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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: De-magnetising
24/10/2020 18:34:51

I made this one from the same ME article as John.  I used a power transformer that came from a Peavey guitar amp that had come in for repair. I had replaced it as the thermal fuse in the primary had opened. It had sat around on the floor of the shop  for a number of years as I couldn't make myself throw it out.  I wired the two secondary windings in series (80 and 30 volts) to the power cord (120 volt supply).  Note the two wood strips where I milled slots in the laminations to break the flux lines in the core, milled down til there was about 1/16" of the laminations left. Works great, de-magnetizes even the largest wrenches in a few seconds, also quite successfully  de-magnetized the 4" 3 jaw lathe chuck. The thin Bakelite sheet glued to the top stops the items to be de-magnetized from sticking to the laminations.


Edited By John Purdy on 24/10/2020 18:37:07

Edited By John Purdy on 24/10/2020 18:53:01

Thread: Highlighting Index Lines
18/10/2020 18:42:49


I do the same as Martin, use steel blackening liquid, in my case cold gun black. After scribing the lines and punching the numbers I use a fine file (#6 cut) to remove most of the raised material then after a good cleaning with a solvent use the gun black, working it well into the depressions of the lines and numerals. Once satisfied I use the file with the part rotating in the lathe to remove the remaining raised material and the black on the smooth surfaces. I initial tried skimming the surface as Martin does, but found invariably some of the lines or numbers disappeared, so now just use the file and find it works better. I have done many this way and find they hold up very well. dscn3394.jpg

Edited By John Purdy on 18/10/2020 18:44:40

Thread: Rumely Oil Pull engine
17/10/2020 18:22:57


E. T. Westbury's Wyvern horizontal engine has a CI cylinder liner shrunk into an aluminum block/water jacket and works fine, at least in mine. If I recall I made the liner .001-.002 larger than the bore of the block then to assemble I heated the block in the oven to about 400F and put the liner in the freezer. The liner just dropped in with no problems.


Thread: Long Keyway
02/10/2020 00:24:05


That idea looks like a possibility. The sleeve I need the keyway in is steel I was planning to use my hyd press to push the broach through so I can see a problem keeping it aligned with the hole. In your picture it looks like in is being held in the drill press chuck therefore maintaining alignment, but I don't think I would want to use the drill press to push it through steel. It could be used in the lathe tail stock as well which might be more feasible. It would certainly be better than a boring bar in the tool post. Certainly food for thought! Thanks.


01/10/2020 18:26:00

Thank you Martin and Jason for your replys. The reason for the query was that I was given to believe that that the max length of cut of a broach is governed by the gullets between the teeth plugging up with the cuttings and the broach jamming. I have had a broach jam in the past and it was a nerve racking process to remove it without breaking the expensive broach or damaging the work piece.

Your suggestions are pretty much what I had come up with, a longer bush to support the broach and do it in multiple passes. An 1/8" keyway with a "B" series broach is normally cut in two passes, first one just using the bush alone and the second with a 1/32" shim in the bush. I was thinking of doing it it say 4 passes by making the slot it the bush 1/64" deeper and using a number of 1/64" shims between passes. Question is how to hold a 1/2" rod to cut the 3/16" wide slot that deep and 1/64" deeper than a standard bush. As can be seen in the picture there is very little meat left as it is in the normal bush. The other possibility I was considering was to cut an initial slot in the lathe by planing it part way then finish with the broach using two 1/64" shims to reduce the chip load.



Edited By John Purdy on 01/10/2020 18:28:11

30/09/2020 17:56:13

I have to cut a 1/8" keyway in a 1/2" dia hole that is ~2.5" long.   A standard series "B" broach will cut a keyway to a max of only 1 11/16" long.   How do I do it?   I have a couple of ideas, I have the broach and the bushing, but any suggestions or ideas would be most welcome. Thanks.

