# flush trimming 3/4" and 1" mdf



## z399y (Mar 14, 2010)

I am working on a project currently, which is essentially a large box made of mdf (a subwoofer enclosure). The way I planned the construction, some of the pieces are made purposely oversized so that they will have a small amount of edge overhang. Some of these are 3/4" mdf and some are 1" mdf. I did this because I think that I can get a more perfect corner by flush trimming than making the pieces perfectly sized to begin with. I am only doing this for some of the joints.


I bought a Freud trim bit that is 1/2" diameter, has two carbide cutting edges of 1-1/2" and a pilot bearing in the end. Is this going to be a good tool for flush trimming those thick mdf edges? Any tips for getting a good result while doing this? Thanks!


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## sofasurfer (May 30, 2009)

I'm building a work bench with a 3/4" thick oak top. I made the top a wee bit larger than the frame. I ran a flush trim bit around the edge. The result is so precisely flush that you cannot feel the joint between the top and the frame. You are using MDF which is softer, so be sure to move slowely so as to not cause tearout and do not try to take of more than 1/16" at a time to be sure of not over working the piece. You'll be thrilled at the result.


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## z399y (Mar 14, 2010)

Here is a video of how the flush trimming turned out.

http://bluesky-sys.com/SubBuild/EdgeTrimming.MOV

One thing I did notice is that on the flush trim bits that I have, the cutter edge is recessed slightly from the line the bearing rides on, so you actually end up with an edge that is slightly proud. With just a little bit of block sanding, I ended up with a very nice corner on this piece.


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## Cochese (Jan 12, 2010)

Once I learned the principle and technique involved, it was one of the greatest feelings I've had. Congrats.


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## z399y (Mar 14, 2010)

I have 3 flush trim bits. One is a Freud, one is a Bosch and one is a Oldham Viper bit. All have 1/2" shafts.

All 3, if you place them flat against a true straight edge, you can see that the cutter diameter is less than the shaft/bearing diameter.

When I make flush trim cuts with these bits, the edge I am cutting is left raised slightly from the surface the bearing rides against.

Is this normal for flush trim bits because its not really flush? Are there any with the cutters set at the same level as the bearing?

Thanks!


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## Check twice! (Feb 11, 2008)

Hi Craig

To my understanding your bit blades and bearing should be the exact same size, hence flush cutting bits. Are you sure the bearing is the right size? I have several flush cutting bits and each one has the bearing the exact size of the cutting blades.

I posted a photo of two flush cutting bits with the bearing the exact size of the outside edge of the blades. Maybe this is the case here wrong size bearing.

You can purchase bearing kits with different size bearings.


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## Hamlin (Dec 25, 2005)

Check twice! said:


> Hi Craig
> 
> To my understanding your bit blades and bearing should be the exact same size, hence flush cutting bits. Are you sure the bearing is the right size? I have several flush cutting bits and each one has the bearing the exact size of the cutting blades.
> 
> ...


+1 Flush trim bits are just that. The bit diameter and bearing need to be equal. Bearing kits are avail. Plus, if the bit has been resharpened, this needs to be taken into consideration as well.


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## z399y (Mar 14, 2010)

I wanted to find out for sure so I just measured the Oldham Viper flush trim bit. So you see the cuter diameter is .007" smaller than the bearing diameter. I think all 3 of my flush trim bits are like that because I see about the same amount of light under the cutter edge when I look at them against a flat surface.


Oldham Viper #135 part number 121-2-FLT

Shaft diameter .4997"
Bearing diameter .5002"
Cutter diameter .4932"


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## BigJimAK (Mar 13, 2009)

That's a QC tolerance issue, and part of what you pay for in premium bits. Different manufacturers have different tolerances and different sampling numbrs.b You can end up with an out of spec from any vendor. My mike is only marked to thousandths so I can only extrapolate to perhaps half a thou, but there was no discernible difference on my Whiteside bit. Now, whether or not being off by 7/1000" is significant is another issue, as the wood will swell/shrink more than that with changes in humidity.


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