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Member postings for Andrew Johnston

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

Thread: How can I make an accurate 90 grind using a diamond whetstone?
27/10/2021 21:04:10
Posted by Jon Lawes on 27/10/2021 20:32:47:
Is it a commercial enterprise?

In an early post, IIRC, the OP was talking about building a prototype of a gadget for use in an office. I assume this is the same project, so it could be said to be commercial, although the approach seems to be anything but.


Thread: Smart & Brown Questions
27/10/2021 11:37:50

Putting a switch between VFD output and motor is fine, if the switch is never operated while the VFD is running.

The output 3-phase bridge in a VFD will use MOSFETs or IGBTs as the switches. These transistors are pretty robust, with the exception of over-voltage. Voltage spikes are the quickest way to destroy the devices. The issue arising from switching the output while the VFD is running boils down to the law of induction:

v = -L(di/dt)

The voltage across an inductance is the self-inductance, L, times the rate of change of current. The minus sign indicates that the generated voltage is such that it tends to oppose the change in current. The coils in an induction motor have inductance. So if a VFD output is disconnected while running the coils in the motor are suddenly open circuit, with a current flowing. The voltage (back EMF) across the coils will rise rapidly as the current decreases in an attempt to counter the change in current. it is quite likely that the generated voltage will jump across the switch contacts and into the VFD bridge.

A secondary problem is that the motor currents are also flowing in the 3-phase bridge. The VFD busbars have inductance, so when the output is disconnected from the load the same problem of back EMF generated by the busbar inductance and change of current may also be enough to over-voltage the output switches. Never mind what the control algorithm may do when it sees the output currents decreasing.

It's best not to court disaster by wiring the VFD output direct to the motor. Direct wiring also makes it simpler to control radiated emissions from the output cables. The only controversial issue then becomes ground the output cable shield at one end, or both ends.


Thread: Mill table wonky
26/10/2021 22:46:42

It would be a help if the pictures were zoomed out a bit. Otherwise it's difficult to see what is being measured. It's not essential that the sides of the table are parallel to the ways, but it's an all round PITA if they're not as then they cannot be used as reference surfaces.


Thread: Guided bus lane
26/10/2021 19:55:05
Posted by Martin Kyte on 26/10/2021 14:07:20:

.......The busses when on the busway are self steering and I would not be at all surprised if they do not enguage cruise control too.

That's the theory. The only time I've used a guided bus (from the railway station to Addenbrookes) it hunted from side to side quite violently. I don't think they have cruise control. The driver was hurtling along! Shortly afterwards there was an accident where a bus crashed off the same bit of the guideway. Intially the driver was hailed as a "hero" for avoiding a cyclist. It turned out that he had been speeding and lost control. Pure luck that the cyclist was knocked out of the way rather than getting squashed.

I'd better not start on South Cambs and the city council or I'll be banned from the forum for bad language!


Thread: Microwave Oven
25/10/2021 16:51:22
Posted by Grindstone Cowboy on 25/10/2021 16:29:10: was either beryllium or boron..........

Technically it's neither, the white insulator is beryllium oxide. Beryllium is a greyish metal. Both are toxic if dust is inhaled.


Thread: Aircraft General Discussion
23/10/2021 11:31:19
Posted by JA on 22/10/2021 12:10:13:

The last time I remember seeing one, for all the wrong reasons, was at the 1970 Farnborough Air Show.

I didn't see the accident as it was a few years before I started working at Farnborough. However, I do remember it. The AAIB report mentions the Helicopter Flight Group at RAE. My father worked at RAE Bedford, starting with NAD (Naval Air Department) and later became head of Structures at Bedford. This encompassed the helicopter group and I remember him talking about the tests and visiting the AAIB at Farnborough. Easy in those days as there was an air ferry from Bedford to Farnborough and back three times a day. If I recall correctly Ken Wallis did some test flying at Bedford as part of the report.

Due to my fathers contacts within the AAIB when I started at Farnborough in 1975 I was lucky enough to get a personal tour of the AAIB facilities.

In later years I used to commute from Bedford to Farnborough on the air ferry on a Monday morning and back on Friday afternoons. The flights were flown by RAF test pilots, mostly using DH Devons, although we sometimes flew on a Dakota. A few times I was the only person on board, so the pilots would lend me a headset and divert slightly off route to show me significant landmarks.


Thread: Wandering mill table
22/10/2021 23:06:24

How do you know that it's the table and not the vice that is off?


