Member postings for John Purdy

Jon

For the top of my lathe bench (Super 7B) I used two thickness of 3/4" particle board glued and screwed together (from the bottom) laminated with Arborite (or similar laminate) on top and the sides. The particle board is harder, stiffer, and more moisture proof than MDF. It has served well for 40 years. The only time I have had to re-level the lathe was after two moves. I check it occasionally and it doesn't change.

John

Andrew

The advice to convert a normal 3 jaw to a poor man's Griptru was published by Professor Chaddock in Part nine of his Quorn article (ME 3 May '74).

I quote

"The answer is simple if you take the advice given to me many years ago by no less of an authority than the late Cmdr. Barker. That is to take the chuck off it's backplate and turn 1/32" off the register. Open out the holes in the backplate by a similar amount and put it all back again. You now have a chuck, like the Griptru, which can be set to run true holding work of any diameter regardless of wear in the jaws or scroll. This outrageous advice, which the editor declined to print on a previous occasion, but I hope he will on this, really does work. The work can be set to run really true to collet chuck limits, and contrary to all belief the body does not slip on the backplate." End of quote.

I have a 4" Griptru which I purchased with my Myford Super 7B in 1977. I find it invaluable. It takes less than a minute to set work to run to 1/2 a thou or less TIR using a DTI on a magnetic stand on the cross slide. The DTI lives permanently at the back of the lathe bench, within a short reach. Usually I find that the limiting factor is the actual concentricity of the work itself.

John

Dave

I did mine the same as Norman states. Mine is a Super 7 but I found the markings at 1/8" increments to be too course so I did pretty much as Norman says. Graduating at 1/32" intervals. I jury rigged up 2 stops for the chuck jaws to come up against as it was rotated to cut the lines, to ensure that the 1/16" and 1/32" lines were all the same length. I made all the 1/16" ones half the length of the 1/8" ones and all the 1/32" ones half the length of the 1/16" ones. I used the lead screw graduated hand wheel to space the lines.

John

My '76 S7B is grey with the lettering on the belt guard and the front of the headstock in cream.

John

Another book that has a full section on designing copper boilers is "Model Boilers and Boiler Making by K. H. Harris". I haven't checked lately but from recollection the formula and calculations are very similar to those in Evans book. Again I believe it may be out of print, but copies may be avail;able. I know there is one in our club library here.

John

Yes, I believe it was a thermite reaction. Aluminum dust mixed with iron oxide dust from previous grindings ignited by the hot sparks from my aggressive grinding of the piece of angle. The fireball only lasted about 1 second but filled the workshop with dense odourless white smoke, and on inspection after getting back from a trip to the local emergency room, the base of the grinder and bench were covered in a white residue and there were globules of steel in the bottom of the grinder. The thermite reaction of aluminum and iron oxide produces white aluminum oxide, molten iron and a very lot of heat.

After the fact I checked the operators manual for any warnings, none, so I wrote to the manufacture ( a well known North American manufacture of machine tools) to tell them what had happened. The reply I got was totally CYA ( American litigation mentality???) . But I have since seen that the operators manuals for these same machines now carry a warning in bold type " Not to be used on metal"

John

NIck and others

A word of warning. I have a 1" belt sander that I use a lot. But from painful experience if you use it on aluminum ensure that ALL metal dust is cleaned out before using it on steel. Unbeknownst to me, my son had used it on aluminum. The next time I used it on steel after a minute or so my hands chest and face were enveloped in a brilliant white fireball, severely burning my hands, setting the front of my shirt smoldering and burning off most of my mustache and eyebrows. ( My eyes were OK thanks to glasses) . See my letter in ME 09 Mar 2001.

I now leave the side cover off and clean out any residue after each use.

John Purdy

I have removed the bracket a number of times by screwing in a 1" 4BA screw and giving the head a good tug with the fingers. Mine are a snug but sliding fit in both the bed and the bracket.

John

I fully agree with assessment that the diode is correct as published. After I wrote that I had second thoughts about my reasoning, as soon as the transistor was turned on there would be nothing to limit the current except the diode and transistor, and as David said probably accompanied by "magic smoke". Sorry about that.

Ian

I have not built the circuit but looking at the schematic there appears to be two errors that would prevent it from working.

The emitter lead of the 2n3055 transistor is not connected to anything, it should be connected to the minus power supply line at the bottom of the schematic.

The diode (D1) at the collector of the 2n3055 appears to connected backwards. As shown it will not pass the plus voltage to the transistor. ( this would explain why you are not reading a voltage here.)

John

I've etched the boards and the clay on the traces caused no problems. Maybe it just adds an extra layer of resist. Over the years I have made probably 50+ boards, On the first ones when I was a young teenager the resist pattern was applied with a paint brush using model airplane dope, that was before the days of ICs or SMDs. The smallest footprint was a 1/2W resistor or TO5 transistor. Most though have been done with hand drawn art work photographed on litho film and used on boards sprayed with photoresist, and etched in ferric chloride. A number years ago I switched to copper chloride as the etchant, it is slightly slower than ferric chloride but has a number of advantages, not the least of which is it never wears out, hence no disposal problems.