Edited By John Purdy on 30/09/2020 17:56:57

Edited By John Purdy on 30/09/2020 17:57:56

Thread: Moving Photos
26/09/2020 18:55:20


Thanks for your reply. From looking around that is what I thought, there is no way to move pictures from one album to another. The reason for the question was, as you suggest, a case of housekeeping, to create individual albums for each subject, as currently everything is under Wyvern. I have all the pictures so I'll sit down and make some new albums upload the pictures and delete the ones in Wyvern. Thanks



Edited By John Purdy on 26/09/2020 18:56:40

Thread: Heat Treating Furnace
26/09/2020 18:42:29


If you go to my "kiln" album you will find my cutting schedule for the bricks. Bricks are standard 9 x 4 1/2 x 2 1/2 insulating fire bricks. I made a rudimentary wood mitre box to cut them using a fine tooth woodworking back saw, they are very soft and saw easily. When on it draws ~7.5A at 240V. The outer case is mildly warm after 15-20 minutes or so. Once the temp reaches the temp preset by the pot on the front it just cycles full on and off at about a 50/50 duty cycle to maintain that temp +-3-5 deg or so. A PID controller would keep the temp to a much closer tolerance.


I would have preferred to use Kanthal elements as they will last a lot longer but I had these ones, so will see how they last. If they burn out I will replace them with Kanthal ones


Thread: Moving Photos
25/09/2020 23:42:13

Is there any way to move photos from a current album to a new one, short of reloading them into the new one and deleting them from the old one?


Thread: Heat Treating Furnace
25/09/2020 23:30:13


When building the sheet metal enclosure I stewed over how to have the door open, up, down or side wise. I had searched a number of kiln manufacturers web sites for ideas including Paragon and side wise lost out as where it was going to sit in the shop was next to the wall and, as can be seen in the pictures, there was insufficient space. I couldn't see how to do it opening upwards, so down it was. My first iteration didn't work, wouldn't stay closed, so back to the drawing board to redesign the hinges. I don't think the heat while reaching in will be a problem as, dare I say it, I have a pair of asbestos gloves for protection that I use for boiler making. With them on I can pick up a red hot boiler and manipulate it with no problem.


25/09/2020 18:49:06

The heating elements are what started it all. I found some replacement elements (Ohm-Rite) for hot plates in a small hardware store in Summerside PEI when I was there in 1980. The ones I used were rated at 660 watts @ 120 volts and I have two in series for 240 volts. They are, I suspect just nichrome wire and are .022" dia and measure 15.4 ohms cold, and the close wound coils (.195" dia) as bought are stretched out to just shy of 23 " to fit the grooves in the side walls. How long they will last I don't know. The walls are made from light insulating fire bricks rated for 2300 deg F bought from a refractory supplier in Halifax after I found the elements.. The elements and the bricks followed me around this country from one side to the other for 25+ years before I figured I better do something with them before the fragile bricks were completely destroyed.


25/09/2020 05:18:49

Many years ago I started to build a small kiln/heat treating furnace (bought the fire bricks about 1980). Finally finished it. It runs on 240 volts and is controlled by a home built controller with a digital temp. readout in deg.C. I've had it up to over 750C and it maintains temp within 3-5 degrees which is close enough for the use I envision for it. Inside dims. are 6"x7"x7" (HxWxD). Before someone says why didn't you use a PID controller the reason is when I originally made the controller the cheapest PID I could find was just under $400 (and up from there). My controller and temp readout cost less than $50. Since I finished it a couple of months ago I have ordered and received a PID controller (REX C100), 40 amp SSR, heat sink, and K thermocouple for $17 delivered to my door!


Thread: Boiler
30/08/2020 19:43:15


Tubal Cain's 3" test boiler was described in ME 18 Feb "77. I made one and you can see a picture of it, and the GA drawing of it in my album. It steams my ST10 quite well on only 10 PSI or so.


