Necessity of hardened crank pins on 3/4” scale

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matthew-s
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Necessity of hardened crank pins on 3/4” scale

Post by matthew-s » Sun Feb 02, 2020 11:43 am

Hi. I’m a newbie building a Kozo A3 per the book (3/4” scale, 3 1/2” gauge)

Kozo calls for hardened crank pins (with tolerances measured in 0.0001!).

My first attempt with machining A2 to this tolerance was an abject failure.

Which makes me wonder ... how common is it to harden this part “in real life”? If common/recommended, what material should I use that can be realistically hardened at home as a beginner?

Thanks!!

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NP317
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Re: Necessity of hardened crank pins on 3/4” scale

Post by NP317 » Sun Feb 02, 2020 11:50 am

Use "drill rod". Commonly available as "water hardening", also as "oil hardening" and "air hardening" (the most expensive alloy).
Water hardening is readily available (often at hardware stores) and can be easily torch-heated for hardening and tempering.
I have used water and oil-hardening drill rod extensively for crank pins, and other such pieces.
RussN

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Re: Necessity of hardened crank pins on 3/4” scale

Post by Harold_V » Sun Feb 02, 2020 5:18 pm

matthew-s wrote:
Sun Feb 02, 2020 11:43 am
My first attempt with machining A2 to this tolerance was an abject failure.
I'm going to assume that you have no provisions for precision grinding, which is not commonly found in the home shop..

I strongly suspect that you don't have the experience that permits one to machine to exacting tolerance. That's not uncommon, and how it can be done may not be obvious to some folks.

One of the first things you must determine is the characteristics of the material choice. In this case it is specified as A2, meaning it is air hardening. It may or may not change size upon heat treating. I don't know, but the maker of the material does know, so you should inquire of them if it changes, or not, as that will affect your choice of sizing prior to heat treat. As an example, it is known that 17-4 PH shrinks approximately .0006" per inch upon heat treatment. That amount must be allowed, otherwise your parts won't be sized correctly after heat treatment.

Once you understand the characteristics of the material in question, your next objective is to determine how much material you must leave so you can size the parts after they are heat treated. Sizing prior to heat treat is NOT a good idea, as they are going to oxidize in the heat treating process unless you have a controlled atmosphere furnace, which, again, is highly unlikely to be found in the home shop.

Once hardened and drawn (tempered), you will size the parts by spin polishing, using abrasive cloth in progressive fineness. It is important for you to understand that polishing a thousandth of an inch is slow business, and that's why it is important for you to leave the required amount of material prior to heat treatment.

It is very easy to ruin parts by not maintaining a straight diameter in polishing. Constant measuring and spot polishing is required, and you must be aware of the temperature of the work, as heat expands the part, which would then end up undersized once it has cooled.

Polishing with abrasive cloth demands that you start with a coarse grade and work progressively with a finer and finer grade until you achieve the desired finish as well as the desired size. You must be patient, but once you get the feel, it works remarkably well.

So then, from my comments you should take away from this reply that it borders on the impossible for many materials to be machined to exact size, as the material simply won't cooperate. It is known to tear, or to skip and then cut, so you end up with an irregular surface. For that reason, one sizes the material by turning slightly oversized when it is in the annealed state, and then rough polished with abrasive strip before heat treat. Sizing is accomplished only after the parts are hardened.
Which makes me wonder ... how common is it to harden this part “in real life”? If common/recommended, what material should I use that can be realistically hardened at home as a beginner?
I can not address what the average person does, as they may not disclose that they didn't follow procedures that guarantee longevity. Some don't. However, hardening of wearing surfaces is critical to long life. Hardened parts will outlive soft parts, and may well avoid tragedies whereby parts that run dry gall, ruining the work accomplished. If you hope to have a reliable engine, heat treatment of critical components is highly desirable.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

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NP317
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Re: Necessity of hardened crank pins on 3/4” scale

Post by NP317 » Sun Feb 02, 2020 6:01 pm

Mathew-s:
Note that the tolerances required to make a locomotive like Kozo's A3 0-4-0 can be pretty loose for the locomotive to operate well.
In fact, I usually machine my model parts too tight, and they required alteration to open clearances to allow the systems to operate smoothy!
However, valve gear DO require close (not tight) tolerances to allow the locomotives to operate "in square" both forward and reverse.
Machining to tenths of thousandths of an inch should not be required at all. But it can be nice, if possible.
I usually aim for .0005" at the best, where it can help operation. Especially with press fits. Other builders may choose looser tolerances, with success.

