It be can be frustrating to choose between an up cut router bit and a down cut router bit until you gain understanding of the reasoning behind the design difference. And because of the difference in design, they can be used to make important differences in your project.
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Differences between up cut and down cut router bits
The basic difference between the two is the direction that the flute runs in. However, if you can’t imagine the rotation in the router, it’ll be hard to tell an up cut router from a down cut router.
No matter it’s an up cut router bit or a down cut router bit, it always turn in the same direction.
Imagine that you’re holding the shank of the bit and point it straight down to an imaginary work piece. If that’s an up cut router bit, when turning the bit clockwise, you’d be able to drill into the imaginary work piece. If that’s a down cut bit, in order to drill into the surface of the imaginary work piece, you have to turn in in a counter-clockwise direction.
The rotation is clockwise with the bit down. In other words, the side of the bit that is farthest from the operator will turn from the left to the right while the side of the bit that is near to the operator will turn from the right to the left. We will want the feed direction and the cutting edges turn in opposite directions, not the same.
When the bit is mounted in a router table, the router will be oriented up with the bit up. The back side of the bit that is farthest to the operator will turn from the right to the left and the side of the bit that is nearest to the operator will turn from the left to the right.
Always feed the wood into the router against the rotation of the cutting edges. This is because, if you feed the food in the direction that matches the rotation, it will lead to a dangerous kick out.
Up cut bits
When a woodworker want to evacuate chips up and out of the slot or the hole that they’re routing, the up cut type of spiral bit is very efficient, especially the hole is ¼ inch depth and deeper.
Making multiple light cuts instead of a heavy one is still the key in this situation. Since the flute spins in an upward direction, the chips will be evacuated and not be built up inside the slot or the hole, eliminating the risks of binding the hole or even breaking the bit.
However, there is one problem when using an up cut bit. Since the spiral bit spins in an upward direction, apart from being able to evacuate the chips, it can also lift the wood up or fray its fibers damaging the cut.
Most of the time, these damages are not really big and can be passed. However in some cases, when the final project is assembled, those edges can be noticed if they’re visible.
The advantage of up cut bits is their depth and how well it can evacuate the waste material; therefore, they’re commonly used to cut mortises. The edges with or without fraying and chipping will be hidden when fitting it to the shoulders of the tenon.
Down cut bits
Down cut bits produce very crisp and clean edge around the groove or hole they cut. Most down cut bits are commonly used to make relatively shallow cuts which minimize the chip ejection deficiency.
The advantage of down cut bits over up cut bits are their clean finish on the edges of the groove or hole. When cutting shallow grooves or dados, down cut spiral bits can be a really good choice, especially when the project is assembled, its edges will be visible.
A slower feed rate will be more appropriate for down cut bits. This is because a slower feed rate will allow a bit more time to throw the chips out and recut them into smaller pieces which will be thrown out from the slot or hole finally.
Achieving the full depth needed by making shallow cuts is always essential. And it’s even more important when using a down cut bit. This is because a down cut bit can suffer from overheating and eventually break due to the combination of chips built up and deep cuts.
As a professional woodworker, it’s best to have both types of spiral bits, up cut and down cut bits in hands. Up cut spiral bits and down cut spiral bits come in sizes from ¼ inch to ½ inch. So if you decide to have both of them in the drawer, you can easily get the right size that you mostly use.
- The most useful type of spiral bit to most woodworkers is the down cut bit, especially those who cut visible slots, rabbets, and dados.
- Some woodworkers who use a hand-held router or table mounted router to cut mortises find up cut bits most efficient.
If the final project needs to have a clean and nice finish on both sides of the work piece, you should consider getting a compression bit, which can be quite expensive.
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