Spiral Bits are the one set of bits that can be confusing for many woodworkers. The main reason for this is the description given for these actually changes depending on where the bits are installed, in either a handheld wood router or CNC router, or ... upside down in a wood router table. 

Watch it on Youtube: https://youtu.be/hTlx3FH39Bs

The nomenclature for router bits is - when the bit is vertical and the tip of the cutting edge is at the bottom of the bit.
Thus a Spiral Up Bit would have the cutting edge at the bottom of the bit and as it rotates in the wood it will drive the wood chips up the flutes and the tear-out will also be driven upward, which if cutting through the wood, will give clean cuts at underneath the wood. 
Conversely and Down Spiral Bit will have the wood chips and any tear-out drive down into the wood or if cutting through the wood, the wood chips and tear-out will be in the bottom of the wood whereas the top of the cut will be clean. 

The first thing to know is what direction the router bit will be traveling in, and fortunately, most routers manufacturers have put little arrows on the base or body of their routers to quickly remind us what direction the bit will be spinning in.

Wood Router Spin Direction

The 4 router bits I will be covering in this section, starting from left to right are

Spiral Up bit          Spiral Down Bit             Compression Bit             Down Spiral Fine Finish Bit 
Watch This Before Buying Spiral Router Bits
If you would like more information, including pricing the following Amazon Links are provided 

Freud Down Sprial 1/4" bit available through Amazon 
freud down spiral 

Freud Up Spiral 1/4" bit available through Amazon 
up spiral

Whiteside 1/4" Compression Bit available through Amazon
compression 

Freud 3/16" Fine Finish Down Spiral Router Bit available from Amazon
3/16 down spiral

The 3 pieces of wood I tested all of these bits on today are ... 
1/8" (3 ply) plywood, often used as Door Skins for making doors;
3/8" Fir Plywood, construction grade
3/4" Pine Boards

All pieces of wood were made the same way, and all labeled front and back so you can see the results. 

1/8" Plywood (this wood is typically used as door skins for making wooden interior doors)
This is the 1/8"  plywood, top. As you can see in the lower left, with Up Spiral, lots of tears, middle-lower cut with Down Spiral the top is very clean, but as you can see in the second picture underneath, the tear-out when using a Down Spiral is on the bottom.  The plywood was almost too thin to use the Compression bit, but I did manage to get it set and it did what it was supposed to do, give us clean cuts on the top and on the bottom.
The Fine Finish bit on the top also performed well. I have not used this bit before but am quite happy with the results. As you can see there is a tiny bit of tear-out on the back of this 1/8" plywood. 

1/8" Top Side
Spiral Bit 1/8 Plywood Front

1/8" Bottom or back Side 
Spiral Router Bits 1/8 plywood back

3/8" Plywood - Construction Grade
I selected this plywood because it is a softwood, is somewhat coarse and readily available in most wood stores in North America, and gives a good general idea of the performance of these bits.

Starting in lower left again the Up Spriral again gave tear-out on the top side, as would be expected with this bit, but nice clean backside as you can see in the lower picture. The middle cut using the Down Spiral, similar to above where the cut is clean on the top but a little bit of tear-out on the bottom, again as would be expected. The lower right cut using the Compression bit was a much better showing of what this bit can do when set up correctly. Clean cuts on both the front and the back, exactly what the bit is designed to do.  

Spiral Bit 3/8 plywood front

 3/8" Plywood Back or underside

Spiral Bit 3/8 plywood back

3/4" Pine Board
I selected Pine on purpose because it is an inexpensive wood, readily available and often gives nasty tear-out results, and this board was no exception. The evidence of the Up Spriral in the lower left with the tear-out is self-explanatory.  The middle cut using the Down Spiral shows almost 100% improvement with the exception of a tiny bit along the top arc. The Compression bit gave another near-perfect cut but this is really a waste for this router bit because it is designed to cut both sides of a board and give a clean-cut, so not really a fair use of this bit, still, it did an excellent job. And once again the Fine Finish bit did another outstanding job on the Pine, arguably the best of the bunch ... certainly a bit to consider if tear-out is the bain of your woodworking. 

Spiral Bit on Pine wood

     

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