Member postings for Andrew Johnston

Hansrudolf: I think you need a new calculator! The ratios I checked came out exactly, as they should be. I'd be interested in seeing the other pitches.

Regards,

Andrew

So about 2 thou per inch error; doesn't sound quite 'perfect' to me? I would have thought that an order of magnitude better, ie, 0.02%, would be acceptable. Of course if you have the 127 tooth gear then the error could be 0%..............

Regards,

Andrew

As has been stated the only exact way to cut metric threads using an imperial leadscrew is to include a 127 tooth gear in the drive train. The number has to be 127 since this is a prime factor of 254. Not only is 127 a prime number it's a Mersenne prime, useless but true! It is usual to combine the 127 tooth wheel with a 100 tooth to give the ratio 1.27, the usefulness of which has been explained by Michael.

A gear with 63 teeth can be used as an approximation, used in conjunction with an 80 tooth gear; 80/63 = 1.269841, an error of about -0.012%. I don't know what gear is normally used in conjunction with a 21 tooth gear.

However, both the 127/100 and 80/63 combinations result in relatively large gears. A ratio of 47/37 uses smaller gears, and gives 1.27027 recurring. This is an error of 0.021% or about 2 tenths per inch.

Regards,

Andrew

I think that red indicates 416, a martensitic stainless steel with good machinability, but poor weldability as it includes sulphur and phosphorus. Silver soldering is not recommended, particularly if the joint is to be subjected to moisture.

Regards,

Andrew

I've been playing around with a few simulations of a coil and switch, with and without a capacitor. I've left off the secondary coil at this stage, once the spark gap breaks down the circuit ceases to be linear. The first circuit is with just a coil and a MOSFET used as a switch. The opamp is in circuit because the freebie simulator I'm using will not do a transient analysis unless there is an IC in the circuit. The MOSFET is driven from a 1kHz square wave. The voltage at VF1 is thus:

The waveform looks remarkably like that in the link referred to by Martin. I agree that this spike is caused by the voltage rising in an attempt to keep the current in the inductor flowing. The magnitude of the spike depends on the factors mentioned by Martin. For simple solenoid type coils the multi-layer Wheeler formula can be used to calculate a value of inductance from the mechanical parameters of the coil and the number of turns.

Now if we add a capacitor across the switch we get a different result:

We still have a fairly high voltage, but the overall waveform is clearly a damped sinusoid. I'd argue that the voltage is now due to the resonance of the series LC circuit, not to the rapid current change in the inductor caused by turning off the switch. As Martin correctly says the impedance of a series resonant circuit, at resonance, is small, and real, being essentially the resistance of the inductor. However, we are not driving the circuit at resonance, we're driving it from a DC source, so with the switch open the impedance seen by the supply is large. While the overall impedance at resonance might be low it tells us nothing about what is happening on internal nodes. There's nothing to say that the internal voltages cannot exceed the nominal supply.

As an aside I learnt this the hard way, as I received a hefty shock picking up a coil that was part of a series resonant circuit. The circuit was being driven at the resonant frequency (30Hz) by an amplifier fed from a 12V power rail, but the voltage on the internal node was nearly 150V.

The diode in the circuit is a fudge. The MOSFET contains a body diode between drain and source, and if the point VF1 goes negative this diode conducts, thus distorting the waveform. So the diode is added to prevent this.

Now for a caveat. Personally I remain highly dubious of simulation results; there's nowt like actually building the circuit and measuring things to be sure. So to this end I have a relay and a selection of high voltage capacitors on order from Farnell, so that I can experiment using the coil that I wound for use in my afermentioned experiments.

Regards,

Andrew

The following is how I think the circuit works. When the contact is closed we have an inductor (L) in series with a voltage source (V), plus the resistance (R) of the inductor. At the moment the contact is closed the full voltage is impressed across the inductor, and current begins to flow. If the inductor was pure, ie, no resistance, the current would increase linearly for as long as the contact was closed. In reality. because we have resistance, the current increases asymtotically to a maximum of V/R. For all practical purposes the first part of the current versus time graph is linear. When the contact is opened the current does not stop flowing. Instead it starts flowing into the capacitor. We now have a series resonant circuit. This is a second order system (two storage elements, the inductor and capacitor) and the differential equations that govern its behaviour can have an oscillatory solution. So what we get is a decaying sinuoidal current as the stored energy is moved between the inductor and capacitor. The voltage across the combination RLC can never be greater than the source voltage V. So how do we get a high voltage on the secondary coil? Simple, because the voltage at the intermediate point of capacitor and inductor can be much higher than the source voltage. It may well be hundreds of volts, and is related to the Q of the circuit, for series resonant circuits defined by:

Q=1/R*SQRT(L/C)

This voltage appears across the secondary, multiplied by the turns ratio, which is also normally large, so we can get peak voltages up into the kilovolts.

I do not think that the equation v=-L(di/dt) is directly relevant in this case. Note that there is minus sign in the equation, as the voltage tries to oppose the current change. The addition of the capacitor actually slows down the waveform after the contact is opened and limits the initial voltage across the contacts at the point at which they open. In this way it does protect the contacts from sparking, but it also plays an important role in the operation of the circuit.

Regards,

Andrew

Addendum: I must get back to playing with my experimental ignition circuit. The aim of it was to get sufficient voltage on the secondary without needing a high turns ratio in the coil. The circuit uses an avalanche MOSFET as the switch and does rely on the equation v=-L(di/dt). It's been a while since I played with it, but as I recall I got about 8kV on the secondary for 12V in and primary/secondary turns ratio of 10.

