Member postings for SillyOldDuffer

Always useful to explore options. My ER collets are mostly used for tool-holding, but every so often they're good for work-holding as well. In my case, mostly when smallish round jobs are moved repeatedly between machines. I have an ER collet chuck for the lathe, and square and hexagonal Stevenson Collet blocks from ArcEuroTrade:

If I did a lot of collet work, I'd invest in a set of 5C collets.  They aren't restricted to round work only, and can grip shorter lengths.  (ER collets need an inch or so of round to grip on.)  ER and 5C both achieve low run-out.  5C are best for work-holding, but costly.  ER are are cheaper because they clamp over a wide range and can be used on both lathe and mill.   Which is most useful depends on the type of work being done, and in that sense run-out is secondary.

Dave

Edited By SillyOldDuffer on 16/09/2023 11:56:04

Small perfectionist point, as Chris is doing a better than average installation, I suggest clipping the screen pig-tails from the gland and carefully removing the remains:

Terminating the screen wires and tightening the gland can cause a few damaged strands to eventually break loose, perhaps much later after the cabinet has been warmed cooled and vibrated by using the lathe. Tidying up the pigtail and carefully removing the remains reduces the risk.

Another booby trap: check that the gland really is earthed to the box. Metal to metal contact is required and a thick coat of paint is a good insulator. You can guess how I know!

Dave

Like beer, tastes vary! Percival Marshall was fiendishly clever when he named the magazine 'Model Engineer'. 'Model' has multiple meanings; it appeals to engineers who build to scale and to engineers who wish to be exemplary. It's a broad church. Steam Locomotion is the mother lode, but Model Engineering covers everything from astrolabes to x-rays. To me fretting about accuracy is just one of many ways of enjoying the hobby. So is making kit cars, programming a computer or restoring scrap equipment to full working order.

I admit to being suspicious of motives though. Expecting accuracy from a second-hand Gauge Block Set and an antique tenths micrometer is surely unwise. Triply so if the measurements are taken without understanding the rules! And high-accuracy is probably unnecessary. Better than 1 thou/0.02mm is rarely needed in my workshop because I don't mass produce interchangeable spares. My work is mostly old-fashioned fitting; parts made close to size are used as gauges to adjust dimensions until they fit together. Fitting is more skilled and much more fun than knocking out duplicates with jigs and fixtures.

Dave

Ordinary cable for the input unless there's a specific reason for screening it, which is unlikely in an ordinary workshop.

If an EMC problem is bad enough to suggest the input cable needs screening, then a proper analysis of the problem is needed. Fitting a screened input cable out of context costs time and money and could easily make the problem worse.

Dave

I like to think about Requirements before buying anything. In this case, a dirt cheap lathe to 'play with'.

I guess the desire for a Drummond is simply that they are a well-known make, a cut above the many cheaper British hobby lathes sold before WW2. Many alternatives to Drummond if the requirement is for cheap lathe to play with - don't box yourself in. Well-known names tend to push prices up when all that really matters is condition. The availability of spares and accessories is also a concern.

Second-hand lathes sell for whatever people are prepared to pay for them. £200 is a problem because plenty of people are prepared to pay more. At auction stuff only goes for a song if no-one else wants it, and £200 isn't much when others are bidding. Deceased workshop sales are a good bet, because executors are often in a hurry to clear the house to sell it. Not having the time or inclination to sort grandad's much loved tools means the whole lot can be sold very cheaply or even dumped.

My chief regret about getting into the hobby was the time wasted dithering about which lathe to buy. Eventually I decided I was serious and coughed up for a new mini-lathe. I never regretted it. It's vices and virtues taught me a lot. Chief failing was it turned out to be too small for the type of work I wanted to do. Had I bought a WM250-sized machine, I'd probably still be using it. As is I replaced the mini-lathe with a WM280, which is the biggest I could get into my workshop.

I suggest learners are better off starting on a lathe in reasonable condition. Second-hand is risky - the lathe may be too far gone. It's the machine's history that matters, not who made it. Not all owners take good care of their tools, and many lathes were worked hard in the past.

A Chinese Lathe bought in the UK from a reputable supplier may be somewhat rough, but can be sent back if a lemon turns up.

Dave

A 100uF motor start capacitor. One of these. If brave, 260Vac working. If nervous 450Vac working.

Dave

What voltage is the battery?

30a 240vac 36vdc refers to the current carrying capacity of the contacts. A hls-t90 (15f) 30a 240vac 36vdc relay can safely switch 30 Amps at up to 240Vac or 30A at up to 36Vdc. (There's probably an additional 20% safety factor.)

If the battery is less than 30V, a 30Vdc 30A relay will be fine. If the battery is 36v, a 30V relay might arc until the contacts burn out, more likely it will work OK except that the contacts wear out sooner rather than later. You could risk it.

