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Member postings for Andrew Johnston

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

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Thread: Gauge Plate
06/01/2011 15:46:45
Chris - Quite so, never be afraid to ask questions.
 
Gauge plate is a high carbon steel ground to thickness and sometimes, but not always, to height. Tolerance on thickness is generally around +/-0.001". The composition, taken from some I happen to have on my desk next to the computer is:
 
Carbon: 0.85-1.05%
Manganese: 1.00-1.40%
Silicon: 0.15-0.4%
Chromium: 0.4-0.6%
Tungsten:0.4-0.6%
Vanadium: 0.15-0.3%
Iron: the remainder
 
The important feature of gauge plate, indicated by the high carbon content, is that it can be hardened and tempered by relatively simple means (compared to HSS). Hence it can be machined soft, and then hardened and tempered to produce cutting tools, springs etc.
 
So, it definitely isn't mild steel (!), and it's a bit tougher to machine too, even when soft.

Regards,
 
Andrew
Thread: Which slideway oil is best?
30/12/2010 21:19:20
I can recommend Hallett Oils too. I've been using their oils and coolant for several years. I tend to buy the 5 litre cans, as they're a bit cheaper per litre. Last time round the loop with them, discussing a slideway oil for a CNC milling machine with Turcite ways, they recommended their 'Slideway68D', which presumably is similar to their SL68. I've been very happy with it. Now that I've nearly finished my 1 litre can I shall buy a 5 litre can from them, plus some more coolant. The coolant (Biokool14) lasts well and doesn't seem to go off. At least it doesn't seem to smell, even although it may be in the sump for up to a year.
 
Regards,
 
Andrew
Thread: gas filler valves
30/12/2010 19:34:05
Hi Nick,
 
I've read the regulations, and as far as I can see private individuals using the equipment for pleasure purposes are exempt. So, for the glider, at least, I don't need to worry. At least until the bureaucrats realise that glider pilots are fitting, and using, high pressure oxygen equipment without a license or EU regulation in sight. My system runs at about 2000psi and it normally sits just by my right thigh. It is supposed to be tested every five years, and I make damn sure it is tested!
 
Regards,
 
Andrew
30/12/2010 16:11:16
As I understand it the acetylene is dissolved in acetone, which is itself held in a porous material. The porous material used to be kapok. This explains why acetylene cylinders are disproportionately heavy. Nice stuff acetylene, that's what you get from a triple carbon bond!
 
Regards,
 
Andrew
30/12/2010 13:48:30

BTW it is illegal to transport portable cylinders on the british highway with the equipment being attached
 
Oeeer, better not mention the oxygen cylinder in my glider then! I don't disconnect it before towing on the road.
 
I always transport my acetylene cylinders vertically; oxygen cylinders I tend to lie flat. Argon cylinders I have delivered, as they won't fit into the car, and they're too heavy for me to lift anyway.
 
Regards,
 
Andrew
Thread: Rulers - my pet peeve
29/12/2010 09:54:36
As a rule I would post in this thread, but I won't, as I'm working to rule.
 
Regards,
 
Andrew
Thread: Surface Rusting in Workshop
27/12/2010 13:45:33
Hi Ant,
 
It should be possible to make an estimate of the heat needed for the workshop as follows. Select an outside temperature and a preferred inside temperature. You can work out the heat flow per degree K by taking the surface areas of the workshop walls, floor and ceiling and an estimate of the U value of said surfaces. That, multiplied by the temperature differential will give you the total heat flow. Multiply that by the cost per kWh and that's the cost in £ (always assuming that you can make any sense of your electricity bill ).
 
I'm intending to keep my workshop at about 8-10°C. I don't like hot buildings, my bungalow is normally at about 15-16°C. Experience has shown that a temperature of 8°C upwards is comfortable in the workshop. When it is below 3-4°C, then you only need to touch the machines to numb the hands. Henceforth it's butterfingers all round! I tend to make stupid decisions when I'm cold, so that's another reason to get some heating organised.
 
Regards,
 
Andrew
Thread: Aircraft General Discussion
27/12/2010 12:10:53
Howard: Yes, I did come across some notes on the internet while checking the facts in my post on gluing, saying the Australia had banned the use of Aerolite. However, both Aerolite and Aerodux are widely available in the UK for use on wooden aircraft. Notes from the LAA (used to be the PFA) indicate that they are happy with Aerolite, Aerodux or some of the expoxies that are more common in the US for wooden aircraft structures.
 
In a similar vein I've dug out a research paper from my files by the UK Building Research Establishment on the loss of strength with time of glued joints. The basic summary is that all glued joints lose strength over time, but that resorcinol and phenol based resins are better than urea based glues. Oddly enough, for close fitting joints, casein was generally better than urea and phenol resins, especially in the wet!
 
