Up spiral.. Down spiral.. What the heck?

[Jim Hall]

I was looking at spiral router bits on the internet and see they have two flute up spiral bits and two flute down spiral bits. I'm confused. What the heck is the difference between these two types and for what purpose. I can imagine use for up spiral.. chips are extracted.. but down spiral? Why.. Thanks..

[todd]

IIRC, the down spiral is used when cutting something like laminate where you want to minimize chipping.

todd

[Jim Hall]

Got it..!! Thank you very much.. Now it makes sense..

[CW]

The idea behind down spiral is to reduce chipout on the surface. Particularly good for plywood. These spiral bits are direct copies of two flute metal cutting end mills. End mills are what I use. They are less expensive (about $25.00 to $30.00 dollars for a 1/2" end mill as opposed to $50.00 to $60.00 for a spiral router bit) and work great. The only real advantage to the spiral router bit is shank size. They are ground to fit the average router. The most common sizes in spiral router bits are 1/4, 3/8, and 1/2 and are ground to fit either a 1/4 or 1/2 collet. With end mills, about the only thing available to fit a 1/4 collet is a 1/4 cutter. 3/8 has a 3/8 shank. 1/2 is of course 1/2 shank. In between sizes have the next size larger standard shank. Example: a 5/16 end mill will have a 3/8 shank. Most routers have 1/4 and 1/2 collets. If there is a 3/8 collet available for your router (there is for PC routers), you have the size range covered from

1/2 down. End mills are also available as up spiral or down spiral. Use the down spiral only if neccessary. They tend to lift the router out of the work and pack chips into the cut.

[Jim Hall]

Interesting.. Thanks for cost saving tip..

[Edwin Pawlowski]

Keep in mid that if the router is mounted in a table, the down spiral is now an up spiral when looking at it so plan your cuts accordingly. Up spiral bits will help remove material better when cutting a groove too.

[pat]