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Flush Trim bit leaves bearing indentation

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Forum topic by jtrz posted 12-16-2017 05:07 PM 589 views 0 times favorited 21 replies Add to Favorites Watch
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jtrz

83 posts in 1047 days


12-16-2017 05:07 PM

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.


21 replies so far

View MrRon's profile

MrRon

4566 posts in 3118 days


#1 posted 12-16-2017 05:33 PM

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.

View Mike_D_S's profile

Mike_D_S

382 posts in 2089 days


#2 posted 12-16-2017 05:37 PM

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

-- No honey, that's not new, I've had that forever......

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Mike_D_S

382 posts in 2089 days


#3 posted 12-16-2017 05:39 PM

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

-- No honey, that's not new, I've had that forever......

View bilyo's profile

bilyo

94 posts in 977 days


#4 posted 12-16-2017 05:45 PM

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.

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jtrz

83 posts in 1047 days


#5 posted 12-16-2017 06:26 PM

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.

View Andybb's profile

Andybb

648 posts in 478 days


#6 posted 12-16-2017 06:40 PM

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.

View LesB's profile

LesB

1448 posts in 3317 days


#7 posted 12-16-2017 06:54 PM

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.

-- Les B, Oregon

View Kazooman's profile

Kazooman

914 posts in 1826 days


#8 posted 12-16-2017 07:31 PM

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!

View jtrz's profile

jtrz

83 posts in 1047 days


#9 posted 12-16-2017 07:37 PM

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.

View John_H's profile

John_H

87 posts in 1580 days


#10 posted 12-16-2017 07:45 PM

My thoughts – is the bearing turning freely?

View Kazooman's profile

Kazooman

914 posts in 1826 days


#11 posted 12-16-2017 07:58 PM


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.

View JackDuren's profile

JackDuren

368 posts in 834 days


#12 posted 12-17-2017 03:20 PM

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….

View splintergroup's profile

splintergroup

1822 posts in 1096 days


#13 posted 12-17-2017 03:30 PM

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

View Mike_D_S's profile

Mike_D_S

382 posts in 2089 days


#14 posted 12-17-2017 03:38 PM

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

-- No honey, that's not new, I've had that forever......

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JackDuren

368 posts in 834 days


#15 posted 12-17-2017 03:43 PM


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

- MikeDS

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….

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