John: I agree, it is always essential to sand plywood before gluing. I also thought it was to do with clearing the crushed fibre ends. According to the LAA though, it's mainly to remove the waxy press release agents. Either way, if you don't do it, you'll get a weak joint, and a self-dis-assembly kit.
 
Regards,
 
Andrew
Thread: Surface Rusting in Workshop
27/12/2010 11:42:58
The rusting is not directly related to the RH%. It's more to do with temperature changes. What is important is the temperature of the machines relative to the air temperature. The worst case occurs when the machines have had a cold soak at a low temperature. If warm air is then introduced, and the cold machines are below the dew point of said warm air, then condensation, and surface rust will occur. The dew point of a parcel of air will depend upon the RH% and temperature of the air, but the formation of rust will depend upon how cold the machines were to start with. The sort of weather that causes rust in my unheated workshop is a cold, clear spell followed by a warm front.
 
I expect that a dehumidifier will have some effect, but personally I think that a better way is to stop the machines getting cold in the first place. That's my plan anyway; finish insulating the workshop, and then add a couple of radiators coonected to the central heating system.
 
Regards,
 
Andrew
Thread: Aircraft General Discussion
26/12/2010 21:33:42
The DH110 crash happened some years before I was born, but I have heard about it first hand, as my father was at Farnborough on that day. He was also working at de Havillands at the time, at Hatfield, so there were some ramifications at work. Sad as it may seem, in the 50's the test pilot was a derring-do hero, and unfortunately a number were killed while pushing the boundaries. Some of my parents friends were killed in the crash of the prototype BAC1-11, after which the phenomenon of deep stall was more closely investigated. These days, with the advances in the theory and practise of computational fluid dynamics test flying is less of a risk than it used to be.
 
Coincidentally, at the time of the DH110 crash, my grandfather was also working for de Havillands. I assume that he wasn't directly affected by the crash as, having been chief draughtsman at Airspeeds which was taken over by de Havillands in 1951, he was working near Portsmouth. This brings us neatly back to model engineering, as Nevil Shute Norway, one of the founders of Airspeeds, was a well known model engineer.
 
Regards,
 
Andrew
24/12/2010 23:22:13

To elaborate on some of the previous comments in this thread here is a short note on glues for wooden aircraft.

Early wooden gliders were built using casein glue, based on the protein found in milk. There have been some problems with glue failures in early gliders built with casein glue, in the UK, but other, similar, gliders are completely unaffected. This appears to be related to the susceptibility of casein glue to dampness and micro-organisms.

I believe that the early Mosquitos were built using casein glue. After a number of unexplained structural failures in the tropics it was found that casein glue was unsatisfactory for use in hot, high humidity environments. Hence a synthetic replacement was identified. This was based on urea formaldehyde. The great advantage was resistance to micro-organism attack and the ability to withstand water. Later Mosquitos were built using these urea formaldehyde glues. Further developments include resorcinol resin glues which have high dry and wet strength, and high temperature resistance.

Both urea formaldehyde and resorcinol resin glues are currently available for use as aircraft glues. The most common urea formaldehyde glue is Aerolite 306, and the most common resorcinol resin glue is Aerodux 500.

Aerolite is a whitish powder that is mixed with water to form a paste before use. This is applied to one side of the joint and a liquid hardener (usually formic acid) is applied to the other side of the joint. When the parts are brought together the chemical setting reaction begins.

Aerodux consists of a dark reddish-brown resin and a liquid hardener. In use the two parts are mixed together and applied to both of the parts to be joined. Like araldite the setting reaction starts as soon as the two parts are mixed.

Over the years I’ve used both Aerolite and Aerodux for glider and light aircraft repair and rebuilds. Generally I prefer Aerodux, as it has a longer working time and, since both sides of the joint are spread with the same mixture, there is less uncertainty about coverage.

Regards,

Andrew

24/12/2010 20:28:13
Being a pedant (who me?) the Lightning was designed as an interceptor, not a fighter. I agree that it's performance is incredible, even now. It was designed for speed, acceleration and minimum time to height. On the downside it's endurance was dreadful.
 
I've sat in a Lightning (at ETPS) but the b*ggers wouldn't let me fly it; spoilsports. They wouldn't let me fly the MRCA (forerunner to the Tornado) either, even though I was working on it.
 
When I was a kid I used to visit RAE Bedford regularly for their 'families' days. One of the aircraft there in the early 60's was the SB5, built to test the low speed performance of the radical sweptback wing arrangement of the Lightning, and whether a high or low tailplane was best.
 
Regards,
 
Andrew
Thread: Late delivery of magazines due to adverse weather
24/12/2010 12:05:45
It all seems very inconsistent, I got my MEW on Monday (20th) and ME on Tuesday (21st). I wonder if the distribution company use the Royal Mail for the whole delivery, or uses one of the competitors for distribution and the Royal Mail only for the 'last mile' of local delivery?
 
