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Member postings for Kiwi Bloke

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

Thread: diameter calculation
13/10/2019 06:05:02

SOD. ' In practice, dividing a circle isn't an accurate operation, in fact the best way to do it is to calculate the tangent to as many places of decimals as needed, and then to lay out the angle over a large baseline. Dividing Y by X is the tangent of the angle, and tangents can be calculated to as many places of decimals as reqiired.'

I don't agree that tan calculation is a good way to divide, because as X tends to zero, the calculation becomes difficult to handle in practice, as the tan tends towards infinity.

I think the key is 'in practice', especially when engineering, rather than theoretical maths is concerned. In practice, we would use whatever hardware we have at our disposal to do the job. Polar to Cartesian translation makes sense if our machines can only cope with rectilinear movements, but when a rotary table is available, or something like a BCA or Boley UFR milling machine can be used, it would probably be sensible to use polar co-ordinates, if that's how the job has been specified. The accuracy we can achieve in practice is far less than the calculation accuracy achievable by cheap scientific calculators, so in practice, translating between co-ordinate systems isn't going to damage accuracy. Theory is another matter...

13/10/2019 05:48:00

'My goodness what a waste of time. The OP just wanted to know a shop grade calculation (and got one early on) .'

So the thread served its original purpose, and quickly. Now it's moved on to an intellectual discussion about other, but still related things. What's wrong with that? Whose time is being wasted? If a few old codgers wish to spend time in a virtual pub, close to a virtual fireplace, bouncing ideas around, should anyone complain? If this forum confined itself to answering questions, it would be a dull place indeed. I have posted to ask questions, to inform, to challenge, to entertain and to get people to think. I don't intend to stop. Naturally, not all of my posts will have been appreciated by all watchers. That probably goes for all other posters too. Too bad. I hope the thought police aren't welcome here.

12/10/2019 01:18:44

Blowlamp: 'I would far rather make a division plate using a DRO on a milling machine than use a rotary table or dividing head if I wanted the best accuracy.'

I wouldn't, because the problem is effectively specified in polar co-ordinates, so errors are introduced by converting to cartesian. Plus, it's far more likely to suffer from operator error (well, at least in my case...).

Thread: Resistance Soldering question
12/10/2019 01:10:32

Patrice, just how restrictive is your house insurance? I can understand acetylene being frowned upon, perhaps also big bottles of propane, etc., but are you allowed a gas cooker, barbeque, gas cigarette lighter (and is it size-limited?), paraffin (kerosene) blowlamp, spirit blowlamp, jeweller's spirit lamp (or candle) and blow-pipe? Just trying to think of other sources of pre-heat...

I have done naughty things in the past with welding transformers. You might be able to get the job done by making very good electrical contact with the small and large part (thus avoiding local over-heating), so the current flows across the joint area. Electrical resistance here will be high, relative to other areas, so will heat up. It might be quite spectacular, because you're less than one step away from an improvised spot-welder (which is another avenue to explore...), so current-control would be a necessary addition. A 12V car battery may also work. I think you may be approaching suggestion-overload... Apologies for adding to this, but it's an interesting problem, and it's making people think!

Thread: diameter calculation
12/10/2019 00:45:39

Duncan: 'But if you work out the diameter for 3 holes on a pitch circle I suspect you get an irrational number whichever way, it's most unlikely to be an exact number'

Most unlikely, yes, but there's a couple of solutions that spring to mind that are rational. I'll leave it as a teaser, for others to find...

12/10/2019 00:29:10

This has got complicated and no doubt I'm out of my depth...

It seems to me that, if the problem is specified in Cartesian co-ordinates, such as could be the case for absolute hole positions, then Cartesian co-ordinate geometry is the way to go, even for problems involving positions on a circle. It's all done without Pi or trancendental functions poking their awkward noses in. However, if relative positions are specified, in terms of lengths and angles, then polar co-ordinate geometry is appropriate. Perhaps that's what Michael and Andrew are saying... For practical workshop applications, especially with simple co-ordinate tables and DROs, Cartesian rules, doesn't it?

