Turning o ring to fit bore

[Donleybill]

With out going through a lengthy discussion as to why I have this question here is my question
For model steam engine piston ring my best plan is to turn down a standard 2” OD ring to a size of 1.946” dia.
Has anyone done this before? Should I freeze the o ring first on a mandrel. Turn with a razor sharp cutting edge. Need to make two each and my problem solved. Waiting for the standard rubber o rings to be shipped.
Bill

 

[TonyM]

So without going into the why's and wherefore's.
In my experience grinding rubber is the only way to change its size easily. It would need to be put on a mandrel without distortion / rolling and gripped on the sides. You might be very lucky but it's not going to be easy.

 

[Charles Lamont]

I agree about grinding. Dremel, or equivalent, mounted on you your toolpost will suffice.

Why?

 

[larryg]

Looks to me like you want a 50 mm o-ring. 50mm = 1.968"

You may also want to work with the durometer of the material to get what you need

lg
no neat sig line

 

[Donleybill]

Ok. Im going tool up to grip the Oring on its sites and using the tool post grinder I try machining the Ring OD.
Waiting for rings to be shipped. once I have the o ring in hand I’ll make an arbor to fit with one Fixed and one movable side wall to clamp the ring.

 

[wespete66]

But why would you need to alter the diameter of an o-ring? As it is elastic, an o-ring will change (within reasonable limits) to fit the groove it is placed into. The trick is to adjust the ID and/or the OD of the groove, as allowed, to accommodate a given o-ring while maintaining the desired amount of radial squeeze on that o-ring. Then look at the width of the groove. A total fill of 85% (ratio of o-ring volume to groove volume) seems to be ideal for hydraulic purposes, and may be a good starting point.

 

[Tim Wescott]

I suspect this is a partly-right and partly-wrong method, but I've always made O-rings fit by selecting a ring that's a bit undersized, then making a groove into which the thing fits while a bit stretched.

Then I tune the depth of the groove to tune the O-ring OD.

I suspect one could even (gasp!) calculate all of this beforehand, if one where of the mechanical engineering persuasion. I've always just winged it.

 

[Donleybill]

I can certainly mess with the o ring groove width and depth. The piston and its fit with the cylinder is not going to change.
If this engine had a cast iron bore and cylinder I would be making split cast iron rings. The materials are cast brass and bearing bronze. Using O rings as a piston ring for this combination of materials is common. Model builders who don’t want mess with splitting cast iron rings use o rings.
My fix to this issue was to cut down a 2” dia standard ring. The suggestions I received may or may not work but we will see. What I learned once again “look twice before making parts to model drawings”. Never asssume the drawings are free of error. In this case I’m stuck looking for a simple component that isn’t available in the size I want.

 

[Dubi]

I can certainly mess with the o ring groove width and depth. The piston and its fit with the cylinder is not going to change.
If this engine had a cast iron bore and cylinder I would be making split cast iron rings. The materials are cast brass and bearing bronze. Using O rings as a piston ring for this combination of materials is common. Model builders who don’t want mess with splitting cast iron rings use o rings.
My fix to this issue was to cut down a 2” dia standard ring. The suggestions I received may or may not work but we will see. What I learned once again “look twice before making parts to model drawings”. Never asssume the drawings are free of error. In this case I’m stuck looking for a simple component that isn’t available in the size I want.

Hello Donleybill, I have been watching this post with great interest. I design underwater equipment including manned Wet Submersibles thus I use O-Rings to keep water out. Your application is something I have never come across before however I am wondering if a rubber O-Ring is the answer.

What is the shore hardness of the O-Rings you have orderd and have you considered machining an O-Ring from one of the heat resistant plastics?
Some of the Polyimide resins include about 15% graphite, look up type SJ-115.

Stay safe,
Dubi

 

[Steamchick]

Agree with your method Tim, or buy a 50mm O-ring.

K2.

 

[SailplaneDriver]

If you are in the US, both Grainger and McMaster Carr have 50 mm OD o-rings in various materials and thicknesses. Some are even available on Amazon.

