looks like it would work as well as the MLCS docs say it will plunge.
I guess the moral of the story is to check your bit end to make sure it can plunge before doing so. Make sure the entire end is capable of cutting and there's no gap in the center where it leaves an area uncut.

Brian H, do you have any comments on the types of spiral bit cutting ends available?

[cabinetman]

You may have a bit that is left-hand spiral... They come that way, but I'm not sure for specifically what purpose. Perhaps somebody else here will know.

One explanation I came up with in searching was that left hand bits are intended for use with tools/machines that are reversible...like shapers and CNC.
.

[AlanWS]

I've also run into spiral carbide bits that did not plunge well. If you sweep as you plunge, they can cut cleanly without burning. That's easy hand held, though trickier on a router table. I had bought it for mortising, and that's what made me switch back to ordinary straight bits for the purpose. Pat Warner is not a fan of spiral bits.

[capncarl]

Left hand sprial bits and cutters are used in gangs of cutters / drills that are belt or gear driven where ever other one rotates the opposite direction.

capncarl

[Norm in Fujino]

I have MLCS spiral upcut bits that I bought specifically for plunging--when cutting mortises. They've worked well for me, the proverbial hot knife through butter.

[LCHIEN]

So anyway I finally made the cut I wanted, a stopped dado.

My first experience was not good, I cut too deeply (the entire 1/4" or so) and tried to plunge the bit in the whole way by dropping the piece against a fence with stops in both directions. It chattered and burned even after I cut a starter hole for the bit that was not capable of plunging 1/4".

The next run I set the same bit so the cutters would penetrate but the non-cutting part was still below the table top - which meant the cut was to be only about 1/16th-1/8". I just sighted the bit along the table top from the side as I raised the bit.

Then I pushed the piece flush to the fence and dropped the piece and it cut readily; I pushed it left and rigt to make the shallow cut. I then raised the bit abot 1/8 inch ata time and made successive passes by holding the piece against the fence and moving it sideways as I lowered it onto the bit, making a quick cut right and left movements to the stops. It was like a "sweeping plunge" as AlanWS called it.

Anyway it came out OK except the right end had some long stringy fuzz at the top of the cut - maybe the result of using the upcut bit. A downcut or compression bit might not have left it, and i had to trim it off with an exacto knife andsand the edge.

[drumpriest]

Interesting, I've never had a problem plunging my spiral bits. I usually just get them from Rockler, though I have some from Holbren as well, all of mine have worked fine. I use them for mortising when I need something more significant than a domino allows.

You can see it in action in this tutorial video, linked here.

http://bt3central.com/showthread.php?t=27419

It just works for me, I wonder why yours is giving you troubles....