11/10/2019 11:50:41

Michael: 'I think it's useful to remember that this is an example in polar geometry'. You mean 'R - theta' geometry? Not really, although I agree a bit of construction is required to get to this, 'sine-formula'-like answer. Co-ordinate geometry is good, if only because it's easy to transfer that approach to machining co-ordinates.

Thread: Tingling from Myford Super 7
11/10/2019 11:45:15

Don't focus all attention on the lathe. I suspect that it's innocent, and just allowing you to experience the fault, which is likely to be common to all power sockets in the house, because the house earth is not good enough. It's potentially dangerous - get the house earth checked!

Thread: diameter calculation
11/10/2019 11:37:45

Dave (S.O.D), look up 'sine formula'. Not all trig needs right-angled triangles...

Thread: Tingling from Myford Super 7
11/10/2019 10:50:29

Colin, it's good to hear that you escaped electrocution. I'm worried that there may still be a problem with your wiring.

If the insulation between live and earth had partially broken down, I think that your house earth must be poor, if earth wires' voltage was 'pulled up' enough to cause a tingle. You were, in effect, acting as a better earth (lower impedance) than the house's earth. I suggest you get the earth impedance checked.

My early childhood was in a house which was wired exclusively (as far as I can remember) without any earthed sockets - they were all two-pin. It was a 'normal' to get a tingle or a buzz, when a metal-cased appliance, such as an electric fire, was brushed against. Also, child-sized fingers used to be able to touch the plug pins when inserting or removing plugs, with expected shocking consequences. I think I'm lucky to have survived - along with millions of others...

Thread: Resistance Soldering question
11/10/2019 10:30:54

I apologise if what follows is rubbish, but I haven't looked up details about your particular resistance soldering equipment.

My limited knowledge about this technique is that the heat is produced in a resistive carbon 'lump' that is held onto the surface to be heated. Heat is transferred from carbon to workpiece, therefore there should be good thermal contact, to avoid local over-heating. Current flows across the interface. I think there may be another type, where current flows through the carbon 'lump' only. Either way, it looks as though your workpiece is getting locally over-heated, whilst the bulk of the workpiece remains cold. In other words, too much temperature, not enough heat. It may be possible to reduce local overheating by slowing the whole process down (lower current), to allow the temperature to build up in the large heat-sinking workpiece.

If current is flowing across the carbon / workpiece interface, over-heating here should be reduced by ensuring the electrical resistance at the interface is minimised. This means getting the surface area of the interface to be as large as possible and making good contact all over. Yes, this means shaping the electrode, however unpalatable.

Can you change the order of assembly so that this awkward joint is made early, when you could, for example, cook the whole thing up in an oven, without fear of melting already-soldered joints?

Thread: diameter calculation
11/10/2019 10:06:06

I'd do it using co-ordinate geometry. Not sure if this is what Andrew is suggesting. I'm a lousy mathematician, and I bet there's a far easier way (no, not CAD).

The perpendicular bisectors of two chords intersect at the circle's centre. You can easily find the co-ordinates of the mid-point of a chord and its slope. The equation of the bisector of the chord follows, straightforwardly. It's then a few small steps to solve the two equations of the two chord bisectors, to get the co-ordinates of the centre. The diameter is the distance from the centre to any of the three points. The equation for the circle follows.

Incidentally, the radius of the circumcircle of a triangle, R = a / 2 * sin A, where a is the length of the chord opposite the angle (vertex) A. Sometimes useful...

Oh, Nick just beat me to it...

Edited By Kiwi Bloke on 11/10/2019 10:07:27

Thread: Creating Heat for Hardening
08/10/2019 11:16:27

X-Acto blades are pretty hard. My guess is that your cheapo chisels are made from chinesium and are not heat-hardenable. You could probably case-harden them, however. Or buy good'uns...

