Why are flushtrim bits different diameters?

[Brian Penning]

Maybe I'm having a "duh" moment but why are router flushtrim bits different diameters?
I have one over 1" in diameter and another maybe 1/4" in dia.
Both do the same job, no?
TIA for any and all replies...

[Stephen Edwards]

For straight work they both accomplish the same thing (though, in my opinion, a larger diameter bit is better for thicker hardwoods).

If you're doing pattern work and you have an inside "curve" somewhere in the pattern that is a smaller radius than the the diameter of the bit....your larger bit won't make that "curve".

Another example is installing a router plate with rounded corners into a router table top. To get a perfect fit, you need a pattern bit that is the same diameter of the radius of the router table plate, assuming that you're using a flush trim bit, and a pattern that's the same size as the opening.

Hope this helps.........

[Dan Friedrichs]

Another example is installing a router plate with rounded corners into a router table top. To get a perfect fit, you need a pattern bit that is the same diameter of the radius of the router table plate, assuming that you're using a flush trim bit, and a pattern that's the same size as the opening.

Hope this helps.........

Huh? Wouldn't a smaller bit give you the same thing?

[Matthew Hills]

Huh? Wouldn't a smaller bit give you the same thing?

I'd guess: square corners on the template; insert with rounded-corners

Matt

[glenn bradley]

The recommendation I read and hear the most (and follow); use the largest bit you have that will do the job. On straight cuts larger bits definitely leave a cleaner cut with less tendency to burn for me. Smaller bits allow tighter curves. I have them in 1/4" through 1-1/4".

[Stephen Edwards]

A smaller bit would work unless you're trying to perfectly match a particular radius, as in a router table plate insert.

If you have a router plate that has corners with a radius of 3/4" and if you use a 1/4" or 1/2" radius flush trim bit the hole that you cut will be larger in the corners than the router plate itself. You'll have gaps in the corners. This is assuming that you're using a pattern with square corners.

[Rod Sheridan]

Two considerations;

- rigidity of the bit, a larger diameter bit is going to be stronger

- cutting geometry. Shapers run large diameter cutters which provide optimum cutting angles. As your router bit gets smaller it can't cut as cleanly as a large diameter bit can, or as well as a large shaper cutter.

So always use the largest bit possible......Regards, Rod.

[Josiah Bartlett]

The larger diameter bits chatter less on thick stock because they are stiffer, and they also reject heat a bit better so they don't burn. They also tend to smooth out any surface irregularities because the larger diameter bearing doesn't fall into the irregular surface of whatever its trying to follow as easily.

[Jerome Hanby]

Just from the geometry, the cutting edge will be moving faster on a larger bit. Assuming similar construction the travel distance from cutter to cutter would be greater, so I'm not sure that the number of cuts per second would be that much different, not sure if the greater velocity is desirable...

[Rod Sheridan]

Normally you select a rotatonal speed to yield a certain tip speed.

Feed speeds are dependant upon rotational speed, the number of cutting wings and the desired chip thickness.

Regards, Rod.

[Peter Quinn]

I can't spin an 1 1/4" flush trim bit in my trim router, so I asked them to make a smaller one!::cool: No, seriously, I think the variations in sizes add to the versatility of the router. There are two many situations to mention where no one bit would cover everything. Bearings top, bottom, or both, various lengths, various diameters, all have their place for some operations.

I tend to use the shortest bit with the greatest diameter I can in any given situation on the router, except freehand, where I use the smallest bit I think will effectively do the job.

[Keith Christopher]

Good explainations, but also the thinner bit will tend to vibrate more than the thicker/thicker shank ones. this leads to cleaner smoother cuts. I also tend to move slower with the thicker ones.