Member postings for Martin Whittle

As I basically stated in my posts yesterday, this will not work!

A bunsen burner uses a small jet of fast-moving gas within a much larger tube, which sucks in air from an airhole which is then mixed with the gas. The tube is large and there is minimal back-pressure.

A gas burner on a cooker, or a camping stove, works much the same way - except there is no tube as such, but the flame burns externally, not at the jet, after mixing with air., but without an actual 'tube'

If you have a narrow constricting pipe after the 'air hole', the back-pressure is going to be substantial - the gas will leak out, and air will not come in (why on earth should it? - it needs negative relative pressure for that to happen, and we have positive pressure).

So the solution is either:

• A properly scaled version of a bunsen, using a (microscopic?) jet in a much larger but still tiny tube
• Or: is there merit in grinding some sort of shape (double-sided taper or more complicated?) on the tip of the burner tube to mix/swirl some air in?

Note one cannot expect designs etc to scale with size. Drop a mouse off a table and it will run away; drop an elephant off a table and it will probably be fatal.

With this in mind, should the pressure at the jet be increased very substantially and the jet made even smaller within the tube: maybe not relevant, but the pressure inside a soap bubble is in inverse proportion to the bubble radius: small bubbles have high pressure!

The jet would need to be very small, and also much smaller than the needle diameter for the air hole to work. I cannot see an air hole near the lower end of a needle working.

Incidentally I re-jetted a bunsen burner to work using a 37mBar regulator supply from a propane cylinder a while ago. I recall I drilled the jet probably around 0.5mm diameter for a 'regular' size bunsen: a micro burner will need something a lot smaller.

Dispensing syringes are available from **LINK** in sizes from 0.15mm to 1.7mm (but packs of 50). The plastic sleeved end might not appreciate getting too hot, but this may not be an issue since the flame is small.

With repect to shaping the tip: if a hypodermic needle were reground with a taper on both sides instead of one, what would this do in terms of oxygenation of the flame, while at least keeping the flame symmetrical?

Edited By Martin Whittle on 23/08/2017 10:32:40

I have some difficulty in seeing how an air inlet can be created.

In a bunsen burner, the jet is used to restrict the flow of gas and to create a jet of gas which sucks air into the airhole.

Without re-jetting the burner, the flow will be restricted by the needle, not the jet, Therefore the pressure lower down in the needle will be above atmospheric, and not below: therefore gas would escape via the 'airhole', rather than air coming in.

Maybe with a very small fast flame, it will burn efficiently without an airhole?

NB could be best to use a dispensing needle (not sharp and cut straight across the end), not a hypodermic needle, unless the taper of the needle end is beneficial in terms of mixing air into the gas - would abrading/reshaping the tip help?

Edited By Martin Whittle on 23/08/2017 08:48:04

Agree absolutely with NDIY. You refer above to a 150mm width slab cutter - I take it that you are not trying to drive anything like with this mill. I question whether it would fit, but whatever motor power you have on this mill, it would be insufficient.

I saw a video somewhere on youtube, showing use of bleach to get rid of black mould on bath silicone sealant. Basically, it requires a long soak time, it is no good just wiping it on. I guess the mould takes a long time to grow, and goes deeper than just the top of the silicone, similarly it takes a while for the bleach to get in.

So I used toilet paper folded to make a small fillet on the silcone between the bath and tiles, soaked with a modest spray of bleach using a syringe (it only needs enough to get it reasonably wet) and pushed it place - WEAR GLOVES!

In 10 - 12 hours, my mouldy silicone was changed to almost pristine white - there were a couple of very minor spots which could be helped with a longer soak, but I was very pleased with the result! I had done a trial run on a few inches of sealant previously, just to check what would happen.

I don't know how long the silicone will stay white, but it is very good for now. Bleach used was 'real' bleach, Domestos brand in a blue bottle, this is a somewhat viscous formulation, so helpfully it does not immediately run away.

