Member postings for Andrew Johnston

Oooops, looks like I was getting confused between engineering and modelling regarding the steam chest.

Andrew

If you do the maths you'll find that the size of the steam chest does matter. It acts as reservoir to minimise pressure drop when the inlet ports are open. Of course if the supply pipe from the source to the valve has a much larger area than the inlet port then the size of chest is less important as the supply pipe takes over the reservoir function. But the pipe might look a bit non-scale.

Andrew

I started off with covers on the lathe and vertical mill, but stopped using them fairly quickly. Unless there is a heater under the cover they just accentuate the rust problem by stopping air from circulating. I spray all the machines with WD40 now and again, and the slideways are oily anyway. I don't generally have a problem with rust. My workshop is fairly well ventilated as there's a gap top and bottom on the garage door due to cowboy installation.

Andrew

+1

Almost certainly something wrong with the tool or setup. The only caveat is what type of brass? The most common type, CZ121, should be easy with carbide or HSS, but some alloys can be problematic.

Andrew

It's sad that yet again I have to wonder why a new member thinks it's necessary to insult forum members. The OP has broken the old adage - it's better to remain silent and be thought a fool than speak and remove all doubt.

Andrew

Yes, it's a bit of an overstatement; the theoretically correct value is 153lbs.

I suspect the question doesn't do what you expect. If the interviewee doesn't know the density of air then any value given will be a pure guess. After the values are given it is an exercise in simple arithmetic. Some of the better interview questions involve the design of something. It allows the interviewer to see how the problem is approached, what extra information is asked for and what ideas arise.

The question on power output is incomplete, for how long is the output to be sustained? For an hour or so I'd expect 75-100W, longer then nearer 50. But it will be highly dependent upon the individual.

Andrew

Me neither, far too many vices.

Andrew

in the early 1970s, when it looked like my school career was going to end in a major car crash, an apprenticeship was considered, at W H Allens in Bedford. They made steam turbines, diesel engines and pumps. My father knew the apprentice master through the IMechE so we visited one Saturday morning for a factory tour and I informally sat the entrance test.

Ultimately I didn't follow the apprenticeship path and also ignored the advice of my school not to go to university. In both cases that was a good choice. No idea where I'd be now if I'd followed the apprenticeship route. Certainly not at Allens as within a few years they'd gone out of business, sold the factory and it's now a housing estate. I'm glad I followed the academic route which gave me experiences an apprenticeship wouldn't have been able to do. I picked up the practical skills anyway starting as soon as I could hold tools. I had a small workshop with a lathe and pillar drill while I was at school and it's progressed from there. It varies from year to year but I make about 20% of my income from machining, usually in conjunction with some mechanical design work.

I've always been amazed by the questions asked in interviews. I've been asked to draw an inverting and non-inverting opamp! One of the more intelligent questions was describe a microcontroller, microprocessor and DSP, and discuss where each one would be used. Another was discuss the range of TTL families and talk about cost versus performance trade offs. Another was about metastability in digital circuits. Of course i knew what it was and the consequences, but the point of the question was to kick off a discussion about how to characterise it and how that could be used to minimise the problem in the design phase.

As for the weight of air question that was simple as I know the density of air at STP.

Andrew

Been there, done that, in most companies where I've worked.

It takes a certain type of person to be a contractor, flexibility is key. Contractors are a bit like immigrants. You get the restless mavericks at one end who do excellent work in short bursts and then move on (that's me) and the unemployable losers at the other end who want it all but aren't prepared, or able, to work for it. What you don't get are the middling, but stolidly competent, people who are happy to plod along doing the hours and getting paid a steady but unspectacular salary.

When I want to annoy a millennial I tell them I got paid to go to university. I did a thick sandwich course with MoD. A year at RAE Farnborough, three years at university (working in MoD or their suppliers during the summer) and a final year working in MoD establishments with the aim of joining the PTO grades, ie, chargehands and up. At university I didn't get an LEA grant, but got paid a regular salary. And since the university was a long way from my home base (RAE Farnborough) it was classified as detached duty so I got travel allowances. Even better MoD would pat half the cost of any textbooks on the official list. So I bought all of them on the grounds that when you really needed them (at exam time) the university libraries would have dozens of students fighting over limited copies.

