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Member postings for John Purdy

Here is a list of all the postings John Purdy has made in our forums. Click on a thread name to jump to the thread.

Thread: Mallable Iron
17/07/2019 18:47:57

Well I attempted to straighten them with a hammer on the anvil and it worked OK but it took a lot harder hits than I thought it would, they definitely weren't soft, the hammer left no marks on them.. They are not perfectly straight but close enough so that they will clean up. Just hope there are no unseen fatigue cracks that will cause them to break when it machine them.  Probably might have been better to put them in the kiln and heat them to red heat and then hammer them straight. Hindsight is great!


Edited By John Purdy on 17/07/2019 18:51:00

16/07/2019 19:37:54


As I said the when I machined the current one the swarf came off as small little cures more like hard brass and there was none of the black graphite dust you get from normal cast iron. The castings are listed on the Stuart parts list sheet as "mallable iron" as were the con rod, crankshaft, and the piston rod/cross head. The reverse gear parts list lists the eccentric rods, expansion link, drag link, connecting link and reversing lever all in "mallable iron".


16/07/2019 19:02:56


Thanks for the link, it basically confirms what I thought I knew about mallable iron, and why it is more ductile than cast. I was thinking someone here might have had worked with forming mallable iron and could tell me their experience. I will try and straighten them and if not successful can always carve them out of solid!


16/07/2019 17:45:57

I have a set of original Stuart Turner (not the current Stuart Models) castings for the reversing gear for the #1 engine. The eccentric rods are supplied as mallable iron castings and they are both warped to the extent that they won't clean up to the desired 5/32" thickness as required, as there is minimum machining allowance on them. My question for any one who has worked with mallable iron is, are they ductile enough to hammer strait or are they going to snap like normal cast iron would.

These two appear to be identical castings to the one I have just finished machining for my #1 (castings of the the same vintage). It was only slightly warped and machined beautifully, much like mild steel except that the chips came off as little chunks, much like cast iron or hard brass.dscn3207.jpgdscn3211.jpg

Thread: Stuart No. 2 centrifugal pump - machining help needed
04/07/2019 16:49:54


The series in ME was by Tubal Cain and was in Vol. 170 and ran from issue #3942 2-15 Apr 1993 to #3950 6-19 Aug 1993.  I've sent you a PM.


Edited By John Purdy on 04/07/2019 17:08:22

Edited By John Purdy on 04/07/2019 17:16:20

Thread: Myford mystery hole
28/06/2019 17:30:35


When I go my Myford S7 back in '77 I very quickly got fed up with the Myford oiler which seemed to put oil every where except where it was wanted. So I modified a standard pump oil can with a nipple as in the photo. The end is reduced to slightly smaller than the hole in the oil nipple and is drilled #60. I also filed a groove across the end with a knife needle file to allow the oil a passage around the sealing ball in the oil nipple. Disregard the bevel on the end of the nipple, that's just wear. I've actually replaced it a couple of times as the small end tends to get banged up. The oil now goes only into the oil nipple not all over the place but where it's not wanted.dscn3201a.jpg


Thread: combustion chamber testing
21/06/2019 18:22:46

I used a very similar rig to yours to do the same thing and I sealed both ends with a sheet of rubber from an old inner tube. That worked well. It's soft enough that it seals the tube ends even if they aren't all exactly the same length. I have copies of all EIMs and seem to recall the article you mention, will try and find it.


Thread: Slitting saw arbour
17/06/2019 20:58:08

My arbor is almost identical to Neil's above ( but with a #2MT shank), except that I made the cap spigot 1/2" long and the flange 3/16" thick. This allowed me to recess the 1/4" hex socket retaining screw fully into the cap which allows the unit to be used very close to the vice jaws or other surface.


17/06/2019 17:32:45


I made one very similar to Dave's many years ago but with a MT #2 taper to use in the lathe. I didn't put flats on it and tighten and untighten it up once mounted in the spindle which I found works OK. The clamping washer is 1 1/4' so only has 1/8" of clamping surface but holds the saw securely and if it does slip I consider it a safety feature. I now use it only in the vert. mil mounted in a #2 to R8 adapter, with the draw bar into the thread in the morse taper. If you use a morse taper ensure you use a draw bar as they tend to come loose if subjected to side pressure as in this case. I have used mine many times in all types of metal with saws from 1/32 up to 3/16" and have found it more than satisfactory


Thread: mazak casting
15/06/2019 17:19:29


I've just read your post. Have sent you a PM.


Thread: ST #1 Problem
12/06/2019 18:06:55


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.



11/06/2019 22:05:55

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


02/06/2019 18:50:02

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

Thread: Fred Cottam article
29/05/2019 17:18:26


I have sent you a PM.


Thread: Taper pins
28/05/2019 18:50:31


Check out the previous thread on this topic " Taper Pins 24/09/2017". There is a full table of drill sizes for taper pins from 7/0 up to 10 in "Machinery's Handbook" .


Thread: Injector Testing
27/05/2019 17:51:28

Well the new shuttle valve to the drawing made no difference. Still getting a reading of about 20 psi on the gauge. Also tried two other pressure gauges and they both read essentially the same. The whole unit is now to the drawing. I have measured it all round and all dimensions are within a couple of thou of the drawing dimensions. So why are we getting such a low pressure reading where others are getting a reading of boiler pressure plus?? I'm at a total loss.


23/05/2019 23:04:04

We've double checked the pressure gauge and it is as accurate as can be expected for a standard commercial gauge

I've had another go with the test unit but before I did I took the valve apart ( with the owner's permission! ) and checked it against the drawings in Brown's book. All dimensions are correct and the pressure gauge and drain pipe connections are in the right place. The only difference is he has replaced the coned seal on the piston with an "O" ring seating against the sharp edge of the inlet bore.

On the test today I tried restricting the outlet of the drain pipe by blocking it with a long piece of wood pressed against the end of the pipe (is that water ever HOT! ). As soon as there was any restriction the indicated pressure rose and when almost completely blocked rose to just under 150 psi, this was with a boiler pressure of about 90psi.

I think I will make a new shuttle valve to the drawing with the cone seating and see if that makes a difference.


Edited By John Purdy on 23/05/2019 23:06:10

Edited By John Purdy on 23/05/2019 23:06:48

22/05/2019 22:51:40

Yes Paul, that's the first thing I did and between 60 and 100 psi is within a couple of psi of a known accurate gauge.


21/05/2019 23:35:56

This is a question for anyone who has built and used an injector test unit to D.A.G. Brown's (Bill Carter's) design. A club member has built one and I have borrowed it to test a couple of injectors. On testing with a known good injector the output pressure gauge only reads 35-40 psi with the injector delivering dry and with boiler pressure of 100 psi applied to the boiler inlet end. The fellow that built it has tested a good many injectors with it, and talking to him he says that is the result he gets and he thinks this is normal. From all I have read I was under the impression that the output pressure gauge should read 5 -10 % above boiler pressure ie. in this case ~105 - 110 psi. I'd be curious to know what other people's experience is.


Thread: Myford Clutch Lever fouls the Belt Cover
14/05/2019 18:41:42


That is right, the drive is disengaged when the clutch lever is to the right and is engaged when the lever is moved to the left. The point that Mike was making was that the cover hits the lever when it is raised with the clutch engaged ie. to the left. In normal operation with the clutch engaged and the cover closed the lever is well clear of the cover.


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