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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.

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Thread: Silver or Stainless?
22/10/2011 12:08:49
The temperature at which an iron-carbon 'mix' changes from ferromagnetic to paramagnetic is known as the Curie point. For pure iron it is about 770°C, and varies as the carbon content increases. The Curie point is not related to the phase change from BCC to FCC.
 
For low carbon steels the change in magnetism with increasing temperature is a useful indicator of the phase change temperature, but is only an indicator. The two effects are unrelated; they just happen to occur at similar temperatures.
 
Regards,
 
Andrew
19/10/2011 22:09:13
Just to confuse the issue; austentite is magnetic. It is paramagnetic, ie, it has a positive susceptibility to an external magnetic field. But the effect is so small that it cannot be detected with the sort of equipment found at home.
 
What most people call a magnetic material exhibits ferromagnetism where the forces involved are easily observed.
 
Just about every material interacts with a magnetic field; it's just a question of the strength, characteristics and observability of the interaction.
 
Regards,
 
Andrew
Thread: A Shattering Experience.........
05/10/2011 17:37:12
Here are a few further comments on the machining side. Sorry, if I missed anybody out.
 
JohnS: Yes, I was using a key. When I put the slitting saw on the arbor I did hesitate, but decided that since this was a 6" cutter, and I would be taking reasonable cuts, 10mm, it seemed better to use a key. May be I was wrong. In my limited experience when a slitting saw spins on a deep cut it breaks anyway, as I'm not quick enough to knock the power feed off before catastrophe.
 
Norman: I agree, but the casting is 1.5" thick at worst; I'm not that fit!
 
KWIL: Mating with the table was pretty good, I did run a file over the bottom of the casting, but it wasn't really needed. The third clamp played no part, as there wasn't one. What I will say in my defence is that it was never my intention to completely separate the casting with the slitting saw.
 
JasonB: Well spotted, there's my mistake, no wonder they're called saws rather than cutters. At no time did the saw get hot, or even warm. I dropped the clutch after each pass to check, and when I was picking up the pieces they were all cold. The cut surface is pretty flat; I can just get a 1.5 thou feeler gauge between a steel rule and the surface if I push a bit.
 
Les: Sound theories but I don't know which one might be correct either, although I lean to the second one.
 
RichardP: I can assure you that the clamps were correctly fixed. My mill will run in either direction; the control panel makes no claim as to which it considers forward or reverse. Even if I reversed the spindle direction, and the saw, surely it would still tend to lift the casting. The only way for the saw to push the casting down is to climb mill. I don't understand the comment on the rule; the numbers are purely arbitrary, the ruler is just to give a feel for scale. It happens to be at about 6" because that's where the rule was in balance on the casting.
 
Let me re-iterate that it was not my intention to separate the two parts of the casting with the slitting saw, but simply to ease the effort required by the hacksaw. Here's what I think happened. On the final cut I didn't leave enough metal, and as the cutter reached the end of its cut the casting moved, possibly a la Les's theory, and momentarily trapped the cutter, causing it to shatter, before the casting itself factured. On the second part I left about 6mm of metal and everything worked perfectly. No funny noises, hot blades or shattering experiences, just a nice clean cut and a couple of minutes work with the hacksaw to finish off.
 
Regards,
 
Andrew

Edited By Andrew Johnston on 05/10/2011 17:39:15

Thread: Taps & Dies
04/10/2011 12:08:48
DIP? Oh, yes, DIPs, I remember them from the distance past. SMT is all very well, but when you get to BGAs you start to need lots of holes; makes the PCB look like a colander. For BGAs with a 0.8mm pitch you can just about get away with 0.3mm vias between balls and 4 thou track and gap. But when you get down to 0.5mm pitch you need 0.1mm vias in the pad; that's a small drill!
 
Regards,
 
Andrew
 
Tongue in cheek comments really; I have just finished designing and building an experimental circuit on Veroboard for a client using mostly DIP ICs. Essentially it was an IQ baseband receiver. If it works in the field I'll probably get the job of converting it to SMT and laying out a PCB for it - no vias needed!
Thread: A Shattering Experience.........
04/10/2011 11:48:03
The RH crankshaft bearing housing and cover for my 4" scale traction engines come as one casting, and need separating before machining can commence. An easy job for a slitting saw. Here's a picture of the final cut on the first part:
 

Thirty seconds after I took this picture the saw shattered. Here's a picture of the bits:
 

Where's the rest of it? I don't know, I haven't found all the bits yet! The furthest, and largest, bit was found by the opposite wall, a good 12 feet away.
 
