Member postings for Andrew Johnston

That's because engineering is as much an art as a science.

Regards,

Andrew

I'd be inclined to change the design to something more practical. There are several options:

1. Thread one hole, and the pin, with or without a shoulder on the pin, and assemble with or without thread lock

2. Put a shoulder on the pin and loctite the smaller end in place

3. Put a shoulder on the pin and lightly rivet it in place

No doubt there are other options.

Regards,

Andrew

Cheaper, smaller, less heat dissipation, more reliable?

Andrew

You did ask; they're a bit small for both lathes and one of my mills, and too large for the other two mills. I use 5C collets on the 4th axis of the CNC mill.

Andrew

I understood that ER collets were originally designed for hold tooling, whereas 5C collets were intended for work holding. Of course there's nothing to stop you holding what you like in any variety of collet.

In theory I suspect that ER collets can be bought that are better than 5C collets. A quick 'google' found ER collets with TIRs of 0.0002" or better, whereas the best I found for 5C collets was 0.0004". However, none of the suppliers operate in the ME market. For the ME user I expect that where the collets are purchased from will have a far greater impact on accuracy and concentricity than any theoretical accuracy standards.

Regards,

Andrew

Edited By Andrew Johnston on 08/05/2013 14:40:30

Norman,

I reckon a 500km flight will take me at least 6 hours, so it's all good practice! The weather is now distinctly iffy, so work (engineering) today.

Yesterday was underwhelming to start with; 'square' thermals, weak climbs and a low cloudbase. Things didn't improve until we got to Milton Keynes. Even then it took us a while to realise that the tatty looking clouds often produced good climbs. Our low point was Oxford (Headington roundabout), surprising really what with all those wannabe politicians reading PPE, one would have thought that there'd be a surplus of hot air. The car factory at Cowley produced the required thermal. The best run was from Melton Mowbray to Bury, 120km in about an hour and twenty.

Regards,

Andrew

I don't have anything in the workshop. One I wouldn't be able to hear it properly when the machines are running, and two I'd find it a serious distraction with safety implications.

Regards,

Andrew

Never mind engineering, the sun was out today! So a nice jaunt in the glider instead, from Cambridge to Oxford, Melton Mowbray, Bury St. Edmunds and back to Cambridge. About 375km in a bit under 5 hours. I'll get back to engineering later in the week when the weather is iffy.

Andrew

Neil,

The diode symbol shown is what I would use for a transient suppressor diode. So I assume it's an attempt to show that the device is avalanche rated, and that the diode can play a useful role rather than just being an annoying parasitic component. As I'm sure you're aware the Schottky effect occurs at a metal-semiconductor interface, whereas the datasheet talks about a pn junction diode, which is what you'd expect from a conventional MOSFET structure.

As Neil correctly says, the ratings are normally given at a rather unrealistic 25°C. For those that are curious consider the following. From the datasheet a continuous rated current of 86A and a Rds(on) of 8mohms gives a device dissipation of 59W. Now the 'thermal resistance' junction to ambient is 62°C/W, so the junction will be at 59x62=3658 degrees above ambient. Clearly the magic smoke will have long since gone! If we assume an ambient of 25°C and the maximum junction temperature of 175°C, to dissipate 59W we need a heatsink with a 'thermal resistance' of about 1.4°C/W taking into account the 1.14°C junction to case 'thermal resistance'. For air cooled, no forced flow, that's a pretty big heatsink. It's actually worse than calculated since Rds(on) is strongly temperature dependent and as soon as the device heats up it will dissipate more power and so on (see Fig10 in the datasheet).

The slightly sneaky part of the datasheet is the 86A continuous under maximum ratings. See the little note in brackets afterwards? Silicon limited? In other words that's the limit of the MOSFET conduction channel on the die. The next, lower, specification of 75A is case limited. So the upper limit is taking the case beyond it's rated current, even though the die itself is capable. This is reflected in Fig9 of the datasheet. In practise the device will not immediately fail if you pass more than 75A through it, but the longer term reliability will suffer.

Confusing isn't it!

