Not an important question but I am just curious

[lcm1947]

I was reading up on router bits and noticed that they sell straight bits that are like 7/16 or 5/16 in diameter. Now I can see the need for a 1/4, 1/2, & 3/4 in diameter since that's the various thickness's of sheet goods but where would one use say a 7/16 in diameter bit for. Now I realize that there are plywood size bits but I'm talking about regular straight bits not the plywood ones. I am just curious but cannot think of a reason. I am also puzzled at the different diameters of flush trim bit sizes. I mean why are there even different diameter sizes in this particular bit. All you are doing is trimming off the excess like on laminate on a table top. Why the need or better yet where would you use any of them other then a 1/2" diameter? Would appreciate any replies as while this isn't the most important questions I've ever asked I sure am curious.

[rbfunk]

The "odd sizes are for plywood as you surmised. 1/2 inch plywood isn't a 1/2 an inch thick, that's what they all the nominal dimension. Plus Europian plywood is metric so it is not actully 1/2 an inch. The same applies to 1/4 and 3/4.
Now you are right that trimming a table top laminate can be done with a half inch bit but what if you are trimming something with an inside corner with a small radius? You need a smaller bit.
Bob

[LarryG]

There are several reasons for those "odd-sized" bits, most involving practicality, or aesthetics, or both. A woodworker might have some rough-sawn lumber than could be resawn and milled into pieces 7/16" thick and used for something like drawer bodies on a small-scale project, on which bodies using the more common 1/2" bodies would look clunky. Or when making something like a fluted column, a 5/16" wide groove might look just right where 1/4" would be too narrow and 3/8" too wide.

In addition to the inside-corner issue Bob mentioned, varying the diameter of a bit varies the tip speed: the larger the bit, the faster the tips are moving in MPH at any given router RPM. There's a complex relationship between tip speed, feed rate, and species of wood (that I don't pretend to fully understand) in order to produce the best cut. Also, the larger bits have more mass and more physical room for the carbide flutes to be brazed on. Generally speaking the smaller the bit the more delicate it will be; the larger, the more robust.

[lcm1947]

OK, thanks Larry. I guess I thought something like that but you never know so thought I'd ask. I guess there's reasons for everything. The mass verous speed thing makes sense but I guess you'd really have to know your stuff and be very experienced to take that into consideration. Anyway, thanks so there are reasons. [?]