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Member postings for Billy Mills

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

Thread: George Daniels-Horologist
04/12/2012 13:37:40


According to the MDS for WD40 it is white spirit, light machine oil and a perfume. Perhaps the WD40 has attacked a previous coating ? Petrol might get it off but you do need to be careful...


Thread: Been playing with me laser.
27/11/2012 09:35:39

Hi Sir John,

Have used celulose as a resist coat, laser off the paint then etch. Fill the etch with paint then wipe off the surface. Also useable for PCB's. Would agree with the comments about the "industry standard aerosol". We do a lot of perspex signs, as you say the backside paint then engrave and contrast coat works great. For illuminated signs you can paint a ground colour ( say red) then overcoat white then engrave through the lot. The white reflects back the light into the box giving more even and brighter lighting.

Your warping stainless might be down to too much power and too slow movement, it is always good to start low and work up rather than using cutting power but I have not used the molly route myself.

I am always very careful about just what gets exposed, few months back witnessed the effects of cutting PVC in a CO2 machine, everything was corroded and the control board was a write off.

It is a good idea to check the lens after blasting at something unusual, debris will significantly reduce power in some machines where the air assist is not so good.


Thread: Compressor
07/09/2012 21:42:40

That's a real issue with the thin walls on current imports. Old models had an inspection port and very thick walls so you could get a clue as to their condition. Nowdays you can't look inside much and you can't tell if the inside is well painted. Perhaps the only easy thing is the state of the drain water, if it is rusty then worry.


Thread: Mercury Battery [233]
07/09/2012 21:32:27

Yup, it's a linear variable inductor sensor, cut down version of a linear variable differential transformer. It has a uniformly wound coil tapped in the middle. A core -about half the lenth of the coil- slides inside the coil. When the core is in the middle of the coil both halves have the same inductance so when you form the bridge with two equal resistor arms the bridge balences and there is no output from the tap and the middle of the two resistors. When the core moves it increases the inductance in one half coil and decreases the other so there is an output from the bridge, the magnitude and sign give the displacement of the core. It is a half bridge transducer because the two resistor arms are in the box- not in the probe.The bridge is driven with typically 1-10V at 5-50KHz.

The arrangement is very simple and very widely used in go/no go rigs. The sensitivity can be very high because you can use a lot of amplification and phase sensitive detection, the limits are thermal, seismic noise, air movement and other exotics. Spliting a micron is simple. The three cell supply will probably work fine, the sensor does not make precision displacement measurements, it is a comparitor, plus or minus from the centre position so you would never want to null the endstop position. LVI's and LVDT's are very easy to make, even home made ones work very well and can be made with good linearity by fiddling the pitch of the windings.



Edited By Billy Mills on 07/09/2012 21:34:55

07/09/2012 20:08:19

Michael you have a PM .


Thread: Advice on grinder selection
07/09/2012 20:06:07

Much safer and better to go diamond, no risk of flying stones and you can put a much better edge on HSS or cobalt or carbide than white or green ever will. But the use is limited, you take off small amounts to maintain ( or greatly improve) the edge so rests and jigs are very desirable. The diamond wheel lasts and lasts and lasts......

Diamond wheels are the same kind of price as stone age jobs but they do take a bit of getting used to, you carefully centre the ali wheel- no dressing to centre. Arc Euro have a good note on diamond wheels on their web site- well worth looking at.

I have a couple of Tormeks for sharpening HSS wood chisels works great, but remain quite shocked at the price of their drill sharpening device! I will stick to the Vertex T&C with diamond as that works on cobalt and carbide too.


Thread: Teflon Spray ?
07/09/2012 19:40:10

According to the Manufacturer's MDS for WD40 it is a mix of white spirit, light machine oil and an odoriser. ( this is a MDS from some years back, the more recent sheets use more exotic terms) No PTFE or anything else.

So it can lubricate and displace water and has a nicer smell than plain white spirit and 3-in-one oil. It can "penetrate" nooks and cranies and then leave a little lubricant behind after the volatiles have gone. However it can also flush out heavier oils and greases (oils with soap) which might then need replacing.

There is also the little detail of flamability, is it a good idea to spray cloth with white spirit then put it away in a confined space? As others have said there are far better corrosion protections than WD40.


Thread: Buzz Coil Condenser/capacitor
24/07/2012 11:44:50

Andrew, I'll go along with a simple model to start with but you do need to load the secondary. If you had a step up transformer with loose coupling and a diode in series with a voltage source and a resistor of about 10K then you would not observe the early ring, you would get the sloping plateau which terminates in a ring when the spark goes out. There is also the early pre-spark spike.

