Flush Router Bit Diameters

[schloff]

I'm wondering what the benefit of a larger diameter flush trim but would be? Most sites offer a vast selection of diameters for their bits, and I was wondering why a person would spend (1/3) - (1/2) more for a 3/4" dia bit over say, a 3/8" bit? Perhaps double even, depending on the cutting height.

I can't seem to come to a logical explanation, other tham maybe heat transfer? But I can't see the carbide heating up on a bit like this enough to make any negative side affects. (If used properly)

All of the bits I have been looking at are double fluted, if that makes any difference in the answer.

If anyone has any input, I'd appreciate it. Looking at getting one for some Toys For Tots projects that I have bouncing around my melon.

Thanks, folks.

Dan
WestPoint, TN

[cabinetman]

I don't use trim bits any wider than 1/2". That width works well on most hardwoods, and substrates. The small diameters I use are 3/16", 1/4", and 3/8". The small diameters are good for trimming Formica type laminates. They cut fast as they don't have to take out much material (because of their diameter).

For flush trimming, its best to size the overhang to the least possible needed for trimming. As in cutting out to fit a pattern, leaving 1/8" or 1/4" overhang trims very easy. The two flute will cut faster. The three flute will cut smoother than a two flute.

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[LCHIEN]

well clearly if following a complex template then a smaller diameter bit will allow copying finer detail. This is not important if trimming a straightedge like a countertop for example.

A larger bit would allow trimming a larger overhang with fewer passes and less strain on the bit. A large overhang might result when you roughcut a larger shape using a jigsaw or saber saw. usually smaller items you'd cut more closely.

I think as a very offhand rule (I just made this up), you would use a bit diameter thats equal to or greater than twice the maximum overhang.

So if your overhang is less than 1/8" it would be fine to use a 1/4" diameter flush trim bit.

But if your overhang is as much as 3/8" then go to a larger, 3/4" bit.

[cabinetman]

I think as a very offhand rule (I just made this up), you would use a bit diameter thats equal to or greater than twice the maximum overhang.

So if your overhang is less than 1/8" it would be fine to use a 1/4" diameter flush trim bit.

But if your overhang is as much as 3/8" then go to a larger, 3/4" bit.

Not a bad rule, but it may depend on the material being routed. It may be thick, or particularly hard, and not necessarily protruding that much, where a small diameter bit would be at task in relation to the mass being routed.

The problem with the larger overhang theory is that a larger diameter bit (like 3/4") has more to hog out, whereas a 1/2" bit would work more easily. I've never had a need to go to 3/4".

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[LCHIEN]

Not a bad rule, but it may depend on the material being routed. It may be thick, or particularly hard, and not necessarily protruding that much, where a small diameter bit would be at task in relation to the mass being routed.

The problem with the larger overhang theory is that a larger diameter bit (like 3/4") has more to hog out, whereas a 1/2" bit would work more easily. I've never had a need to go to 3/4".

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that's why i said its a very offhand rule. Modify as per other factors that affect it including thickness. Trimming a bit of 1/32 laminate is no sweat for any bit but a 3/4" thick x 1/2" overhang maybe you want a stouter bit with longer contact.

Pretty much for everything I have done, 1/2" dia. is fine, there's no real harm in using a larger bit (except for following curves tighter than the bit diameter). I guess Cab man has more experience.

[woodturner]

I'm wondering what the benefit of a larger diameter flush trim but would be?

A larger diameter bit cuts a "flatter" cut, due to the larger diameter. As a result, it gives a smoother cut. In addition, the larger diameter bit is "stiffer", allowing a deeper cut without deflecting from the template.

[cabinetman]

Thought I would add that flush trim bits can be used for procedures other than flush trimming. If you have a pattern and want a defined overhang left after trimming, a larger bearing can be substituted.

For example: If you want a 1/4" overhang left larger than the pattern/template, and you have a 1/2" diameter bit, you could add a 1" bearing.

