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Old 02-21-2011, 08:53 PM   #1
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Default Stuart triple - types of metals to get

Hi,

I need some help in choosing the right types of metals to obtain for a Stuart Triple Expansion engine that I am just about to start making.

First some background on me as I am new here.

I am just getting my home hobby metal shop going. I have done some metal working 30 years ago, but worked more in wood since then, so I have much to relearn. After searching for roughly 6 months, I was able to get my hands on a nice used Hardinge HLV lathe and building up the tooling for it. I am still on the lookout for a vertical mill.

My first real project will be a Stuart Triple Expansion for which I have the castings, but I am currently making a couple very simple Stuart Oscillators to make sure I am familiar with the new to me lathe, reacquaint myself with machining metal, and to make sure I can achieve the high quality and precision I wish to achieve when I dig into the triple. The HLV is a way nicer and more sophisticated lathe than the tired South Bend’s and larger tool room lathes I’ve previously used, and I am very impressed with how easily the HLV is allowing me to achieve very nice cut quality and equally good precision. I don't care how long it takes me to make the triple, but the goal is to do a very good job of it. The triple is being built as a high quality and attractive show piece which I intend to run, but realistically it will probably not see a whole lot of use.

Enough with the quick background.

I need help/suggestions on what types of “mild steel”, “stainless steel”, and “silver steel” I should be getting.

The mild steel parts include the crank shaft, vertical columns, reversing gear. Most of these pars require a fair bit of machining. It kind of looks like 1018 would be a good pick for most parts, and perhaps 1144 for the crankshaft. Is this reasonable, or should I be using very different alloys?

The stainless steel is for the valve and piston rods, for which little machining appears to be required. I don’t have a lubrication system and have not machined stainless before, so I’m a little afraid of the unknown. What types should I be looking at?

The “silver steel” is for the reverse shaft and taper pins. I am not sure what the North American equivalent is for silver steel. The silver steel parts don’t seem to require much machining.

I think I am ok in picking out the required brass and phosphor bronze, and have had experience cutting bronze.

I welcome any suggestions you can provide, and I promise to provide some pictures along the way, though it might be spread out over a bunch of time.

Thank you



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Old 02-21-2011, 09:40 PM   #2
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Default Re: Stuart triple - types of metals to get

Hello hankey,
The Triple is an ambitious first project, good luck with it. The first thing to say is if you are in the USA, our equivalent of "silver steel" is drill rod. Simply substitute drill rod wherever silver steel is called for. The water, air, or oil hardening variety all machine about the same.

For built-up crankshafts my preference is for drill rod or ground stainless steel for the main shaft and throw journals, and whatever steel I have scraps of for the throws. As for mild steel, 12L14 is a leaded free-machining alloy and gives a batter finish than 1018 and is popular for that reason. The general preference is for either 303 or 304 stainless, I can't remember which, 304 I think and if I want steel parts to stay bright without oiling them I substitute SS for mild steel. SS, especially a leaded free-cutting SS, can be machined just like mild steel, with or without cutting lubicants, your choice. Hope this helps.



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Old 02-21-2011, 11:29 PM   #3
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Default Re: Stuart triple - types of metals to get

Thank you Harry for you suggestions. I am in North America. And yes, I appreciate I am taking on a very ambitious first project, especially given that I want to build the engine to a high standard.

What would your recommendation be for a crankshaft made in one piece? I would prefer to avoid the built-up method. I will most likely add the counter weights as separate pieces, per the plans, but am still tossing around thoughts of machining the counter weights as part of the crankshaft, and probably won't make a final call until I see how it is working out. I appreciate a built-up crankshaft might be easier for a shaft that is roughly 9" long by 3/8" diameter and with 3 throws.

I will investigate a lead free SS and 303 & 304 SS, for if SS is as easy to machine as mild steel without lubricants, then that is an easy decision to make given I want the piece to have a bright finish and not be a rust magnet.

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Old 02-21-2011, 11:50 PM   #4
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Default Re: Stuart triple - types of metals to get

hankey,
I don't know the latest and greatest of steels metalurgy but for what it is (a model, not intended for hard work) you could actually use just about anything. But one important factor will be to select something which will have minimal distortion when it's machined - and that would NOT be a cold rolled. Although I'm not an automotive guy years ago (and maybe still) competition crankshafts were turned from 4140, and no doubt there was a good reason for that, so that might be one suggestion. I'll bet someone on the board is far more up to date on this than I am and will have a suggestion.

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Old 02-21-2011, 11:54 PM   #5
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Default Re: Stuart triple - types of metals to get

For turning 303 and 304L are both fine, but 304 can be quite difficult to mill. 303 would be the best choice all around. 304 is also very hard on bandsaw blades.

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Old 02-22-2011, 01:02 AM   #6
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Default Re: Stuart triple - types of metals to get

What KVOM said! 304 is UGLY......303 is fun....
I think it's the nickel content, but I'm not sure.
Dave

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Old 02-22-2011, 01:12 AM   #7
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Default Re: Stuart triple - types of metals to get

If it were me, and I wanted a strong crank, I might go for 1144SP. If your concerned about rust though, 303 would be just fine.

I think 1144sp machines better and has fabulous properties....like nearly 100000 psi tensile,but is free machining.

Dave

PS the crank on my 12" to the foot boat engine is 303 SS.

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Old 02-22-2011, 11:27 AM   #8
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Default Re: Stuart triple - types of metals to get

Thanks to all. It sounds like I should use 303 SS for all the mild steel and SS needs, including the one piece crank shaft. Most of these parts will need as much milling as turning, whihc on that count would seem to rule out 304 based on what you guys describe. I will stck with the drill rod for the reversing shaft and taper pins, as I presume they specd that to avoid it twisting at all. I figure I will turn the crank down in stages over a span of a month in the event the material I use has any unequal stresses.

Once again thank you all, and I'll try to provide some progress pictures along the way but I will be taking this project slowly.

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Old 02-22-2011, 11:45 AM   #9
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Default Re: Stuart triple - types of metals to get

What everyone else has said especially on the 304 Stainless. Give it a wide berth. It is nasty stuff.

"Bill Gruby"

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Old 02-22-2011, 01:27 PM   #10
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Default Re: Stuart triple - types of metals to get

Quote:
Originally Posted by rhankey
I will stck with the drill rod for the reversing shaft and taper pins, as I presume they specd that to avoid it twisting at all.
hankey,
IMHO specifying hardened (or hardenable) pins is more a tip of the hat to tradiiton than actual need. The shear forces present in the engine certainly wouldn't be great enough to require that. I like pinned and cottered and keyed connections, these were the oldest traditional way of making connections, and something you might look into is that the clockmaking world uses pinned connections so small tapered pins and reamers will be available from clockmakers suppliers.


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