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Member postings for John Olsen

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

Thread: Replace Speaker Surrounds
16/01/2022 06:35:44

I wonder if the problem with the early transistor amps that SOD mentions was actually due to residual crossover distortion? Most transistor amps were class B or at best class AB, to make the most of limited power ratings. There was a lot of work done on this about 40 years ago as MOSFETS started to emerge. Wireless World around that time described a transistor class A amplifier which was claimed to give excellent results, but at relatively low power.. If a Class B or AB amplifier is not set up quite ideally, there can be a little glitch in the waveform as it crosses over from one output device to the other, and this is a particularly harsh kind of distortion.

One problem with the transistor stuff was transient intermodulation distortion, where the early stages of an amplifier can go into overload until the slower power stages catch up. The solution is to try to reduce the reliance on large amounts of negative feedback around the whole amp and instead to try to get low distortion in each stage, as well as trying to have good high frequency performance in the output stage

The way they measured transient intermodulation distortion was to apply a fairly large square wave (but not enough to clip) along with a lower level higher frequency sine wave. At the output, the sine wave would be liable to disappear for a bit after each transition of the square wave. An ordinary distortion test with a sine wave input will not show up this sort of problem.

The first place I worked was the Civil Aviation part of the local Ministry of Transport. They used to say to us trainees that 5% distortion was perfectly acceptable, and so of course I made myself unpopular by pointing out that they were applying this to every item in quite a long chain, the mike in the aircraft, the amplifier in the transmitter there, the same in the receiver on the ground, then any amplifiers needed to drive the landline to where the controller was sitting, and his amplifier and speaker. To me the end result has always sounded like Donald Duck, and I have never been able to understand what he was trying to say either. Just as well I never tried to be a pilot! Of course this sort of thing was probably a factor in the Teneriffe accident.


Thread: shaper cross feed
12/01/2022 19:53:56

The usual feed system uses a ratchet wheel that has square teeth. The spring loaded pawl that engages with it has an angle on the end, and can be rotated so that the ratchet drives in either one direction or the other. There is a slot in the tube that engages with a pin through the pawl. So in normal use, when the cutter has passed the end of the job, you pull the pawl back against the spring, turn it through 180 degrees and drop it back into the slot again. Usually you then need to rotate the manual handle to take up the backlash so that it starts driving again to pass back over the job in the opposite direction. At some point while the cutter is past the job, you can wind on a bit more cut. Unless the shaper is running very fast, you can generally do this without stopping the machine.

When you don't want the feed to drive, you pull the pawl back out of the slot and turn it through 90 degrees, this stops the pawl engaging with the ratchet wheel.

Some machines have provision to change the stroke of the feed system, so that the pawl picks up one, two, or even three teeth on the ratchet, giving a coarser feed.


Thread: Slowing lathe RPM
10/01/2022 01:00:48

It depends on the diameter of the aluminium, but 435 rpm does sound a bit fast for the minimum speed. It would help if we knew what the actual lathe is too.

If the existing motor is a single phase induction motor, then you are not going to be able to change the speed of that. You would need a three phase motor with a VFD, or a universal or DC motor with some sort of electronic control. You can buy a plug and play setup for the Myford, for instance from Newton Tesla.

It might be easier to do it mechanically, for instance an extra countershaft with stepped pulley to allow lower speeds. For example, the Myford ML7 that I have came with three steps on the spindle and a countershaft, but also had quite a large stepdown between the motor and countershaft. (It also has a back gear.)

Bear in mind that just changing speed electronically has its limitations. When you reduce the speed, you will still only get about the same maximum torque. So say you double the diameter of the job, you would halve the speed to get the same cutting speed. To take the same depth of cut, (same force on the tool) you would now need to double the torque, but with an electronic control you can't. Whereas if you halve the speed by changing pulley sizes, halving the speed will double the maximum available torque, so you would be able to take the same depth of cut. This means that you would actually be able to remove the same volume of metal in a given time.


Thread: Stuart Turner Twin Launch engine
31/12/2021 06:43:47

You may already know this, so sorry if I am teaching my grandmother how to suck eggs here, but it is important for the holes in the bedplate and the cylinder block to line up. One way to do this is to make a jig plate which is just a bit of flat stock with the holes drilled in it. You use it twice, once to do the holes in the bedplate and then again to do the holes in the block. If both sets of holes match exactly, the columns will go in without side loads, which makes assembly easier.