Cutting parameters were 41rpm, depth of cut per pass ~10mm, and a feedrate of 80mm/min, giving a tooth chip loading of 0.05mm. So what went wrong? I'm not sure, but here are some possibilites:
 
1) The gullet of a tooth got jammed with swarf due to an over-ambitious feedrate - seems unlikely since the cutter shattered on the last cut, and I've subsequently split the second casting using the same cutting parameters with no problem
 
2) The cutter wandered and then jammed - possible, but I haven't experienced this when using a slitting saw in the horizontal mill
 
3) The part being separated moved - this is my favourite; the cutter shattered when the final cut was almost finished. There is 'gouging' in the part (see picture below) that suggests that at least one tooth had broken before the cutter shattered. I suspect that the part moved, a tooth broke and that then precipitated the beak up of the saw.
 

When I split the second part I didn't try and cut all the way through with the slitting saw. I left about 6mm and used a hacksaw to finally separate the parts.
 
However, the purpose of this post is not to prove that I was an idiot, I'm sure you know that already, but to highlight the fact that when the saw did shatter it spread bits all round the workshop, even though the rpm wasn't high. So, when it says on the box 'Caution, cutter may shatter, wear eye protection', it means it!
 
Was I wearing eye protection? Actually I was, a proper pair of goggles from the local professional tool shop. They're £1.45p + VAT a pair; an absolute bargain! I highly value my sight, especially so since I've come close to losing it in the past for medical reasons.
 
My advise is to get some goggles and wear them, if you don't already do so.
 
Regards,
 
Andrew

Edited By Andrew Johnston on 04/10/2011 11:50:08

Thread: Taps & Dies
03/10/2011 18:51:55
Hi Neil,
 
Interesting; likewise most of my taps and dies are random buys, mainly on Ebay, with these exceptions. I have a complete set of ME taps and dies, a birthday present from long ago, although I rarely use them. Most of my metric coarse and BSP taps and dies have been bought new. Otherwise BSW, BSF, UNC and UNF are mix 'n' match! I'm probably in a minority of one in that I almost never use BA, so I have a very limited range of BA taps and dies.
 
Blimey, 1/32" drills; that's a bit big for PCBs isn't it!

I hope you have recovered your sang-froid and are now safely ensconced in the workshop.

Regards,

Andrew
Thread: A Challenge - How Would You Machine This Part?
03/10/2011 18:37:01
The denouement; here are all the parts for the complete differential:
 

And the assembled differentails; for scale the OD is about 12 inches.
 

Regards,

Andrew
Thread: MEW 182 - Wot no ...
01/10/2011 23:19:17
Posted by Stub Mandrel on 01/10/2011 19:48:28:
Congrats to Mick for taking on a tough task. Being largely self taught, I find articles by tme-served engineers/machinists really useful - there's always something to learn.
 
 
Quite so, but it's also interesting to read articles by non-professionals. Sometimes the trouble with professionals is that they 'know' it can't be done, but the amateur has no such knowledge or training, so he goes and does it anyway.
 
Regards,
 
Andrew
Thread: A Challenge - How Would You Machine This Part?
01/10/2011 23:12:19
I'll have you know the plants were all of the edible variety. Turns out that the courgettes were a bit of a disaster, but the peppers and chillies that are in the picture are still going strong in the greenhouse. Great in a salad and useful to give a curry a bit of zip!
 
Main problem with 'elf 'n' safety is that I'm not strong enough to lift the boilers onto the worktop. It was bad enough getting them out of the car and into the kitchen, even with the help of my neighbour and his son, plus a decent sack trolley.
 
I'm not even going to show the tool and measuring equipment store; sorry I mean the dining room.
 
Neil: Gears made using the parallel depth tooth method I assume? I'm planning to use the same to make the mitre gears for the governors on the traction engine. Partly because they'll be much too small to be practical by CNC and partly because I haven't used the method before, so it will be instructive.
 
Regards,
 
Andrew
 
Thread: Taps & Dies
01/10/2011 22:58:20
Likewise, I tend to use Tubal Cain's drill sizes for about 65% thread depth. However, for stainless steel I might choose to be nearer 50%.
 
Neil: I'm curious, who made your metric HSS set? On the whole I've only got odds 'n' sods bought off Ebay, except for metric and BSP where I've bought new as I need them.
 
Regards,
 
Andrew
Thread: MEW 182 - Wot no ...
01/10/2011 17:37:27
Posted by Peter G. Shaw on 01/10/2011 17:14:04:
 
What is a "honset" slip please? Is it a new type of slip gauge?
 
 
I think it's a new type of Chinese banana skin with added magnesium, or should that be manganese?

Regards,

Andrew
Thread: Taps & Dies
01/10/2011 10:44:30
Posted by Adam Gregory 1 on 30/09/2011 21:22:09:
Hi
May be a daft question but when using taps do you have to use the second or botton to get to final thread size or should the taper get to final thread size
 
 
An interesting question; and one on which it has proved difficult to get a definitive answer. The only reference I can find to the taper tap being undersize is in the WPS book 'Drills, Taps and Dies' by Tubal Cain. None of my other books, or the manufacturers' websites, mention it.
 