Full sized steam locomotives always operate better on poor track when they are aged and worn. Derailments happen less frequently!
Having helped restore and operate 4 full-sized logging locomotives, I speak from some personal experience.

Drive and connecting rod bushings are perfect examples. Due to the vertical motion of drive axles, the rod bushings MUST have seemingly too-large clearances to prevent the rods from binding when rolling down rough tracks. As the axles rise relative to each other, the connecting rods effectively need to become longer. The bushing internal diameters provide that ability. Full-sized, and our models. Listen to the clanking sounds from any full sized steam locomotive (especially when they are "drifting") to prove that operational requirement.

Others here can attest to these concepts, I'm sure.
RussN

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Bill Shields
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Re: Necessity of hardened crank pins on 3/4” scale

Post by Bill Shields » Sun Feb 02, 2020 8:31 pm

hardened pins not needed...i would not bother
Too many things going on to bother listing them.

10 Wheeler Rob
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Re: Necessity of hardened crank pins on 3/4” scale

Post by 10 Wheeler Rob » Sun Feb 02, 2020 9:30 pm

You can get 4140 half hard, which you can machine and finish to fine surface finish with a file. It wears good.

Rob

matthew-s
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Re: Necessity of hardened crank pins on 3/4” scale

Post by matthew-s » Sun Feb 02, 2020 10:17 pm

Thanks for input so far. The idea of polishing post hardening is one I had not considered.

4140 half hard is also interesting. I only found machinability data for fully annealed. I’m stuck with my sieg 7x16 lathe, so need to work within its capabilities. I’ll consider this path too.

I’m diligent and good at following instructions, but have limited hands on experience, and fewer heat treatment tools. Whatever the approach, I hope I can make do with a MAPP torch, fire bricks, and “salad oil” as Kozo calls it! Perhaps I missed the spot where he gives guidance on machining tolerance and material selection. The book, as amazing as it is, could stand an index!

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JBodenmann
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Re: Necessity of hardened crank pins on 3/4” scale

Post by JBodenmann » Sun Feb 02, 2020 10:20 pm

Hello My Friends
Case hardening is a good choice. You can use cold or hot rolled, and most L series free machining steels can be case hardened. Tool steels require annealing after hardening and this is the touchy part. If you don't get it right you get brittle or soft material. This is not an issue with case hardening as you only Harden the surface. Kasenit, or Cherry Red are two case hardening treatments that are well suited to the home work shop.
Jack

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dnevil
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Re: Necessity of hardened crank pins on 3/4” scale

Post by dnevil » Sun Feb 02, 2020 11:53 pm

I have tried repeatedly to find a supplier of Kasenit, to no avail. It appears to no longer be made and sold. Have I missed something?

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NP317
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Re: Necessity of hardened crank pins on 3/4” scale

Post by NP317 » Mon Feb 03, 2020 1:07 am

Amazon carries Cherry Red case hardening powder.
RussN

matthew-s
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Re: Necessity of hardened crank pins on 3/4” scale

Post by matthew-s » Mon Feb 03, 2020 8:14 am

Hmm watching YouTube videos case hardening “in a can” looks to be the most approachable option. McMaster Carr has Cherry Red. I needed to order something else from them.

I have 1144 stressproof on hand. I presume that is too high carbon content? Can you case harden leaded steel (12l12), or is it best to use plain 1212?

Do you need to provide machining allowance on case hardening, or can you go to the dimension from the start?

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JBodenmann
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Re: Necessity of hardened crank pins on 3/4” scale

Post by JBodenmann » Mon Feb 03, 2020 10:17 am

Hello My Friends
I have case hardened leaded steel many times. Its great for pins in valve gears and throttle linkages and such. No dimensional changes happen.
Jack

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