Arcs strike whenever a switch opens and the contacts are damaged if the arc persists for too long. The gap an arc can bridge is proportional to the voltage. However AC arcs tend to self-extinguish because a 50Hz voltage drops to zero every 20 milliseconds. DC arcs persist until the gap between opening contacts becomes too big for the voltage to bridge. Even though DC switches are often spring loaded to open with a rapid snap action, the voltage has to be kept down when breaking DC.

Dave

Got some nervous players on the forum I see. Well, can't disagree with better safe than sorry, but here's the case for the defence: a 230Vac capacitor rating takes the peak voltage into account. Not necessary for the customer to calculate anything. Anyone buy 400V light-bulbs from their supermarket, or are we all happy with the ordinary 230Vac types?

Hard question, in the circuit below, when V=230vac and the centrifugal switch is closed whilst starting the motor, how many volts are across C?

Dave

It depends on the windings. As far as I know there isn't a simple way of working out what the capacitor should be by looking at the plate. To be certain the motor's dataheet is needed or the capacitor's value and working voltage might be marked inside the protective outer tube.

If it were mine, I'd try 100uF 260vac. Make sure it's a Motor Start Capacitor. Many sources, this example is Farnell. Until reliability is proven by not going bang after several stop starts, assume the capacitor might go pop! Although far from spectacular exploding capacitors sometimes spray hot chemicals about. Eye protection, gloves and old clothes.

I had to leave in a hurry after a big capacitor in an old radio blew up and ruined a friend's new carpet. It was clear his house-proud wife wasn't going to take any prisoners...

Dave

Dave

Isolating a pendulum from the environment is difficult. This graph is from my clock's log just after the severe earthquake in Morocco at 2023-09-08 22:11Z. The epicentre is about 2400km from me. Might be a coincidence, but I think it caused my house to wobble.

Dave

The two are identical except the GT has a plain top whilst MT has a chip-breaking edge.

Chips are easier to clear than ribbons by ny experience suggests inserts have to cut fast and deep before the chip-breaker works properly.

Dave

Too hot hot and tired to think clearly about wavelengths, but a period of 5uS is very much in the electromagnetic slow lane - 200kHz. 5nS is still fairly pedestrian - 200MHz. 5 picoseconds is more like it - 200GHz. Visible light is about 400THz

Really interesting question though - what limits the reliability of an IR beam break? Hmmm...

Dave

Neither can I, which is why I said your reluctance is friction, in the sense extra force is needed to get a mass moving. Remanence and stiction are both wrong and I'm not happy with drag. I expect the Greeks had a word for it!

Dave

As I understand it, in the UK the Local Authority is responsible for approving and registering street names, and street numbers or house names. With a new build, the developer should contact them and pay a small fee. After approval the Council tell the Post Office who allocate a Post Code. The Council also approve street-name and street number changes, which are special, and individuals can change the name of their house if it has one rather a number. The street number and post code or house-name and post code must be unique.

My local council's website has all the details, and the charge is about £50.

Personally I think house-names are a risky affectation in the crowded UK. They're only easy to find when the number of homes in the area is very small. And this can change! Isolated named houses often find themselves surrounded by housing developments, with Bide-a-wee Cottage going from local landmark to anonymous in short order.

I have mild address trouble because the road I live on has two street names. One covers houses to the North, which were built in the 1950s, whilst the other is for houses added on the south side 20 years later. Both start with No 1 at the end, so delivery drivers have a 50-50 chance of choosing the house opposite. They usually get it right though!

If there were bother with deliveries here, I'd add my What3Words location to the delivery address. Very precise: it divides the surface of the world into 3m squares. Longleat House front door is "inserted.surprised.risk" and the Souvenir Shop is "painters.blubber.gymnasium".

Dave

Edited By SillyOldDuffer on 07/09/2023 11:51:42

No problem, a forum quirk. I was able to fix it by setting table properties width to 500 pixels fixed. Shouldn't have been necessary: your tables are correct in edit mode, but not when displayed in the thread.

Dave

Formula are models attempting to simulate the real world. They are correct, or at least "good enough" provided their limitations are recognised.

The period formula assumes that the pendulum is rigidly mounted on solid ground, has a frictionless suspension, temperature and pressure are constant; and there are no other external forces in play. My cat is a menace!

Suspending a pendulum from a magnetically attracted roller is likely to cause trouble because, even if it causes the bob to swing isochronously, the suspension is unlikely to be rigid or low friction. (Reluctance is Friction)

The energy lost due to friction whilst bending the suspension scuppered Huygens cheeks, even though the geometry produces isochronous beats:

Easier to build a clock with an ordinary pendulum and run it at low amplitude (red example above). Near bottom dead centre the curve followed by the bob is nearly isochronous. It's a good practical compromise because the amplitude error is small. I believe Fedchenko is the only person to make a satisfactory low friction isochronous suspension. So far!

All practical clocks break the pendulum formula to some degree because real-world engineering is always imperfect. Mathematical models are still useful though. Comparing the behaviour of an engineered solution with that predicted by formulae perfect in theory often reveals where engineering effort on improvements is most needed. Or the experiment might reveal that the formula isn't quite right, and it has to be changed. Practical men are far too inclined to dismiss theory, leaping to the conclusion that a few failed misunderstanding prove conclusively that 'expurts' know nothing. Practical men get stuff wrong too.

