Member postings for Andrew Johnston

XD351 is correct, most off-the-shelf lithium based cells/packs have in-built protection. Manufacturers don't want to be sued if a battery catches fire; even if the user was being a wally! The protection should cover short-circuits and over-discharge as a minimum. They do; because I've managed to do both in the process of designing units that use lithium cells.

The standard lithium charge cycle is as follows; the numbers might be incorrect as I'm too idle to check with datasheets. If the cell is below 3V then the charge is constant current, usually at about a tenth of normal charge rate. Once the battery reaches 3V the charging changes to constant current at the specified charge rate, often around C/2. When the cell gets to around 4V the charging changes to constant voltage until the cell reaches the endpoint, around 4.2V. The end voltage in particular needs to be well controlled, better than 1%. This caused a bit of a shake up at the semiconductor manufacturers - achieving <1% over process and temperature variations required them to up their game.

In my experience cells that have been shut down by the internal protection due to over-discharge do recover, although it may take some hours of charging.

I'm not sure why the batteries in the OP's case are phut. One of the quickest ways to kill a lithium cell is to over-voltage it, but as I understand it the hack charger was a lower voltage than required?

Andrew

I'm glad that people liked the article; no point in writing them otherwise.

In my defence I was only following orders! I have an email from our esteemed editor where he says that the "One Man and His" articles are as much about the author as the machine, so start with a bit about oneself. Which is what I did.

Andrew

Me! I don't do all of them.

But on the other hand I do several that are not shown.

Andrew

Last time I looked at Fourier analysis adding in-phase odd harmonics of reducing amplitude to a sine wave approximated a square wave.

I had a quick go with the simulator; compared to some it is easy to use. I didn't register or login, but I didn't try and save anything. I have an AD login, so it's possible my computer remembered it anyway.

Warning: Skip this bit if maths isn't your thing!

It's interesting the way the simulator switches between filter characteristics without so much as a by your leave. Like all simulators they can be useful if one understands the theory, but if one doesn't then they can lead one into a cul de sac. Via Wikipedia I've just read the original paper by Butterworth from 1930. Of course that was long before opamps and RC filters, so everything is valves and L, C and R. The intent of the original paper was to design filters that were maximally flat in the passband without compromising the roll off. In other words the amplitude characteristic is monotonic and flat in the passband, which it appears previous filters were not. It's interesting to note that the original article only refers to second order sections. The concept of single real poles isn't mentioned, so the original Butterworth filters could only be even order.

In order to understand filters an appreciation of the complex s-plane, and poles and zeros is useful. On the s-plane the x-axis is sigma, a measure of how a signal decays and the y-axis is j times omega, complex frequency. A pole is a point of inifinite value and a zero is just that, zero. If a rubber sheet is stretched over the poles, and nailed down at the zeros, then a section along the y-axis will give the frequency response of the filter. There are some constraints, for stability all poles must be in the lefthand half, ie, sigma is negative. Poles always come in complex conjugate pairs. A special case is when omega is zero in which case the two poles are coincident, and real, on the x-axis.

The filter simulator starts off with Bessel filters, which are maximally flat in group delay, ie, signal distortion in time is minimised. This is at the expense of roll off. Pole position is determined from Bessel functions, hence the name. As more roll off is required the program switches to Butterworth which as stated is maximally flat in amplitude in the passband. So no amplitude distortion and faster roll off, but at the expense of non-linear group delay giving time distortion of the signal. A Butterworth filter only has poles, and those poles are equi-spaced, and lie on, a circle. As yet more roll off is required the filter characteristic becomes Chebyshev, which allows ripple in the passband in return for faster roll off while still being monotonic in the stop band. Technically these are type 1 Chebyshev fillters. Type 2 Chebyshev filters are flat in the pass band but have ripple in the stop band - never seen them used. A type 1 Chebyshev filter is also pole only, and the poles lie on an ellipse. In the limit as the ellipse becomes a circle the passband ripple reduces to zero and the filter becomes Butterworth. Although not used in the simulator the fastest roll off is with an elliptic filter that has poles on an ellipse giving ripple in the pass band and zeros on the y-axis giving ripple in the stop band.

It took me a while to find the design where the capacitor values are uniform; in the multi-feedback circuit. The Sallen-Key arrangement is the classic filter circuit mentioned in all text books. It's ok but can have problems implementing high Q sections. There are better, but more complex, circuits available such as the biquad.