Personally I wouldn't admit to using a 555 in public, there are limits!

Actually all the time. If nothing else, it makes parting off a cinch.

Regards,

Andrew

Errrrr, where did the '4' come from; I make 90psi more like 6.2 bar?

Andrew

Hi Wolfie,

Excellent, great feeling I bet, to see all the bits come together and run. And you're one up on me 'cause you finished it and got it running!

Regards,

Andrew

Ah well, that rules me out then..........

Andrew

PS: I think that the quote ascribed to Henry Ford actually comes from 'Slide Rule', Neville Shute Norway's autobiography.

Richard: Thanks for posting the instructions. It looks like I've been on the right track. It is slightly odd that the taper dowels are not mentioned in the re-fitting process. I'd had thought that they should be tapped home between operations 5 & 6?

Regards,

Andrew

Paul: I've just recieved a summer special offers flyer from Cutwel that includes the CCGT-AK inserts in H01. I'll get some to see how they perform.

Regards,

Andrew

Plenty of food for thought here. Ultimately there's no substitute for actually seeing a finish that is described as 'good' in order to make a comparison with ones own efforts.

I tried, and failed, both the tests mentioned above. For milling I used a 2 thou depth of cut, using a carbide end mill running at 2000rpm, on the side of a 22mm thick slab of hot rolled steel. I was climb milling. The finish at a feedrate of about 10mm/min was definitely worse than at a speed of about 200mm/min. The finish at the higher speed was smooth, but not a mirror finish. For turning I used a piece of 7/8" diameter EN3 from the scrap bin. Turning at 1200rpm, depth of cut 5 thou and feedrate of 2 thou/rev I stopped and started the feed a couple of times. A witness mark was just about visible.

Still, I'm not downhearted; I normally expect to achieve a finish on the order of 1 to 2µm Ra, sometimes better, sometimes worse, depending on material and the finish I actually need.

As an aside, last weekend I designed and machined some jigs for a company to allow them to hold a part for coating. As part of the design exercise I normally get the part and its associated drawing to allow me to design the jig. One area of the part had quite a coarse finish, almost like a fine thread. For that area the drawing actually called out a specific insert tip radius and feedrate. First time I've seen that.

Regards,

Andrew

The saga continues........

I called the UK distributor and had a brief, but unsatisfactory conversation, along the lines of the person concerned didn't set the prices, it was a straight euros to pound conversion, no added value, take it or leave it. So I left it; companies like that deserve to go out of business.

However, I do have a cunning plan, so watch this space.

Regards,

Andrew

Hmmmm, not sure if I'm allowed to answer the original question as I don't consider myself a model engineer. My background is professional engineering (read my profile for more details) and I am building some model engines. However, I also earn part of my living from the workshop, so I have a different perspective than some. My emphasis is on the engineering, rather than the modelling.

I don't belong to a model engineering club, but I do find this forum useful, as there are some very knowledgeable people here, and it is always helpful to throw ideas out and be able to ask questions.

Regards,

Andrew

Edited By Andrew Johnston on 07/07/2012 12:39:42

Personally I wouldn't want to see articles on models in MEW; I subscribe to ME for that. That's not to say that models can't be used to illustrate a technique or use of a tool, but that's not the same as a construction article.

Sadly the recent 'Scribe a Line' correspondence in MEW is rather naive. It is very difficult, if not impossible, to give a blow by blow account of how to make a part. Everybody has different equipment and tooling at their disposal, making it hard to cover all eventualities. For instance even making a simple bush is fraught with problems. Instructions to turn to diameter 'x' for distance 'y' are one thing, but should the speeds, feeds and number of passes be dictated? Arguably a beginner might have more problems with the latter than the former. But suppose a particular beginners lathe chatters at the stated speeds and feeds? What then?

Ultimately one has to try something, fail or suceed, experiment if needed, and learn from it. Engineering is an art, not a science, and cannot be achieved simply by following a 'recipe'.

Regards,

Andrew

KWIL: So there is, I wondered what the extra hole was for. One learns something every day!

Paul: Thanks for that. A while back I did try fine cuts with a high rake insert intended for aluminium but the results were inconclusive. However it is instructive that the Cutwel catalogue states that the CCGT-AK insert is for both aluminium and for finishing on steel. I'll have to buy some and repeat the experiment. It's a shame that I had a letter from Cutwel recently offering me a 10% discount as I hadn't ordered from them for a while, but the offer ran out last week.

Regards,

Andrew

Richard: It seems an oversight on the part of Harrison not to mention re-fitting of the gap piece. I guess that way back when it was assumed that the foreman would have the knowledge anyway. If you get a reply from Harrison then I'd be interested!

I have in mind that for some far eastern lathes removing the gap piece invalidates the accuracy limits. I assume that this is because once the gap piece has been removed it cannot be easily replaced in the correct position. Anybody know if this is true?

Regards,

Andrew

In a fit of shopping mania on Sunday morning I ordered the power drawbar and electronic tool setter from the US. I got an email today to say it's been shipped. The next task is to go and buy a compressor to run the drawbar!

Regards,

Andrew

Here's a picture of my shaper:

In has the name 'Invicta' cast on the side panel as well as in the base. It's a 4M, which translates as 18". Sadly it's a broken at the moment as the main drive crank casting is broken, due to a combination of poor design and poor casting. I have a repair scheme sketched out; it's just a case of convincing myself I need to keep the shaper and then finding the time to repair it. Getting the broken crank casting out is going to be a challenge too.

Regards,

Andrew