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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: Tears as last Harrier jets leave Ark Royal
25/11/2010 08:48:51
Ah, but who cancelled the TSR2?
 
Regards,
 
Andrew
Thread: Chinese lathes
22/11/2010 22:59:17
To pick up the points made by Steve, I assume that most of the electronic goods that are assembled in China are actually designed by the relevant (western world) company. The goods are assembled in China and then sent to that company for sale by them. While rigorous quality control is required, the Chinese side is essentially assembly only.
 
I further assume that most of the UK retailers of Chinese origin machine tools do not design those tools, but simply import and sell what is supplied. There may well be some feedback and requests for certain features, but that's not the same as being the original designer.
 
Regards,
 
Andrew
 
PS: The Chinese assembly companies I've worked with are very, very keen to learn about the design process. When they do get to grips with it, that's when we really need to start worrying!
22/11/2010 21:32:00
Just a quick note to second what was said above: the ISO9000 system is not a quality system, it's a consistency system. It simply says you have a set of procedures, and that you stick to those procedures. This, in itself, is a good thing for production. But it has nothing to do with quality. If there is a flaw in the assembly procedures, the system simply ensures that that flaw is present in each unit.
 
Regards,
 
Andrew
Thread: Clarkson Chuck - removing the centre
18/11/2010 23:00:11
Chris,
 
I believe it is still possible to buy centres from Clarkson, or their successors, for some models of chuck. If you're interested I'll find out the details from a friend who tracked them down in an attempt to get a new centre for his old style 'C' chuck. They did send him one, but it didn't fit. In the end I made him a replacement from silver steel.
 
Regards,
 
Andrew
Thread: adjusting gib on machine vice
18/11/2010 22:55:07
I'd second the use of oil for lubrication; use slideway oil. Grease is much too thick, and the swarf will stick to it like s**t to a blanket.
 
I used to spend ages adjusting the gibs on my machine vice to minimise jaw lift, and then it was stiff to operate. In the end I bought a vice that doesn't have the problem, or at least I can't measure it.
 
Regards,
 
Andrew
Thread: Things they don't tell you in the books so I'm asking you lot
18/11/2010 11:47:18
Ian: Pheweeee; had me worried there for a moment. For roughing on 2" diameter soft steel I'll use around 370rpm, 0.1" DOC and 0.01"/rev feedrate. That gives a power required at the tool of 2.3hp; my lathe is a nominal 3hp. Certainly makes the motor change note!
 
Sam: I concur that OD means outside diameter; at least it's not one of those annoying TLAs
 
Adam: Welcome to the forum and congratulations on your academic success; even if it was at the 'other place'. Having supervised undergraduates for the Cambridge Tripos I can confirm that the Tripos is also heavily mathematical.
 
I might be inclined to say that, to some extent, any situation amenable to a closed analytic solution probably wasn't worth analysing in the first place.   A corollary of that is that what is taught on engineering courses is perhaps influenced by what can be easily examined at the end. But then I'm just an old cynic.
 
One thing that doesn't seem to be taught are the real aspects of engineering design, which include:
 
You've got X time to get it designed
 
It has to cost less than Y
 
We've only got 32 slots on the assembly machine, so you can only use 32 different components

We can't get that part or material in the time available, use something else
 
We just moved the goalposts - it gets much more 'interesting' when the shape of ball, and hence the whole set of game rules, gets changed
 
Ain't engineering fun! 

Regards,
 
Andrew

Edited By Andrew Johnston on 18/11/2010 11:47:47

17/11/2010 11:36:31
Hi Ian,
 
Blimey, 0.2" DOC on steel! I'm obviously being a big girls blouse when it comes to roughing cuts on the lathe. What sort of speeds and feedrates are you running at that DOC? And how many horses power your lathe?
 
Regards,
 
Andrew
Thread: Which New Lathe; choices, choices...
16/11/2010 22:09:55
Hi Edmund,
 
Ah, didn't think about landing fees. I've done my bi-annual renewal flight a few times from Bourn, but I've only flown in there once from somewhere else. That was in the Pawnee, to drag a glider out, and being in the evening, there wasn't anybody there to collect a landing fee!
 
I'll PM you with contact details.
 