Andrew

LEA = local education authority

PTO = professional and technical grade as opposed to the parallel scientific grades

What the point is of joining a forum and immediately denigrating members is a mystery to me.

Those of us who have to work for a living get paid what our employers, oe clients, think we are worth. Whether one thinks that rewards the skills we have is another matter. If we think we are underpaid there are two choices. One, do something about it, like expand skillsets, ask for a rise or find another job. Two, sit back and moan.

Andrew

Oooopsie, and it wouldn't be the first time.

Andrew

My M300 sits against a wall with the headstock end next to the garage door. I open the gear train once a year for oiling. In 20 years I've never needed to change the gears, and I do a lot of screwcutting. Likewise I've only opened the electrical cabinet a few times to make adjustments. There is about a foot between the end of the lathe and the garage door. So enough room to poke metal through the headstock. If I ever needed to turn something longer than 4 or 5 feet, or properly access the electrics, I can always open the garage door. I don't recall ever having removed the tailstock.

Andrew

Cross-sensitivity is an ongoing problem. There is some evidence that it can be reduced by altering the hotplate temperature for different gases. The sensors we work with have sening elements consisting of a mix of transition metal oxides.

Andrew

We hold some patents on metal oxide gas sensors and designed a circuit some years ago that would allow AC interrogation of same. There is some evidence that additional information can be obtained by looking at the complex response of the sensor rather than making a simple DC measurement. We have a business tie up with a company that makes gas sensors, including metal oxide. Our current products use electrochemical, NDIR and PID sensors and an optical particle counter.

Andrew

Yep, been self-employed, as a sole trader, for the last 20+ years, and for short periods before that in between working for a variety of companies ranging from engineering and scientific consultancies to electric vehicles and high end motor racing. I've also been an academic doing research, mostly mathematically based. So, in the gospel according to SoD I must be practically useless. Also a director of a small company doing gas sensing equipment for air quality and medical research.

Andrew

That must be one of the daftest statements ever made on this forum, and there's been some pretty stiff competition.

It presupposes that an engineer with academic qualifications is, ipso facto, not practical. Of course some aren't, but conversely some are. The reverse is also true, some practical people are hopeless at basic arithmetic, but others are good.

The best, and best paid, people in both categories are those that are comfortable with both aspects.

Andrew

The few carbon steel taps I have bought are for ME threads and are only used on copper alloys. I do a lot of machine tapping on the repetition lathe and the vertical mills. Consequently I tend to buy machine taps (mostly spiral flute) in preference to the traditional sets of three hand taps. If I do buy hand taps I only buy taper, and bottom in case of blind holes. Of course machine taps can be used perfectly well by hand and are often easier to use as they have better swarf clearance.

The original question is unanswerable as no context is given. So it's impossible to say X is better than Y, without even considering Z.

Andrew

As standard I use 0.4mm. The 0.2mm is a bit delicate so I use it only when I need a smaller fillet. The 0.8mm inserts get used for roughing.

Andrew

Correct, there will be a main winding and a start winding connected to the capacitor. The springs will be associated with a centrifugal switch which disconnects the start winding once the rotor reaches a set speed. Connecting a VFD as suggested will not work. As discussed VFDs create a 3-phase output, so the phases are inherently 120° apart. Since the output is 3-phase there is no neutral connection, so you couldn't run the windings phase to neutral, only phase to phase. Many industrial systems do not use neutral. All my ex-industrial machines are connected with 4 wires - 3 phases and earth.

Furthermore the outputs of the VFD are not sine waves, but are pulse width modulated signals where the pulse width is changed cycle to cycle to simulate a sine wave.

Andrew

Edited By Andrew Johnston on 06/07/2021 15:53:59

Just to clarify; a VFD can be single phase or 3-phase power in, but is always 3-phase out. So a VFD can be used to run a 3-phase motor from a single phase supply.

A single phase induction motor is not self-starting as there is no rotating magnetic field. So capacitors/start windings are added to give the motor a nudge on start up. If a single phase motor is run on a variable frequency the parameters of the capacitors and extra windings will be incorrect. Single phase motors are one huge fudge.

On the assumption that a true 3-phase supply is not available I'd use a 4-pole single phase motor along with a countershaft and belt drive.

Andrew