Allen models Fitchburg Northern build by first timer

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rmac
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Re: Allen models Fitchburg Northern build by first timer

Post by rmac » Wed Dec 08, 2021 8:40 pm

Greg, even when the system contains some steam and some water, the pressure within the system is the same everywhere (but see note below). If it wasn't, then fluid (water and/or steam) would flow from the areas of higher pressure towards the areas of lower pressure until equilibrium was established. So in the situation you describe, the brake cylinder is filled with water at 125 PSI, which exerts 125 pounds of force on the 1-square-inch piston, just the same steam at 125 PSI would.

As Bill said a few posts earlier, "pressure is pressure". It doesn't matter if the fluid is steam, water, Bud Light, or some combination thereof.

The key mistake in your analysis is that you confused pressure with force when you said:
So the pressure on that is .0274 x 125 = 3.425 lbs.
That should be, "The force is .0274 x 125 = 3.425 lbs."

But since that 3.425 pounds of force is acting on an area of only .0274 square inches, it still results in a pressure of 125 PSI in the water, which then results in a force on the 1-square-inch piston of 125 pounds.

-- Russell Mac (not a physicist, but I'm pretty sure I've got this straight)

Note: This ignores the fact that the pressure within a body of fluid increases with depth due to gravity.

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Greg_Lewis
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Re: Allen models Fitchburg Northern build by first timer

Post by Greg_Lewis » Wed Dec 08, 2021 9:14 pm

Thanks, Russ. Makes sense except for the Bud Light. Anything is better than Bud Light.
Greg Lewis, Prop.
Eyeball Engineering — Home of the dull toolbit.
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mcjustis
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Re: Allen models Fitchburg Northern build by first timer

Post by mcjustis » Wed Dec 08, 2021 9:42 pm

Got my apprentice working on something today so I had some time to fool around. I got the brake arms all done. To make these I cheated quite a bit. I tried to make them from the castings, but they were just meh...kinda looked a little off, so I decided to make them from some solid bronze I had. I first cut the side profile out on a mill /turn CNC lathe that I run. After that I made a fixture to hold them so I could cut the outer profile that is 3/4" on one end and 1/2" on the other. Here's the fixture in the mill. I made a threaded post to both locate the 3/8 bore and tighten it down with a nut. I also put in another 1/4-20 socket head cap screw on the other end to hold the part in place.

brake arm fixture.jpg
Once that was set up I put the part on the locating pins and tightened it down. I then cheated a little more and ran that profile on the 2 axis CNC mill.
brake arm machining.jpg

Once i ran both of those profiles I set the fixture up straight and touched off the edge and made the 5/16" slot with a 1/4 " endmill.
brake arm slot.jpg
Both parts all done. I like the looks of these way better than the cast parts. Two more parts down!
brake arms.jpg
And I would have to agree on the bud light.

Martin

mcjustis
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Re: Allen models Fitchburg Northern build by first timer

Post by mcjustis » Tue Jan 04, 2022 7:57 am

Just noticed it's been a month since I updated here. Happy New year to everyone.

I have some more parts all finished and have been slowly making my way through this build.

I have the brake cylinders all done. I still need to mount them permanently to the frame sides.
brake cylinders.jpg
I ordered some tubing for the boiler so was able to get the smokebox cut off that piece. I cleaned up the ends and turned the id on both ends to fit the boiler and the front end cover.
smokebox.jpg
I set the tube up on the frame just to see what it looks like. Made me feel good anyway. Getting there.
tubeon frame.jpg

I ordered some more of the castings needed a few weeks back. I wanted to start on the reversing quadrant. The bronze casting was warped pretty bad to the point that it wouldn't work as intended. I thought I'd try heating it up and see if I could bend it to get it a little closer to shape. That ended in an epic failure. Oh well, I guess I'll order the cast iron one next time.
brokenquadrant.jpg
I did get the exhaust pipe and nozzle finished up too. Nothing particularly exciting there but it's one more piece done. I still need to make the ring that mounts on the top of the nozzle.
exhaust nozzle.jpg
I have also have the cross heads done. They call for 2-56 bolts to hold in the top slider piece, but I may change over to 4-40 as the 2-56 just look too small to me. We'll see.
cross head.jpg
That's all for now.

Martin

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NP317
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Re: Allen models Fitchburg Northern build by first timer

Post by NP317 » Tue Jan 04, 2022 12:50 pm

Looks like fairly fast progress!
Going nicely.
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Re: Allen models Fitchburg Northern build by first timer

Post by Pontiacguy1 » Tue Jan 04, 2022 3:53 pm

I'm interested in seeing how you hold your quadrant casting for machining. I've got a couple of them that I need to get to eventually.

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Re: Allen models Fitchburg Northern build by first timer

Post by Harold_V » Tue Jan 04, 2022 4:33 pm

mcjustis wrote:
Tue Jan 04, 2022 7:57 am
The bronze casting was warped pretty bad to the point that it wouldn't work as intended. I thought I'd try heating it up and see if I could bend it to get it a little closer to shape. That ended in an epic failure.
Oh, yeah! You should have tried making the correction without the use of heat. It's well known that many of the copper alloys are hot short and crumble with hot working. Is that what you experienced?

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

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Greg_Lewis
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Re: Allen models Fitchburg Northern build by first timer

Post by Greg_Lewis » Tue Jan 04, 2022 4:39 pm

What about just fabricating the quadrant? It doesn't have to be finished to .001 so it looks to me like some work with simple tools and some bits of steel would do it.
Greg Lewis, Prop.
Eyeball Engineering — Home of the dull toolbit.
Our motto: "That looks about right."
Celebrating 35 years of turning perfectly good metal into bits of useless scrap.

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Bill Shields
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Re: Allen models Fitchburg Northern build by first timer

Post by Bill Shields » Tue Jan 04, 2022 6:10 pm

Be aware that the wrist pin is a high wear area.

My go to solution is to put delrin AG or molydisulfide impregnated nylon bushings in there from the get go and forget it for life.

As suggested..you will find it easier to make a quadrant than fool around with the casting.

A few minutes with a band saw and sander and you can have one quicker than you can make the widgets necessary to hold and square up the rough casting
Too many things going on to bother listing them.

Harold_V
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Re: Allen models Fitchburg Northern build by first timer

Post by Harold_V » Wed Jan 05, 2022 2:53 am

I think Bill made a typo----Delrin AF, not AG.

Bill?

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

mcjustis
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Re: Allen models Fitchburg Northern build by first timer

Post by mcjustis » Wed Jan 05, 2022 6:46 am

Harold, I tried heating it up. There's no way it would have worked as I received it. I didn't even think of trying to bend it without heating it up. Lesson learned. Luckily it was only a $12 casting. I think what I might do is use the top piece and make the lower half of it. If that doesn't work I can just fab up the whole part.

Bill, dumb question... which wrist pin are you referring to?

Thanks guys!

Martin

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Bill Shields
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Re: Allen models Fitchburg Northern build by first timer

Post by Bill Shields » Wed Jan 05, 2022 10:05 am

the front end of the main rod...where it connects to the cross head (guide...)
Too many things going on to bother listing them.

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