An equivalent circuit for the coil would be very useful, from what I can remember of compairing the primary and secondary voltages some 30 years ago ( using a capacitative divider on the plug lead) there is quite a big difference between the primary and secondary waveforms. The inductance of the secondary is very high and the distributed capacitance adds to the complexity.


22/07/2012 01:03:50

The seconary and primary resonances are not very significant. The basic point is that the stored magnetic energy produces a very high voltage accross the points ( and distributor gap) which then -after a short delay- ionises the plug gap. The spark lasts 1-2mS untill terminated by the points closing or the stored energy being depleted to gap extinction.

As a rough transformer, the secondary voltage is approximatly a multiple of the primary voltage so the gap(s) voltage is reflected in the primary voltage however the onset of ionisation is a very violent event, the di/dt is gigantic and much RF energy can be produced at this time. The suppressor resistors in series with the plug are around 10-15K ohms, they greatly reduce peak currents and radiated noise and can produce a longer burn by low passing the step function.

When the points open there is no voltage accross them- the capacitor has been shorted, the inductor's charging current then ramps the voltage up on the capacitor. Providing that the points open sharpish the gap opens as the voltage rises and sparking is minimal or zero. The voltage rise is a damped sine which is a complex product of Primary C and L and the less than perfect coupling to the secondary which has the spark gaps accross it!

An important detail is the leakage inductance between the windings, the centre core has a massive gap so is an undercoupled bandpass transformer. So putting the "Kettering" capacitor accross the points mainly affects the primary. The secondary winding has a very much higher inductance and a much higher distributed capacitance than the primary because of the turns ratio however the secondary never really gets to ring early on because the large flux change induces a sharp "Kick" to the secondary so the voltage goes to around 20Kv before the gaps break down then burns for some time at a much lower voltage, dumping the stored energy.

The ring at the end of the burn is the secondary parallel resonance, the ring shows a Q of 2-5 - which illustrates the effect of the loose coupling. Because the coupling is loose the Kettering capacitor's value is not critical. As you increase the value from say 10nF the point's sparking will reduce then cease as the gap opening is faster than the slowed points voltage, the engine runs slightly smoother too. Increasing the capacitance further may increase the output slightly- some coils can show a resonance when the primary is tuned however in my experiments 30 years ago on car engines the effects were small, about 200nF worked well on the cars that I looked at, but the coil is a complex component. They all make you leap in the air when you hold onto the EHT lead but the height varies! ( that is a thought experiment, not a recomendation).


Thread: Great Idea Guys
19/07/2012 14:56:26

Have a google for Cyber Knife. One has been installed at Mount Vernon Hospital, was a gift from a grateful patient ( £10,000,000). Destroys tumors that are inoperable. Hope for those who had none.

The CNC bit allows the beam to be rotated around and converge on the target without the operators being anywhere near. It is a step up from the linear accelerator technology that has been used for mostly one axis rotation.


Thread: Suds pump
18/07/2012 23:39:42

God bless your colon Neil. I have had a couple of views inside mine and a bit of maintainance in the bore, so far so good. The pink tube is pretty clever but it is the weirdest thing when you watch a monitor displaying the trip through your own pump. Glad they clean the fibre optic tube between uses!

Just happened to see that Mike Cox had an article in MEW 181 Oct 2011 for a suds system- he used a windscreen washer pump.


18/07/2012 19:47:38

Martin, you didn't say what was being pumped. In our aircon application we have 20 pumps which have been used for over 14 years. They are pumping dust containing water. Four of the pumps are in a server room which runs 24/7. The building was empty for 18 months without the rollers moving. No tube issues at all. The tubes are thin walled, the roller pressure is just sufficient to close the tube.


17/07/2012 19:09:56

Air conditioners produce a lot of condensate, often there is a small plastic sump with a magnetic float. When the float rises it operates a reed switch causing the pump to run. An old aircon pump may have issues, the silicone rubber peristaltic tube often is OK but it is plugged into a plastic pipe assembly which tends to break up after many years. You can fix the pump by replacing the plastic pipes with brass tube. The motor is normally a small shaded pole job with gearing down to around 100rpm. Regrease the gears and soak the shaft bushes for another ten years of life.

If you are in a DIY mode peristaltic pumps are easy to make if you can get some suitable silicone rubber tubing and a slow running motor. No valves, always self primes and will deal with all kinds of stuff that would jam other kinds of pumps.

(Have bodged quite a few aircon pumps, got fed up with paying silly money every time another old pump leak flooded the ceiling, refurbished all of them for less than the cost of one service call.)


Thread: The 3 phase question
14/07/2012 20:19:47

Perhaps the relative cost and convenience of 3 phase v. converted single phase is more important than other factors to most M.E.'s . If it cost say £500 to supply and wire the workshop with a 3 phase supply then that would be around the cost of a suitable single phase alternative. The actual cost of having a 3 phase feed, meter change and new 3 phase wiring into the workshop ( to current regs) is likely to be above the cost of single phase conversion.

The three phase supply is simple and 100% reliable, you can run more than one machine at a time and the tools are designed to work on a 100% duty cycle- suits someone working from home.

The single phase solution suits the hobby workshop-one machine at a time- by swapping connectors.

100A supply is the norm for single and three phase supply, has been or a long time.


Thread: 12 Volt PMDC motor armature
14/07/2012 13:50:52

Brushes are made of different mixes for different uses. A low power mains motor can use a mainly carbon brush, a low voltage high current motor may have 90% copper or silver content so that it has a much lower resistance than an all-carbon brush. So fitting a mains brush to a low voltage motor is not a great idea.

The brush is intended to contact the commutator across the whole of the brush end, if the contact is only along a narrow area and a lot of current is sent through the brush then rapid wear will occurr, arcs to the copper commutator will cause years of wear in seconds. So brushes are shaped before the motor is heavily loaded. Please NEVER emery! the emery can cause extensive damage to the commutator, brushes and bearings. Garnet paper around 220 grit with the paper against the commutator will do a good job of shaping the brushes. The commutator is best skimmed with a lathe, the motor shaft often having countersinks for turning between centers.


Thread: Optical Encoders
01/07/2012 17:11:28

HP have not used steppers in low cost printers for many years, just cheap brushed DC PM motors and printed plastic encoders. There is some complex gearing in some models with one way devices so that you feed paper one way and do cleaning chores the other way- that's why they often make busy noises without any paper movement after power-up.

Some models have a six channel peristaltic pump which transfers the inks into holding tanks then back- that way you can measure optically how much ink is actually left. You might also find a ccd camera in some all-in-one models.


Thread: Uses for old hard drive platters?
08/06/2012 12:57:55

Most recent HDD's have cast ali frames not zinc alloy which went out about a decade ago. You can give the frames a quick skim in a mill to give a thin rectangular slab which can then be used for all sorts of milling setups, if you need an extra hole or two then drill it!.

However before scrapping the drive it is worth trying recovery software such as Spinrite 6 to find out if the drive is beyond help. Mostly they are recoverable. Commercial recovery services do remove the platter from a failed drive to prevent further loss from damaged heads or electronics. They then mount the platter in their own equipment.

Drives made in the last year are unreliable, brand new drives from well respected brands often fail within a month so it is more important than ever before to run a RAID config AND to do independant back ups.


Thread: Linear division in early 20th century
01/04/2012 20:42:18

Sorry Michael but you CAN improve on the internal reference by adding complexity. Maudsley's template corrected leadscrew was one such example as was his method of using an auxillary screw to compensate for errors in the average pitch of a leadscrew.


Thread: What's inside a Digital Micrometer?
05/03/2012 14:38:43

The micrometers that I have seen use a capacitative encoder like the digital caliper but circular, not an opto or magnetically coupled encoder which would use a lot more power. The stator has many electrodes that capacitativly couple to a rotating pcb .

With this scheme the counting circuits are powered all of the time so that the device "remembers" the number of turns from zero to save the user having to mechanically reset the micrometer to the anvils together position.

So turning the device on only powers up the display but it will draw a very small current when switched off, Mitutoyo's have a very low standby current compaired with some other brands.


Thread: 3 phase motor rotation direction
02/03/2012 00:31:42

Yes there have been alarming posts on electrical matters however the same is true of chemical, fire and mechanical risks on this forum. Pointing out the risks and explaining what is sensible seems a kinder route than "get an electrician/fireman/chemist" , it should then be up to the individual to decide what to do after getting the info they seek in a HOBBY context.

I would agree with John that many "professionals" get it wrong. There is the classic normalisation of red/blue/yellow/ black to black /brown/ grey/ blue or just all black with number sleves. That progress is retrograde-it has already killed electricians in the UK. If you have ever seen a blown armoured cable sliced up down a clay hole or worked under bad lighting you are left thinking why we abandoned bright colours- including red- for the darkest possible combinations. That's my rant for today.



PS well done AMR! at least you asked the question to find out WHY.


Edited By Billy Mills on 02/03/2012 00:34:37

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