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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: Boring Problem
22/08/2019 22:41:14

Yes Michael, that is what I thinking when I said in the next section of that post of mounting it on an angle plate on the mill so that it is vertical and plunging down with end mills till it is almost the right dia, then boring both sides to final dimensions.

I just dug out the two series in EIM on building the engine ( the Cygnet Jun 81 and the 5A Aug 04) and noticed on a picture of the rough soleplate that the openings for the bearings are cast in as semi circles. I wonder if the caps are the same. That would make boring simple as you would be starting with a round under sized hole.


22/08/2019 19:32:03

Thanks for the replies. The ideas are somewhat like I had in mind except I hadn't thought of doing it like Jason suggests, that's very similar to the way the main bearings of the #1 and #9 are, among others. The problem I can see with boring it out between centres, or in the mill, is that you are trying to bore out a solid half circle .468" deep over a .468" deep open half circle. Obviously can't be done in one pass, (not with our machinery anyway), meaning that the boring bar plus bit has to fit through a .468" hole in the lower half, not practical on the lathe but could be done in the mill by successively increasing the dia of the sweep of the the bit in the bar and moving the bar towards the centre of the final bore. The other option I can see in the mill would be to successively remove the excess part of the upper cap with a series of increasing sizes of end mill till almost full size then use the boring bar to go through both sides increasing the bore dia slightly to ensure both are in line, and make new brasses to suit. I figured I'd be making new brasses anyway as I only have one, and have some suitable material. Of course the other much more expensive option would be to buy a new soleplate an bearing caps and start over!! ( currently 134.50 + shipping half way around the world X 1.7 to convert to Can.$)


21/08/2019 21:50:01

I have a part built ST 5A that I would like to finish. Most of the machining has been done. The sole plate has already been bored out for the bearing caps and the bearings, but one of the bearing caps and one bearing are missing. I can get a new cap casting from Stuart Models but my problem is if I machine it to fit the existing seat in the sole plate and fasten it down how do I bore out the bottom of the new cap to match the existing cutout in the sole plate and still be in line with the other side ? Anyone got any suggestions?


Thread: Stuart Turner #1
17/08/2019 17:06:04

Thanks all for the complimentary comments.

George, the colours, I mixed up from scratch as I couldn't find anything like I liked in either paint tins or spray cans from builders or auto supply shops. The brown is what I'd call a "bright chocolate" mixed up from a quite dark brown with some red added and then a little yellow to lighten it up. The cream was white with a little yellow to make a bright cream, then a drop or two of the chocolate to tone it down. It actually is a little darker that shows in the pictures, the colour balance has made it come out quite a bit whiter. There was much mixing up of drops of paint on a sheet of paper till I got what I liked ( then trying to remember what I had used!).


Edited By John Purdy on 17/08/2019 17:08:16

16/08/2019 19:31:39

    Finished my Stuart Turner #1. Bought the castings second hand in 1996, They were initially bought in the late '60s or early '70s, and when I got them they had been through 2 or 3 (or more) owners. Everything was there except the rings were broken. Finally got around to starting it on 5 Oct. last year.
    Have run it on air for around an hour to run it in and it now will run down to 12-13 psi. The rings are producing quit a bit of friction in the bore. I noticed that at the slowest speed it runs quite unevenly. After a bit of head scratching and watching it was apparent that it was slowing down over TDC the once about 30 deg beyond it would speed up down and over BDC only to slow down again approaching TDC.
    So as an experiment I removed the rings and the piston rod packing and found it turns over extremely freely (the piston has about 2-3 thou clearance in the bore).  When turned over slowly by hand and the piston gets to about 20-30 deg after TDC  it  rotates by it's own weight down to BDC, slightly beyond then back to BDC. So it would appear that the slow speed unevenness is due to the fact that there is no counterbalances on the crank to compensate for the reciprocating mass of the piston, rod, crosshead and con rod. Might look at fitting some in the manner of the ST#5 as an experiment.


Edited By John Purdy on 16/08/2019 19:32:52

Edited By John Purdy on 16/08/2019 19:36:11

Edited By John Purdy on 16/08/2019 19:41:39

Thread: Steam powered RC aircraft
29/07/2019 18:44:00

The series in ME that the aircraft in the link in the first post in this thread was based on, (and in ME 4248), was titled  "Comet"  and was a three part series by David Parker starting in vol. 162 #3848 5-18 May 1989 and was a full construction series detailing the engine and boiler for a model aircraft.  It also included outline drawings of the aircraft.



Edited By John Purdy on 29/07/2019 18:46:03

Edited By John Purdy on 29/07/2019 18:59:01

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.


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