Thread: Quill feed milling machine
22/10/2021 09:31:26
Posted by Ady1 on 20/10/2021 13:13:48:

Can you tell us how you cut between the drill holes?

I didn't need to. The number of holes and drill size, on a pre-determined PCD, were chosen so that the holes overlapped by a few thou. After drilling a sharp tap with a nylon faced mallet was enough to break any remaining webs and for the core to fall out.


Thread: Help with a broken Sieg Super X3
21/10/2021 21:37:20
Posted by JasonB on 21/10/2021 20:47:07:

I'm not sure if you can test the Brushless motors in the same way as the Brushed.........

I'm fairly sure it isn't possible. A brushed DC motor uses brushes and a mechanical commutator to switch current between colis. So to test, simply apply DC and the motor should turn. A brushless DC motor dispenses with the brushes (!) and commutator and relies on external electronic switches to switch current between coils. So if one applies DC direct to the coils nothing much will happen except there's a good chance of overheating the coil(s).

The Hall effect devices are there to monitor the rotor position so that the coils can be switched at the appropriate points in each revolution. A speed reading can obviously be derived from the output of the Hall devices but it's not their primary purpose. it is possible to dispense with the Hall devices and use backemf from the non-driven coils to determine switch timing. Of course that doesn't work when the motor is stationary as there is no backemf. So on starting the controller starts at an arbitrary position and if the motor doesn't run properly it makes adjustments until the motor is running correctly, with a bit of luck.


Thread: Britan Lathe - New Lathe Day
21/10/2021 21:22:35
Posted by websnail on 20/10/2021 22:51:47:

I'm still lurking on here, so if I can help in any way, just ask.

Dave: That's good to know, thanks. thumbs up

I've never seen one of the Britan 4 wheel grinders but for ages there was one on Ebay. It seems to have disaapeared, but I don't know if it was sold or the seller got fed up.

I'm aware that I'm not pushing the Britan anywhere close to it's capabilities. The last items I made were for work; a batch of custom lockscrews for D-type connedtors converting from a Japanese standard M2.6 thread to the more common 4-40UNC. The next job will be custom flat head rivets for the chimneys on my traction engines.


Thread: Hello from East Northamptonshire
21/10/2021 21:14:53

Welcome to the forum. One couid probably build a 2" scale traction engine with a Myford, but for 3" there would likely be a few parts (final drive gears and flywheel) that would be too large. I'm building 4" scale engines but my lathe will swing 18" in the gap. It is perfectly possible to build at least the smaller scale traction engines without a mill, but having a mill makes life a whole lot easier. Irrespective of the capability of the lathe for milling, having a separate mill avoids the need to set the lathe up time for milling, and then take it all apart again for turning.


Thread: Help with a broken Sieg Super X3
21/10/2021 20:36:58

I think the hi-torque description implies a brushless DC motor, so the light bulb test, and motor across 12V, will not work. To see what is going on one would really need an oscilloscope.

My bet would be that the electronics is phut. Unfortunately one of the ways to cut costs is remove overload protection circuitry and/or use components that are only rated for normal use, not overload conditions. It may just be that a fuse has blown, but it's difficult to know without seeing a picture of the board in question. A definite sign would be any component, or part of the board, that is darkish brown or otherwise discoloured, meaning that something has got very hot.


Thread: How to Mount Collet Closer Chucks ?
21/10/2021 19:51:48

I'm not convinced that the OP has two complete sets of collets and closers.

The upper set don't look like the Burnerd multisize collets I have. What I assume are the metal fingers are tapered, so they couldn't slide in slots like conventional multisize collets. They look more like rubberflex collets. Same result as sliding metal fingers but different construction. Rubberflex collets were often made by Jacobs, as were the ones I use in my auto reversing tapping heads. The lower set of collets are dead length collets.

The two types of collet work in different ways. Rubberflex collets work by contracting as they are pushed back towards the headstock. So the collet moves with respect to the headstock when it is closed. A corollary is that the work also moves relative to the headstock as the collet closes. Dead length collets are closed from behind, away from the headstock, towards a fixed stop at the front of the collet holder. A consequence is that the collet does not move relative to the headstock when being closed, nor does the work. Hence the name dead length. It's a significant advantage on repetition and capstan lathes.

It would be interesting to know if the two holders work in the same way, or different ways. The internal tapers should be different as well.

