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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: What to drive a J & S with ?
11/09/2021 10:25:39

I can't help with the original query as I run my surface and cylindrical grinders from a true 3-phase supply.

A VFD that inputs single phase 240VAC and outputs 3-phase 415VAC is conceptually simple from an electronics point of view. However, I've never seen one for sale commercially. One issue would be that the maximum power rating would be fairly low due to the limited power that can be drawn from a single phase outlet. I also suspect that there is little, or no, industrial demand for such a device, so they're not produced.


Thread: cutting threads
11/09/2021 09:41:11

The OPs lathe is stated to be a Student, so I'd agree with DC31k, most likely a 6mm pitch leadscrew. My imperial lathe (Harrison M300) has a 4tpi leadscrew, the metric version is 6mm pitch.


10/09/2021 22:03:39

It must be a metric lathe? Metric thread pitches are not necessarily simple multiples of the leadscrew pitch. So a metric thread dial indicator has a choice of gears, five on the metric version of my lathe (Harrison M300), to simplify the maths. The chart on the indicator should list the gear to be used for a given pitch as well as the numbers at which the half nuts should be engaged.


Thread: origin of CAD
10/09/2021 14:10:28
Posted by John Alexander Stewart on 10/09/2021 12:39:14:

......Quaternions for at least 150 years.......

To be precise 1843, introduced by Hamilton. When I was messing about researching on the computation of the FFT I looked at quaternions (which can be viewed as 4-dimensional numbers) in order to simplify the maths for higher radix implementations of the FFT. It turned out to be a duff idea as quaternions are non-commutative in multiplication. I also looked at Cayley numbers (8-dimensional) but they're non-commutative in addition as well.

I am old enough to have started in 2D with pencils and then Rotring pens and then 2D CAD. I had no trouble at all in transferring to 3D CAD. It's far more logical than 2D CAD. The world is inherently 3D while 2D only captures limited representations which can be prone to mis-interpretation.


Thread: What Did You Do Today 2021
10/09/2021 12:44:48

Finally got around to machining the underside of the flanges on my traction engine cylinders:


The piece of steel sheet against the angle plate is to tilt the cylinder slightly so that the vertical centreline of the pistons lines up with the crankshaft. In the CAD model the angle is 0.64°, but I settled for anywhere between 0.6° and 0.7° on the digital angle gauge. The backward tilt of the cylinder mimics that of the full size engines, where the designer presumably ran into the same problem of things not lining up.

Roughing cuts were 40 to 50 thou deep and 10 to 15 thou feed per rev at 54rpm. On the finish cut feed was reduced to 4 thou per rev, which took for ever feeding by hand.

Fortunately when mounted on the engines both cylinders slope back by 0.6° indicated and the measured distance from flange to top of the cylinders is within a couple of thou of the CAD model value of 8.07". So there's no excuse for everything not lining up.


Thread: Hi
10/09/2021 12:24:29
Posted by Squarepeg on 09/09/2021 16:39:45:

.....making them multiple times due to lots of schoolboy errors in machining...

That happens to me too. I'm building two identical engines, but produce enough scrap to have made three. That's due to mis-reading drawings, making dumb mistakes or set ups not working the way I'd hoped. At least using CAD prevents what I'd call avoidable mistakes such as holes in mating parts not lining up, or parts interfering with each other. It also helps eliminate errors in the official drawings, and there are a lot of those!


09/09/2021 16:28:39
Posted by Squarepeg on 09/09/2021 15:10:01:
Being a design engineer means it's easy for me to quickly model stuff up and draw it rather than have to figure it out as I go along, I can deal with most issues on the screen and then just make the parts.

Welcome to the forum.

That's how it should be. Sort it all out on paper, or in CAD, first to iron out any problems and then parts only need to be made once.


Thread: It would be nice to know........
09/09/2021 15:01:58
Posted by Mark Simpson 1 on 09/09/2021 14:55:40:

You would have thought than an idler somewhere would have been cheaper than the switch gear......

An OTS switch is way simpler and, cheaper, than trying to fit an idler into the head, especially without making the head bigger. Why do import lathes/mills use variable speed drives? Not because they're inherently better but because electronics is way cheaper than gears or belts and pulleys.


09/09/2021 10:10:09

All my milling machines have reverse and all the above apply. I use reverse for tapping on the CNC mill. I have reversing tapping heads for the vertical mill, although I have used reverse and handraulic control when It wasn't worth setting up a tapping head. I also use reverse on the vertical and horizontal mills depending upon which way it is convenient to run cutters and boring heads, like this:


My vertical mill has a single stage backgear reduction, so to keep tool rotation the same the motor needs to run in reverse when backgear is engaged.


Thread: Multimeter recommendations
08/09/2021 09:33:58
Posted by Joseph Noci 1 on 08/09/2021 07:56:08:

........must be a real lapse in focus to measure volts with the meter set so........

We can't all be perfect. embarrassed


08/09/2021 04:01:11
Posted by Paul Kemp on 08/09/2021 00:24:12:

.........Long story short there was a massive flash and a bang and leccie was on the floor still holding the two undamaged leads...........

