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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: Injector Testing
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.


13/05/2019 18:13:04


On my Super 7B (1976) the belt cover also hits the clutch lever when opened with the lever in the fully left position and has always been like that since day one. But as Peter says that is when the clutch is engaged and the spindle turning, so I'm not sure why you would want to open the cover with the spindle running. I think this is normal.


Thread: Lubrication of Quorn grinder spindle
08/05/2019 20:37:09

Further to my last, using the method in the article I calculated that the quantity of oil that I needed was 2.2cc. When I built my spindle I used a stack of 4 Belleville springs instead of the coil springs and spring box that Chaddock used, so if made to the original drawings the quantity would probably be slightly different. Glad I could be of some help.


08/05/2019 18:26:20

There was an article in MEW # 62 "Compendium of the Quorn" pg 20,21 that covered the amount of oil to use in the spindle,including the calculations to determine the volume of oil required. This is what I used when I made my spindle and it has been perfectly satisfactory.  If you can't find the article send me a PM.


Edited By John Purdy on 08/05/2019 18:27:33

Thread: Leaf spring deflection
27/04/2019 18:05:07


Tubal Cain's series on spring design in ME ran  from issue #3783 vol. 157 1986 to # 3807 vol. 159 1987 for a total of 10 installments.  


Edited By John Purdy on 27/04/2019 18:07:38

Thread: Casting resurrection and soldering on
21/04/2019 18:39:42


I had a similar problem a few months ago with a cyl. casting for a ST #1. The side of the valve face opposite the exhaust port had a large chunk knocked out of it well into the valve face. I machined it back by about 3/32" and using JB Weld fastened on a piece of 1/8" steel held on by 2 countersunk screws with the heads well below the surface, and filled with JB Weld. Then machined the surface back to the correct dimension. Once painted the fix will be invisible.


Thread: ST #5A Reverse Gear
06/04/2019 22:18:24

I think I have figured out what it is. It's an extended valve chest stud to hold the slotted link for the reverse lever. Thanks to everyone for baring with me.


06/04/2019 22:02:42


Thanks John but I don't think that is what this is as there are no other parts to go with it detailed. The method used to hold the reversing lever it place in these drawings is a slotted link pivoted on the corner of the valve chest with a screw through the link into the reversing lever and done up with a knurled nut to hold the lever in position (parts # 8, 12, and 13 on these drawings).


06/04/2019 20:12:05

Here's a question for anyone who has built the reversing gear for the ST #5A engine. On the plans I have (issue date 15 Jan 1969) there is an item, listed as #3, which for the life of me I can't see where it fits. It's a 3/16' dia rod 2 7/16" long, threaded at one end 2 BA for 1/4" and at the other turned down to 5/32 for 1/2" and threaded 4 BA for 3/16". There is also a 2 BA thread 1/4" long just above the turned down part on the 3/16" dia. Does anyone know where it fits? (It's probably staring me in the face). It's definitely not the valve rod as that is 5/16" dia. and 5 9/16" long.


Thread: Myford S7 Power Cross Feed Apron Adjustments
27/03/2019 17:32:29


The two screws (LA8) lock the gib to the apron casting and have to be slacked off to allow adjustment of the gib, once adjusted they are tightened up to lock the gib in position.

I have had the carriage off my lathe many times and the way I adjust it is before putting the apron back on I slack off the two screws LA8 and adjust the gib so that the half nuts move easily with no play when opened and closed with the lever, tighten the screws LA8, hold the apron up to the saddle and put in the 4 screws (LA49) in the  the saddle to hold the apron on but don`t do them up, leave them loose so the apron can still be move a bit ( the holes in the saddle are over size to allow this) , re-install the lead screw, position the carriage as far to the right as possible, then engage the half nuts and with them engaged solidly on the lead screw tighten up the 4 screws holding the apron to the saddle. This ensures that when the half nuts are engaged with the lead screw the lead screw is not bent out of alignment. As far as the adjustment of LA6, what I do is with the half nuts closed on the lead screw just to screw it in till it just stops by feel. That`s always seemed to work.

Just a note, by far the easiest way to remove the apron and saddle is to first remove the lead screw by removing the two screws and two dowels in the bracket at the right hand end of the lead screw and withdrawing it totally out of the apron to the right.


Edited By John Purdy on 27/03/2019 17:36:19

Thread: Dial indicator probe threads
18/03/2019 17:38:36

I have 5 dial indicators, 4 of which ( one US one UK and two Chinese ) have 4-48 threads, the other Chinese one has a metric thread. Not sure what the metric thread is but a 4-48 probe will screw in 3 or 4 threads, so is very close to 4-48. 4-48 seems to be the standard for american made units.


Thread: Spring Calculation
26/02/2019 18:29:06

There was also a 10 part series in ME by Tubal Cain on spring calculations. It started in 1986, Vol. 157, #3783 an ran to 1987, Vol. 159, #3807. While I don't have his book from the Workshop Series I suspect that a lot of the material is the same.


Thread: print-offable and laminatable chart
06/02/2019 20:20:55


Yes your tool will have to be very sharp. The theoretical tip radius for a 569 TPI Whitworth thread is only .000247" and the flat on a 60deg. tool for a UN thread is .000225" wide!    Good luck measuring that, at least with our normal measuring kit.   About all you could do is to sharpen it to as fine a point as you can and as the theoretical thread depth for a sharp Whitworth thread is only .00173' and a 60 deg. one is .0016 there wouldn't be much of a tip load!



Edited By John Purdy on 06/02/2019 20:38:11

05/02/2019 17:50:11

The last half dozen or so lines on the inch chart are intended as fine feeds. I only added the TPI out of curiosity!  120 TPI is UNF 000 and 160 is 0000 but obviously a thread of 569 TPI is a little over the top and not much practical use, at least for the kind of work we do!


Edited By John Purdy on 05/02/2019 18:05:18

04/02/2019 22:37:15


I found that the chart in the gear cover was hard to read with the lighting in my shop so I printed it out 8 1/2 x 11, laminated it, and it hangs on the wall behind the lathe were I can read it while standing it front of the lathe.

I have it as a PDF file and can send it to you if you pm me your e-mail address. You can then print it out and laminate it.



Thread: Model Turbines
15/01/2019 18:13:50

The article on a turbo generator that Ian refers to was by a R.E. Rowbottom of South Africa and commenced in the 11 Nov. 1954 issue of ME and ran for a couple of installments.

   There was also a very similar one in 3/4" scale described in the American publication " The Miniature Locomotive" starting in the Feb 1952 issue.


Edited By John Purdy on 15/01/2019 18:19:16

Thread: Con Rods ST#1, #5A
28/12/2018 19:34:10

Thanks for the replies.
Jason the rings were bought from Reeves in Mar '93 and were listed as 2" dia. Your 5-7 thou gap is about what I thought would be about right. I'll go for that and we'll see what bore I end up with.
My measuring is quite possibly suspect so I will just slowly open out the bore till the ring just goes in then a little further to get the 5-7 thou gap.

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