Author Topic: A Wallaby of my own.  (Read 252217 times)

Offline stevehuckss396

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Re: A Wallaby of my own.
« Reply #930 on: January 01, 2019, 01:19:01 AM »
Wow.

Nikcole makes inserts that are .027, .032, .035, .043, .051 if that will help. I try to work around these sizes. I can send you the dimensions so you can make a holder. I have made a few for the sherline's.

No pressure.
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Offline steamer

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Re: A Wallaby of my own.
« Reply #931 on: January 01, 2019, 01:35:57 AM »
Wow.

Nikcole makes inserts that are .027, .032, .035, .043, .051 if that will help. I try to work around these sizes. I can send you the dimensions so you can make a holder. I have made a few for the sherline's.

No pressure.

Sounds like a plan to me!   You think 0.032 wide will work well   x2.   ?
« Last Edit: January 01, 2019, 01:39:55 AM by steamer »
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Offline steamer

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Re: A Wallaby of my own.
« Reply #932 on: January 02, 2019, 01:03:24 AM »
Insert Grooving tool being made.   The body is nearly done, just needs the hole drilled and tapped.

Thanks to Steve Huck for the reference! 

Dave
 
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Offline dieselpilot

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Re: A Wallaby of my own.
« Reply #933 on: January 02, 2019, 02:15:58 PM »
Old engine designs, full and model scale, used very wide rings. Most production model engines (RC) have 1mm rings, up to 27mm bore or so. Then, only 1.1-1.2mm.

Steve, do you cut the pocket at the compund 1° angle to the holder? I've given up on Nikcole as the pockets are week. Maybe in a hardened material they would hold up. Also their threading inserts have much too large a radius for small threads.

Offline Admiral_dk

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Re: A Wallaby of my own.
« Reply #934 on: January 02, 2019, 08:54:45 PM »
I think I know why the ring sizes are that way and for that reason I wrote a bit about reducing friction to gain power and reliability. In the end I had to Google a bit and almost the first hit was this one :

<a href="https://www.youtube.com/watch?v=iXhjytn860w" target="_blank">http://www.youtube.com/watch?v=iXhjytn860w</a>

Though it does not explain why as such, it shows that no one has really used (that kind we have access to) cast iron for decades. These more modern materials allows slimmer rings, reduces friction and allows higher rpm's.

Argh - I just posted and discovered that I can't see my own post with video on IE11  :killcomputer: - it works on Chrome.

Offline stevehuckss396

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Re: A Wallaby of my own.
« Reply #935 on: January 02, 2019, 09:36:41 PM »
Old engine designs, full and model scale, used very wide rings. Most production model engines (RC) have 1mm rings, up to 27mm bore or so. Then, only 1.1-1.2mm.

Steve, do you cut the pocket at the compund 1° angle to the holder? I've given up on Nikcole as the pockets are week. Maybe in a hardened material they would hold up. Also their threading inserts have much too large a radius for small threads.

Nope, 90 degrees to the part

I have never had any trouble with the inserts. They seem to last me a long time. I mostly cut brass, aluminum, and 12L14 and 1144 steel. They work great for ring grooves. The .019 wide inserts don't like interupted cuts but not many carbide tools do. Most are C2 grade carbide so not the best for steel but i stick to leaded steel and they work great. Other steel I think you can find C5 grade nikcole inserts.
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Offline steamer

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Re: A Wallaby of my own.
« Reply #936 on: January 02, 2019, 09:59:13 PM »
I did a bit of research on rings today....ill share in a bit
Steve is on the right track.
Dave
"Mister M'Andrew, don't you think steam spoils romance at sea?"
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Offline steamer

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Re: A Wallaby of my own.
« Reply #937 on: January 02, 2019, 11:28:39 PM »
I think I know why the ring sizes are that way and for that reason I wrote a bit about reducing friction to gain power and reliability. In the end I had to Google a bit and almost the first hit was this one :

<a href="https://www.youtube.com/watch?v=iXhjytn860w" target="_blank">http://www.youtube.com/watch?v=iXhjytn860w</a>

Though it does not explain why as such, it shows that no one has really used (that kind we have access to) cast iron for decades. These more modern materials allows slimmer rings, reduces friction and allows higher rpm's.

Argh - I just posted and discovered that I can't see my own post with video on IE11  :killcomputer: - it works on Chrome.


Piston ring geometry
Piston ring geometry has been a subject of much print and consternation....I'm not going to dig a new trench here....there are miles and miles of them, but I did get a glimpse of the crux of the previous conversation on this thread.   The subject of thickness of the ring.

OK so what I did was model up 2 piston ring, and subjected them to evenly distributed external pressure, and checked the results,   and what I found, at least to this engineer, makes a bit of sense
I modeled a 0.040" thick ring and a 0.027" thick ring leaving the rest of the ring geometry identical.

