Straight or Spiral - Popular Woodworking Magazine

Straight or Spiral

 In Shop Blog, Woodworking Blogs
Down with the upcut. Spiral upcut bits are useful in lots of instances, but are more expensive and not as versatile as straight bits. For this reason I recommend adding them to your core set if, and when, you need them.

Down with the upcut. Spiral upcut bits are useful in lots of instances, but are more expensive and not as versatile as straight bits. For this reason I recommend adding them to your core set if, and when, you need them.

In the November issue of Popular Woodworking Magazine I wrote an article outlining the router bits I consider to be the core of any powered shop. The workhorse bits of that core set are the straight bits.

Straight router bits come in three varieties: Spiral-upcut, spiral-downcut and straight-cut bits. The choice of which to use comes down to the specific job and your preferences.

The difference between straight and spiral bits is not only the helical shape of the cutting surface, but also how they function. With a helical shape, you get a shearing cut, which leaves the surface smoother with less tear-out. They’re great for trimming surfaces of highly-figured woods. 

A straight bit planes squarely across the surface of the wood, which makes its cuts subject to greater tear-out. Also, because a straight bit strikes the surface of the wood perpendicular to the direction of rotation, you’ll also get more chatter. A spiral bit stays in constant contact with the surface, resulting in less chatter. 

You can use the helical design of spiral-upcut bits to help hold the router and the work in position. The force from the cutting action tends to draw the router and workpiece tighter together. With a little creative thinking, you can use this action to hide potential tear-out or hold the router tight to a pattern on a piece where a straight cutter might leave things feeling less than stable. 

Spiral-downcut bits have the reverse effect, pushing the router and the work apart. This may not sound like an advantage, but when trimming or grooving a veneered surface, the down-cut bit causes less lifting and tear-out of the veneer.

It might sound like I am suggesting you only have spiral bits in your core group; I am not. In fact, none of the bits in my core set are spiral bits. I think they have their uses, but they are considerably more expensive and, in my opinion, not necessarily the best choice for every job. 

The spiral nature of the bits makes removing larger amounts of waste a more tedious and laborious job. I’ve found you just can’t take as deep a cut with them as you can straight bits; I own just a few spiral bits and I use them sparingly.

For more on what I consider to be my core set of router bits, and why, check out the November 2014 issue arriving in your mailbox soon. Not a subscriber? You can fix that by clicking here.

—Chuck Bender

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Showing 10 comments
  • cagenuts

    “Straight router bits come in three varieties: Spiral-upcut, spiral-downcut and straight-cut bits”. There are actually four varieties as you forgot about the compression bit that combines the up-cut and down-cut helical cutting edges to give a lovely edge when trimming double sided laminate. One of the big advantages of an up-cut router bit is for mortising.

    • Chuck Bender
      Chuck Bender

      You are correct, sir. I had forgotten about those bits. Thanks for bringing them up. I do, however, put them in the same category as any spiral bit – buy only as needed.

      The chip ejection qualities of upcut bits are a definite advantage when routing mortises. And, as you’ll notice when you read the article, I suggest you look at the type of work you do when building your own core set of bits. If you do lots of mortising with a router, a spiral upcut bit might be on your shortlist, and probably ought to be.

  • lcm7293

    I have heard that spiral bits are more prone to snap off when used to make a plunge cut. Is this true?

    • Chuck Bender
      Chuck Bender

      That has not been my experience. Other’s mileage may vary. I’ve actually found it easier to snap regular straight cutters than spiral bits, but that might just be me.

  • HawkeyeGuy

    Great info as usual from Chuck.

    A fellow woodworker with a day job as a machinist turned me on end mills as an option. You get a much greater selection of shear geometries, number of flutes, length of bits and cutting lengths, but are far less costly than spiral “router” bits. I paid $25 for a 1/2″ D 2 flute carbide end mill instead of $70 for a freud bit with the same specs…

    I’ve still got my straight bits when a task calls for a bottom piloted bearing bit, but for anything I can use a pattern bit (bearing toward the collet) I throw in a spiral bit and run the shank of the bit on the pattern.

    • Chuck Bender
      Chuck Bender

      End mills were all the rage for a while early in my career. I tried them and always felt like they were wandering about on me. That probably wasn’t the case, I was just used to regular router bits. I’ll have to give them a try again at some point.

      • gumpbelly

        Chuck it would be my belief your earlier experience with “end mills” was spot on. I’ve yet to see an end mill where it was classed for wood cutting. Pretzel logic could get you that if it will cut metal, it will surely cut wood……………… The difference and the reason you felt they were harder to control was speed. An end mill is made to run at 800 to 1600 RPM. I’m not aware of any variable speed router where that can happen. Because the router is going much faster the cutting geometry is all wrong, and you end up with a whole lot of willy nilly. Folks looking to save a dime on their purchases will often opt for the lower $$$ options, sometimes at the expense of safety.

        Now if you routinely do a lot of plastics, well check. Because plastics are much denser laid fibers than wood, ahhh yes, similar to metals, well they cut plastics like a knife through hot butta. Using a wood router bit on plastics is a short life, due to heat build up.

        This is all very sad, because I agree a straight cutting bit is an often used bit, and through a machinist buddy I can get end mills dirt cheap, oh well………………

        • okrejci

          Per personal experience, solid carbide end mills worked better than helical carbide router bits because of a lower tendency to grab and self feed beyond the start and stop points when cutting mortises. RPM is determined by cutting material versus cut material, which for carbides versus most woods is beyond 10,000. The drawbacks are that carbide end mills usually have the same shank as the cutting diameter, restricting most to 1/4″ and 1/2″ without special collets, and picking the right mill, generally three-fluters for aluminium work best. Good luck and enjoy.

    • cagenuts

      You also get bottom bearing spiral bits. I think mine is a Katana.

      • fpiano41

        James Krenov suggested end mill bits in one of his books, had much success,
        never snap, found a 4flute that works fine using my table saw arbor
        approximate rpm 3700.

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