 

[Tim Wescott]

MSC Industrial has a very good selection of O-rings, too. If they're not better than McMaster, they're just as good, and the price is way lower.

 

[Donleybill]

McMaster Carr had the size I was looking for. Too easy I must say. I searched by OD and there it was. Ordered and was delivered the next day.
I appreciate everyone support.

 

[HMEL]

With out going through a lengthy discussion as to why I have this question here is my question
For model steam engine piston ring my best plan is to turn down a standard 2” OD ring to a size of 1.946” dia.
Has anyone done this before? Should I freeze the o ring first on a mandrel. Turn with a razor sharp cutting edge. Need to make two each and my problem solved. Waiting for the standard rubber o rings to be shipped.
Bill

I don't mean to be snarky, or obnoxious but it seems to me trying to turn down o- rings is a bit like beating a dead horse. The material is a polymer and will not shear in the same way metal does. There are kits that allow you to custom make o rings of specific diameter and a few videos will show you how to calculate the cut dimensions. I have worked with air compressors that use Teflon rings and in my mind might have greater promise. I probably would not recommend a custom cut o ring for high pressures or where seals are critical but I don't see the pressures the piston will see to be beyond what a custom ring could hold.

 

[Ramon]

Bill,

A bit late in the day now as it looks like you have now solved your problem, but an 'O ring is an O ring is an O ring' - full stop. I'm afraid to say that the part very much requires to be made to accomodate the O ring and not the other way round.

Based on what engines I've made so far I consider a much better material for piston packing is PTFE impregnated yarn. This allows a free-er piston to liner fit but provides an excellent seal whilst providing an almost frictionless fit.
It also means the tolerances in the groove in the piston, unlike that for using an O ring as a seal are much more relaxed. The tolerances for an O ring need to be dead right to get the 'perfect' fit for the job in hand.

Hope that's of use at this late point

Tug

 

[Larry G.]

trying to turn down o- rings is a bit like beating a dead horse. The material is a polymer and will not shear in the same way metal does.

GRIND, not turn. The roller industry has been grinding every type of rubber, elastomer, and non-metallic material for ages, in sizes and to accuracy that are remarkable.

Measuring them is another neat trick. Pi-tape, anyone?

 

[HMEL]

trying to turn down o- rings is a bit like beating a dead horse. The material is a polymer and will not shear in the same way metal does.

GRIND, not turn. The roller industry has been grinding every type of rubber, elastomer, and non-metallic material for ages, in sizes and to accuracy that are remarkable.

Measuring them is another neat trick. Pi-tape, anyone?

Some clarity is in order. When you make a custom o- ring you measure the part diameter or slot depth where the o ring is to be placed with a micrometer. Then you measure the diameter of the o ring material. These two numbers are used in a formula to calculate the length of material. The material is laid out in a linear length and cut. An epoxy glue is applied to each end and then the ring is formed. No Pi tape is needed.

An o ring is designed to seal. It will deform when pressure is applied. Much different then the elastomers used for rollers which are harder. Whether you GRIND or turn the o ring will deform as soon as contact is made. The technique is impractical and more likely borders on insanity!

 

[TonyM]

An o ring is designed to seal. It will deform when pressure is applied. Much different then the elastomers used for rollers which are harder. Whether you GRIND or turn the o ring will deform as soon as contact is made. The technique is impractical and more likely borders on insanity!

On the contrary. Printer rollers are ground to very high tolerances and they are invariably around 40-50 shore hardness. O rings are typically 70 shore hardness, so much harder. I don't disagree that grinding an o ring is impractical and is technically difficult but grinding polymers in no way borders on insanity. It is a normal production process in many industries.

 

[Peter Twissell]

As a former manufacturer of print rolls, I should point out that they consist of a layer of elastomer bonded to a steel roller. Although the elastomer can be quite soft, it is well supported.
That said, it shouldn't really be necessary to machine o rings. If you look around, o rings are available in a very wide range of section diameters and ideally, one would machine the groove to suit the o ring, not the other way around.