Northern 4-8-4 Build 3/4”

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bushav
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Location: Bainbridge GA

Re: Northern 4-8-4 Build 3/4”

Post by bushav »

Another good day. I’ve finished the pistons with the exception of facing off .015 from each end. I’ve got to figure out how to chuck the piston without scratching or distorting them. I left .015 on the back end to true each one up after parting off. The extra on the front ends was just a lack of attention :shock: I also need to add a little dished out recess on each face as per the drawings. I am going to buy some copper pipe fittings at Home Depot in the morning and see if I can fabricate some chuck jaw covers out of them. That should let me chuck up the pistons without damaging them.
Pistons parted off
Pistons parted off
EDE45D55-1637-4A2A-BEC9-2D089713C5A6.jpeg (1.66 MiB) Viewed 2489 times
The great news is that nothing is binding. My pistons are pretty close tolerances with the cylinder bores. I may need to take another .001 off in the future. For the time being I’m going to press on because I think the tolerances will loosen up as I run it in on air. I can always take some metal off but can’t easily add it back!
First test fit of pistons
First test fit of pistons
328E9F6A-2C82-4CC5-98E9-A605005AA389.jpeg (1.4 MiB) Viewed 2489 times
Getting close to making some rings. Can’t wait. It’s going to be fun for me I think.
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Building 3/4” 4-8-4 Northern &
1” 4-6-2 Pacific
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Bill Shields
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Re: Northern 4-8-4 Build 3/4”

Post by Bill Shields »

Grab them by the rods in a collet.

Do not make the pistons too tight or they will stick.

.010 is my minimum clearance
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rmac
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Re: Northern 4-8-4 Build 3/4”

Post by rmac »

Bill Shields wrote: Grab them by the rods in a collet.
Or wrap a strip of aluminum cut from a soda can around them???
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Bill Shields
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Re: Northern 4-8-4 Build 3/4”

Post by Bill Shields »

Copper is better...but is concentricity a concern?

I sometimes make stub arbors to use in place of the rods for this type of work...start with a piece of hex stock, turn the end to fit the piston
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bushav
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Re: Northern 4-8-4 Build 3/4”

Post by bushav »

Bill Shields wrote: Sun Feb 19, 2023 8:28 pm Grab them by the rods in a collet.

Do not make the pistons too tight or they will stick.

.010 is my minimum clearance
I would like to use my collet chuck but that will not let me face the entire surface (the pistons aren’t threaded internally ). I do plan on using that method for dishing out the reliefs and taking a few thousands off the diameter if that’s needed. I checked this afternoon and I’ve got about .005 of runout in my collet setup when I chuck the pistons using the rods. The runout is not in my pistons and rods. It is actually in the chuck. Cheap Chinese chuck.

I can easily use my Bridgeport mill to face the pistons down to dimension.

.010 is a lot of gap in my small scale! The plans specify piston should be .001 less than cylinder bore. I can slide a .001 feeler into gap but can’t slide a .002 into gap (almost goes). I’ll revisit this after I have slept on it and studied my books. No hurry to make an adjustment as I still need to finish up my valve section and get it running on air.
Last edited by bushav on Sun Feb 19, 2023 9:25 pm, edited 1 time in total.
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Building 3/4” 4-8-4 Northern &
1” 4-6-2 Pacific
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bushav
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Location: Bainbridge GA

Re: Northern 4-8-4 Build 3/4”

Post by bushav »

rmac wrote: Sun Feb 19, 2023 8:53 pm
Bill Shields wrote: Grab them by the rods in a collet.
Or wrap a strip of aluminum cut from a soda can around them???
I see BlondiHacks using soda can on YouTube quite often. Seems like a quick and easy solution. The only issue is keeping it all positioned as you tighten the jaws. To start with I’m going to give the copper a shot and see if I can make something that holds itself in position and is reusable. I’ll snap a picture if it works out. If not I’ll resort to Coke Zero!
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Bill Shields
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Re: Northern 4-8-4 Build 3/4”

Post by Bill Shields »

.010 is what I use on that scale for 40 years and never had a leakage problem. I know we will get discussion about this, but I cannot tell you how many locos I have seen that run fine on air and lock up tight as a bull's butt when hit with steam because the piston expands and sticks.

I use .020 or greater on 1.5" scale. Why do you think you need a small clearance?

Remember the piston is not sealing and must not drag on the bore...and there will be times when the piston is at steam temp and the cylinder is cold.

The piston is not like in a car...it is not a bearing surface ..guidance is provided by the packing and crosshead....it's not going to rattle no matter how loose you make it as long as the rings fit properly.

Make an arbor and loctite the piston on and you can cut the face. Then toss it in the oven and it will fall apart.

Or you can make / purchase an od cup collet and hold the piston that way.

You will need the od collet anyway if you are making rings

Soda can aluminum is fine, but I do not drink sodas, and have buckets of scrap copper bits left over from years of boiler building.
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Harold_V
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Re: Northern 4-8-4 Build 3/4”

Post by Harold_V »

bushav wrote: Sun Feb 19, 2023 8:11 pm I am going to buy some copper pipe fittings at Home Depot in the morning and see if I can fabricate some chuck jaw covers out of them. That should let me chuck up the pistons without damaging them.
Is your three jaw chuck one with master jaws (the jaws are held in place with cap screws if so)? A previous picture you posted suggests it is. If your chuck is so equipped you'd be far better served to use soft jaws. They will solve all of the gripping problems, including holding the pistons in the proper attitude, and without leaving any marks of any description. Give it some thought before spending time making something that isn't going to serve you well.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
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bushav
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Re: Northern 4-8-4 Build 3/4”

Post by bushav »

Pistons finished. I’m excited with the way they turned out. Mounted on the chassis everything is moving smoothly in regards to the rods and pistons. I ended up facing them to length on the Bridgeport which worked very good. The dimensions are correct and the surface finish is pretty.
Setup for facing pistons
Setup for facing pistons
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To add the face reliefs I chucked the pistons in my collet chuck using the rods. I think this is called trepaning? I had to grind a tool blank to the proper radius and use plenty of oil.
Cutting the reliefs per plans.
Cutting the reliefs per plans.
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I think the finished product honors the original builder’s intentions nicely. I wish there was a way to discover the history behind this build.
Completed pistons in 3/4” scale
Completed pistons in 3/4” scale
2EFDE77E-FCBB-47C2-9FFC-C0D1DF957F73.jpeg (1.49 MiB) Viewed 2385 times
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Building 3/4” 4-8-4 Northern &
1” 4-6-2 Pacific
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Bill Shields
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Re: Northern 4-8-4 Build 3/4”

Post by Bill Shields »

Yes. Trepanning is correct.

Looks very nice
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steamingdon
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Re: Northern 4-8-4 Build 3/4”

Post by steamingdon »

That BIG nut is going to fit in cylinder without striking the head cover?
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Bill Shields
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Re: Northern 4-8-4 Build 3/4”

Post by Bill Shields »

That is why God made counter boring tools
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