Just for info. - the Curie temperature is that temp at which magnetic properties are lost. Conveniently also the temp you need for hardening. The 'magnet test' (above) detects that temperature, far more reliably than misleadingly-named colours.

Thread: Tingling from Myford Super 7
08/10/2019 11:07:31

Well, I'm puzzled too. The 'tingling' implies a.c., so energy stored in the motor's start capacitor can't be responsible. It was well-known, to schoolboys, decades ago, that a torch bulb could be connected between neutral and earth, and would glow, variably, providing free light. This demonstrated that the neutral line floated around a few volts from ground potential, but nothing like enough to cause a tingle. The wall switch should switch the live line. It's possible that it has been installed so that the neutral is switched instead. Surely, if the wall switch is OFF, and assuming it's correctly switching the live line, there is either a persisting high-impedance connection to the live line (because the switch is faulty) and a poor earth, or stray coupling to the neutral line and a poor earth. If you have a high-impedance voltmeter, with the switch OFF, can you detect significant volts around the machine, between L or N and earth? And can you do the same test using a known good earth?

I have seen a domestic light switch in which the insulation between the contacts had carbonised, allowing a small current to flow across the open contacts.

Never trust earth connections!

Edited By Kiwi Bloke on 08/10/2019 11:07:57

Edited By Kiwi Bloke on 08/10/2019 11:09:37

Thread: Creating Heat for Hardening
08/10/2019 10:45:37

Well, it depends on what sort of cherry we're talking about. A common error when hardening is to not heat to a high enough temperature. The correct temperature (for silver steel) has been described as 'cooked carrot', considerably hotter than morello cherry. Food-based terminology is colourful but unreliable and ambiguous. Safer to heat to the Curie temperature - it's objectively testable.

Thread: Resistance Soldering question
06/10/2019 07:18:49

Hi Patrice. Wow! That's impressive craftsmanship. Perhaps you could find time to describe some of your methods - I'm sure many of the forum members would be interested. I'm sure you'll receive a warm welcome here.

As to your problem: It seems that the 'larger piece' isn't going to be easy to heat from what is almost a point-source of high temperature, but not much heat energy. The piece will act as a heat-sink. I'd imagine that you will need to deliver considerable heat (rather than temperature) to it, possibly over a large enough area to avoid local damage, and this sounds like a job for a small blow-torch. If the 'larger piece' can be brought up to near-soldering temperature, the resistance soldering equipment may be able to complete the job. Sounds like it may be a three-handed job though...

Thread: Mechanisms in modern engineering design Artobolevsky
05/10/2019 22:03:01

I have have the complete set of seven printed volumes. If anyone's wondering what they're missing, Vol 5 pt 2 contains Section 33, Simple Electric Mechanisms, Section 34, Lever-type Electric Mechanisms, Section 35, Toothed Electric Mechanisms, and Section 36, Complex Electric Mechanisms.

I'm puzzled as for whom this work was intended. Like the other volumes, there nothing about the mechanisms' underlying fundamental principles of operation; instead, it's a collection of increasingly elaborate, but very old-fashioned and often laughably inelegant mechanisms, often with difficult-to-follow explanations and GA or isometric drawings of the whole shebang, presented in no obvious order. Perhaps these were the 'go to' reference volumes for 'designers' working in a regime where thinking for one's self was dangerous. Perhaps these are the Party-approved design approaches.

I particularly like 'No. 4737, Electrohydraulic Window-opener Mechanism for an Automobile'. It operates four windows and one partition glass, using hydraulic cylinders, scissors mechanisms, 'powerful springs', an electric motor driving a gear pump and electromagnetic valves controlling the flow of 'brake fluid'. In spite of the powerful return springs, the pump motor is reversed to lower the glass. It seems that occupants of the vehicle (Party officials and chauffeur only?) would have to agree amongst themselves whose turn it was to move a window, since opening one whilst closing another is prevented.

