Yes, the spiral bit to the left is old and ugly, but I didn’t have a nicer one to show. So please bear with me on the image.
A spiral bit treats material like a private jet treats the sky, but like a private jet, the price tag on a spiral bit is not for everyone. A straight bit is more like a coach class ticket: you’ll get where you need to go but for much less of an investment.
So when should you pay the extra cost for a spiral bit? There is one advantage shared by all spiral bits that puts them above straight bits: the angled edges slice cleanly into wood instead of chopping at it. The end result is a cleaner cut, since part of the bit’s cutting edge is always touching the wood. Spiral bits are also made completely of carbide, unlike straight bits which are carbide cutters attached to the steel body of the bit.
The solid carbide manufacture means that the bits can have wear-resistant formulations, to stand up against the abrasive nature of plywood, fiberboard, and particleboard.
Types of Flue Patterns on Spiral Bits
1: The Up Cut. The spiral rises from left to right around the bit. These bits are designed to quickly pull out chips from inside deep cuts, similarly to a twist bit. They can lift up the veneer when making a plunge cut into sheet goods, but when they are used in a table-mounted router set with the good side up they push down on the veneer.
2: The Down Cut. These spiral bits are designed to press the veneer down when making plunging cuts for grooves, rabbets, and dadoes. However, they don’t clear out chips, so you’ll need to use multiple quarter-inch passes or use a table saw to cut in relief kerfs. Be careful, however, as the downward force of the bit can lift your handheld router right off your workpiece.
3: The Combination. In a combination bit, the up-cut and down-cut flutes meet near the middle of the bit, which allows the bit to compress veneer on both sides of a panel, holding it against the substrate.
Other than these three, there are also specialty spirals. These include bearing guided up-cuts, down-cut flush-trims, and combination bits that have longer down-cuts and shorter up-cuts.
When Do You Need a Spiral?
Spiral bits are the best for woods with complicated grain patterns that would otherwise be difficult to machine, like curly maple, and for sheet goods with a veneer. The speed and cut quality of a spiral bit won’t be cheap, but the price is a lot easier to swallow when you are looking at a significant waste reduction in an $80 sheet of oak-veneer plywood.
You can source straight bits on the Internet for a lot less than spiral bits. For example, one seller has 1/2 x 1 1/4″ straight bits for just $18, while an up-cut spiral of the same size is $47. Specialty bits are even worse. A 1/2 x 1 1/2″ bottom-bearing flush-trim straight bit runs $21, while the comparable down-cut spiral bit is $111.
Not only are straight bits cheaper, but there are some wide cuts where they actually work better. The all-carbide manufacturing of a spiral bit means it is limited to a width of half an inch, while straight bits can be as wide as 1 1/2 inches. This makes for easy single-pass clean up of rabbets and dadoes. So before you invest in an expensive spiral bit, be sure to push your straight bits as far as they can go. Pick the right bits based on the job, not on cost or perceived reputation of a bit style. (The bit on the left is a straight bit.)
DO YOU HAVE a woodworking project or tip that you would like to submit to WooDesigner and get it added to our site? Then go to the contact tab below OR at the top of this page and let me know. Just make sure you are able to give us step by step instructions on it, as well as images as you see here, and we will put it on this site just like you have read on this page! As long as it is clear and concise like you just saw, it will go up. Your name will be credited to the project, as where you live. (Please include that.) Also include the type of lumber you use for the design. We look forward to hearing from you! (Take note that the tip and images and instructions have to become ours. I do hate that so much but in today’s society so many will sue, and yes, you can sue for a lot of money over images.) The tips will need to include YOUR IMAGES that you take. They can’t be images from somewhere online. Remember, we have to go through these images and if they aren’t your own, we will not be able to use them. This is illegal and it won’t be tolerated and you will be banned from this community by being blocked. We can get sued for this, and just one bad apple can ruin our community.
Ted Leger –
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