The shortcomings of formula as models are the subject of much humour in the scientific community, notably the many variations of the Spherical Cow joke. This example pinched from Wikipedia.

Milk production at a dairy farm was low, so the farmer wrote to the local university, asking for help from academia. ... Shortly thereafter a theoretical physicist returned to the farm, saying to the farmer, "I have the solution, but it works only in the case of spherical cows in a vacuum."

I think the magnetic suspension idea has merit, but it will be hard to make it work.

Dave

My electromagnet is mounted side on so I can pulse the bob at any point after the beam is broken. I had the idea that pulsing the electromagnet as the bob approached top of swing (max amplitude) would lift the bob a little higher by it flying into an area of apparently reduced gravity, thus reducing the jolt. As the strength of a magnetic field weakens rapidly with distance, I thought the grab would accelerate the bob softly.

However, George Airey did the maths about 200 years ago and showed least disturbance occurs when the bob is travelling at it's fastest, ie when it passes bottom dead centre. So I measure amplitude, and if it is below an experimentally determined value, I pulse the magnet on the next beam break, with the beam set to trigger at BDC*. Pulse when is easy, I don't know about a timing a sinusoid to energise a bob..

Dave

Due to a build error, the electromagnet fires after BDC at the moment. Hopefully explains why my pendulum is noisy...

An interim progress report, and some clues about what's going wrong/ Maybe.

Clock's been running for nearly 53 days and is 16 seconds slow at the moment. The rate is wandering:

Looking at myclock vs reality(NTP), I wondered if I was seeing the start of a 40 day cycle. No idea if such a thing exists in nature so I added kilometres from the Moon in a forlorn hope the curves would show an astronomical influence. Nope, and I didn't really expect it to. Don't believe in star signs either.

The wander may be due to a different problem. The clock assumes that its pressure and temperature correction factors are both straight-line graphs, i.e. the relationship between period and temperature/pressure is accurately predictable. These graphs aren't straight. Temperature shows a distinct bad news curve and pressure only becomes fairly straight after starting with an anomalous kink:

Maybe the sensors are faulty - the device is a clone, not made by Bosch who invented it.

Dave

Now the clever people have answered, here's my take.

The curve followed by a pendulum bob isn't isochronous, so its period varies with amplitude. The bob's curve is nearly isochronous at small amplitudes, so most pendulum clocks operate with amplitude less than 5°. This reduces amplitude error considerably.

I can use the effect to decide when more energy is needed. Measuring period with high resolution allows small changes of amplitude to be detected, and used to govern impulses. The goal is to keep amplitude constant whilst minimising any disturbance of the bob. (Over impulsing is causes many problems!) I've tried two strategies:

• Impulsing on every beat, but reducing impulse power until the pendulum just runs reliably. Graphing period at high resolution shows both over and under-impulsing. I have not found it possible to eliminate impulse disturbance entirely.
• Impulsing whenever amplitude drops below a set value. Allowing the pendulum to swing freely for 'n' beats minimises impulse disturbance, but the cost is the extra disturbance caused by the powerful impulse needed to increase amplitude enough to beat free 'n' times.

My first clock ran best with impulse every beat. The second runs better with n about 3. My hypothesis is that:

• the first clock needed plenty of energy to keep it going because it was low Q and crudely built. Amplitude fell so quickly that the best strategy was to impulse it little and often.
• the current clock being better made and higher Q loses amplitude relatively slowly, allowing it to maintain a governed amplitude with less frequent pulses. The best strategy is a moderately powerful impulse every so often.

If I'm right there is no instantly obvious physical law because how best to achieve constant amplitude with minimum disturbance depends on how the pendulum is constructed and what it is running in. A pendulum suspended in air inside a tight box is very different to the same pendulum swinging in a hard vacuum. I guess a well-made pendulum in a vacuum will perform better with 'n' beats per impulse, whilst a competently made pendulum swinging in air might do better with 1 impulse per beat.

I have a notion of applying graduated impulses. When more energy is needed, do a succession of weak impulses rather than one big one. Not done it yet - I'm already up to my armpits in pendulum problems!

Dave

Interesting: that isn't true of the Arduino chips. They pulse the counter/timers at 16MHz, straight off the system clock, so the resolution really is about 62.5nS. Accuracy is less satisfactory.

The Arduino's Achilles Heel is its oscillator. Most Arduinos are fitted with a 16MHz ceramic resonator - rather unstable. The Uno, Leonardo and Pro-Micro all have crystal oscillators - considerably better, but not temperature compensated, so much inferior to an OCXO.

The big advantage of Arduino vs PICpet is that the Arduino can be programmed to do more than just timing - reading sensors, logging to Serial or an SD card, driving the electromagnet, and displaying HHMMSS etc. I expect the more powerful PIC chips could do the same but I fell out of love with PIC way back when it was hard to upload firmware. Since then I've forgotten everything!

Dave