The tolerance feature of the simulator was useful, and very quick. Not sure how it worked, seemed a bit fast for a proper Monte Carlo anaylsis?

I had to look up the Linkwitz circuit. Looks a bit odd with T-sections in the input and feedback paths. It's not high enough on my priority list to do a proper analysis. In the past I've used Tina from Texas Instruments as a simple circuit simulator. I expect it is based on Spice internally but has a graphical input interface. Ah, I see Kiwi Bloke is using Linux, in which case he's probably out of luck for a simple simulator.

I'm with Bob Pease on circuit simulators. They have their uses, but are only as good as the component models. Ultimately prototyping trumps the theory and simulation.

Andrew

I've just got my issue of MEW286 in the post. Unfortunately there are some errors in my article on the Bridgeport mill.

Someone has added a short paragraph at the beginning. Unfortunately it's incorrectly punctuated and not well stated. Later in the article Adcock is mispelt. That's my fault, as the error also occurs in what I originally wrote. For photo4 (showing the milling head) I supplied a photograph carefully annotated with the positions of all the controls. As requested I also supplied an unannotated copy. Sadly that's the one that has been used, which makes it difficult to follow some of the article unless one is familiar with the Bridgeport miil. In which case one wouldn't be reading the article anyway! It's all a bit disappointing.

Andrew

Soooooo, we should be offering ill-considered advice based on things we know nothing about?

Andrew

Not really, if I can do it I'm sure you can. It's just three angles and as long as you understand why the angles are the way they are (the precise values are not critical) everything should be fine.

I've never really understood the obsession with grinding jigs. While I have the wherewithal to accurately shape HSS if needs be almost all my HSS tools are shaped freehand, not even using the piddly rest on my bench grinder. I use HSS exclusively on my repetition lathe, and that's a production machine, so the tools get run hard.

Andrew

I wouldn't get a set of any sort of cutters. There'll be some you never use and quality isn't a given. That's another mistake, buying cheap because you're a beginner. That way lies frustration. Personally I'd buy some HSS blanks, a cheap bench grinder, and experiment. Grinding a basic lathe tool is simple, just three angles and you don't need to hone it within an inch of its life for it to work.

For aluminium the CCGT polished inserts are useful as they reduce the likehood of aluminium building up on the tool. Likewise inserts may be good for stainless steel. But both can be turned with HSS tooling.

You don't say what size the lathe is, but drilling 22mm holes, certainly in stainless, going to require a lathe with some grunt. At those sorts of diameters you're much more likely to be using a boring bar after drilling a smaller hole.

It also helps to say where you are, a lot can be achieved by a short face to face meeting and practical demo.

Andrew

In both cases the metal is removed with a shearing action, it's simply a matter of degree in terms of speeds and feeds. I get swarf that looks the same irrespective of whether it's cut with HSS or carbide inserts.

Silver steel can be cut with HSS tooling. These silver steel taps were roughed out with carbide, but the threads were cut with a HSS tool and the flutes were milled with a HSS form cutter.

Andrew

My best advice would be don't bother.

Your lathe simply isn't going to have the power or speeds to make best use of insert tooling on stainless steel. Even on industrial lathes the finish is highly dependent upon the type of stainless, assuming that we're talking austenitic here. Grade 303 turns well as does 316, but 304 is horrid stuff. Even then getting a good finish can take some experimentation in terms of speeds, feeds and coolant or not.

As an aside stainless steel seems an odd choice for pivots? I thought pivots normally used pivot steel, ie, hardened and tempered carbon steel.

Either way you should be able to cut both materials with HSS tooling without any problems.

Andrew

Ooops, forgot to mention methanol. In the mad days of working in motor racing we did a lot of work on Indy cars, which run on methanol. Of course unlike the Merlin the engines were small capacity, high compression, fast revving and turbocharged. At the time, early 90s, the engines were routinely running at 18.000rpm and testbed engines were pushing past 20,000rpm.

Methanol is horrid stuff. It's more volatile than petrol, is more poisonous, especially to the optic nerve, and corrodes aluminium in high concentrations. It also burns with an invisible flame, so the first you know about a fire is things getting hot.

My first project was a fuel gauge for an Indy car, which inovolved much experimentation. The H&S lady had a blue fit when she discovered I had a 40 gallon drum of methanol in the wooden garage behind the offices and was pumping it back and forth to a real fuel tank in a fixture that allowed the tank to be tilted in two dimensions.

