Let's Talk Flush Cutting Bits

[LeftyTom]

I understand the concept of a top bearing (hand held router) and bottom bearing (router table), and those bits with both a top and bottom bearings.

What befuddles me is the bit diameter. :dontknow:Whether the diameter is 1/4" or 3/4", it will still trim flush. :eusa_thin Are the wider diameter bits used for denser materials?

 

[Alan in Little Washington]

I understand the concept of a top bearing (hand held router) and bottom bearing (router table), and those bits with both a top and bottom bearings.

What befuddles me is the bit diameter. :dontknow:Whether the diameter is 1/4" or 3/4", it will still trim flush. :eusa_thin Are the wider diameter bits used for denser materials?

There may be other more educated replies, but here is my take on the differences.

First, bearing on top or bottom, whether it is a router table or hand-held router, and whether it is designed to make a straight cut or a shear cut as in a up cut or down cut- distinguishes a flush trim bit from a pattern bit (which is also a form of flush trim bit.)

As far as bearing diameter, as long as it is the same as the cutters it will make a flush cut.

Now as to the size of both- a smaller flush trim bit/bearing will get deeper into and make a smaller radius inside corners and contours, and the opposite is true.

A small bit/bearing will also be more affected by surface irregularities where the bearing rides and more likely to transfer those irregularities to the edge being trimmed. A small bearing on soft wood can actually make its own irregularities - depressions- if you are not careful.

I'm sure there are other reasons like tangential cutter speed and cutting resistance, etc. Usually taller flush trim bits have smaller diameters to reduce amount of stock taken in one bite and hence reduce the demand on the router motor.

 

[LeftyTom]

A small bit/bearing will also be more affected by surface irregularities where the bearing rides and more likely to transfer those irregularities to the edge being trimmed. A small bearing on soft wood can actually make its own irregularities - depressions- if you are not careful.

I'm sure there are other reasons like tangential cutter speed and cutting resistance, etc. Usually taller flush trim bits have smaller diameters to reduce amount of stock taken in one bite and hence reduce the demand on the router motor.

Thanks, Alan. That sheds light on areas I had not considered (inside curves).

If I am flush material 1/2" thick, and the cutter is 1" long, is it safe to use double-sided tape to stack the material 1" thick?

 

[fergy]

It also has a lot to do with chip load, and the material being cut. I've got some 1-1/8" bits that I like for really smooth cutting, especially in plastics. The mass of the bit also has to do with that. The edges will look like they were cut with a jointer.

But, for general work, I just use a 1/2" bit, unless I'm template routing with a bunch of intricate cuts, and I'll pull out the 1/4" spiral flush trim.

 

[fergy]

If I am flush material 1/2" thick, and the cutter is 1" long, is it safe to use double-sided tape to stack the material 1" thick?

You can, but you're probably not going to get as smooth of a cut as if you did each piece separately.

 

[Bigdog72]

There may be other more educated replies, but here is my take on the differences.

First, bearing on top or bottom, whether it is a router table or hand-held router, and whether it is designed to make a straight cut or a shear cut as in a up cut or down cut- distinguishes a flush trim bit from a pattern bit (which is also a form of flush trim bit.)

As far as bearing diameter, as long as it is the same as the cutters it will make a flush cut.

Now as to the size of both- a smaller flush trim bit/bearing will get deeper into and make a smaller radius inside corners and contours, and the opposite is true.

A small bit/bearing will also be more affected by surface irregularities where the bearing rides and more likely to transfer those irregularities to the edge being trimmed. A small bearing on soft wood can actually make its own irregularities - depressions- if you are not careful.

I'm sure there are other reasons like tangential cutter speed and cutting resistance, etc. Usually taller flush trim bits have smaller diameters to reduce amount of stock taken in one bite and hence reduce the demand on the router motor.

That really cleared up a lot of my questions.:banana::banana::banana: