Member postings for SillyOldDuffer

Exactly what the switch looks like varies, but after taking the wired end the motor off, it will be found mounted on the axle something like this:

As the motor gets up to speed, a weight, or pair of weights, pulls against a spring and eventually move far enough to open a contact. Every time the motor starts, the contact sparks and they eventually wear out. Or the lever gets out of adjustment so the contact doesn't move far enough to open and close correctly. If the contact can't close when the motor stops, the run winding gets no power, and the motor won't start.

The contact is usually easy to see, and it may only need a clean or adjustment. If it's burned out might be possible to replace the contact points, or to source a replacement centrifugal switch. It depends on the age, make, and type of motor. I'd expect Bridgeport spares to be available, but not necessarily cheap!

The centrifugal switch and capacitors are a weak point in single-phase motors and they vibrate. The advantage of single-phase motors is they can be plugged into ab ordinary mains socket: everything else about them is shades of grey! If the motor has to be replaced, consider 3-phase and a VFD.

Dave

This sort of post does nothing for my inferiority complex! Loads of good ideas, extra well-made, and Joe is another one who must work at least twenty times faster than me.

Disappointing results from the accelerometer were predictable, but good to see it tried, especially in a pendulum design that allows that sort of experimentation. Though I like sensors inside the bob, I worry about wires flapping about inside the rod and adding friction where they exit at the top. (Causing much more friction than the knife-edge?)

May be worth adding a humidity sensor. Although almost completely insensitive to temperature change, my samples of solid Carbon Fibre rod were humidity sensitive. I believe humidity acts on the matrix and alters its elasticity. If running the pendulum for a long time shows anomalies, humidity might be the cause. (In the UK humidity varies a lot: I've no idea how much it changes on a Namibian sea-shore!)

Can't wait to see how it performs.

Dave

PS Poor Joe. After resisting the pendulum bug for ages he's caught it badly. Worst case of pendulitus I've ever seen. Welcome to the dark-side...

Certainly is. Stephen Fry did the powder explosion on QI. Moving up the scale, Tony Robinson did a series demonstrating the effects of replica WW2 German Bombs on typical British homes. From memory his smallest explosion was 50kg of an un-named High Explosive, distinctly more violent than custard powder! Mythbusters (made in the US where they have more empty space for big bangs), filmed many explosion based myths and realities. Gas igniting inside a Portaloo (don't smoke), up to a cement truck filled with quarry blasting explosive.

Modern TV is increasingly coy about the exact 'how to' details. No desire to make it easy for those who want to cause explosions in crowded public places. All countries have large 'Sh1t for Brains' communities!

Dave

Dave

Spurious sensor readings are causing my pronblems rather than the computer as can be seen in this graph, where 3 giant errors cause the clock to jump:

I don't know what caused them. May be a coincidence they all occur just before midnight.

Zooming in on the data shows thirty odd more much smaller anomalies:

Although the smaller glitches balance out, they shouldn't be there. Best thing would be to find and fix the cause, but I've thought of filtering them out in software. Hard to find the cause of only 40 apparently random errors in 2.9 million readings, so it's tempting!

I can't get away from using a computer because I'm experimenting with a statistical clock. In this experiment the actual pendulum period doesn't matter provided all swings are normally distributed. Instead, detecting a swing causes the microcontroller to calculate and count what the period should be after compensating for temperature and pressure.  The period calculating formula is derived from statistical analysis of a long pendulum run during which the pendulum is compared with a much better clock. (Bog standard NTP at present but I can also use GPS).

Despite progress, I'm not getting the precision I long for!

On the graph above the straight green line shows what the clock should be doing, i.e tracking NTP within about 100mS. The blue line shows my clock is wandering, and other graphs show the rate changes are not temperature or pressure related. The wandering blue line is bad news - straight lines are easy to compensate, random wandering is a horrible mystery. Roughly:

• Gained 23 seconds in 7 days, then
• Lost 2 seconds in 2 days, then
• Gained 8 seconds in 9 days, then
• Lost 1 second in 8 days, then
• Gained 1 second due to a sensor error, then
• Lost 14 seconds in 6 days.

After a month, never been more than 29 seconds wrong, and is currently only 17 seconds fast, not awful except I'm hoping for milliseconds per year, not half a minute per month! In theory my clock is working brilliantly: in practice it's below average. Unfortunately the experiment requires I persist with the computer!

Dave

Edited By SillyOldDuffer on 15/08/2023 21:00:31

I watched it. Wish that chap had taught me maths at school; he explains every step.

I wonder if the same approach could be applied to model boiler design. What we have are scaled down versions of full-size boilers tweaked to meet practical construction needs rather than optimised efficiency. LBSC started the coal-fired boiler ball rolling over a century ago by imitating full-size practice. His methods were informal.

Though the Battle of the Boilers was controversial, coal-fired became most popular. Between 1922 and 1970-ish, a few boiler configurations were tried without a clear winner emerging. Since then, I don't think much has been done, probably because boilers are expensive, and failed experiments severely punish the wallet! Also, all a model loco needs to do is work reasonably well - top-rate thermally efficiency isn't essential.

CAD offers a cheap way to experiment with boiler design without needing to physically make a real one. Now it's possible to design a model boiler for thermodynamic performance, and add the mechanical details later. For example, given a ⌀70mm boiler, what's the optimum:

• boiler length
• number of tubes
• tube arrangement
• tube outer diameter
• tune inner diameter
• firebox capacity
• firebox dimensions
• firebox grate design

At present, I think all these factors are decided by rule-of-thumb, and although that might have given us the best of all possible designs already, maybe modern engineering methods would result in worthwhile improvements. Especially at IMLEC.

My CAD experience hasn't taken me much beyond basic mechanical engineering. Thermal design is a step too far at my age. Anyone else up for it?

I like the kind of engineering where the designer meets a requirement to cut polystyrene by calculating how much energy is needed to heat a given wire to the correct temperature rather than guessing and hoping for the best! Especially when the same sums lead to hot-wire anemometers and home-made boilers!

Dave

My set would make an interesting study because the drill grinds are all over the place! Down to 1.5mm, the bigger diameters are almost all OK, but as the diameter drops, an increasingly high percentage of the smaller drills are ground asymmetrically. Something like a third of the smallest diameter in my set are acceptable, the rest are junk.

I got value for money out of the set because it was delightfully inexpensive, but sure is an annoying time-waster to have to check drills with a loupe before using them. Not keen to try another set - they're too inconsistent.

Dave

As always a huge amount depends on the requirement. Two extremes:

• Noise transmitted through the floor is annoying neighbours and they are threatening legal action or shoving dog poo through your letter-box. This requires the best anti-vibration measures you can arrange, and it's not simple and is unlikely to be cheap.
• The operator would like to listen to a radio whilst turning, and the lathe is a bit too noisy. Almost anything rubbery under the lathe will help, but anti-vibration mounts and pads will do a better job.

Given the amount of trouble it is to lift a lathe to put anything underneath, I'd be inclined to cough-up for purpose made anti-vibration mounts rather than mess with unknown alternatives. The problem with car-pads and similar is we have no idea what their anti-vibration properties are. Like as not a car pad is too stiff to be much good at absorbing vibration, but the only way to find out is to fit them. Being too soft is just as bad: if the weight of the lathe compresses the pad flat, it won't absorb much noise.

Andrew suggested a noisy lathe might need attention. What's the nature of the noise? Problem might be worn bearings etc. On my Chinese machine the cooling fans are far noisier than the lathe itself. The change gears cause a fair old racket but it's much reduced by spacing them properly. (Not too close or too far apart, roughly a double thickness of A4 printer paper on my machine.)

Dave

It's why precision pendulum technique is so interesting!

I'd be very surprised if any mechanical system outperformed a light beam or magnetic detector. The advantage of the electronics systems is they react orders of magnitude faster than a mechanical trigger, and they don't absorb any of the pendulum's energy. They're much less intrusive.

Far from perfect though! But keeping it in perspective, we're discussing noise, that is small deviations from ideal behaviour, detected by electronic methods. I'm worried not because my pendulum is obviously badly behaved, but because I don't know how much of the noise measurement is genuine pendulum misbehaviour, and how much is sensor error.

Various problems with Fedchenko for perfectionists. He used a permanent magnet on the bob to excite current in a coil as the bob flew past BDC-ish. The resulting current hump triggers an electronic circuit not much different from the optical equivalent. Not clear to me that magnetic noise performance is better than optical noise performance. And a problem having a magnet generate current is that it takes energy from the bob, and replacing it with an impulse disturbs the bob slightly, causing noise!

I don't see technologies as being more pure than others. In my view, the one that does the job most effectively is the best.

Dave

Makes sense to me, except maybe the other way round. I imagine the springs spread the pressure out from the centre towards the ends. The middle of the silver bends first, and the spring action causes slightly less force to be applied at the ends. It would make the die less likely to damage the ends.

Dave

I'm surprised hobby quantity cutting oil is causing a problem, so maybe it's something else.

In the UK at the moment Hayfever, colds, and a new COVID variant are doing the rounds. (Don't panic, the COVID is a mild variant, only causing cold/flu-like symptoms and most of us are still vaccine protected.) And, rarely, some individuals are sensitive to the chemicals. Short or a rebreather, masks are unlikely to help because they catch particles, not fumes.

Unless a lot of hard cutting is done, cutting fluid may not be needed at all. Brass and cast-iron are cut dry, and I cut Aluminium with occasional brushed Paraffin, or sprayed WD40. Cutting fluid is reserved for threading, in tiny quantities.

The main purpose of cutting fluid is lubrication. It helps cool the tool too, but getting it too hot causes nasty fumes. In quantity, a flow of cutting fluid washes swarf away, so it isn't minced under the cutter.

For bulk cutting suds are often more useful than cutting fluid. Applied in quantity, suds cool and wash swarf away. Lubricates too, but that's secondary. Suds is an emulsion of a little soluble oil in water. Water is cheap and an extremely effective coolant. The soluble oil provides lubrication and prevents rust.

In the good old days suds was a serious health risk because nasty bacteria grow in soluble oil, and - before antibiotics - getting it in a cut could end in amputation or fatal sepsis. And it stinks. Modern suds contain disinfectants, but they don't last forever.

As flood-cooling is messy I avoid using it unless a lot of steel has to be machined. And carbide inserts reduce the need for cooling somewhat because carbide can be run hotter than HSS.

Industrially, mist systems are most likely to cause health problems, for which reason they are operated inside a sealed cabinet. Last time I read about it, industry (machine centres) seemed to be moving towards compressed gas systems. Something cold like liquid Nitrogen is sprayed accurately into the cut: the freezing gas blows swarf away and keeps the carbide cutter cool. There is no lubricant. A big advantage is it keeps swarf clean, greatly increasing the scrap value.

How much cutting fluid are you using. Try reducing it dramatically! As for advice if cutting back causes different problems.

Dave

A logical error I think SK : where I come from ignorance of the law is no excuse!

I know what Michael means, and might try it. Or a variation. On a raspberryPi with a camera module fitted, the program libcamera-still allows long exposures. If, in the dark, I open the shutter and flash an LED at the bob once every millisecond, then close the shutter after 1 period, I should get a single exposure showing where the bob is at each flash. The frame rate doesn't matter, because the frame is only read once per beat. Should give a decent measure of actual amplitude because the bob effectively stops at the end of each swing to be photographed.

Dave

Noise not pertinent? On the 11th: 'In the measured scenario here, the rise and fall times are ~3ms (as I am currently seeing with a slit in front of it). That's 3,000,000 times slower (!), just because the pendulum is so slow. Now, as the voltage is very slowly crawling up, any superimposed noise on that signal (I'm seeing a lot) will also be amplified by the op-amp, comparator, etc., and will cause the output to trigger either earlier or later than the ideal switching point (threshold). Over that 3ms rise time, even a little noise will cause the output to trigger earlier or later potentially by many us - which is what we are seeing with the Sharp opto. It may even bounce many times as it crosses a noisy threshold (a latching comparator or Schmitt trigger might be needed).'

Cause of Jitter in my measurements, I don't know - very much still under investigation.

• Previously suspected to be mechanical, but suspension and other improvements didn't reduce it.
• Might be the beam sensor - a hot topic as per this thread!
• A build error due to a CAD mistake. The plastic holder supporting the sensor and electromagnet is too short to allow it to be positioned to break the beam at BDC. Thus the bob is always impulsed late, a known cause of disturbance. (John posted a graph.)
• Possible design error: may be the plastic holder isn't strong enough to resist flexing when the electromagnet is pulsed. Glued at the moment instead of being bolted down, so even if strong enough could be moving slightly on the base when It's supposed to be rigid.

For simplicity I'm an aperture man too! A potential advantage of a lens is they gather more light, if needed. Don't think it is, there should be plenty of signal inside a clock case even from a weedy emitter struggling to poke energy through tightly spaced slits because the beam only has to travel about 300mm maximum. I think a 1mW laser would be detectable several hundred metres away.

Dave

Don't want to start a row, but Card Carrying Vegetarians aren't allowed custard of any type. Custard has egg in it.

Only poachers turned gamekeeper claim eggs aren't really flesh. In my opinion all those who have betrayed the movement should be boiled, fried and scrambled. It will teach them to take a yolk.

Dave

Looks like a dog clutch to me rather than gears, and I have guessed they were 'good enough'.

Is the loose circlip on the shaft top right naughty? Almost certainly meant to clip into the groove above the lower right hand side gears to stop them walking along their shaft. Possibly it not being in place allows the gear to move enough to foul the dogged gear you suspect is worn.

Dave

It appears the sensor in my cheap Arduino module works better than SK's.

The module uses this circuit:

The photo-transistor is decoupled by a 0.1μF capacitor, and so is the supply.

With an oscilloscope connected between ground and the top of the photo-transistor, waving my hand produced this output:

The phototransistor is 240mV above ground
Noise level is about 80mV
Signal level is 1580mV above ground

Testing wasn't straightforward. I did the experiment on my dining table next to a south facing window. Warm and cloudy here and the light level is very changeable today. Even with a curtain drawn, sunshine causes havoc. I think strong ambient IR from the sun floods the unshielded phototransistor from all sides and causes its sensitivity to vary wildly. IR and light sensors have to be shielded, with peculiar results if they're not.

Dave

Dave

Um, I'm not convinced of that at all. Many pendulum clocks have been built with light beam detectors, and others with permanent magnets inducing current in a coil, with similar rise time characteristics. Fedchenko used a permanent magnet / coil detector, and his clock worked quite well!

Most likely something is wrong with SK's experimental set-up. I think the noise level is far higher than it should be.

Having said that, my experimental pendulum is noisy, and uncertainty introduced by the beam detector as described by SK might explain it.

I'm going to measure how noisy the sensor in my cheapo Arduino module in hope I can do it quickly. (I don't have much spare time at the moment and the earlier question about whether of not my data has a normal distribution has dropped me into deep water. Q-Q plots, Shapiro-Wilk and Kolmogorov-Smirnov - yer wot? Also John has sent me a couple of papers to read, and am struggling with the maths. Years ago I attended a class called 'Maths for Scientists'; should have been called 'Last Hope for Boys Too Dim to Understand Calculus'.

If my Arduino module's sensor is as noisy as SK's I'll have to think again. If the sensor is quiet, he will!

Dave

That's what you should get! Although digital calipers have a resolution of 0.01mm, they are only accurate to ±0.02mm. Even the expensive models!

Assuming it's physically in good order - gibs adjusted, no wear or damage etc, the fundamental accuracy of a digital caliper depends on the granularity of the track and the spacing of the sensor, These are easy to ensure during manufacture.

In action, much depends on the operator. First he must ensure the battery is OK, the correct buttons pressed, and the instrument is clean, especially the jaws. Second, he must align the jaws correctly with the object being measured. Alignment is easier said than done especially on curved items, where a rocking motion is needed. Don't forget round items may not be circular and/or could be tapered. Thirdly, the reading depends on thumb pressure, which is an acquired skill requiring a lot of practice and is still easy to get wrong on the job.

With my clumsy paws it's hard to apply thumb pressure consistently; you may be more talented! To get the feel, I practised with a set of precision ground parallels until the caliper always agreed with each parallel's nominal size. This way of learning is a bit suspicious because though my parallels are parallel, they're only specified within ±0.02mm. The feel I've developed could be 'off' because my caliper and training parallels are both up to ±0.02mm 'out'.

Test how good your feel really is! You need a mixed set of precision sized blocks and cylinders, a blindfold, and an honest assistant to read the display. The assistant hands blocks out randomly, you measure them by feel only, and, without comment, the assistant writes down what's on the display. After 15 minutes take off the blindfold and look at the written record. The readings should be slightly off as Mike describes.

In my experience accuracy isn't the main difference between expensive and cheap calipers. Rather, the expensive instruments are smoother (making it easier to apply consistent thumb pressure), and presumably better-made: less likely that the jaws move sideways; more resistant to wear; and sealed to keep moisture and dirt out of the electronics. The feel is better and results more trustworthy, making it less necessary to take repeat readings, and the batteries last longer!

When accuracy matters, switch to a micrometer. For best results micrometers also require a 'feel' to be practised, but the ratchet gets much better results than a caliper. Round stock is still harder to measure accurately than square.

Dave

Be an interesting thing to study. The tubes are cooled by water boiling and the engine consuming steam to do work. If the tubes were 100% efficient, the smoke-box gases would be at boiler water temperature, at 150psi about 185°C. Impossible to be 100% efficient, so the smoke-box will be hotter, depending on how much heat the whole engine loses by radiation and convection.

The tubes cool exhaust gases less efficiently when the blast is on because the increased vacuum causes combustion gases to flow faster, allowing heat less time to pass through the tube wall into the water. Therefore the smoke-box of a labouring engine with blast will be hotter than a cruising engine.

I've an account somewhere of the smoke-box of a full-size engine being seen to glow dim red, which must be at least 410°C. It was pulling a heavy load up an incline after dark.

As model sized locomotives can't be well insulated and have a large surface area compared with their volume, the rest of the engine will keep the smoke-box cool. And as models are rarely thrashed up long steep hills, their smoke-boxes are unlikely to get as hot as a full-size might.

Assuming coal rather than Oxy-Propane, 120°C to 400°C seems a likely range, but I guess very high temperatures would be unusual, perhaps only the result of protracted bad driving.

Dave

Edited By SillyOldDuffer on 12/08/2023 22:15:27

The noise is very high, seems constant and isn't related to the light input, so I suspect the DC circuit. Could the reverse bias voltage be too high causing the diode to break down like a Zener or Avalanche type? It's done deliberately to generate RF noise.

Or has the diode been accidentally forward biased?

Dave

I also came across Estill I Green, (1955) "The Story of Q", a not too mathematical history of Q and its wider adoption.

This comparative table is useful:

and it mentions the relationship between Q and Power Factor, which I hadn't twigged must exist.

Dave