Regards,
 
Andrew

Edited By Andrew Johnston on 16/11/2010 22:10:22

Thread: Things they don't tell you in the books so I'm asking you lot
16/11/2010 16:37:11
Peter,
 
Just because you can feel the ridges on a surface finish, doesn't mean that the finish is inadequate. It's what is appropriate for the circumstances.
 
If you look in my odds 'n' sods photo album there is a picture of a Rubert surface roughness scale. On the little rectangles in the bottom right, for milling and turning, the first few blocks have clearly visible ridges. I doubt the makers would have bothered to do this if those kind of finishes were not acceptable in some cases.
 
Surface finish is as much an art as a science, which is why there are so many ways of defining it. As I've recently found out even professional machinists don't always understand it. I think it is very difficult to convey the different surface finishes via pictures. The 'fingernail' test is remarkably sensitive, which is why the Rubert scales were produced in the first place. See this link:
 
http://www.rubert.co.uk/Comparison.htm
 
(I don't seem to be able to post it as an actual link, grrrrrrrrrrrrrrrrr!)
 
Regards,
 
Andrew
16/11/2010 13:19:29
Hi John,
 
May be I'm cynical, but in my experience a good number of professional engineers are completely clueless when it comes to actually making things.
 
Only yesterday the managing director of the company that does some of our electronics assembly was telling me that they had turned some assembly work away as it simply wasn't possible to assemble the items as designed. Well, not without the aid of a sledgehammer. Even more worryingly the engineers that had 'designed' the assemblies couldn't see what the problem was until it was forcibly pointed out.
 
Regards,
 
Andrew

Edited By Andrew Johnston on 16/11/2010 13:20:09

Thread: EN1a Copper Soft soldering
16/11/2010 12:21:08
Lead free solder is 'orrible! It doesn't flow and wet the surface when molten. While a bad joint looks dull a good joint looks ...... dull. I was talking to our electronics assembler yesterday and he hates lead free too!

Within Europe any new electronic product to be placed on the market must use lead free solder. However, medical, military, large scale industrial and safety critical automotive systems are exempt. So, if it's a safety critical product and/or requires high reliability, it can use leaded solder; go figure that one out!

The regulations do not apply to any product on the market before the stipulated date. Nor do they apply to the repair of such products. Prototypes, experimental and hobby kit are also exempt.
 
So it is still perfectly legal to buy and use leaded solder for electronics as well as other uses.
 
Regards,
 
Andrew
Thread: Things they don't tell you in the books so I'm asking you lot
16/11/2010 11:52:46
Just out of interest, while having my 11am coffee break, I've measured a few of my turned surface finishes. The range was from 0.2µm Ra to 6µm Ra. This was on a mix of aluminium, stainless, EN24 and cast iron. A fingernail test against a Rubert scale confirms that the results for the rougher finishes seem about right. I'm not surprised about the rougher finishes; I'm deliberately using fairly high feedrates, about 0.2mm/rev, but the finish is appropriate for the purpose.
 
I've learnt the hard way that to get a good finish on steel with carbide inserts you have to run at the appropriate surface speeds, otherwise the material tears.
 
The provenance of the material is also important. The first steel (EN1A from company X) that I turned on my lathe gave a very poor finish. I tried varying everything, speed, feedrate, depth of cut, coolant - no effect. The material would turn nicely for a inch or more, and then tear. I was really disappointed and upset, having just invested a lot of money in the lathe. Coincidentally, at the time at work I was liasing with a local machine shop to get some CNC milling done. I asked them about metal suppliers and they said, unprompted 'oh we don't use X, their steel is rubbish, we use Y'. I bought some EN1A from Y, and what a difference! Nice finish and consistent along the whole length, about 500mm. Companies X and Y are both trade suppliers. You live and learn!
 
For roughing I'll use a DOC of between 0.04" and 0.15" and a feedrate of about 0.01"/rev. Finish cuts anything from 0.001" to 0.04" and feedrates from 0.002" upwards depending upon the finish and accuracy required.
 
Right, back to work and PCB layout.
 
Regards,
 
Andrew
15/11/2010 22:27:45
Surely the surface finish should reflect that required by the function of the part?
 
