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Member postings for Billy Mills

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

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Thread: Finished model too good
11/10/2010 12:44:33
Some very early cars were built by "coachbuilders" on chassis by car makers. So they might have been brush painted on a very small scale. I doubt if any medium scale commercial outfit a hundred  years ago would use brushes when spray painting was available. Spray painting being fairly widespred by 1900.
 
Perhaps it is the paint that has changed, paint chemistry has changed a great deal over this period. Rubbing down and multiple finish coating were common in the 1960's but rising labour costs stopped that. It was fairly common at that time to have a complete car respray after accident repair or even just to improve the finish. It could take 1-3 weeks to  strip, prime and then colour coat with all of the rubbing down in between and drying time so the labour costs would be astronomic today.
 
Came across a "realism" spray last week at the NEC It appears to be watercolour based paint in an aerosol can. After you have finished your brand new beauty you spray on some grott to make it look used. Hardly a new idea! At least you can wash it off. The BBC used to use cold coffee solution to "dirty down" on location.
Regards
Alan.
Thread: A strange fluid
11/10/2010 01:48:45
When you publish a circuit it should:-
a) work as stated without tinkering.
b)be reproducable with readily available components
c)be  readable immediatly
For a non-specialist readership, simplicity is also very important.
 
Having built a couple of sinker EDM's for producing punch tooling, building a worthwhile machine is not difficult.  The electronics are very simple, for a sinker the aim is to keep the tool-workpiece gap constant as errosion continues. So you use the gap voltage as a too close/too far indication and drive the motor accordingly.  Capacitors are not a problem but the circuit produces HUGE amounts of electrical noise. If you don't know -or are not  told- how to control  the noise then you might have all sorts of problems. The layout and wiring is very important. Flushing the active region is also very important for longer cuts.
 
A key mechanical element is to  minimise backlash. The working gap is around 0.1mm for some jobs so you need to position within -say- 0.01mm to get good gap stability and cutting rate. If there is too much backlash then you end up with "Hunting" as the gap opens and closes excesivly with only the odd spark instead of a steady stream.
 
It is absolutly silly to have 200Hz vibration. When working correctly you should have hundreds of thousands of sparks per second each cutting away at the workpiece in a controlled way. Opening up the gap at 200Hz rate totally messes up steady cutting.  ULTRASONIC drive is used - at around 200,000Hz by some to cause dielectric pumping- to get rid of the conductive particles out of the gap. There is loads of info for the interested on the web.
 
Regards,
Alan.
 
 
 

Thread: Rhoestats
08/10/2010 23:13:10
Ray
Would you be better off running a constant current supply instead of a constant voltage with dropper resistors? One way of arranging this is to have a number of -say- 1A current sources, you then switch in as many as you need to get the current desired. Or if you wanted to be digital have 1A,2A,4A, 8A sources seperatly switchable.
 
 For a 1A module you might use  something like a TIP120, output from the collector , emitter to -ve supply via 0.68 ohms  1W . BC184 or any other small NPN , emitter to negative supply, base to TIP120, collector to TIP120 base. A 4k7 from the last node to + supply completes the job. You can mount all TIP120's without insulators on a common heatsink. Wire a switch between -ve supply and the TIP120 base to turn off that current source. Dirt cheap, simple and reliable.
As you might be warming a heatsink a fan might be good news. Nowadays instead of spending a fortune on finned ali extrusions,  people are using smaller ali heatsinks and blowing air over them ( the heatsink should get the airflow FROM the fan for best cooling as the flow is turbulent).
Regards
Alan.
 

Thread: A strange fluid
08/10/2010 18:04:46
I would agree with Howard that the aggro over one misspelt word is a lot of effort over very little. EDM is however an important topic in that:-
You can build a useful machine at home with little outlay.
You can machine fully hardened metals as easily as soft metals
You can cut  splines or rectangles or any other complex shape.
The  EDM process is potentially very precise.
There are still lots of facets of EDM to be explored.
 
