cutting a groove for an “O” ring

[martin107Participant @martin107]

What is the best thing for cutting a groove for an "O" ring I have a Quick change tool post with a 10mm tool holder on a ML7.

Cheers

Martin

[martin107Participant @martin107]

[not done it yetParticipant @notdoneityet]

A parting-off tool?

[JasonBModerator @jasonb]

Yep parting off tool, does not have to be the exact width of your groove so just move tool sideways and plunge in again to get what width is required.

[martin107Participant @martin107]

Thanks Lads

[Sam Longley 1Participant @samlongley1]

Apologies for changing the subject slightly but the heading suits !!!

I have a pair of 1 inch silicon "O" rings for the piston of my steam engine but I am unsure how big to make the groove.

Is there a simple rule based on the thickness of the ring or is it guesswork?

Edited By Sam Longley 1 on 28/12/2017 08:10:31

[JasonBModerator @jasonb]

What section is the ring?

There are tables of suitable groove sizes for model use in things like Reeves catalogue and Model engineers handbook. Most online tables are for too tight a fit so you will get too much friction.

[J HancockParticipant @jhancock95746]

As important, is how you treat the 'O 'ring , getting it ' into and out of ' the groove without damage.

You have some reading to do on the subject , all on-line, or manufacturers ref' books.

[RobinParticipant @robin]

Steam is relatively low pressure so I suppose the O ring might actually retain it's shape rather than becoming a squidged up smear of rubber driven in to the low pressure corner. Could you actually use one as a sliding seal? That seems a bit unlikely but maybe there is a trick to it?

[Sam Longley 1Participant @samlongley1]

Posted by JasonB on 28/12/2017 08:12:18:

What section is the ring?

There are tables of suitable groove sizes for model use in things like Reeves catalogue and Model engineers handbook. Most online tables are for too tight a fit so you will get too much friction.

The ring section is 3.5mm

have looked on Reeves site but have not found anything re groove sizes though !!

I chose these as they were cheaper than CI rings & at last years Alexandra palace ME exhibition one of the display chaps told me that his traction engine had run on silicon rings for a long time with no ill effect.

From Robin's comment above – have I made a mistake?

[Anonymous]

The PDF handbook listed on this page:

**LINK**

will tell you all you need to know about O-ring applications, groove sizes and surface finish requirements.

Andrew

[JasonBModerator @jasonb]

No Mistake, perfectly acceptable method, I have O rings in several of my engines.

So You have a nominal 1/8" section O ring which is actually 0.139" or 3.53 cross section. This needs to be compressed slightly in the groove to give a good seal which will make the round section of the ring into an oval shape so the groove needs to be shallower than the 3.53mm but also wider and you also allow a little width so the ring can roll slightly.

For a piston in a steam engine you need the groove to be 0.160" 4.1mm wide and 0.132" (3.35mm) deep also deduct any clearance between piston and cylinder from this depth. 

This is quite a bit looser than the standard SAE sizes you find in a lot of online charts as I said earlier which will just make for a tight engine that needs excessive pressure to run. Typically 1/2 to 1/3rd the compression of the ring from Andrews link

 

J

 

PS it's in the Reeves paper catalogue not on their site.

 

Edited By JasonB on 28/12/2017 19:32:26

[SillyOldDufferModerator @sillyoldduffer]

Posted by Robin on 28/12/2017 09:33:51:

Steam is relatively low pressure so I suppose the O ring might actually retain it's shape rather than becoming a squidged up smear of rubber driven in to the low pressure corner. Could you actually use one as a sliding seal? That seems a bit unlikely but maybe there is a trick to it?

Robin's question led me to a 'well I never' moment – turns out o-rings are counter-intuitive. According to this paper it's harder to seal them at low pressure because they don't squish enough to form the 'D' shape needed to make a proper seal. And by 'low pressure' they mean less than 400psi!

Yes O-rings are good for sliding seals. The main problem seems to be sticking unless they're moved regularly.

Dave

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