# Flush Trim Bit Top Bearing vs. Bottom Bearing



## steveD (Oct 21, 2008)

Hi folks,

Looking to get a Flush Trim bit. I understand that on some project you might need a top bearing and on some you might need a bottom bearing. But is there a preference among you at least for my first one? Use on more than the other? Also, I was looking at a Freud bit that had both top and bottom bearings. Does this solve anything? 

Thanks.


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## kp91 (Sep 10, 2004)

Steve,

If the bearing is on the bottom, that is usually a flush trim bit. They are quite usefull even when not using the bearing because they are a reasonably priced straight cutter with a slight shear angle and they leave a decent cut. 

If the bit is on the top, it is a pattern bit. They can come in handy as well when hot using the bearing because they can plow out grooves, dados, rabbetts as well, things that you cannot do with a bottom bearing bit.

I think the flush trim bit is probably the best way to go for someone who can only have one, but if you go to grizzly.com you can find a set of 3 bits, 1 flush trim and 2 pattern for about $20.

http://www.grizzly.com/products/Straight-Flush-Trim-3-pc-Set-1-2-Shank/H5567

It's a decent set, and I've gotten pretty good use out of it.


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## cbsjoez1935 (Mar 14, 2007)

Hi Steve,

The most simple explanation I can give is this. If you are following a pattern template, that is below your workpiece, use a bottom bearing flush trin bit. If the pattern template is sitting on top of the workpiece, then use a top bearing bit. This all assumes you are working with a table mounted router. There are probably a myriad of other reasons and methods, but I would suggest a top bearing shear angle flush trim bit for your first one.
They cut nice and clean since the cutting edges are angled and slice rather than chop.

Joe Z.


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## Drew Eckhardt (Aug 2, 2008)

steveD said:


> Hi folks,
> 
> Looking to get a Flush Trim bit. I understand that on some project you might need a top bearing and on some you might need a bottom bearing. But is there a preference among you at least for my first one?


A double bearing bit with shear that's 3/4" in diameter with a 1" cutting length (assuming you use 3/4" stock) and some shear angle.

Otherwise it would depend what you're doing. A flush trim bit with the bearing on the end is better for making pieces over-sized (like a box top) and trimming to fit and can still be used for pattern work which involves through-cuts. Obviously you couldn't do inlays or route rabbets with a flush trim bit.



> Use on more than the other? Also, I was looking at a Freud bit that had both top and bottom bearings. Does this solve anything?


Yes. Routing into the grain like this -> \ often catches and breaks the piece (example attached - even the clamped block of wood on the end and trimming within 1/16" of the pattern isn't enough to avoid breakage). The best way to avoid the problem is to flip the piece over. When the pattern is stuck to the piece with double sided tape that means you go from needing a top bearing to a bottom bearing. With the double bearing bit you only need to change the router height 1/2".

You can also put toggle clamps on your pattern to hold symetric work pieces in place so you can flip the work with the pattern staying on the same side , or make a sandwich with patterns on the top and bottom.


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## bobj3 (Jan 17, 2006)

This is just my 2 cents 

I like the bearing on the top,, that way you can keep most of the cutter down in the hole, it's bit safer or if use in the hand type router it has a way to ride on the stock.

The one I like best is the one with 2 bearing on it..
Spiral Flush Trim Router Bit
http://www.mlcswoodworking.com/shop...l/pages/bt_solid.html#spiral_flushtrim_anchor

=======


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## steveD (Oct 21, 2008)

Thanks guys! Makes a whole lot of sense.


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## matthewd5 (Nov 7, 2008)

just my $ .02 but i have both the single bearing top or bottom but the freud bearing on top and bottom is last bit of that sort that i have bought and i find myself using that one virtually 99% off the time.

matthew


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## AlanZ (Aug 21, 2008)

I'm considering getting a bit with bearings at the top and bottom from MLCS.

They have an interesting video that explains how using a two bearing bit avoids tearout when routing an arch.