Thread: GHT crosslide micrometer collars for the myford S7
28/08/2020 21:42:22

I too have made a number of GHT's designs, including the cross slide micrometer mod and the top slide retracting mod. Pics of both can bee seen in my album, 12th pic down. The cross slide mod was made from scratch using the instructions in the original article in ME of Nov. '77 (It is also described in his Workshop Manual book). I highly recommend it, it is an absolute pleasure to use and I wouldn't be without it. It's been a while since I made it but I don't remember any real problems. The retracting top slide was made from a Hemingway kit ( I baulked at cutting the 3/8" left hand Acme leadscrew and nut!). Initially I was going to make the design without the retracting feature as I wanted it more for the better micrometer dial and the tailstock clearance but in the end went for the full retracting version. I'm glad I did as makes screwcutting so much easier and faster. Again I highly recommend it.


Thread: three cylinder aero engine
19/08/2020 21:10:51


There has been no fettling done on the castings other than cutting off the feeders, as can been seen in the pictures below, so there quite a bit of moulding flash to remover before any machining can be done. Actually I prefer it that way, other than trying to correct for the over zealous use of a grinder at the foundry. There is a little porosity evident in a couple of them.. On the ones I've checked there is plenty of machining allowance. The only potential problem evident is the core in one of the pistons was displaced to one side but there is sufficient machining allowance on the outside to compensate. Overall the castings appear to be quite good.



19/08/2020 01:16:13


I just received the castings this morning and the OD of the cyl. barrel is 1.975+- and the plans call for the steel liner to be 1.690 OD so that would leave a wall thickness of about .140. The plans call for the liner to be a press fit and "Loctited" in so once the liner is installed the whole could be mounted on a mandrel and the lower fins cut. The only problem would be the flat at the top of the barrel for the valve cyl. mounting. Maybe the upper ones could be cut using a small dia. slitting saw with the assembly mounted vert. on a rotary table on the mill?



Edited By John Purdy on 19/08/2020 01:18:18

Thread: painting - preparing to paint
14/08/2020 20:00:19


Can't help with the paint but to give you an idea about the number of parts in a loco when I built my 5" gauge 0-4-0 ST "Gemma " (to the design in EIM) I kept track of the number of individual parts and it came out to 1970. Since then I have added a few more bits and pieces.


Thread: three cylinder aero engine
07/08/2020 17:50:28


By coincidence I just ordered the castings last week, awaiting delivery. I'll let you know the dimensions of the barrels when I get them (so you can decide if there is enough meat on them to make fins), if you haven't already ordered the castings. Actually I think finning to provide cooling on a steam engine is the opposite of what would be wanted, more like insulating to keep hot.


Thread: Stuart Turner No.9 Slide Valve
09/04/2020 18:35:31


I've sent you a PM.


Thread: A free ride to hospital
13/03/2020 17:19:03

Yes it VERY DEFINITELY can happen, I know from personal experience as it happened to me. Unbeknownst to me, my son had ground of the heads of a number of aluminum pop rivets on my 1" belt sander. Later while fairly aggressively grinding the sawn edges of some angle iron, a large bright white fireball engulfed the sander, my hands and the front of my shirt. My hands suffered fairly sever burns with the skin hanging off the heals of both hands and the ball of my thumbs. The front of my shirt was smoldering. I lost most of my mustache and eyebrows, My face looked like I had a good sunburn ( except around my eyes which were protected by my glasses). and the workshop was full of white smoke. Luckily the burns weren't quite bad enough to require plastic surgery. As has already been said, essentially what you have made in the grinder is thermite with the mixture of aluminum dust and the iron oxide from grinding steel, ignited by the hot sparks. After it happened I wrote it up in ME (09 Mar 2001) as a warning to others. I have pictures of my burned hands somewhere. My son has been educated and I now leave the side cover off the belt sander and brush out any residue on a regular basis. Also never use it on aluminum!


Edited By John Purdy on 13/03/2020 17:23:24

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