what to look for in flush trimming bit?

[Tyler Davis]

I need to replace a cheap flush trim bit with a new one. 1/2" shank is my only requirement so for...

1. I know you can do top/bottom/both bearing location. Is it better to get one bit with both options, to include both table routing and handheld pattern trimming?

2. there are straight flutes, mild shear flutes, or full spiral flutes. Do the new fancy ones really reduce tearout significantly?

3. there are 2,3, and 4-flute cutters. Is this to reduce tearout, or wear on the bit? I know bearing bits are not sharpenable since it changes the radius. Is it better to invest in one expensive bit that does it all or a few disposable specialized ones?

[Karl Brogger]

These aren't cheap but they are awesome. I think I paid $80 for mine about 5 years ago. I have no idea what they cost now. The knives are about $1 a piece, are double edged and last a really long time. Cheaper than sharpening, the bit doesn't get smaller from sharpening either. I think they have a pattern setup as well with the bearing on top. I uss the ICK-30, the knives are about an inch long. 30 is probably a reference in mm, so that is 1.18"

edit-Sorry, this is from Amana's web site.


http://www.amanatool.com/bitdetails/rc_1000.jpg

[Gene Howe]

The mild spiral is likely to give you an acceptable cut. A more acute spiral would be best used for jointing. I find the bottom only style bearing on either bit more advantageous.
Consider a 1" cutting length bit, as well as longer ones.
Flush trim bits are not that expensive. Don't worry about sharpening them. But, even if you sharpen them and discard the bearing, They are still useful in the table, with judicious fence adjustment.
Bear in mind that sharpening a spiral bit may be more costly(+-) than a new bit.

[Prashun Patel]

I prefer bottom bearing bkz i use it for flush trimming on the table and handheld. The only reason to get a top bearing is if you're doing patterns with a handheld with the pattern on top of the piece.

Straight flute is the cheapest.

Shear angle better for preventing tearout.

IMHO, the best are the spiral bits. They minimize tearout. They're also the priciest. Don't get yrself confused w/ upcut and downcut if getting a bearing one and yr using this for flush trimming alone. Get one with a nice long cutting size, obviously since you're going the full depth of the routed piece. On a table without an extension, this is always a pain, right?

Tearout is a HUGE issue around corners and on end grain. You can try backer pieces and climb cuts to minimize this, but the best (and safest) is to use the right type of bit.

[Bill Huber]

I have had straight flutes and they worked very well and then I got a mild shear and the difference is like night an day.

When you think about cutting a tomato with a knife, if you put the width of the blade on it and try and push it though it is very hard to cut. If you put a small part of the knife on it and then slice it goes right though. It is the same with the shear bits.

I am not sure and have never had a bit with 4 flutes on it so I really can't address that part of the question but I do know that I will never go back to a straight bit again.

This is my go to trim bit, I use it in the router table for templates and it works really great.

http://eagleamerica.com/down-shearflush-trim-bits/p/117%2D0842/

[Charles McCracken]

The only reason to get a top bearing is if you're doing patterns with a handheld with the pattern on top of the piece.


Not really. The top and bottom bearing bits allow for template following with either bearing, depending on the grain direction. So if you encounter grain that may tear out you can flip the piece and template to continue with the other bearing in the opposite direction.

[Paul Demetropoulos]

+1 on Mr. McCracken's not really- on top bearing bits.
I find them quite useful especially for jointing long boards with a straight edge on top, boards that would be difficult or impossible to work on the jointer. I use the down shear as well, no comparison.

[Tyler Davis]

OK, so can anyone point me towards a down-shear top-and-bottom-bearing flush trim bit?

do these exist? - I can't find any in a search

[pat warner]

"What to look for"? Alot, but one issue overlooked here:
Cutter diameter. Cutter must be smaller than the bearing lest the templet, often quite dear, turns to dust.

Routers (http://www.patwarner.com)

[John Lucas]

"What to look for"? Alot, but one issue overlooked here:
Cutter diameter. Cutter must be smaller than the bearing lest the templet, often quite dear, turns to dust.

Routers (http://www.patwarner.com)


Pat,
I know you know all router stuff cold, so I am confused by your last answer. I would say that bearing must be exact same diameter as cutter diameter. Am I mis-reading your entry?

[Paul Atkins]

Correct, John. Hence the name 'flush' trim bit. I use both top and bottom bearing bits depending on the job.

[pat warner]

JL and ladies:

Don't know it cold but do know that a cutter whose (cutting diameter) CD = or > than the bearing OD will nick the templet.
Subbase, templets , substrates, armature axis, bent castings and so on, usually have enough stack up error to tilt the router slightly to seriously.
That tilt is what causes a cutter to cut into the templet.
A cutter whose radius is 2 -.003" < bearing diameter is often enough to prevent the cutter from nicking the templet.
A precision templet can be quite expensive, A Keller e.g. Use it 10x with a big cutter and it's history.

[Prashun Patel]

+1 on Mr. McCracken's not really- on top bearing bits.
I find them quite useful especially for jointing long boards with a straight edge on top, boards that would be difficult or impossible to work on the jointer. I use the down shear as well, no comparison.


Sorry; didn't mean to imply that top bearings are not useful, nor that patterns-on-top are a rare op.

[hank dekeyser]

Hmmmmmm- all this talk of bits, yet no-one has asked the million dollar question -

What exactly are you using the bit to trim ???? Without that vital tidbit, one can write a book on trim bits and "hope" you give the right answer.

For general pattern work I use both top and bottom bearing straight flute bits, depending on the pattern and material I am machining.

For plastic laminate work I use a solid carbide single flute cutter. I get the Bosch 7-1/2 degree bit and change the depth of cut to vary between "flush" and "beveled" A shot of generic furniture polish on the edge keeps the bit from burning or scratching the surface. I've been doing it like this for many years. (I used to do "commercial" laminate cabinets also, so YES this is how it's done)


So Tyler, what is it you are "trimming" with the flush bit in question ???

[Tyler Davis]

Hank - good follow-up

I am flush trimming a mix of hardwoods and softwoods, usually 4/4 thick. Cypress for adirondack chairs, cleaning up mahogany rabbet joints, etc.

I usually am working off a 1/4" MDF pattern or following another hardwood surface on the piece.

Since I can't find the double-bearing bits with downshear, I will just get a bottom bearing and flip when necessary

thanks all

[John Lucas]

JL and ladies:

Don't know it cold but do know that a cutter whose (cutting diameter) CD = or > than the bearing OD will nick the templet.
Subbase, templets , substrates, armature axis, bent castings and so on, usually have enough stack up error to tilt the router slightly to seriously.
That tilt is what causes a cutter to cut into the templet.
A cutter whose radius is 2 -.003" < bearing diameter is often enough to prevent the cutter from nicking the templet.
A precision templet can be quite expensive, A Keller e.g. Use it 10x with a big cutter and it's history.


Pat,
Interesting and I stand corrected. I can't speak for the laadies.