Regards,

Andrew

Hi Murray,

Impressive stuff; I'm sure you could flog the swarf as art to some unsuspecting tourists. Now all you need is a decent milling machine.

Regards,

Andrew

Hi Paul,

I wonder if my last couple of emails went into your junk bin too?

I hadn't seen the CNC lathes from Axminster, although I've just had a quick look. They look ok, but seem quite expensive. I've just bought a repetition lathe, so any CNC lathe I'd be interested in would have to offer something over and above that. The previews I've seen from Tormach look very interesting, especially with the revolving tool carousel. I can't see any reason why it couldn't, in principle, be used for live tooling, which would be a big plus for me. It'll be interesting to see when it finally makes it to market, and how much!

Regards,

Andrew

I make my filing buttons from EN1 or EN3, usually from the offcuts bin, leave them soft, and throw them away after use.

Andrew

Not according to the datasheet it won't, but you have to know how to 'read' the datasheet to get the real story.

By the way the drain-source body diode isn't a Schottky diode, it's an ordinary pn junction diode. But it's a pretty good one, and the MOSFET is avalanche rated, so there's no need to add external diodes.

Regards,

Andrew

Well this weekend I was going to (redacted due to activity being off topic) but, since the weather was so awful, instead I made 560 aluminium spacers with an M3 tapped hole through the middle. All they need now is a quick wash to get rid of any remaining swarf and coolant before bagging up and handing over. Now I've got them finished I can get on and start cutting the spur and bevel gears for a fellow modeller who is building the same traction engine as me.

Andrew

Hi Norman,

Congratulations on reaching the milestone; if nothing else it isn't half a lot of typing! I've certainly valued your posts.

I don't know about 'fizz', but I agree that the forum has changed over time, possibly reflecting a change in the make up of members.

Regards,

Andrew

'Strike While the Iron is Hot' by Guy Lautard

Andrew

Must be spring at last; I've just watched the first aerobatics display of the season by a Spitfire over my house. It was a later Griffon powered variant.

We also had a Chinook fly over the end of our road yesterday; he was certainly pushing the 250 foot limit, if not below it. I hope he was in contact with Bourn airfield, as he was well within their ATZ.

Andrew

In theory designing a semiconductor NVR for mains voltages is straightforward. A couple of back to back N-channel MOSFETS with the gates connected together would do the trick. An isolated gate drive would be needed, which is a bit more involved. I don't think that the voltage detection would need to be that complicated; all it needs to do is power up in the off state, and have a means of turning it on, and keeping it on as long as there are volts on the downstream side of the switch. It doesn't need to know anything about the current.

While the open frame contactors and NVRs do have problems, such as dirty pole pieces and contact pitting they also have some practical advantages:

They're pretty simple

When they're off, they're off, no leakage currents or anything else

Unlike semiconductor devices they're pretty robust against all the cr*p that comes down the mains, especially in an industrial environment

Most of my 3 phase machines already have relay style NVRs built in, with the exception of the shaper and surface grinder, where I've replaced motors and switch gear with DOL starters but no NVR.

Regards,

Andrew

As a generalisation DC relays don't care which way round the DC is applied to the coil. They work equally well either way round. So in an AC relay the contacts close on each cycle irrespective of the applied voltage. However, an AC relay will tend to open as the applied voltage goes through zero. So AC relays have a shading pole that tends to retain some residual magnetism in the core as the voltage goes through zero, thus keeping the contacts closed until the applied voltage rises again. That's why AC contactors 'chatter' if the pole pieces are dirty and don't quite mate, thus reducing the maximum magnetism. The residual magnetism isn't then quite enough to keep the pole pieces closed, so they start to open, before being closed again as the voltage rises.

Regards,

Andrew

I use inserts intended for aluminium too. The key to machining plastics is low surface speed and high feedrate, otherwise all you end up doing is melting the plastic. For 22mm nylon I'd be down below 1000rpm and a feedrate of up to 10 thou/rev. I've never used coolant when machining plastics. For turning I let it get on with it, and for CNC milling I hover with the vacuum cleaner going.

Regards,

Andrew