Look to find 5% sodiun hypochlorite in the ingredients, igore the ******** anionic surfactants!.

Hope this helps

Martin

Que?

While (not quite) this subject:

**LINK**

Martin

Very impressive indeed, you have convinced me that I have to make one!

Yet another one of many projects for my round tuit list

But thanks David - I shall have fun!

Martin

As others have mentioned above, I would be concerned about the abrasive effect of the clay particles being embedded in a soft material and abrading the harder material in the bearing.

On a house move, we once acquired a pond including a pump to run a fountain. This ran extremely roughly - on investigation, it used an impeller using a stainless steel shaft located in cylindrical recesses in the thermoplastic enclosure. The plastic recesses were not significantly worn, but the stainless steel was worn down to near half of its original 1/4" diameter!

So don't expect SS to wear well if the application does allow clay slurry into a softer bearing!

Many years later, I still have 3 off 24V 50W pond transformers attached to 50W halogen machine lights (bought cheaply at the Midlands model engineering exhibition a few years ago) to light my machines. The pond pumps are history.....

Martin

Further to the excellent quidance from Russell:

For loss in synchronously tuned filters, in the time when I was (less) gainfully employed, a couple of extremely useful reference works were written by Seymour Cohn, specifically see ‘Dissipation Loss in Multiple-Coupled-Resonator Filters’, Proc. IRE August 1959 if you can find access.

I don’t now have access to my paper copy, but I remember something close to:

Filter loss in dB = 4.343*(reciprocal of fractional BW)*sigma(gi/Qi)

Where Qi is the Q factor of the i’th resonator, and gi is the normalised value of the i’th resonator (for the low pass prototype at a frequency of 1 radian/sec and impedance 1 ohm). I cannot remember whether the fractional BW relates to the half-sided BW or the full BW as a ratio of centre frequency.

But anyway, the passband insertion loss in dB is directly proportional to the inverse of the resonator Q factor (simplifying to assume the resonators are all the same). The stopband performace is not significantly affected by the Q factor.

Note that silver oxide is electrically conductive (hence silver use in switch contacts and general electronic use), but copper oxide is significantly less so, see use in copper oxide rectifiers, **LINK**

I agree that coat of lacquer will give much of the long-term benefit of silver plating

Martin

Edited By Martin Whittle on 26/03/2017 20:53:29

I have been very satisfied with several orders of cutters from http://richontools.com/ - I think I may have first found them from this forum: They have a good range of cutters etc, and prices are much lower than UK. Several delivery options depending on how fast you want it : delivery is at a significant price in relation to the basic goods price, but the overall package is very good..

Remember you could be liable for VAT and import duty (and administration/carrier surcharges) for any orders above the duty-free allowance of £15 for goods imported from abroad. I have not yet incurred this (on a modest number of orders), but you may not be so lucky.

Martin

Edited By Martin Whittle on 19/03/2017 20:56:04

Agreed

To return to the original photo, the counterbore on the parts is not deep - on the contrary, it is very shallow. There is little exit for the swarf, so it will completely choke up before 5mm depth is achieved. So more for shallow spot facing than a counterbore for a cap head.

But it does demonstrate a very simple way to make a custom counterbore cutter!

Martin

Edited By Martin Whittle on 03/03/2017 10:23:44

The speed of an electromamgnetic wave (so radio, light, x rays, gamma rays etc) in vacuum is approximately 300,000km/sec.

In a cable the speed is then dependant on the velocity factor, which is a ratio of the speed of the wave in the cable related to that in vaccuum. In most typical cables or other electromagnetic media (fibreoptic cable, lenses, etc) the speed is proportional to the square root of the dielectric constant of the material (also the permeability for case of magnetic materials). So most cables have insulation of plastic (polythene, PTFE, etc) having dielectric constant around 2 to 2.5, so the velocity factor is around typically 70% for solid dielectric, or maybe 75% for air-spaced plastic dielectric.