Click here for the video


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## Drew Eckhardt (Aug 2, 2008)

AlanZ said:


> I'm considering getting a bit with bearings at the top and bottom from MLCS.
> 
> They have an interesting video that explains how using a two bearing bit avoids tearout when routing an arch.
> 
> Click here for the video


Page 118 from _Ultimate Guide to the Router Table_. I've had problems trying to link to google books so here's a screen grab.


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## bobj3 (Jan 17, 2006)

Hi Guys

Just my 2 cents 

I have not found a bit yet that will not rip out the stock once you run the bit uphill with the grain of the wood,,,it's nice to have two bearings on the bit but using a bit with a skew angle and running it at hight works well,,,the real key is to cut the stock with in 1/16" of the template/pattern.
And once you get to the grain running uphill flip the stock over so you are going down hill grain of the stock..

Bob & Rick show how to do this on the RWS show with a jig and two hold downs.

=========


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## thistlefly (Dec 2, 2008)

So, a simple question: The meaning of 'top' and 'bottom' of the bit in this context presumes that the router is handheld, not table-mounted, right?

I've only done a few routing projects, and that some years back, so have little experience to go on. I can envision times when I'd want to flush trim projects with either the table-mounted or handheld router, and would think that having the 'bottom' bearing would be best for handheld and the 'top' bearing bits (at the 'bottom' of the inverted router) would be best for table use.

Does this make sense in any way whatsoever? It's not an idle question, since I'm still trying to assemble my perfect set of 'general-use' bits. This forum is a wealth of information on that, by the way...


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## bobj3 (Jan 17, 2006)

Hi thistlefly

This just my 2 cents.. 

They both works very well,, it all comes down to what type of jig/fixture/template you are wanting to use.. 

see below

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thistlefly said:


> So, a simple question: The meaning of 'top' and 'bottom' of the bit in this context presumes that the router is handheld, not table-mounted, right?
> 
> I've only done a few routing projects, and that some years back, so have little experience to go on. I can envision times when I'd want to flush trim projects with either the table-mounted or handheld router, and would think that having the 'bottom' bearing would be best for handheld and the 'top' bearing bits (at the 'bottom' of the inverted router) would be best for table use.
> 
> Does this make sense in any way whatsoever? It's not an idle question, since I'm still trying to assemble my perfect set of 'general-use' bits. This forum is a wealth of information on that, by the way...


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## thistlefly (Dec 2, 2008)

Cool response as usual, Bob... thanks.


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## Skyglider (Nov 2, 2008)

thistlefly said:


> So, a simple question: The meaning of 'top' and 'bottom' of the bit in this context presumes that the router is handheld, not table-mounted, right?


The reference to top and bottom bearings has confused me also, though I’ve used those terms myself. I think the better way to refer to bearings is “tip” and “shank” which I hope folks will adopt.

I don’t know if it makes any sense since I’m a newbie but I bought a 7/8” diameter, ½” shank combination bit that has bearings on the tip and shank. Dual bearings to be able to route downhill on the grain like the MLCS video AlanZ referenced. 7/8” diameter since Whiteside recommends bit diameters larger than ¾” when using bearings mounted on ½” shanks. Something to do with the fragile nature of bearings when there is only ¼” difference between shank and bit diameters. --- Also, my understanding is that larger diameter bits cut smoother than smaller diameter bits.

Skyglider


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## TWheels (May 26, 2006)

AlanZ said:


> I'm considering getting a bit with bearings at the top and bottom from MLCS.
> 
> They have an interesting video that explains how using a two bearing bit avoids tearout when routing an arch.
> 
> Click here for the video


I have several such bits from MLCS and Katana. The MLCS 3/4 in (19 mm) diameter, 2 in (50 mm) cutting length #8814, and the Katana counterpart is probably the one bit I use more than any other. After many uses on pine, my first such bit seemed to be getting dull, so after buying another, I tried cleaning the older one (using a kit from Rockler) and it fully restored its usefulness as far as I could tell.


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