Internal O-ring Groove in Small Bore

[Baumium]

Hello,

I need to make a couple o-ring grooves in a small bore, and I was wondering what the best practices were.

The part in question will be used to house a piston shaft as part of a fountain pen I'm working on. The bore is 0.1" diameter in 303 stainless that a matching 303 stainless shaft rides in, and I need the seal to withstand mild vacuum as the piston is depressed to draw ink into the pen.

From what I can tell, finding such a grooving tool is not easy. Micro100 seems to only sell internal grooving tools down to 0.205" min bore diameter. Am I best off grinding my own grooving tool that can fit such a small diameter (all I have are 1/4" hss blanks)? Also, are there any resources for preferred geometries for such a tool?
Or better yet, is there a better solution to this problem of a sliding seal for small diameter shafts?

Thank you,

David

 

[pontiac428]

Or better yet, is there a better solution to this problem of a sliding seal for small diameter shafts?

Depends, maybe the seal can ride in a groove in the piston. Else I would retain it with a light press-fit backing piece and forget the retaining groove in the bore.

Definitely grind your own tool if you decide to move forward. Take your time and finish it by hand with a honing stone. after roughing it with a powered grinder.

 

[extropic]

Micro 100 has square shouldered grooving tools that you could use.

Standard-Grooving Tools-Retaining Ring-Square-Right Hand-Miniature

www.micro100.com

 

[Flyinfool]

O rings do not have to have a radiused bottom. They will seal just fine on a flat bottom groove. They are already sealing just fine on a flat shaft surface.

 

[woodchucker]

grind your own. I have quite a few that were ground by older machinists that I bought, and I have ground one as well. It is a lot of work to remove all the material, but a coarse wheel makes it go much faster, Or a cut off blade might work if you are good enough to remove material to a consistent depth and finish with a grinding wheel or even a narrow belt sander. I would be tempted to use my HF narrow belt these days with a quality coarse belt. I think it's 1/2 in wide.

 

[durableoreo]

That's a challenge. My first thought is to grind a tool on a D-bit grinder. I would use a carbide shank and diamond wheel. Don't let the geometry stop you. Just follow the basics that you already know---correct shape, enough relief in the radial and axial directions. Approach the cut with caution. If something is wrong, you'll get burs, heat, and noise.

 

[Ischgl99]

If you decide to purchase something, the Micro 100 tools are excellent quality. I have some Red Line carbide tools that are also just as good for a little better price. I have found some of my Micro 100 tools new on Ebay for half the cost of retail.

The Parker O-ring handbook is a great resource for anything related to o-rings. This is what the geometry should look like, so any tool that can do that would be appropriate. Note the side walls are 0 to 5 degrees, so straight sides is acceptable.

 

[Jimsehr]

grind your own. I have quite a few that were ground by older machinists that I bought, and I have ground one as well. It is a lot of work to remove all the material, but a coarse wheel makes it go much faster, Or a cut off blade might work if you are good enough to remove material to a consistent depth and finish with a grinding wheel or even a narrow belt sander. I would be tempted to use my HF narrow belt these days with a quality coarse belt. I think it's 1/2 in wide.

Where are you going to get the O-ring that small?
And what size is the Ring.
I have ground hundreds of O-ring tools but never that small.

If I were to make that part I would bore the ring master ring size in the end of .1 bore and press fit a bushing with a .1 bore for the back side of the ring wall.

 

[RJSakowski]

As Pontiac suggested, put the O ring gland on the shaft. It's much easier to machine and it's easier to install or remove the O ring.

 

[Baumium]

Thank you all for the suggestions! Going in order here:

Depends, maybe the seal can ride in a groove in the piston. Else I would retain it with a light press-fit backing piece and forget the retaining groove in the bore.

As Pontiac suggested, put the O ring gland on the shaft. It's much easier to machine and it's easier to install or remove the O ring.

There are actually two seals: the piston itself and an additional seal between the piston shaft and the body of the pen (which has the small bore in question). The piston seal is intentionally leaky in one direction due to how the ink filling mechanism is designed (see: vacuum filler pens), so the shaft seal is necessary as well, as ink can make it past the piston head. Long story short: the o-ring has to be retained in the bore instead of the shaft.

I did consider a backing piece as you mention, but I wasn't sure how reliable a seal that would be. I'll keep it as an option!

Micro 100 has square shouldered grooving tools that you could use.

Looks like I didn't search hard enough... I'll keep this one in mind.

The Parker O-ring handbook is a great resource for anything related to o-rings. This is what the geometry should look like, so any tool that can do that would be appropriate. Note the side walls are 0 to 5 degrees, so straight sides is acceptable.

Thank you for this resource, very helpful.

Where are you going to get the O-ring that small?
And what size is the Ring.

The o-ring I have on hand is McMaster P/N 9452K13: 0.101" ID and 0.241" OD. The page I'm looking at has o-rings down to 29 thou ID -- very cute.


Given all the advice on grinding my own part, I think that's the first option I'll try out. Seems like the most satisfying. I'll report back!