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ST #1 Problem

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John Purdy02/06/2019 18:50:02
189 forum posts
59 photos

I have finished machining both the con rod and the piston rod for the ST #1 and have run into what appears to be a discrepancy between the drawings of the con rod and the piston rod.

According to the dimensions on the drawings when the piston rod is assembled to the con rod with the crosshead pin, the con rod cannot rotate on the crosshead pin to allow for the crank rotation as the bottom of the crosshead hits the curved sides of the slot in the head of the con rod.

Looking at the drawings below you will see that the bottom of the crosshead is shown as 1/4" below the centre of the 9/32" hole for the crosshead pin, but the bottom of the curved slot in the head of the con rod is shown as 17/32" below the centre of the hole for the cosshead pin which puts the start of the curved section at the bottom at 17/32 - 9/32 = 1/4", the same as the crosshead, therefore any rotation of the crosshead on the pin causes the bottom of the crosshead to hit the curved sides of the slot preventing rotation.

#1 piston rod.jpg

#1 con rod.jpg

When I assemble mine I do get do get a few degrees of rotation, but crank rotation requires 16 or more degrees from the vertical to allow rotation of the crank.

My drawings are dated 1975 so if any one has more recent drawings, how do the relevant dimensions compare?

I have come up with a number of possible work arounds, remove some from the bottom of the crosshead, use a 1/2" or 3/8" end mill to change the radius of the curves at the bottom of the slot to allow more clearance, bevel or radius the corners of the bottom of the crosshead to allow clearance.

Has anyone else who has built the #1 run into this problem and if so how did you get around it?


Edited By John Purdy on 02/06/2019 18:52:23

John Purdy11/06/2019 22:05:55
189 forum posts
59 photos

I solved my problem by mounting the piston rod/crosshead on my small 4" G. Thomas rotary

table, setting it 15 deg. from the vertical both ways and milling off the bottom edges .050" deep

on both sides, as in the picture. This now gave just over 20 deg. of rotation which is more

than enough to allow crank rotation.dscn3174.jpg

Here is my progress so far. Still have to complete the front column, steam and exhaust

flanges, most of the valve gear, slot in the crank for the flywheel key and the key. Then the

job I look forward to the least --- painting.dscn3196a.jpg


David George 111/06/2019 22:18:59
1010 forum posts
320 photos

Nice looking job. I am looking for something to make perhaps one of these, what do you run your models on air or steam.


John Purdy12/06/2019 18:06:55
189 forum posts
59 photos


I run most of mine on air as I only have a 3" vertical test boiler to Tubal Cain"s design. The exception is my ST# 10V that I run on steam. It is mounted on a base with the boiler and feed pump as a unit like this.



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