Slightly OT, I have some insulation for the workshop on order, that should have been delivered last week. Rang up yesterday to ask why it hadn't been delivered, fully expecting the snow to be given as an excuse. Turns out the depot 'lost' the delivery note, so the order became invisible.

Regards,

Andrew
Thread: Aircraft General Discussion
22/12/2010 23:44:40
Norman: Feeling smug here that I guessed the right aircraft! I have flown a Pilatus B4, but not the fully aerobatic version. I did an inspection on one and advised the buyer as to whether he should buy it or not. Which he did, and payment was a flight in it once it was back at our local club. I think I had about an hour and half in it; easy enough to fly, but not one for scraping in weak thermals. Interesting that the official BGA tech sheet claims -4.79g as the negative limit? I haven't flown many metal gliders, the others being a Blanik, IS29D, IS28 and one no-one will have heard of, the VFW FK3.
 
I reckon on a decent thermalling turn being about 15-20 seconds per circle. Certainly in the UK, if it's blue, then you'll need to turn more steeply. And in the mountains, especially low down, you need to turn really steeply. If you don't you hit the mountain!
 
John (RJW): In my experience composite gliders don't creak that much, unless they're made by Schempp Hirth, in which case the canopy frame continually squeaks! When I was learning to power fly we regularly had to demonstrate steep turns. Easy really, just line up the flying wires on the cabane struts with the horizon, and you've got about 60° of bank. Also, although spinning wasn't part of the CAA syllabus at the time, the CFI's view was that, since the aeroplane we used would spin, it was necessary to demonstrate three turns of a spin in each direction to his satisfaction.
 
At least in a glider you don't need to worry about the engine stopping! Funny thing, but I'm happy going places in the mountains in a glider that I would never, ever consider in a power 'plane. Oddly, I've also been higher, further and faster in a glider than in a power 'plane.
 
Stub: Ah yes, the Merlin, sounds just like a bag of nails, and always on the point of stopping! Quite unlike the Griffon. I am fortunate to live a few miles from Duxford, and in the summer we regularly get all sorts of WWII aircraft practising overhead. I spend more time in the garden than working!
 
Right, enough rambling for one evening, bedtime calls. I'm working tomorrow, but on the plus side I'm getting paid to play in the workshop.
 
Regards,
 
Andrew
22/12/2010 16:13:28
Good grief, +7g and -4g placarded! Must have been a fully aerobatic glider? Given the clanking I'd guess metal, possibly a Pilatus B4?
 
Regards,
 
Andrew
Thread: TCT LATHE TOOLS
21/12/2010 12:28:12
Errr, not quite sure why there is a picture of my surface roughness measurement meter at the top of this post, pretty though it is!
 
John: By TCT I assume you mean tungsten carbide tipped, ie, a brazed tip? If that is so you'll need a silicon carbide grinding wheel (colloquially known as 'green grit'). If you just want to clean up the edges then a diamond 'hone' will be fine.
 
By the way, in forum speak all capitals is taken as shouting, and may be considered not quite cricket.
 
Regards,
 
Andrew
Thread: Clutch in push rod to disconnect drive?
20/12/2010 11:10:28
Not surprising really:
 
In theory, theory and practice are the same, but in practice, practice and theory are completely different.
 
Regards,
 
Andrew
Thread: 5C or ER Collet Chuck
16/12/2010 19:01:20
Errrr, 5C offers square, round and hex?
 
Confused of Cambridge!
Thread: Anyone used an Inverter with a Colchester Student
16/12/2010 00:07:28
Definitely not the norm, at least in the UK. As I understand it the normal household incoming fuse is at least 100A these days. When I bought my bungalow 10 years ago the incoming supply was rated at 60A; the surveyor marked this down as an issue that would need to be sorted out.
 
When I upgraded to a three phase supply the electricity board automatically installed 100A fuses per phase.
 
Where are you based Jon?
 
Regards,
 
Andrew
Thread: HSS v Carbide Tipped Tools
15/12/2010 23:24:53
Traditionally carbide tooling has been recommended within the ME community for jobs such as turning iron castings. Here the 'skin' may be hard, or have sand embedded in it. A carbide tool will handle this, whereas a HSS tool will blunt quickly.
 
However, to get the best out of industrial style indexible carbide tooling you need to run at fairly high speeds and feedrates. Generally the required rpm is within the range offered by ME type lathes. But the high speeds and high feedrates imply a high rate of metal removal, which in turn implies a lot of horsepower; which may not be in the range offered by ME lathes.
 
Carbide inserts are relatively blunt compared to HSS tooling and rely on adiabatic shear bands (google it) to achieve a good cutting performance. At the shear band the metal is very hot (often red heat). To achieve this requires a lot of power.
 
In essence the problem is probably not lack of rpm, but possibly lack of power.
 
Regards,
 
Andrew
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