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[schloff]

If following a pattern, I wouldn't think the diameter of the bit would make any difference. As far as I know, the flutes aren't any "taller" on a larger diameter bit, the body of the bit is what's larger.

I'm not just trying to be frugal, I am genuinely curious about this. I will most likely purchase the biggest/baddest bit I can find and a smaller diameter as well, as my router can handle most anything. Heck yea!!! Gotta love overkill...

Anyway, all I can see being any difference in diameter bit is to get into tighter corners with a smaller bit.

[JR]

. As far as I know, the flutes aren't any "taller" on a larger diameter bit, the body of the bit is what's larger.

Be careful about this assupmtion! There are many different cutter lengths available and it can sometimes make big difference in your work.

When you are making grooves, as opposed to following the outside of a pattern, you may be following the inside of a dado guide or other patern. Instead of taking the last bit of the outside off, you're making several passes at hogging out the groove. Now you need to worry about the bearing resting on the pattern while the bit is not embedded too far in the material. So having a long cutter may be a problem in some circumtances.

Depending on the depth of cut desired you also may have difficulty plunging the bit deep enough if you choose a short cutter length. I've had to change from short to long cutters when hogging out a deep cut.

Other than that, I think you've got it right. The diameter of the bit is an issue mainly when you are routing in cofined spaces. I have been doing a lot of inlay work, so I want small diameter bits that require less clean up in the corners.

JR

[vaking]

C-man already said that flush trimming bits are sometimes used for other tasks. Router sometimes is used to prepare jointing edge. For this operation the bit needs to have cutting length more than wood thickness. Specifically for this task I have a flush trimming bit with 2" long cutting length. A bit that long must be able to take heavy stress and heat. You definitely want to have 1/2" shank and the bit itself should be heavy, ideally more than 1/2" cutting diameter. Mine is 1/2" diameter bit because I could not find 2" long bit bigger.

[JimD]

I don't remember if I have one bigger than 1/2 inch right now but I have in the past. I like a 1/4 because it goes into corners better. I would most typically use a 1/2 inch bit with 1/2 inch shaft. But with a 1/2 inch cut, 1/2 shank the weak spot is in the cutting portion. A bigger diameter bit will leave more "meat" in the steel where the carbide is attached and be stronger. A bigger bit is also traveling faster (at the point where the cutting occurs) and will follow a template guide better in my experience. I like a chair design from Woodsmith (modified a little for height, their designer must be short) where the back legs are made from a template on the router table. A bigger than 1/2 bit - a 5/8 or 3/4 - seems to result in less tendency to chatter.

Jim

[charliex]

A large diameter bit will take more of a bite, true. They can bite into a large overhang, throw the wood clear across the shop and bust a big chunk out of an almost finished molding. DAMHIKT. Moral of the story, Never use a large diameter bit to "hog off" material. Trim very close to the line before routing.

[schloff]

Great point, Charlie. That would make for a pretty unhappy me.

[LCHIEN]

If following a pattern, I wouldn't think the diameter of the bit would make any difference. ...

Anyway, all I can see being any difference in diameter bit is to get into tighter corners with a smaller bit.

That's what I meant.

[cabinetman]

well clearly if following a complex template then a smaller diameter bit will allow copying finer detail.

Anyway, all I can see being any difference in diameter bit is to get into tighter corners with a smaller bit.

That's what I meant.

That's what I understood you to say. The downside of the smaller diameters is that it will follow detail more accurately than a larger diameter, and also follow any variations (bumps, gouges, and rough areas) i n the edge used for the bearing, or pilot.

Patterns and templates used to make copies should be prepared as smoothly as possible. Patterns with corners, like a 90 degree corner, will leave a rounded corner (from the bearing or pilot) that would have to be filed, chiseled, sawed, cut, or sanded to match.

If following a pattern, I wouldn't think the diameter of the bit would make any difference. ...

The ramifications of that was pretty well covered I thought.

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