# Flush Trim bit leaves bearing indentation



## jtrz (Mar 10, 2015)

This is a bit of a noobie question but it is frustrating nonetheless. I glued together two mdf sheets for table top and was starting to clean it up buy flush trimming with my router. The problem is that the bearing of the bit makes a groove in the mdf and as a result I'm left with an edge like this:



















You can see that it did trim it flush however it's only flush with the indentation on the bottom piece. The rest of the bottom piece slightly protrudes now. The entire edge is not flush which seems to defeat the purpose.

What greenhorn mistake am I making. Is the MDF too soft?

The bit is one of those diablo bits from a big box store and it seems to be aligned correctly.









Any help would be appreciated.

Thanks.


----------



## MrRon (Jul 9, 2009)

Going by the picture, you are taking too much of a cut. Maybe you can turn the sheets over and repeat the trim. I would use a saw to trim as much of the MDF before using the router.


----------



## Mike_D_S (May 3, 2012)

My best guess is it's a combination of things. Some of which are the MDF is not hard, but it's not really that soft and secondly you may be pushing too hard to try and keep the bit up against the guide piece.

I have two pieces of advice:
1. Try and take as much of the waste off before the final cut. Rip a thin 1/16 or 1/32 strip with a TS or band saw and then tape that to the bottom so the bearing can ride on it. Make a first pass to get the bulk of the material off. Then come back and make a skim finish pass.

2. Let the bit and bearing do the work. You don't get burns on MDF very easily, so no need to be in a hurry. It's likely you're pushing too hard trying to get it flush. Keep the router base flat and let the bearing do it's the work. You shouldn't have to apply much pressure to get the bit to touch. If you're pushing into the piece hard enough to get good positive feedback that the bearing is touching firmly, you're probably pressing too hard.

If you take pressing hard and taking aggressive cuts where the cutting forces are pushing the bearing into the piece, you could be getting that kind of groove.

Mike


----------



## Mike_D_S (May 3, 2012)

I'd add that in general if i have to muscle a cut with a router I'm probably taking off too much. Sometimes you do it on purpose, but for cuts with guide bearings or using bushings, ideally you wouldn't need to be really forcing the cut.

Mike


----------



## bilyo (May 20, 2015)

My first thought is that the router and bit are getting tilted. On second thought, I wonder if you are simply pressing too hard to keep the bit against the work. That is the only thing I can think of that would make the guide bearing leave that mark. I notice that you are using a bit with only a 1/4" shank. If it is not inserted all the way into the collet, it could be flexing a little also. Having said that, don't really put it in all the way. After it is pushed in as far as it will go, then pull it out about 1/8" and tighten. Also, make sure that the guide bearing is turning freely and smoothly.

Put just enough side pressure on it to hold the bearing against the work. You may end up with places where it didn't get quite flush. No worry. Just make a second pass to clean it up.


----------



## jtrz (Mar 10, 2015)

Thanks guys. I think I was trying to take too much off with the router and as a result put too much pressure on the MDF instead of just letting it glide.

I'm going to trim some of the excess off with my circular or jig saw and give the router another go with a gentler hand.


----------



## Andybb (Sep 30, 2016)

How big is that piece? Too big to just run it through a table saw? That slab looks thicker than the bit. It doesn't look so much like the bearing denting the mdf as it does the bit is not contacting the lower board. The bearing needs something to register off of. You could also try clamping a board slightly shy of the top board and using that as a register, but then, again. the piece looks thinker than the bit which is why I suggested the table saw.


> I'm going to trim some of the excess off with my circular or jig saw and give the router another go with a gentler hand.
> - jtrz


As long as you can get a dead straight jointer-like edge with either of those, otherwise the bearing is gonna follow whatever line the saw cuts. Again, that's why I asked about using a table saw.


----------



## LesB (Dec 21, 2008)

It seems obvious that you may be pushing too hard. 
The other thing I would check is the bearing. Is is turning properly? It could be seizing from the pressure or just be defective and sticking. I treat my bearings with a lubricant (made for router bit bearings) almost every time I use them.


----------



## Kazooman (Jan 20, 2013)

There is a spacer (a washer) between the bearing and the cutters of the bit. From your picture it looks to me like that is what is scoring the MDF. I see a narrow line, not a line anywhere near as wide as the bearing. It is hard to see clearly from your picture of the bit, but I do think I see the spacer in your photo. On all of my flush trim bits this washer is slightly smaller in diameter than the bearing so it does not contact the work piece. Is that true for your bit? It actually looks like the washer might be a bit proud of the bearing, but I can't tell from the picture. Also, the washer is not flat. The center section is raised on one side. This dimple faces the bearing and it keeps the spinning part of the bearing from contacting the washer.

So….. from all the advice you have so far I think you should check to see if the spacer is too large (defective bit), be certain that the bearing turns freely and perhaps lubricate it, be certain that the spacer is installed properly, take a shallower cut, and don't push so hard. Should work like a champ!


----------



## jtrz (Mar 10, 2015)

I was worried that there was too much space between the bearing and the bit. Can't return it now as it's too late and the receipt is long gone.


----------



## John_ (Sep 23, 2013)

My thoughts - is the bearing turning freely?


----------



## Kazooman (Jan 20, 2013)

> I was worried that there was too much space between the bearing and the bit. Can't return it now as it's too late and the receipt is long gone.
> 
> - jtrz


There does need to be some space so the bearing can rotate freely. If the spacer is installed incorrectly the flat side can be in contact with the bearing and hinder the rotation.

Here is a picture of a 1/2" Freud flush trim bit (2 1/2" cutters). You can see the bearing, the spacer washer and then the body of the bit. The picture of the disassembled bit shows the spacer and you can probably see the shiny inner ring that is raised, This side goes towards the bearing. The bearing is 12.70 mm in diameter. The spacer is 11.98 mm.


