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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: 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

Thread: Filter Design Wizard
30/09/2019 11:40:47

Thanks Neil and Joe. I need to clarify. I think I need something that plots the frequency response, given the component values, rather than a component value calculator. The schematic does not give the intended filter parameters, just the component values, and these seem strange. From memory, pairs of Rs and Cs are supposed to have identical values, but not in this schematic, so I'm wondering why... Sorry to be vague, but electronics isn't my field, and I'm, perhaps rather cheekily, only posting because the original post piqued my interest.

30/09/2019 10:28:55

Interesting. Anyone know of a simple to use* online simulator/analyser that can cope with a 'Linkwitz transform' filter? I have a schematic with some funny component values - different from what the text-book formulae suggest. Not sure whether the differences are typos or intentional. The circuit's designer is not available for interrogation. I'm using Linux, hence the request for an online app.

* I'm far too lazy to learn how to use a Spice clone.

Thread: Cutting tools - what type is most suitable?
30/09/2019 10:21:40

Take the advice above to learn to grind (and hone) your own HSS tool bits. There's plenty of advice and instruction out there. You can easily experiment with shapes, angles and tip radii. You'll learn how surface finish, stock removal rates, chatter, etc., etc. alter with tool configuration and you'll understand why things are happening. You're then only a few steps away from being able to make tools from gauge plate or silver steel, for odd and special jobs. However, if you start equipping yourself with tipped tooling, it'll cost a lot, you'll need to navigate the arcane coding system for tips (most are unsuitable for lightweight, low-powered machines), and you'll need to understand what tip parameters are required for each application. Also, you'll cry when you find out how fragile they can be, especially in the hands of the inexperienced lathe operator. I'd also suggest that a tipped-tool-only user won't understand anything like as much about cutting as one who has gone through the HSS-grinding apprenticeship.

If you're determined to go the tipped-tooling way, I'd suggest going to someone like Greenwood Tools (no connection) who supply amateurs with a pre-filtered, small selection of holders and tips, which are suitable for 'our' machines and materials.

Another important consideration is the cost of a handful of toolholders and a selection of tips, compared to a bundle of HSS blanks. The considerable difference can be put to good use - on beer.

Thread: Drill running off course
17/09/2019 12:30:48

Had to have a rant about misuse of centre drills... Now, back to Eric's question.

Hopefully, all the good advice that has been given will result in success. However, it hasn't been mentioned that if swarf is packing in the hole, horrible things can be expected to happen. Are you trying to drill the hole in one pass, or are you 'pecking', i.e. very frequent complete withdrawal of the bit from the hole, to clear swarf? With a drill that size, you will need to peck every few seconds. It's tedious...

17/09/2019 12:09:58

I wish the 'tradition' of using centre drills to start drilled holes would die. Centre drills are for making centres - that's why they have the little bit on the end - the bit that breaks off easily. Spotting drills are robust and stiff and make good drill-starting dimples. One with a 90 degree point angle can pre-countersink or chamfer the hole too (if it's going to be less than the spotting drill's diameter). Try to get one without a chisel-edge, i.e. four-facet grind, and you don't need a centre-pop (which is a bit of a problem if you're working to co-ordinates...).

Thread: Dewhurst reversing switches
17/09/2019 12:00:14

There's recently been some discussion about Dewhurst reversing switches in MEW and this forum. In MEW No. 283, Glyn Davies advocates the only sensible solution - throw the Dewhurst away! There's no need for these expensive abominations these days.

We all owe it to ourselves and our families to take basic precautions to preserve our health and well-being. Serious machines really should be controlled by a DOL starter / no-volt release. I don't believe the Dewhurst switch was designed to switch machine motors on or off - the make and break action is too slow and the contacts are rather feeble. It is fine as a switch to select forwards or reverse*, but not to start or stop the machine. Unfortunately, this seems to be its most popular application, and so the poor thing eventually burns out...

* Well, clearly it wasn't in Glyn Davies' case.

Thread: MEW No. 285 Gear Cutters and Gear Cutting
17/09/2019 11:32:58

I've just received MEW No. 285. The idea of incorporating relief into single-point gear cutters by using conical milling cutters to make them is superficially attractive. However, it seems to me to be an unnecessary elaboration of a simpler technique, and it comes with two problems: 1, the need for conical cutters, depth-setting collars, look-up tables (or real maths), etc., and 2, most serious, the resulting cutter is not form-relieved, thus it can't be sharpened without changing its shape.

If, as the author suggests, it is acceptable to make a cutter with profiles that are slightly part-elliptical, rather than part-circular (and it should be), then a form-relieved cutter can be made using cylindrical milling cutters (rather than conical ones), by tilting the blank nose-down (or -up) in the way that has been suggested for providing positive rake, towards the end of the article.

The advantages are: 1, it's easier; 2, the resulting cutter can be sharpened by grinding its top surface and 3, there's no need to buy conical cutters, so you can spend the money saved on beer.

OK, this method also comes with problems: 1, the form-relief is not arcuate, as, for example, a Eureka device would produce, but, providing the 'tilt' chosen for its manufacture provides enough heel clearance in use, that doesn't matter; 2, the correct diameter cylindrical cutter will probably have to be made, but that's easy; 3, you still have to look up or calculate the diameter of that cutter.

Thread: Which oil and oil gun
28/08/2019 11:50:15

'Cleverer people than me will know the whole answer, but I believe modern engine oil is not suitable for your lathe. Those oils are designed to work at a much higher temperature. I suspect that the various additives in them wont help either!'

What he said!

'Hydraulic' oil is the stuff to use, eg Nuto 32, in gearboxes and nipples, with 'way oil' on slideways.

Thread: Glass cutting
26/08/2019 22:03:58

Decades ago a bottle-cutting gadget was marketed. It was simply constructed from strips of flat stock. One end had a lump that was able to rotate in/on the bottle's neck, the other held a glazier's wheel. The bottle was scored, then the score line tapped until the thing broke into - hopefully - two pieces. Presumably any other means of guiding a wheel would be as effective.

I think I read about another technique in which the bottle was filled with oil (of some sort) to the level required, then a red-hot poker was plunged into the oil. The bottle was supposed to fracture at the oil-level. Sounds like Health & Safety might not approve...

No experience of either. Good luck.

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