From a practical point of view I have never had a problem using just the taper tap to produce a 'to size' thread, thru-hole or blind. If thread depth is limited in a blind hole, for soft materials like aluminium, low carbon steel or cast iron, I follow the taper tap with the bottoming tap. I rarely, if ever, use 2nd taps. If a blind hole is too shallow to start with a taper tap then I use a spiral flute machine tap by hand. This will cut a thread to within one or two pitches of the bottom of the hole in one go. I do quite a lot of machine tapping, so I have spiral flute taps to hand anyway.
 
The 'progressive' taps mentioned by Mike are also known as serial taps. As stated they cut a deeper thread with each tap. So it is essential to use all the taps in sequence. I've
never had the need to use serial taps, but I understand they are intended for tough materials, like some stainless steels. The only place I am aware of that sells serial taps in common sizes is ArcEuroTrade.
 
Regards,
 
Andrew

Edited By Andrew Johnston on 01/10/2011 10:46:17

Thread: A Challenge - How Would You Machine This Part?
26/09/2011 11:33:52
KWIL: Looks a lovely job; I just hope my worm and worm wheel look as good. I don't have a hobbing machine, so will be relying on gashing and then free hobbing. But I must finish the differentials first.
 
Springbok: I hardly think 'er indoors is likely to ask me to do something in the house, when the kitchen looks like this!
 
 
Andrew
24/09/2011 10:46:43
Finally, after delays and interruptions due to having to work for a living, the complete set of bevel gears has been finished in cast iron.
 

Due to problems with the drawings, I threw away the castings for the bevel gears and machined them from cast iron bar. The spur gears were part of the original casting. The only thing left to do is cut a keyway in one of the bevel gears. It's a bit of bottom clenching job swinging the ram round on the Bridgeport, so I'll do it in one big session when I have other items ready that need keyways.
 
Regards,
 
Andrew
Thread: electronics
18/09/2011 22:12:35
Steve,
 
As a starter have a look at the notes in my photos area, under 'Semiconductor Devices'. It summarises the pros and cons of various power semiconductor devices up to moderate power levels, say a few hundred kilowatts. Above that thyristors rule OK!
 
If you're interested in further details PM me and we can continue, without me boring the masses.
 
Regards,
 
Andrew
 
PS: I'd vote in favour of an electronics sub-forum
18/09/2011 14:05:45
Posted by Steve Withnell on 18/09/2011 13:39:51:
Any books good on the design of the input stages to power / mos FET type devices?
 
 
For linear amplifiers or for use as switches? And what sort of power levels?
 
Regards,
 
Andrew
18/09/2011 12:29:53
I'll second the 'Art of Electronics'; good practical design notes and plenty of example circuits. I haven't seen the 'Student Manual', so I can't comment.
 
For trouble-shooting advice 'Troubleshooting Analog Circuits' by Bob Pease (ISBN 0-7506-9184-0) is excellent.
 
There are also a lot of very good apps notes and design guides available on the web from the semiconductor manufacturers. There are good analog design guides available from Analog Devices, plenty from Bob Pease at National Semiconductor and anything written by Jim Williams (late of Linear Technology) is excellent.
 
Regards,
 
Andrew
Thread: benchtop power supply
10/09/2011 18:16:36
Ah, my mistake! I read it as referring to the size of the reservoir capacitor on the input of the regulator; and I couldn't for the life of me see how that would affect the regulator.
 
Oh well, where's the dunces cap?
 
Regards,
 
Andrew
10/09/2011 12:13:54
Posted by Steve Garnett on 10/09/2011 02:09:41:
 
In fact a good argument could be made that having a larger cap on the output makes it harder for the regulator to work properly.
 
 
Steve: I'm not sure I follow that; can you elucidate?
 
Regards,
 
Andrew
Thread: Suitable high speed spur gears
10/09/2011 12:11:15
Posted by Richard Parsons on 10/09/2011 10:23:46:

The teeth are just 60° prisms an the do not slide as involute gears do
 
Oh dear, I thought that one of the advantages of the involute tooth form, in theory at least, was that the contact was a rolling, not sliding, one? Theoretically, for spur gears I thought that the rolling contact was a line, and for parallel helical gears it was a point.
 
I thought that one of the reasons that helical gears were used in high speed drives was that more than one tooth pair was in contact at a time, so they should run more smoothly than spur gears, where only one tooth pair is in contact at a time. If the spur gears were perfect this shouldn't matter, but with the inevitable backlash it leads to small impacts every time the line of contact is passed from one tooth pair to the next.
 
Regards,
 
Andrew
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