Pattern/Flush Trim Router Bits -Large Diameter vs. Small Diameter

[Glen Blanchard]

I have often wondered about this, have never seen it discussed, and I have never previously asked….. so I will now.

In a situation where (let’s say) either a ¼” or larger diameter pattern or flush trim router bit will perform the task at hand, is there any benefit to using larger rather than smaller (or vice versa)? I mean there is a difference in the physics – the speed of the cutters will be moving faster in a larger diameter bit (assuming the bit is not large enough to warrant slowing the motor down, which I think is suggested beginning at 1” if memory serves) . What about the angle of cut? I have not really studied straight router bits - is the angle of cut something other than perpendicular to a tangent? Does it change as the diameter increases? Does the likelihood of tear-out vary with varying diameters (everything else being equal)?

Bottom line, I am just wondering if there is any practical difference, and in circumstances where many different diameters can be used, if there is a reason to choose one diameter over another?

[pat warner]

Major subject especially if you add in shank bearing guided vs. end of cutter bearing, guided bits.
To be sure, use the shortest flute with the largest practical diameter. Deflection/vibration is your enemy. Skinny cutters, independent of grind angles design etc, bend and vibrate.
And that will spoil your cut.

[glenn bradley]

I knew I'd heard or read that from someone who's advice I respected. I have found (following the advice of folks who know more than me) that using the largest diameter possible gives me the smoothest results. I will even change bits on complex patterns so I only have to use the little ones for the details. I have also become enamored with the top and bottom bearing style as you can flip the piece and always route with the proper grain orientation.

[David DeCristoforo]

Larger diameter = cleaner cuts w/ less tearout potential.

Smaller diameter = the ability to cut a smaller radius.

The choice between top or bottom bearing should be made on the basis of what will be most convenient in the particular application.

[Larry Edgerton]

I have often wondered about this, have never seen it discussed, and I have never previously asked….. so I will now.

I. What about the angle of cut? I have not really studied straight router bits - is the angle of cut something other than perpendicular to a tangent? Does it change as the diameter increases? Does the likelihood of tear-out vary with varying diameters (everything else being equal)?



Draw a pair of circles, one 1 1/4" and on 1/2". Now draw a line through both circles at say 1/8", representing your cut depth. Notice the different angle of departure on the two circles. You will notice that the 1/2" bit is leaving the cut on a steeper angle, and as such is more likely to lift the wood fibers rather than shear. Along with this if possible you want to precut close to the line in some cases to keep the angle of departure as low as possible.

I commonly use Whiteside 1 1/4" pattern bits for this reason, and will clean up corners with another router with a smaller bit or a chisel if necessary.

Also there is the advantage of mass, a large bit is less likely to be upset by grain variations, and the 1/2" shank is less prone to wobble, as well as the larger bearing rides the pattern more uniformly.

[Scott Wigginton]

Just get the behemoth bit from William Ng (http://www.wnwoodworks.com/contact.php) ($130) and be done with it :D


135548


Just make sure you have a 3 HP router and only use it when in the table :rolleyes:

[Ed Griner]

Scott, That is a great looking bit,could you tell us a little more about it?

Ed

[Denny Blewcrowe]

One difference would be the circumference of the guide wheel. While a smaller guide will pick up more detail and work closer to stopped corners, on the other hand, it will also pick up more defects (should any exist) in the pattern itself.

[Scott Wigginton]

Scott, That is a great looking bit,could you tell us a little more about it?

Ed

Other than I drool over it in my dreams? :p

I saw it on the Wood Whisperer when he took a class at William Ng's school and since then he's made an episode just on flush trim bits (haven't had time to watch it yet). I ended up getting a steal on an unused '85 craftsman shaper so there went my savings/need for a 3 HP router/lift, maybe one day...

If you really want to find out more checkout the thread a few weeks back on flush trim bits (http://www.sawmillcreek.org/showthread.php?p=1261705), Sam Layton has one of those bits and should be able to tell you more (turns out he knows William Ng to boot)