I think Martin is correct that some of the dead length collets are hexagon. I can vouch for their usefulness when making batches of nuts and bolts:

more bsf nuts and bolts.jpg

Burnerd multisize collets will hold hexagons:


But I'm not convinced the upper set of collets will, as I think the rubber between the blades would get in the way.


Thread: Britan Lathe - New Lathe Day
20/10/2021 22:10:08

Thanks Michael, long before I bought my Britan. So they must have sold on the spares, but I can't find any trace on the internet.


20/10/2021 19:37:23
Posted by Baz on 20/10/2021 17:28:07:

I think Britan are still trading.............

They're long gone. The name on my Britan manual is Britcam Tools and an address in Cambridge on what is now Castle Park. They shut down and formed Britan Machine Tools in 1979, who I think were based in Newmarket. There was a further sale to Arden Machine Tools, based in Birmingham, in 1985.

When I bought my Britan there was a company (may well have been Arden) who had spares, although you had to look hard on the website to find the information. Arden now seems to be dissolved, although I don't know when. So it looks like there is no longer any support.


Thread: Help with a broken Sieg Super X3
20/10/2021 13:52:42
Posted by Nathan Adamson on 20/10/2021 10:45:52:

c) ...........and also have the torque issue fixed?

No idea about a new board, but the torque issue is inherent to the design. The fundamental equation is:

Power (W) = Torque (Nm) x Angular velocity (radians/sec)

Simple motor speed controllers have constant torque as the speed decreases. Consequently spindle power decreases in proportion to the reduction in speed. The 1000W motor may only be providing 100W at the lowest speed. That's why industrial machines have stepped belt or geared drives. IIn those cases as the speed is decreased the torque goes up and spindle power stays constant. A simple PCB is much cheaper than a belt or geared drive head which is why it's done.

Some newer variable speed mills have a "hi-torque" capability which provides more current to the motor at lower speeds and hence more torque, and power. That helps but can't completely compensate for the loss of power as the increase in current causes more heating within the motor, which of itself can be terminal.


Thread: Quill feed milling machine
20/10/2021 11:38:26

I do 99% of my drilling on the vertical mill and it would be a real PITA not to have a proper quill feed. I use the knee when milling to precise depths, but not for drilling. My mill has a quill handle like a pillar drill, a precise feed wheel and power downfeed on the quill. I don't use power downfeed very often, but it is useful when there are a lot of deep holes needed in order to remove waste material:

roughed liners me.jpg


Thread: From where I might be able to source some 1300 micron (1.3mm) mild steel sheet?
19/10/2021 15:18:16


Low carbon steels, such as mild steel, aren't particularly magnetic. Electrical and transformer steels have much higher permeability and are likely to be available in a wider range of thicknesses than bog standard cold rolled sheet.


Thread: Oxy propane welding kit
19/10/2021 14:16:57
Posted by fizzy on 19/10/2021 13:58:48:

BOC and Air liquide will no longer allow you to load acetylene into a car - has to be a commercial vehicle with an all steel cab partition.

Don't know about BOC but certainly true of Air Liquide. However, it's a company rule not a legal requirement. The HSE website states there are exemptions for private individuals using private cars, albeit with quantity limits. It's one of the reasons I dumped Air Liquide as a supplier.


Thread: Load reactors for VFDs
17/10/2021 20:14:28
Posted by SillyOldDuffer on 17/10/2021 12:46:05: inductor in series with a capacitor......

A series capacitor? I think not, as it would need to carry full load current, equals expensive. All that is needed is a simple low pass filter, Butterworth or not. So series inductors and capacitors across phases, or to a common point which is essentially a star point albeit not connected externally.

I'd be inclined to set the roll off frequency of the filter at 300Hz or so if the VFD is going to be set at 50Hz. The pulse rate should be well into the kilohertz so will still be filtered. If the frequency of interest (50Hz) is well below the 3dB point of the filter then there should be little mismatch between phases (in amplitude and phase) due to component tolerances and filter characteristics.

There are two issues regarding the inductance. One, working out the value and two, designing an inductor with that value that will not saturate at rated current. The theoretical value is straightforward. Designing an inductor with that value will require knowledge of the properties of the laminations, in particular the permeability.


Note: A Butterworth filter is simple, in the sense that it's pole only (no zeros) and the poles lie on a circle in the s-plane. It's beloved of text books and example sheets, but not used much in practice as it tends to use non-standard value components and has significant sensitivity to component tolerances.

Edited By Andrew Johnston on 17/10/2021 20:27:33

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