The other thing that helps is if you can make all the columns themselves exactly the same length between the shoulders that seat against the bedplate and the block.



Thread: Suitable Metal for Electro-magnet Levers
28/12/2021 22:37:13

It seems to me that what is wanted is a material with high permeability and low remananent magnetism. In less technical terms, you want a high amount of suck when the electromagnet is on, and a low amount when it is off. If it retains too much magnetism when the electromagnet is off, it will tend to stay in the same position even when released. Some relays had to have a little brass pip that made a gap when the moving part was pulled against the pole piece as otherwise they would pull in and never release despite the springs.

Anyway, soft iron is generally the most commonly available material that will fill the bill. Transformer laminations are also a very suitable material, and readily available from scrapped transformers, although only in thin sheet form. You may want to stack up a few pieces if you use that source.



Thread: New mini mill or use lathe as a mill.?
27/12/2021 10:27:19

A separate mill will be better, space and finance permitting of course. The trouble with combination machines of any sort is that they are always set up for the wrong function, so you spend a lot of time changing back and forth. Especially when a part needs turning, then milling, then more turning...

On the other hand, the kind of light milling that you can do on a Myford with a vertical slide won't do it any harm. Plenty of good work has been done that way.

Another thing to think about is the kind of work you want to do in the unforeseeable future. A vertical slide will do good work but will limit the size of the jobs more than even quite a small mill.

The usual sort of small vertical mill will act as a drilling machine, so takes the place of another machine you might want later if you went the vertical slide route.



Thread: Drilling brass.
14/12/2021 02:29:17

You want a different rake for drilling brass and bronze, which can be achieved by stoning the cutting edge with a small stone. So not actually blunt, but with the opposite rake to what you would use for steel. On a drill the rake for steel is built in by the spiral angle but it is enough just to stone the cutting edge back so that you can see a small flat on the spiral side, not on the end.

If you think of it like lathe tools, for steel the top of the tool slopes up towards the cutting edge. This peels off those nice spirals. On brass, such a tool will cause a large force dragging the tool deeper into the job, so by instead sloping down towards the cutting edge we reduce the dragging in force. The brass will sheer off in small chips and everything will behave itself.

The rake does not need to be very large, maybe 5 to 10 degrees. Ideally you would have a separate set of drills for brass, and I believe drills have been made with a different spiral angle for brass, but modified normal drills are fine for most amateur work.


Edited By John Olsen on 14/12/2021 02:30:21

Thread: Arduinos and Microcontrollers ref: Rotary Table Mew 249
01/12/2021 05:30:03

I've just been playing with an Arduino myself lately so found some of the above quite interesting.

A few points about speed and stepper motors. They can go quite fast, but sometimes need a few tricks. One is that in order to build up the current and hence the magnetic field quite quickly, the drive voltage can be increased. To avoid overcooking things when the motor is running slow or holding, this is done with either a constant current drive or more simply, a resistor in series. The high initial supply voltage gives a faster rise to the current, but the constant current part ensures that the current never exceeds what the winding is rated for.

Then there is the acceleration thing. Depending on the rotational inertia of the whole drive train, if you just start switching at a high speed, the rotor can lose lock. So the controller needs to be set up to accelerate at a reasonable rate, and similarly to decelerate at a reasonable rate. This requires that the controller knows in advance when it is going to be asked to stop the load, so is not so good for unplanned emergency stops. Really if high speeds and sudden changes are needed, a servo motor become a better solution.

I've just implemented some code from the web to allow using the Arduino to remotely display the output from the usual common digital calipers. The original code only did millimeters, so I have messed around a bit and after a few changes have got it to read out correctly for either millimeters or inches, including reading the fourth decimal place correctly. I might get around to doing a DRO for the Myford yet...


Thread: What is this?
25/11/2021 21:18:11

The wasp in the jampot is that there is nothing to stop the taper centre turning in the taper, apart from the jamming of the taper itself. This is not enough to prevent movement with milling loads, even cutting gears where the load should be end on. Also since the chuck or faceplate screws on, this can also move from milling loads.