I found that the ring deflection didn't change at all for both rings.  I also found the the maximum stress didn't change either.   So whats that telling us?

Well   when the ring changes in thickness, the area of contact changes directly with the change in thickness.   Halve the thickness of the ring, you get half the area in contact with the cylinder wall

the piston wall pressure will not change as that is a function of the width of the ring.   HOWEVER...as there is more area on the thicker ring, the over all normal force to compress the ring is also higher proportionally with the thicker ring than the thinner ring.     The friction force generated by the ring also increases with the thickness of the ring, Friction is a REALLY bad thing in miniature engines!!!!

So that said, going as thin as you can safely make, and count on making some extras to cover breakage and wear is the way to go.

For this engine, I'm going to split the difference and go 0.031" thick x 0.040" wide.

Dave

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Offline Vixen

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Re: A Wallaby of my own.
« Reply #938 on: January 02, 2019, 11:57:06 PM »
Hi Dave,

I understand and agree with your ideas of keeping the rings as thin as possible to reduce contact friction. Do you think it would produce the same results if you were to machine a step on a wide in order ring to reduce the contact area? Just thinking.

Did your research indicate a better or alternative material to cast iron? Many of my racing bike engines seem to have thin, hard chromed, steel rings. They are strong and very flexible, unlike cast iron. Do you think we could get away with steel rings, without the Chrome? Has anyone tried it?

Mike  :thinking: :thinking:
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Offline steamer

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Re: A Wallaby of my own.
« Reply #939 on: January 03, 2019, 12:06:26 AM »
Hi Dave,

I understand and agree with your ideas of keeping the rings as thin as possible to reduce contact friction. Do you think it would produce the same results if you were to machine a step on a wide in order ring to reduce the contact area? Just thinking.

Did your research indicate a better or alternative material to cast iron? Many of my racing bike engines seem to have thin, hard chromed, steel rings. They are strong and very flexible, unlike cast iron. Do you think we could get away with steel rings, without the Chrome? Has anyone tried it?

Mike  :thinking: :thinking:

To answer the first question   NO.    the friction force would be the same as the full width ring.  The wall pressure is a function of the width of the ring.  My analysis was centered around square or rectangular rings, but I think my comment applies.

Materials:
The video in the post before mine talked about various ring materials.   One of them that I think might be worth looking into as a model engine ring material is Ductile Iron.       The hardened steel rings I think are beyond the scope of ME builders I think   I imagine the heat treat on those is tricky to say the least.
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Offline steamer

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Re: A Wallaby of my own.
« Reply #940 on: January 03, 2019, 01:25:25 AM »
Insert groove cutter holder machined.   Inserts will be here tomorrow.

I don't like the piston fixture yet...probably make another one tomorrow night.....

Progress
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Offline Zephyrin

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Re: A Wallaby of my own.
« Reply #941 on: January 03, 2019, 08:20:05 AM »
Hi,
the cast iron rings, slotted are held closed with a thin brass thread on a mandrel which has a screwed washer in front of it to tighten one or more rings and keep them closed; the exact outer diameter required is then finished on the lathe as it is generally made;
But before this finishing cut, I give a very slight eccentricity on this mandrel (about 0.1 mm) on the side where the ring are cut to reduce the thickness at this place, what is expected to distribute the compression of the ring evenly over the entire perimeter. I saw this trick on piston rings of vintage engine (10cm3 Micron sparkie engines) and now I try to reproduce it on my own engines !


Offline Roger B

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Re: A Wallaby of my own.
« Reply #942 on: January 03, 2019, 11:01:45 AM »
Still following along  :wine1: That's a lot of interesting info on piston rings  :ThumbsUp:  :ThumbsUp:
Best regards

Roger

Offline Vixen

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Re: A Wallaby of my own.
« Reply #943 on: January 03, 2019, 11:52:57 AM »
Hi Dave,

I understand and agree with your ideas of keeping the rings as thin as possible to reduce contact friction. Do you think it would produce the same results if you were to machine a step on a wide in order ring to reduce the contact area? Just thinking.


To answer the first question   NO.    the friction force would be the same as the full width ring.  The wall pressure is a function of the width of the ring. 


Hi Dave,

You are quite correct. I had not taken into account the fact that the wall pressure is a sum of the ring's hoop force and the gas pressure behind the ring. Wider rings will produce more gas pressure force onto the cylinder wall.

Mike
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Sometimes, it can be a long and winding road

Offline steamer

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Re: A Wallaby of my own.
« Reply #944 on: January 04, 2019, 01:20:54 AM »
Mike you are quite right regarding gas force increase with wider.rings, though without porting the pistons, its going to be hard to estimate.
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

 

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