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Subject Topic: How do I shrink open hole grommets? Post ReplyPost New Topic
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gregbrown
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Posted: February.02.05 at 9:19am | IP Logged Quote gregbrown

I'm doing an open-hole repad, and the grommets will not hold the pad in the cup (grrrr). I made a shrinking die, but the grommet seems to expand again the minute I press it back into the pad cup. What do other people do?
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Posted: February.02.05 at 11:58am | IP Logged Quote admin

Hi Greg,

First make sure the grommets are not split. This is common. If they are, replace them.

Second, maybe you just aren't shrinking the grommet enough. For a shrinking die I use the Allied rotor valve bearing shrinker. One of the collets is just the right size to also shrink flute grommets.

Some people will use their brass mouthpiece trueing tool to widen the chimney on the key. I have never tried this because I figure I can replace a grommet a LOT cheaper than a key section if things don't go as planned. But it may be an option for you.

You can also usually just order replacement grommets. They usually fit.

Anyone else have any other methods?

Michelle
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marksteenis
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Posted: February.02.05 at 1:24pm | IP Logged Quote marksteenis

at the national napbirt convention last year, i showed a die press for shrinking grommets. ed kraus is working on this project and may have something available soon.

mark
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Posted: February.02.05 at 1:45pm | IP Logged Quote admin

Cool. Can't wait to see it. The big 'ol rotor shrinking tool works but is a little clumsy.

Michelle
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gregbrown
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Posted: February.02.05 at 1:46pm | IP Logged Quote gregbrown

Michelle and Mark,
Thanks for the replies. I did shrink the grommet down to the point where I needed to gently tap it back into the cup - I couldn't push it in by hand. But they seem to just then expand right back to a too-loose state. I wonder if the die I made only shrinks the very bottom of the grommet chimney, which then doesn't have enough strength to "hold" when it is put back in the cup.
I keep thinking there must be a better way of doing this altogether - I mean the grommet system altogether. That's another discussion though.
thanks again for the help. Sounds like I need to call Allied.
Anyone else?

Greg
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GordonPalmer
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Posted: February.02.05 at 1:52pm | IP Logged Quote GordonPalmer

I use the Ferree P61 French horn rotor bearing tool. Is that the same as Allied's?

I have noticed that the result is a slightly triangular cross-section. I improve this by turning the grommet 60 degrees and squeezing again.

My thinking is that if it is a little triangular there is more chance of it leaking. Even so, I use a little 222 Loctite as a sealer after installation.

Damn those open holes!
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Posted: February.06.05 at 5:13pm | IP Logged Quote admin

Hi Gordon,

I thought both the Allied and Ferree's were pretty much the same although I have never used the Ferree's. However, I do not get the triangular problem with the Allied one. So there must be some difference.

Michelle  
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GordonPalmer
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Posted: February.06.05 at 5:54pm | IP Logged Quote GordonPalmer

Hi Michelle

With ANY collet-type shrinking device, unless you shrink to the EXACT inside diameter that was originally drilled when making the jaws of the tool, there will be some non-cylindrical distortion.

With the type of tool we are talking about, the hole through the tool's collet jaw is cylindrical only when the tool is not compressed. As compression progresses, the hole is increasingly non-cylindrical as the curvature in each of the jaws becomes less appropriate.

The more jaws there are, the less is this effect. With three jaws it is quite pronounced. Furthermore, the triangular distortion of the grommet does not readily show up by comparing DIFFERENT diameters with a calliper. (Indeed, the correct triangularised cylinder shape could successfuly be used for roller bearings.)

Whether this slight triangulation actually causes leaks depends on whether there are sufficient forces, when the grommet is installed, to force the grommet back to a cylindrical shape.
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Posted: February.06.05 at 6:42pm | IP Logged Quote admin

Ahh. Now I see what you're saying....I just took a loot at the swedging too. It does indeed start to go into a rounded triangle when closed almost all the way. I'll have to pay more attention to how much the collet closes when swedging the different sized grommets.

Now I'm even more interested in making the "draw-plate" type swedger.

Thanks Gordon!

Michelle
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JeffPeterson
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Posted: March.27.05 at 10:42pm | IP Logged Quote JeffPeterson

One quick and easy way to get them to fit is to stick the tips of a closed pair of needle-nosed pliers into the open hole(from the pad side with grommet installed)
press gently and twist a full turn.
A much better solution is to replace the metal grommets with delrin bushings by Jim Schmidt. The delrin holds much more securely, seals better, and because it prys off easier it does not murder the pad after repeated shimming attempts. He even makes a sleeve for Yamahas to give the bushing a parallel surface to grab against.
They are available from www.jlsmithco.com or www.jsengineering.net.
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