So the taper centre on mine has been tapped on the small end for a drawbar. This is easy enough to do, just grind a little off the end first to take any surface hardness off. Most centres that I have encountered are only surface hardened.

The other fun thing with older Vertex dividing heads is that the taper is Brown and Sharp. I believe later ones might be Morse, and might even have a mandrel nose that matches the Myford thread, but mine doesn't, so has to have its own dedicated chuck.


Thread: The Internet of Things The Internet of problems
22/11/2021 05:11:03

The whole Internet of things is basically a scam. The idea is to sell you things that depend on talking to a remote server at their site to work. When they feel like it, they turn off the server, your IOT device stops working, and they hope to sell you a new one. The same applies with operating system upgrades, where you will often find yourself stuck with a perfectly serviceable device for which updated driver are not available.

Then there is the dreadful security, or lack of, with a lot of these IOT devices, where they are very useful to people who want to use them for denial of service attacks. You might never know that your device has been taken over and used to cause harm to other people.


Thread: Mag chuck dismantling
22/11/2021 05:02:21

Being apart for long enough to clean the chuck out is unlikely to have any noticeable effect on the magnets. It will depend a bit on how old they are, the older materials were not as good.. If you need to have a chuck apart for a long time, rig up some sort of keeper for the magnets, it just needs to be iron or steel that provides a path from one pole to the other. The Alnico material used by Eclipse is pretty good, I'm not sure when they started using it by I think maybe some time after the war. Later materials are even better.


Thread: Unimat 3 drive questions
20/11/2021 20:39:21

In standard form all of the no load speeds are considerably faster than the name plate rating. If I recall correctly, the ratios between them are also incorrect, the lower speeds being higher again than they should be. This doesn't matter too much, unless you set up to do screw cutting, in which case a handle in the spindles is a better bet than using the motor.

I converted mine to use toothed belts about 30 years back, the little ones about 3/8 wide. I think this was published in MEW some years back, although I don't have a copy of that. I can dig out the files if needed. The setup I used fits inside the original belt cover. A later change was to put a countershaft where the motor would normally be, and provide two steps on the pulleys there. Those ones use a poly V belt. The motor I am using with it now is a 1/8 horsepower three phase one with a vfd, so now I have 8 steps of ratio change plus the vfd range of speeds. so I can go from really slow on up to about the original maximum.

The toothed belts will break if things get too hectic, so I have had no problems on that score. It does need a spare kept on hand just in case.

The current setup does not suit the milling attachment, so I have another motor mounting plate cut out and a DC speed controlled motor waiting to be mounted on it. Since I have a much larger 45 size mill drill, this has not been treated with much urgency.


Thread: Another chinese lathe rises through the smoke
16/11/2021 21:04:08

I don't have a Chinese lathe, although my mill drill is Chinese. From what I have seen the actual lathes themselves are fine, good value for the price, especially if you look at what people want for Myfords. For what people want for a Myford in this country (NZ) you can get a much bigger beefier Chinese machine which I think would be better value.

However, the smaller Chinese lathes do have this weakness of not providing much in the way of gearing changes and instead providing a variable speed motor. A variable speed motor, however good, is no substitute for actually being able to change the drive ratio. When you reduce the speed you will still only at best be able to get about the same torque. What you need on a lathe is to be able to change the ratios, so when you turn that large diameter job, you can reduce the speed, increasing the torque, and still being able to take a reasonable depth of cut.

So if I found myself with a lathe with a speed controlled motor that needed replacing, I would be thinking about how I can also incorporate some different sizes of pulleys or gears into the drive train. Not an easy thing to do when your lathe is not working. I did do something like this with my Unimat three, which started out with a four ratio belt drive and a two speed universal type motor. It now has an eight ratio belt drive and a VFD controlled 1/8 horsepower three phase motor. If I was doing it again now, and didn't already have the little three phase motor, I would certainly think about one of those sewing machine motors.


