Spiral Bit Question

[Chris Frazee]

I recently purchased a 1/2" spiral upcut router bit and used it for the first time a few nights ago. I was attempting to use it in the process of making my dado jig and I ran into something unexpected. Basically, I was trying to rout a laminated hardboard sandwich to make a reference guide from router base to bit. So I cut the hardboard long and wanted to use the bit to cut the hardboard back to the correct length. While attempting this, I seemed to be burning through the hardboard rather than actually cutting it. I ended up finishing the cut with a standard 1/2" straight bit which worked as expected.

I hypothesize that what I was trying to do with the spiral bit would be the same as trying to cut with the side of a drill bit. If that's the case, I understand. My question, though, is how is this bit supposed to cut a dado if I'm essentially doing the same thing? The hardboard I was trying to cut was approx 3/8" thick. I can certainly see someone making a dado cut of that much or more. Was it the fact it was hardboard? Or am I missing something else?

TIA

Chris

[Gerry Grzadzinski]

I hypothesize that what I was trying to do with the spiral bit would be the same as trying to cut with the side of a drill bit. If that's the case,

Not the same, as a spiral bit is designed to cut on the side.
The only thing I can think of is that you were trying to cut much too slowly. Unless you somehow managed to get a left hand router bit, or a bit that was very dull?
Normally, a spiral bit is very aggressive, and capable of cutting many times faster than a straight bit.

[Chris Frazee]

Gerry,

Bit is brand new, just purchased from Rockler, first time using it. I don't think cut speed was an issue as it was taking an abnormal amount of pressure on the router to get it to move at all and the hardboard shows burning where that bit did cut (and the smoke lol.) I'll look again, but I did think about direction of rotation on the bit because that's what it seemed like -- cutting from the wrong side -- but I believe that was also correct. The flutes on the bit are sharp. I dunno, I'm at a loss, I'll try it again on different wood and see what I get.

Chris

[Prashun Patel]

A spiral bit can be used to cut on its side (to trim as you are doing) or to plunge into a dado or mortise.

My first guess is that the spiral bit is or has become dull. I trim hardboard and mdf on occasion, and while it's not good for blades, it doesn't kill them as instantly as the Internet would have me believe.

What happens if you try to take multiple passes of say 1/8"? When you say "cut the hardboard back" are you trimming less than 1/2" off the board, or literally cutting the full width of the bit?

Being too aggressive could contribute to burning and not cutting.

[Jerrimy Snook]

Chris, Could you post a picture of the bit?

[Chuck Nickerson]

When I use a router bit to 'cut back' a board, I have much greater success if the amount out wood I'm removing is 1/3 or less of the router bit's cutting diameter.
In other words, when using a 1/2" spiral bit, I've already trimmed the work to within 3/16" of my target line.

I can remove 1/2 the bit's diameter but there are other forces that make it more difficult/risky. YMMV

[Bill Space]

Yes, but he said that a standard but worked properly.

So would not one expect the spiral bit to work as well as the standard bit, all other things being equal, as they apparently were?

Perhaps there is something wrong with the spiral bit? Manufacturing error or ?

[Mel Fulks]

I see the spiral bits and compression bits as being no tear out things. The fastest cutting bits are straight single flute. Which saw cuts faster, 5 point or 10 point ? It's kinda the same thing.

[Gerry Grzadzinski]

The fastest cutting bits are straight single flute. Which saw cuts faster, 5 point or 10 point ? It's kinda the same thing.

It's actually a very different thing.
The more flutes in a router bit (up to about 4), the faster it can cut. Single flutes give you the impression that they cut faster because only one flute is cutting, which means the cutting forces are lower. Multiple flutes can cut faster, but you have to push it a lot harder.

[Mel Fulks]

I said "kinda" because I knew a scientist would post. I don't think the four flutes and mule is practical. I don't know anyone who has tried 2 flute and 1 flute and come to a different conclusion from what I posted. Some suppliers that sell a lot of bits sell NO single flutes except in the smallest diameters. Maybe they think they work TOO well. A sharpener and tool maker described the difference to me the same way I did in my post.

[Cody Jensen]

I have to disagree with more flutes = faster cutting. More flutes usually means smoother/cleaner cutting, which is a result of the cutting edge removing smaller amounts at a time. If you look at hogging endmills, they are not 4 flute.

[Gerry Grzadzinski]

If you look at hogging endmills,

Hogging endmills for metal don't need more flutes, because metal can only be cut so fast.
You can get 3 flute roughing bits for wood, and 4 flute bits that have both roughing and finishing flutes in one tool.
You need to be cutting at extremely high feedrates before you need them, and they are quite expensive. You'd need a $150,000 machine to be able to take advantage of them.

You can only take so much of a bite at a time per flute (chipload), and 2 flutes lets you cut twice as fast as 1 flute at any given chip load. 3 flutes = 3 times as fast. As long as you have room in the flutes to evacuate the chips, more flutes allows the potential for more speed.
https://www.vortextool.com/index.cfm...570CBB03F577CE

[Mel Fulks]

The site cited says says those bits are for CNC and Op has not mentioned them.

[Andrew J. Coholic]

As stated, the more flutes a bit has, the faster the feed speed it is designed to be run at, assuming the same RPM.

I would suspect that you are feeding too slowly, and at too high of an RPM. Spiral bits do behave differently than simple straight bits. I have used a lot of them on both hand held, and my CNC router, and I do believe most people run bits far too high of RPM in a hand held router.

When you go to a single flute, it will indeed perform better at a slower feed speed than a two flute. The more flutes you have, the faster these bits are designed to be fed for optimal cutting. Which requires more force - seldom if ever, will a hand held router be fed anywhere near the feed speed required for "optimal" cutting, and again, at far too high an RPM.

Just for fun, try slowing your router down to between 1/2 and 3/4 top speed, and try and take a bit more aggressive cut. Overly cautious feed speed, at too high of an RPM - is the recipe for burning.

All that being said, without seeing the bit, its all a guessing game.

[Mel Fulks]

I think we have made some progress here in acknowledging that single flute bits are fast and useful for quick work with inexpensive portable electric routers. I now understand that a couple of similar threads seemed overly contradictory in that accurate CNC comments were included. I've seen commercial shops without any appropriate bits for rough cutting, that has convinced me that they are even more unknown in home shops.