Andrew

Pay attention there at the back.

That's not what I wrote. If you run a motor in star and at a phase to phase voltage of 240V instead of 415V then there will be a loss of power compared to running at 415V. That's why when running from an inverter that gives 240V phase to phase you need to change to delta to avoid the power loss.

Andrew

One big reason for staying with petrol was supply. Oil refineries were set up to produce petrol and supply lines were adapted to deal with petrol. It's no good having a fuel that produces a higher performance if you can't get the fuel to the user in quantity. For road vehicles in particular the ability to run on any old fuel is a big advantage when you're advancing rapidly across a battlefield.

One must also consider the technology of the engines. Even the Merlin is a large capacity, slow revving and low compression ratio engine. Much of the increase in output power in WWII was achieved by more, and better, supercharging.

Bill is more or less correct. Basic fuel was 80 octane, dyed red, not just for road vehicles but aircraft too. When I was learning to fly on Tiger Moths we ran them on 80 octane fuel supplied by Carless, sadly no longer available. Higher performance engines ran on high octane fuel, known as 100/130. Basically 100 octane, but 130 octane when used in a supercharged engine. The octane rating was boosted by addition of tetraethyllead. Towards the end of WWII some engines could run on 150 octane fuel for increased power with high boost pressures from the supercharger.

The modern certified aviation fuel is 100LL, ie, 100 octane and low lead. It is dyed blue and is better controlled, with less volatiles, than road fuel. When I started flying the Tiger Moth at Thurleigh we ran it on 100LL and it didn't like it! We had at least one engine failure attributed to the fuel where a cylinder dropped a valve seat jamming the valve open. Once Mogas was approved we ran it on 2 star, if anyone remembers the star rating. We used to collect fuel 250 gallons at a time from the local village garage by towing a tank that was welded to a WWII bomb trolley chassis with the wheel rims modified to take Mini wheels inflated to 50psi. Dunno if any of the local coppers ever saw us, but if they did they probably decided not to get involved due to the paperwork that would be involved.

Andrew

Must be an old motor. The rating plate certainly implies it's a dual voltage motor, but the connection box isn't what you'd expect. I'll leave it for someone with more knowledge to suggest a wiring arrangement.

Incidentally a star connected motor will run just fine off an inverter producing 3-phase at 230V phase to phase. The issue is that the phase currents, and hence torque and power, will be down by a factor of 1.732, aka the square root of three.

Andrew

That's false economy. Small lathes, and the inexperienced, need all the help they can get. Using cheap cutting tools often just leads to frustration. As does the use of scrap material of unknown composition. It's not uncommon for threads to appear on the forum along the lines of difficulties with cutting and getting a good finish. Many of the probems are solved by changing to known material and quality cutting tools.

Andrew

Full size practice is to use rivets, which is what I will be doing when I get round to the tender. I will be making my own section, curved but not half round, from hot rolled steel and bent hot around the various cut outs and bends on the tender.

Andrew

Not in mine! At primary school we had a chart on the wall covering all pupils with coloured stars for each table recited. I was always at the bottom, and never did complete up to 12 times table. My argument was that recitation didn't equal understanding, which was more important. Of course I was also bone idle and stroppy. Amazing really that I are an engineer.

I work quite happily in both metric and imperial, but no-one is ever going to serve me a beer in anything other than a pint glass.

Andrew

For roughing I might leave 0.5-1mm of stock and use a tolerance of 0.2-0.5mm. Even so my CAM software sometimes seems to faff around in places with teeny cuts.

Andrew

As well as the issues mentioned by Jason there is also the matter of how good the software is at fitting a series of straight lines into a circular movement and how well the machine follows them. In my experience an interpolated hole can be anything from 0.02mm to 0.1mm out of round depensing upon size, material, and feedrate. I think Tormach did a video some years ago on hole accuracy and concluded that if you need an accurate, and round hole, use a boring head.

Andrew

Good grief, it all seems way over-complicated for a simple non-precision part. Techniques need to be appropriate for the level of precision needed.

I made this as an exercise some years ago to prove a long forgotten point on the forum:

It was made in less than 15 minutes using no measuring equipment, not even a rule. Everything was done by eye. Precision where precision is needed is good, but precision where it isn't needed is simply a waste of resource.

Andrew