I had heard about using laser printers for the resist pattern but didn't do any thing with it till the #100 issue of MEW came out with a detailed description of the process. It wasn't till a few years later that I got a laser printer and could try it. I had already been using ExpressPCB for other things so it was a logical extension. I have found it much easier and faster than the photoresist method (other than the clay problems), but I'm not sure how you could reliably transfer the pattern on a large board. (The largest I have ever made was 8" x 10" double sided. although I can't see myself ever having to do that again).

Duncan, Peter

What I meant by full port opening is the design port opening which may be less than the actual port width depending on the geometry of the ports and the valve. I should have been more clear, sorry about that.

  As Duncan says the ports should remain open for about 75% of the stroke in full gear, cutting off proportionally earlier as the reverser is moved to mid-gear. The 75% figure is governed by the design of the whole valve gear, and is chosen to allow some expansive working (increasing efficiency) while still providing sufficient power to start a train from stop.

John

Edited By John Purdy on 30/01/2017 20:06:37

Peter

In mid gear with Walschaerts valve gear the total valve movement should be 2x(lap plus lead), so the valve should be just opening the ports at each end of the stroke by the amount of the lead. As the reverser is moved forward the valve will open progressively further till the full port opening is achieved in the fully forward ( or reverse) position .

John

All

Thanks for the suggestions, but I have tried, isopropyl, acetic and citric acids and they don't touch the residue. Maybe it's the particular brand of paper I'm using. For the two I did last week I used a dry VERY well worn "scotchbrite" pad and gently rubbed the board just until all the residue was removed from the copper areas. There is still a fair amount left on the traces but I don't think that should be a problem. I haven't etched them yet so we'll see.

John

Edited By John Purdy on 18/01/2017 00:11:24

As Neil and Brian say it was this high force on the valve, which caused high friction losses and high forces on all the valve gear mechanism that lead to the adoption of piston (and in a few cases poppet valve) systems. But before these were widely used steps were taken to reduce the forces by using what were known as "balanced valves" where the upper surface of the slide valve was sealed against the valve chest cover with a sliding seal which was vented to atmosphere. This considerably reduced the area of the valve exposed to valve chest pressure, reducing the force exerted against the valve face.

Iain

The valve nut should be an easy but slop free fit in the slot at the back of the valve and the valve should be able to move off the port face by a small amount. It is then the steam (or air) pressure in the valve chest that forces the valve into close contact with the valve face to effect the seal. This is due to the difference in in pressure between that in the steam chest and atmospheric pressure in the exhaust port. Hope this makes sense.

John

Here is a circuit for a temp. controller that I have developed for a kiln or heat treating furnace. It's a considerably modified version of a circuit that was published in the "Engineer's Notebook" section of an early 80's issue of "Electronics". The original used some components no longer made, hence the modifications. I also added some extra features of my own. I am using two, one to control an old pottery kiln I was given that I use to melt Al and bronze, the other a small heat treating furnace I built.

The output of the K type thermocouple (0-55 mV) is amplified 100X by U1B, 1/2 of a 358 op-amp chip, to give an output voltage of 0-5.5V. This is applied to pin 2, the inverting input, of the other 1/2 of the 358 which is configured as a comparator. The non-inverting input, pin 3, is supplied with a variable voltage (0-~6.5v) selected by the 10K temperature select pot. If the thermocouple voltage is below that set by the 10K pot the output of the comparator at pin 1 is high which turns on the triac controlling the heating element(s) via the opto-coupled triac driver. It also turns on the LED to indicate the status of the heater. Once the temp rises so that the thermocouple voltage exceeds the preset voltage on pin the 3 the the output of the comparator go low turning off the triac and LED. The cycle then continues. The switch gives a convenient, low voltage/current means of manually turning the heating element off. All resistors are 1/4W except as noted on the schematic. The triac is a 800V unit selected to have a current rating of twice the current draw of the heating element(s).

I hope someone might find this useful, either for a new project or to retro-fit an existing furnace.

For a large view of the schematic go to my album "Weyvern" and select the schematic picture "full size".

Oliver try this. With the back gear disengaged (lever down), rotate the half moon key to disengage the bull wheel from the 4 step pulley, you should then be able to hold the pulley with one hand and rotate the bull wheel and the spindle freely. Then if you hold the bull wheel stationary you should be able to rotate pulley freely. This will confirm if the pulley is seized on the spindle or not.

John

Rod

The top fitting for my 5" gauge "Gemma" (GER 0-4-0 ST) has the same type of top fitting for the water gauge. What I did was make it out of a block of bronze, held lightly in place by 2 small (#4) bronze screws and silver soldered it on when the backhead was being soldered on. I think far superior than soft soldering it . And probably less work as it is done at the same time as the backhead.

John

Has any one found an easy way to remove the clay residue that remains on PCB's after soaking the paper off when making iron on resist patterns using the laser printer /photo paper method? The only way I have found is to gently abrade the residue off with a "Scotchbrite" (or similar ) abrasive pad and water. But in doing this I'm always afraid of damaging the resist pattern, especially on a board with SMD traces.

John