'Thermal bimetallic strip relay 21 and reversing and interlocking relay 9 are provided in the circuit for remotely switching motor 6 on in either direction to raise or lower the window glasses and to protect the system against simultaneously switching on motor 6 in both directions.'

Thread: Filter Design Wizard
03/10/2019 10:11:56

Thanks Russell, I'll look up QUCS. Thanks Andrew for your input - I wish I understood the theory...

Having somewhat hijacked this thread, I've felt obliged to try to follow some of the suggestions - make some effort to solve my own problem. Crikey! The maths is frightening, but not as opaque as the underlying concepts hinted at by Andrew J. OK, I think I should give up, my poor aged brain won't cope with going off in such a complex new direction. Current mental activity is centred on wondering how to manipulate a CertiFlat welding table kit: not disturbing the fit of the components whilst invering it for final TIG welding. It will weigh >100kg.

Before I give up on this filter thing entirely, however, the problem is (if I remember correctly) that the circuit contains various Sallen-Key-like filters, built around op-amps. The usual implementation of these seems to use two same-value Rs and two same-value Cs. It looks as though this may be because it simplifies the maths. The schematic uses different values for the Rs, and I wondered why. I can't remember whether the C values were identical, and now I can't remember where the wretched schematic is. The perils of working from memory...

Thread: Pulley material
03/10/2019 09:42:46

Is the 'yellow coating' in fact passivated cadmium plating? Not the best anti-rust treatment, unfortunately.

Al-alloy pulleys may be OK, but I have seen enough cases where moisture in the belt has caused the metal to corrode, leaving white material on/in the belt surface - presumably abrasive aluminium oxide - and pitted pulleys. This may be less of a problem with modern 'cut' V belts. Just make sure you get a corrosion-resistant alloy, if you go down this route.

Thread: Drill running off course
02/10/2019 11:26:33

SOD said "A starting hole rather less than ⅓ the diameter of the main drill seems about right. (Do others agree?)"

No. Or yes - it depends on the length of the chisel point relative to the diameter...

When a larger drill is fed into an existing smaller diameter hole, what tends to happen is that one cutting edge 'grabs', stops moving, and acts as a pivot, around which the drill orbits - for part of a rotation. Then, the 'pivot' lets go, and the other (or the same) cutting edge grabs again, and the cycle repeats. Eventually, things tend to settle down, but lobed holes are often produced in thin sheet, where the 'settling down' can't happen, because the side of the drill may never engage properly with the side of the hole. This is perhaps the most obvious manifestation of this phenomenon, but 'orbiting' drill bits can be observed when drilling from the lathe tailstock, as they engage an existing hole in the workpiece. In this case, it may be helpful to employ the very old trick of starting the (follow-up) drilling by holding or restraining the free end of the drill, using a bit of metal in the toolpost. This temporarily deflects the point of the drill, which then acts as a boring tool, with only the rearmost cutting edge initially engaging. (Try to set the drill bit's cutting edges horizontally).

This phenomenon varies with the drill stiffness and the relative diameters of the starting hole and the following drill. I suggest the best size of a starting hole is the same diameter as the length of the non-cutting chisel edge of the following drill bit. This, of course, assumes the machine is powerful and rigid enough. The often-advocated idea of using a series of bits of increasing diameter is just asking for trouble.

120 degree, four-facet spotting drills make ideal starting dimples and four-facet drill bits (no chisel point) make drilling so much more civilised, often allowing drilling without the need for any centre-pop or spotting drill dimples.

Having said that, millions of holes have been drilled successfully using all sorts of techniques, many of which are passionately held to be the best...

Edited By Kiwi Bloke on 02/10/2019 11:27:05

Edited By Kiwi Bloke on 02/10/2019 11:28:36

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