The velocity factor also simply the inverse of the refractive index of the material: so for a dielectric constant of say 2.25, the refractive index is 1.5, and the velocity factor is 0.667. (Assuming the material is non-magnetic).

So since the speed of light is 300km/sec or 186,000 mile/sec: if you connect yourself to a mile of cable and then have high voltage power applied at the other end, it will take 5.3 microseconds before you are electrocuted for open wires, or 7 microseconds through typical plastic-insulated cable.

If the the cable were 70 light-years long, you then need not worry

Martin

Edited By Martin Whittle on 24/02/2017 23:06:42

Edited By Martin Whittle on 24/02/2017 23:07:34

Edited By Martin Whittle on 24/02/2017 23:20:06

1. Don’t try this at home (else really at your risk): metal cutting.

I recently finished making a crosscut sled for my home-made woodworking table saw; the final operation was to mount the sled onto the saw in order to cut its own clearance slot for the sawblade. I accidentally found that some idiot had assembled the sled with a woodscrew in the line of the blade: there was a sudden minor shower of sparks, remarkably little mechanical resistance, and the screw was very quickly and neatly sectioned! After digging out the remains, a part of the screw is shown in the middle of the picture below. The screw is a ‘Torx-Fast’ part from Toolstation, and is expected to be reasonably hard.

I was surprised to find no obvious damage to the (fairly expensive) sawblade. It has 80 carbide-tipped teeth, 250mm diameter, spinning at nominally 4350rpm. So approximately 5800 teeth per second at a surface speed of 205km/hr, or 128mph. The cutter speed of over 11,000 feet per minute is rather faster than I normally use for steel! Something had to give way; I am glad it was the screw.

Whilst I ‘got away with it’ this time, I would not recommend using one’s best sawblade for cutting steel, but it might be worth considering for softer metals? I think the very high cutting speed is probably critical for this use.

2. Do try this at home: plastic cutting

I tried using the table saw to cut some pieces of Perspex (acrylic plastic). I was impressed by the quality of the cut surface: remarkably smooth, with a modest level of the expected scoring from the sawblade but otherwise a partially burnished surface, with no sign of melting, or any significant chipping on break-out. It might actually help that I have some unwanted wobble on the blade: the 3.2mm blade tips cut a 4.0mm kerf. For my intended purpose, it was completely unnecessary to further clean up the surface. In future I shall probably use the table saw to cut Perspex, in preference to a hacksaw.

I understand that plastics are generally cut slowly to avoid melting. However it does appear that if you cut them with a VERY FAST sharp cutter, this also avoids melting, except probably on a microscopic scale at the surface, and gives a good finish on the cut edge.

Martin

PS I have found the crosscut sled to be a very useful accessory indeed, for the table saw.

There are many other coincidences of units relating to historical connections:

I have an allotment, '5 rods' in size. A 'rod, pole, or perch' (i.e. a precision reference standard consisting of a wooden stick) is a linear measurement of distance of a quarter of a chain, so 5.5 yards. Alternatively it is also used for an area measurement of the same linear dimension squared. A chain is a historical unit of measurement, also the length between the wickets on a cricket pitch. So using a chain to outline a square yields an area of a rod, (pole, or perch). So a linear rod is only 0.584% off a measurement of 5 metres; the area measurement is just 1.17% off 25 square metres. So my allotment is very close to 1/80 hectare. See **LINK**

Home brewers will also know that 6 wine bottles is the expected yield from a demijohn: 6 bottles of 0.75l is within 1.02% of an imperial gallon.

Other old measurement units show close correspondence:

The French monetary ‘livre’ was a ‘pound’ unit equivalent to a one pound weight of silver (Latin ‘libra' ), hence also the British pound £ symbol. **LINK** .