Thread: china/India - Cop 26
16/11/2021 05:08:54

Heat pumps do work, but only within the laws of thermodynamics. So we can put say a 1kW electric radiator in our room, and get exactly 1kW of heating from it. Or we can use that 1 kW to run a heat pump, in which case we might get something like 3kW worth of heating in our room, for an input of 1 kW of electricity from the mains and 2 kW of cooling the great outdoors. But in the long run, all of the heat in our room will escape outdoors, and either way we will have used 1kW times the number of hours we ran either system of actual power that had to be generated somewhere. So no, the planet will not be frozen down, but we might be able to get by with less power used for heating. The actual output from the heat pump will depend on how cold it is at the outside end, the above example would be typical enough for where I live, where the air temperature outside rarely drops below freezing, but you will get less in a very cold climate. Heat pumps are pretty good but TANSTAAFL still applies. There is a practical limit to how cold the outside end can be allowed to get, since it has to be able to provide energy to evaporate the working fluid. Allowing it to be covered in ice will not help the heat to flow in. (Remember that even freezing point is a long way above absolute zero, so there is still heat available at freezing point, provided it can flow to your outside end.)

Pumping heat to a high temperature to generate power comes under the same general classification as pulling yourself up by your bootstraps. An ideal heat engine running off such a heat pump would generate just enough power to run an ideal heat pump, with no heat or mechanical energy able to be spared for actual use. Of course ideal heat pumps and heat engines are not actually possible.

There is an idea for how to extract energy from heat in the oceans, it would rely on the difference in temperature between the surface, especially in the tropics, and the deep ocean. But there is only a small temperature difference to work with, and done on a large scale it might interfere with ocean currents. This should be a particular concern to you guys in the UK, since the only reason the place is inhabitable is because of the Gulf stream.


Thread: Texas Instruments TI-5020
12/11/2021 16:41:05

Any sort of processor with registers in it is liable to wake up with them in a random state. The usual way of dealing with this is to apply a power on reset. Typically this involves holding a reset line low until there has been enough time for the power line to become stable, eg switches have finished bouncing and so on. A common way of doing this is for the reset pin to have a capacitor to ground and a suitable resistor to the power rail. Once the capacitor has charged enough the system comes out of reset in a known state. This sort of scheme is much harder to apply to a solar powered device, where the power may come up very slowly. So it becomes easier just to rely on the user to press reset.


Thread: GPS as a low-speed Speedometer
08/11/2021 05:54:13

Ady that is no longer true. The feature was called selective availability, it was turned off in May 2000. You can now get the full accuracy that the system is capable of, provided of course that you aren't standing under wet trees.


Thread: Replacement for thunderbird (emails)
04/11/2021 05:15:28

Microsoft is of course renowned for their great security.... /sarcasm mode off.

Thread: Unimat 3 Restoration
04/11/2021 05:12:52

The standard tommy bars are 60mm long by 4mm diameter. So far as I can tell, the material is a bit better than mild steel, they can be bent with over enthusiastic use but normally are OK.

I would suggest though that you look through the old magazines, I had an article published about twenty or so years back about making C spanners to suit the Unimat. These are not hard to make, although the one for body of the three jaw does need an awkward cutout to clear the jaws. and they do end up making things a lot easier. There is one to suit the pulley end of the spindle, two for the three jaw, two for the collett chuck, and one to fit the milling attachment spindle. I made a lever actuated tailstock for mine too, ver useful.

I don't think the belt drive to the fine feed is at any sort of angle, I looked at mine just now and it all looks square. The body of the attachment is a funny shape but when it is on the machine the pulley shaft looks to be parallel to the base and square to the spindle. I've got a mini rotary table made out of one of those. Of course that belt is only an O ring, but it works OK.


03/11/2021 20:20:03

I was hoping that you would put a rack feed on and convert the feed screw to a proper screw cutting setup with change wheels. I could measure up my fine feed if Emco don't come through.

They need a better belt drive too, I mentioned on the current SL thread how I converted mine to toothed belt drive. Poly V would be another good way to go.


Thread: Unimat SL modifications
03/11/2021 20:09:38

I made a Lammas three way tool post for my U3, that works quite well since it is less prone to foul the tailstock than the four way. I had already made the four way from Gerald Wingrove's book first.

The limitation on cut size on these small lathes isn't actually rigidity, it is the belt drives and the motor power, and also the lowest speed tends to be too high. I changed mine to a toothed belt drive, and it also has an extra countershaft where the motor used to be, and a pair of two step pullieys back to the motor. This gives eight speeds altogether, plus the motor now is a very small three phase unit with a vfd. (I think it is 1/8 hp nominally.) Not that you would use it to take really heavy roughing cuts, but it is much more capable now.


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