----------



## JackDuren (Oct 10, 2015)

Your not taking too much off with MDF. Looks like a 1/4. Your either going too slow or pushing too hard,bearing getting bad or it's too dull….


----------



## splintergroup (Jan 20, 2015)

I'll second (ninth?) the motion, pressing too hard. Alternatively, if the bearing isn't in hood contact with the MDF, the outer race will start to spin and cause a groove. This explained why I'd get the same groove on harder woods like cherry and walnut. Check to see that your bearing spins freely


----------



## Mike_D_S (May 3, 2012)

As the OP started with a statement that he was relatively new, I'd like to add something to JackDuren's comment.

He's right in a way, 1/4" of MDF is not too much to take off, but bit size and cut height matters as well. As a general rule of thumb I like to make cuts with no more that 25% of the cutter face engaged.

So for example, a 1/2" bit with 1" long cutters taking a 1/4×1" cut is 50% engaged. Take that down to 1/8×1" and we're at 25% engaged. along the same lines, 1/4×1/2" is also 25% engaged. If I want to hog out a 1/2" by 1/2" cut. I'll go to a bigger diameter bit (making sure I had the corresponding router HP) to get the % engagement down to about 25%.

Same sort of concept applies for plunge routing mortises, etc. I try and keep the 25% engagement rule.

There's no high science here. I just find it makes for less stressful routing with less chatter and less white knuckle thrill ride with the bigger more powerful routers.

So following on the comments above, 1/4" may not be to much for MDF, but may be to much for MDF with that bit.

Mike


----------



## JackDuren (Oct 10, 2015)

> As the OP started with a statement that he was relatively new, I d like to add something to JackDuren s comment.
> 
> He s right in a way, 1/4" of MDF is not too much to take off, but bit size and cut height matters as well. As a general rule of thumb I like to make cuts with no more that 25% of the cutter face engaged.
> 
> ...


The bit looks like a 1/2 and should be fine. I make a lot of round 60+,8/4 round tables with 1/2 plunge bits when the CNC can't handle the size and they do fine as long as there sharp, taking off small amounts and going slow….


----------



## Mike_D_S (May 3, 2012)

> taking off small amounts and going slow….
> 
> - JackDuren


I agree. Controlling the speed and depth of cut, I take full width plunge passes with 1/2 or other size bits in MDF and hardwoods. But you and I have experience and our own way of deciding what "small amount" is about right. And I would guess for a lot of us, that experience comes from trying to make that cut where the router is driving you and not the other way around.

The info above is intended as advice for someone who is fairly new. Following that rule of thumb will generally give them very acceptable results while they build competence and can figure out their own rules.

I wasn't trying to object to your feedback, but just trying to frame it in a way that could be applied objectively.

Mike


----------



## JackDuren (Oct 10, 2015)

I get it Mike. No problem at all. I was asked at work by the owner if I could take a kid with zero experience and make him my apprentice just last week. I gave him the deer in the headlight look and told him if he had cabinet shop experience first only..

I may still be in that conversation and a bit negative..I apologize…


----------



## 000 (Dec 9, 2015)

It's not whether the bit can handle the 1/4" or not as much as it is the more material your taking off the more pressure you have to use to keep it pressed against the side, therefore indenting the bearing mark into the mdf.

Mdf is so soft, 
also the quality of the mdf is unknown, 
it doesn't take much at all to leave the indent.


----------



## Mike_D_S (May 3, 2012)

Jack,

No need to apologize. In my opinion we were having a discussion and that's never a bad thing.

It's funny you mention taking on an apprentice. Recently I've had one of the neighbor kids looking over my shoulder when I'm outside and as he asks questions, I've had to really think about why I do certain things a certain way in order to explain to him.

I've quickly figured out that, 'because that's the way you should do them' is not a great answer for a 13 year old. For some reason he seems to have a complete lack of awe of my legendary woodworking skills….....

Mike


----------



## jtrz (Mar 10, 2015)

I tried another go with another flush bit. This one has the bearing at the top. Not sure what the correct nomenclature is, flush trim bit or pattern bit, regardless I trimmed up one of the other edges with it. It looked like a good cut, no bearing mark, but then I noticed that the bottom piece of MDF was still a bit proud. So I went over it again with a tad more pressure and got the bearing mark and still a proud edge.

I think one of the main problems is my glue up with the MDF sheets (2' x 4' sheets) was a total botched job. I got them all clamped up and then realized my stupid mistake. Because my main work surface was covered with someone else's stuff which I can"t move I was using a much smaller workbench. As a result the clamped up MDF sheets overhung on the sides. I knew right away it wouldn't be flat. I just hoped I could trim it up to the right size and glue some stretchers underneath to help bring it back as well as wrapping the edges.

I just didn't anticipate the warped surface would affect the flush routing so much. I am going to get my friend to move his stuff today and it will free up my my table saw. I'm going to dispense with the router and get my edges as close to flush and square with the table saw. Hopefully some stretchers on the bottom side will help knock the bow out of this thing. I'm not trying to make a masterpiece here, just a simple utilitarian table.


----------



## Mike_D_S (May 3, 2012)

If you're not going to glue this edge to something else, then a simple fix would be to clamp a straight edge (level of similar) to the top of the piece to use as a guide. Use a non-bearing bit (or remove the bottom bearing from the bit you have) and establish a straight edge that you know is perpendicular to the top. Then come back and make the pass with the top bearing bit to flush up the bottom piece.

This should give you a corner that is perpendicular at least which should still be ok when you flatten out any bow you have.

Mike


----------