Almost certainly had it set on DC current, been there done that. Many years ago I went to measure a 730V DC bus with my mid-range Maplin multimeter. Unfortunately I had it set on the 10A range. Despite the range being fuse protected the multimeter literally exploded. The range selector knob was found on the other side of the room.


Edited By Andrew Johnston on 08/09/2021 04:09:12

Thread: Cast iron cylinders
07/09/2021 11:11:45
Posted by not done it yet on 07/09/2021 10:28:44:

Either way, it is generally accepted that wearing surfaces are of different metals?

An exception is cast iron on cast iron due to the graphite. On my traction engines the pistons are cast iron running in cast iron liners. Similarly I've re-designed the slide valves to be cast iron, in a cast iron cylinder, to reflect full size practise.


Thread: Why do designers do this!!
07/09/2021 10:24:50
Posted by ChrisH on 07/09/2021 09:49:16:

Because they are ignorant with absolutely no practical experience.....................

That in itself is an ignorant comment.

I'd agree with Nigel, but go further and say they definitely dance to tune of the end user, who often buys on price.


Thread: Bench grinder - wheel grit
06/09/2021 16:48:18

I use an 80 grit Alox wheel on one end of my small bench grinder for HSS tooling. The other end has an 80 grit 'green' wheel, mainly for TIG electrodes. I also have a larger bench grinder with a 220 grit wheel, but that's really too fine to be useful. After grinding I use a diamond hone to clean up the edges after grinding, and bring to size if needed. If a lot of material needs to be removed I use the mill and a carbide cutter.

Dry cutting only refers to the use of flood coolant, not a toolbit that's been dunked in water to cool it.


Edited By Andrew Johnston on 06/09/2021 16:49:27

Thread: Urgent - opinions of lathe I am going to view/buy
02/09/2021 16:44:06

Posted by Andy Thompson 3 on 02/09/2021 15:34:10:

This Boxford has come up and there is something peculiar about it. I was going to view/buy for £750. Am I wasting money and getting into trouble?


Whatever model it is, it's a basic version with no power cross feed or quick change gearbox. There seem to be some controls missing from the headstock and elsewhere. No evidence of a set of change wheels or quick change tool holders.

It might be restorable, but at the moment it's a basket case. In addition the seller being evasive is always a deal breaker for me. It's possible that his father used it, in the same way that a little old lady used the sports car for sale just for trips to the shops. More likely the seller knows exactly what it is and is looking to palm it off onto an un-suspecting buyer. Personally i wouldn't go to look - it's a powerful pull thinking that if I don't buy I've wasted six hours driving and fuel.


Thread: 5 Rotary Table/Tailstock/Chuck Kit Info/Questions
02/09/2021 11:34:08
Posted by JasonB on 02/09/2021 11:21:43:

Unfortunately said screw would get in the way of machining with the 1.5mm dia cutter..........................

Oh dear, it seemed obvious that the screw would go in from the bottom of the work so I didn't specify same.


Thread: Newman Motor Star To Delta?
02/09/2021 11:29:43
Posted by John Haine on 01/09/2021 22:53:35:

So a star connected motor expects to see three phase wires and one neutral wire..........

None of my machine tools, running on a 3-phase supply, have a neutral connection; just the three phases and earth. In theory at least for a motor running in star there would be no current flowing in a neutral wire connected to the star point, which is why it is not connected.


Thread: Cleaning my lathe
02/09/2021 11:20:18
Posted by petro1head on 02/09/2021 11:06:28

Spring clean, so it looks good as new

Never, but once or twice a year I do a more thorough clean, checking adjustments and oil levels. My lathe is ex-industrial so it's never going to return to showroom condition.


Thread: Using a ginding setup on a lathe
02/09/2021 11:15:14

It is possible to turn hardened silver steel with carbide inserts or, even better, with CBN inserts. See here for results:

Turning Hardened Silver Steel

I've not tried turning HSS, but it's certainly possible to mill HSS with ordinary carbide cutters.

If a material is really tough then I'll use my cylindrical grinder. smile


Thread: 5 Rotary Table/Tailstock/Chuck Kit Info/Questions
02/09/2021 11:05:06
Posted by Dr_GMJN on 01/09/2021 21:47:49:

......the first part I’d use it for is the small (c.30 mm) diameter aluminium output shaft blank for my son’s rc car (as per earlier thread) The Chuck would seem the obvious choice to hold it, but the height restriction could be an issue. What would be the best way of centering and holding a piece like that direct to the table? Make a temporary base plate and screw the bar vertically to that, then clamp the plate to the rotary table? Or something like that?

I'd concede that a chuck might be the simplest solution, but not necessarily the most accurate. My rotary table has a 1" parallel hole in the centre, which is way more useful than a Morse taper. So I'd make a 1" spigot with a flange for clamping to the table and a recess to locate the work. There would also be a hole for a screw into the part to be machined.


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