The French weight ‘livre’ unit was 1.079 UK pounds. The use of the livre was finally revoked by the La loi du 4 juillet 1837, **LINK** but I believe the livre may be still used in French markets. I had thought change was slow enough in UK, having allegedly ditched the antiquated old Fahrenheit measurement of temperature, and inches/yards/poles/furlongs etc of length when I was a child 50 years ago, but it has taken a long time to change.

Also the old ‘French Inch’ (Pouce) is 1.066 imperial inches (or even US inches). Still used in horology, 12 lignes = 1 pouce.

US measurements of volume (US pints/ gallons) and weight ('e.g. 'short' tons) seem to show almost no closer correspondence to UK units than Metric or other European units!

I'll just get my coat...

Martin

Edited By Martin Whittle on 29/01/2017 23:08:13

Edited to remove spurious smilies!

Edited By Martin Whittle on 29/01/2017 23:09:54

A polyester capacitor IS a foil capacitor, but the converse is not necessarily the case.

Foil refers to the electrode material and hence implies the construction, polyester refers to the dielectric.

I don't know details of the application, but in general polypropylene has better ESR and pulse performance than a polyester, and will be physically larger for the same rating, and more expensive. So I am sure polypropylene would be an excellent choice for the application, but polyester would probably be OK providing you are not pushing the ratings too hard.

Martin

In another life, I have used 'Tetra Etch' for etching the surface of PTFE and FEP wire insulation prior to potting electrical connectors. It was then made by WL Gore, but I cannot find now it on their website. On an internet search I have found a source of it on one website for about £200 + VAT for 500ml (!), and polytetra.de also list it The bottle size I used to use was much smaller (50 or 100ml), and you will find many more web links for MSDS (safety) sheets than you will find any supplier information. I believe it is made by reacting metallic sodium with an organic compound; this then gives a highly reactive compound, quote: "A sodium compound in the solution reacts with highly fluorinated polymers to form a reactive film on the polymer surface. Nearly any adhesive can be used on this treated surface. The active sodium in TETRA-ETCH® etchant reacts with the surface molecules of a halogenated polymer to form a carbonaceous film of free radicals".

Tetra Etch was a remarkably evil-looking black liquid having the appearance of a crusty lava-like skin. I recall I used isopropanol to wash it off after giving a bent-back end of a cable a 30 second dunk in the compound (insulation only, not exposing any electrical conductors). It left the insulation with only a slightest surface milkiness on coloured insulation, invisible on white. I believe the useful shelf life of a treated surface may be about 7 days.

On a tear test, pulling a cable out of a thin surface layer of potting compound, it did clearly gave significantly better adhesion than a similar test on an unprepared wire, but I would say it was still not a mechanically strong joint. Surely not helped by the inherent flexibility of PTFE

I suspect that PFTE can therefore be bonded to the degree that a sealed union may be made, but perhaps not a joint that required to suffer significant mechanical strain.

Martin

Back in the 1970's the car spares firm Unipart promoted itself with the (then) well known slogan 'thousands of parts for millions of cars'

Simple maths appears to show it had only one useful part for every thousand cars

Martin

Two years ago I scrapped my 2001 VW Passat 2litre petrol when it decided it could run OK on 3 cylinders at 150k miles - exhibiting one severely oiled plug; RAC man tried new plug on the offending cylinder but it did not help.

I bought a 3 year old BMW 320d to replace it: Initial cost maybe not that cheap, but 161 horsepower and just £20 road tax (£21 renewal this month). I get typically 56/57 true mpg; car display says a bit better at around 60mpg - (I could make comments about how mpg data is averaged may not give true average of MPG, but a bit late for a Satuday night, and it is not that far off), However, annual tax and fuel costs are £500 lower than the passat, so I am a happy bunny!

Martin

On my Reeves Master engine, I made an inlet 'manifold' using a schrader tyre valve body as the inlet connector, silver soldered into a brass block. It runs well off a compressor, but will also run (less evenly!) using a bicycle pump .

Martin