Regards,
 
Andrew
Thread: Which New Lathe; choices, choices...
15/11/2010 22:22:05
Hi Edmund,
 
I'm not directly familiar with the lathes you're considering, but if you'd like to have a cuppa and a chat about lathes, and flying, feel free to drop by, I'm not that far away. You can even nip down in the Jabiru, I'm right on the edge of Bourn airfield.
 
Regards,
 
Andrew
Thread: Things they don't tell you in the books so I'm asking you lot
15/11/2010 13:30:55
Any ordinary screw and nut has to have a certain amount of clearance to work without binding. So, if you're turning a screw one way and then reverse it, the screw will turn a little bit before the nut starts to move in the reverse direction. Hence the cross slide and topslide on a lathe will have a deadband where the dial turns but the slide doesn't move.
 
Points to note:
 
All manual lathes, even as new, have some backlash
 
If you're turning the dial in one direction only then the backlash is irrelevant
 
If you reverse the dial far enough to reverse the slide movement, and then move back in the original direction to the starting number on the dial, then on any half-decent lathe the slide should be back in the same place irrespective of the backlash
 
Some bigger, industrial size, machines have backlash adjusters on the screws; basically two nuts that can be moved relative to each other
 
Some people get their knickers in a twist about backlash - unless the screw (or nut) is so worn that it's about to fail, you don't need to worry about it
 
For reference, on my 30 year old industrial lathe the topslide has backlash of about 25 thou, and on the cross slide the backlash is about 15 thou. It makes absolutely no difference to using the lathe and the accuracies to which I work.
 
Regards,
 
Andrew
Thread: Smileys
15/11/2010 11:47:15
Norman,
 
Nothing going cheep, but I do have a dead parrot for sale, bought as seen, no refunds......
 
Whether my kit is nice, or not, is probably a matter of opinion. But we can say it's a weighty issue, no point in turning up with a transit van, most of my stuff would go straight through the floor, even if it went in.
 
Regards,
 
Andrew
Thread: Things they don't tell you in the books so I'm asking you lot
14/11/2010 22:35:08
Here's my twopence worth:
 
Swarf: I doubt the professional scrap metal dealers will be interested in the amount of swarf that a model engineer will produce; especially as it is likely to be mixed up. When I have reasonable amounts of swarf of one type, say more than 50kg, I dispose of it via a friendly local machine shop. Otherwise I take the mixtures to the local council tip, where they have containers for scrap metal. No-one has objected yet, even when I dumped loads of fine cast iron swarf which blew everywhere.
 
Turning:  If it's not the final cut, I just return the tool while running and accept a fine spiral; it'll get removed on the next pass. If it's the final cut I just back the tool out and that's it. How far do you back the tool out? If you're backing out by half a turn or more, it would seem that the lack of repeatability is unlikely to be caused by backlash.
 
Knurling: Are you trying to cut a straight or diamond knurl?
 
Regards,
 
Andrew
Thread: Smileys
12/11/2010 20:38:42
I knew I wasn't grumpy enough to be a professional machinist, but apparently I'm not grumpy enough to be a model engineer either. Ah well, might as well sell up the workshop and find another hobby, anybody for tennis? And no, I'm not going to add a smiley to indicate if I'm being tongue in cheek, or serious.
 
I'm with Martin and Keitth. Smileys are often overdone, but used sparingly they can add to the message. I still think that the available smileys are limited.
 
Regards,
 
Andrew
Thread: Over the Counter
10/11/2010 20:34:11
Even more worryingly it is not unknown for spares for commercial aircraft and engines to have all the 'correct' paperwork but turn out to be fakes.
 
Regards,
 
Andrew
Thread: Drilling of 34CrNiMo6 4340 817M40 En24?
10/11/2010 11:34:56
Sounds like you're way too fast on speed. Recommended speed for 4340 normalised, quenched and tempered, is around 30-40 feet per minute for HSS. For a 12mm drill that's around 280rpm. Feed rate about 0.15mm/rev. A HSS speed drill should be fine, but you must keep the feedrate going. By hook or crook you'll also need to get coolant to the drill tip. I've has no trouble drilling EN24T with HSS drills.
 
As 'ady' says, low speed, high torque (geared or belt drive head, no VFDs) and keep the feed on.
 
Regards,
 
Andrew
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