There are several published designs that are straightforward and work  well. But there are some issues with the Walter design. Firstly anyone trying to duplicate the design  would find that:-
  1) paper capacitors are obsolete ( for about 40 years)
2) The SAA1027 is a 1970's part also obsolete
3) 48 step 12V motors are virtually obsolete, 200 step lower voltage parts are the norm.
4) there are no mains fuses at all.
5)The ammeter shunt is a hopeless case.
6)The vibrator is ill conceived to say the least
7) The complete lack of EMC filtering is a disaster for those living near.
8) The limit comparitors and bistable are confused and overdone, you only need half the parts to get the same results.
 
It is conventional to use standard symbols for circuit diagrams and to include component values on the diagram so that the diagram can be read- just   "R1" stops us understanding the mind of the designer.
It is completly understandable that stuff happens when you have 3 mags to edit each month. Equally "mechatronics" - the fusion of electronics and mechanics- is increasingly important and interesting. So we want more interesting articles of all kinds- some of which will have some electronics content.  So I would sugggest to David that it would be good to have specialist project parts reviewed by a reader or two who could assist in shaping an article for publication.  I offer to do that.
Regards
Alan.
 
07/10/2010 19:42:13
 I would very much agree with Jeff. The article describes a one-off design that is somewhat old. Paper dielectric capacitors are a very strange choice, they have not been made in any numbers for many decades and are obsolete devices. The basic design is obviously a test bed to explore various concepts- such as vibration. However the basic mode of operation -RC relaxation- is not an optimal choice.
 
The use of vibration is interesting, the advantage is that if the mechanics are very rough  vibrating one electrode results in some sparking even though the gap may be way out of a sensible distance however it does mean that most of the time the gap is wrong , the discharge rate is 10%-1% of what it should be and the surface finish is much rougher that it could be. That suggests that the positioner had excessive backlash, vibration was used  in an attempt to overcome too coarse stepping or backlash.
 
We must await the rest of the series however it is very disturbing to read that the writer requests help with a resistor value- such issues should be sorted before publication. It would have been very interesting to see details of the much more modern design that was at the last MEX.
Regards,
Alan.

Thread: Visit to London
26/09/2010 18:47:42
Four more for the list:-
Imperial War Museum Lambeth
RAF Museum Hendon N London
Dehavilland Museum near J22 M25
London Transport Museum
  
One place only would have to be the Science Museum for non clock people or
Greenwich to see the real Harrisons running ( except H4) . Would agree to read the book however the Channel 4 film Longitude is stunning from the brilliant tracking shot start to the last frame.
 
In 12 hours, Tube to  Science museum then tube to Westminster, boat down Thames to Greenwich, past Cutty Sark remains to see clocks and the meridian lines then rush to airport.
 
Regards,
Alan.
Thread: Ignition coils for small engines
22/09/2010 16:41:10
Flourescent chokes are NOT designed with Kilo Volts in mind, they have pile windings so a varnish insulation breakdown is on the cards. If you want to wind an inductor for High Voltages then you wind in a layer then insulate before winding the next layer or you wind in a former that has slots i.e. small partitions along the core to insulate the windings. Impregnation with a good epoxy is not a bad idea.
 
Backlight invertor transformers ( from LCD displays) produce about 2.5KV pk-pk in a very small high frequency transformer ( 30-60KHz). They might be a starting point for a small ignition system. There are certainly enough of them thrown away by people who service displays.
 
 
Thread: Angled HSS toolholder and tool sharpening angles
19/09/2010 20:50:06
Frank,
Next time you face off anything in your lathe, the centre pip is more exact than several measurements added because it is the real thing!, when you have exact centre height the top tip of the tool will be right through that pip, slightly too low and you will get a minute pip.
 
The  finish you get is down to the finish of the tool edge, rough grinding -lots of scratches that will be partly reproduced in the job because only the high spots leave a lasting impression.Try stoning the edge and compairing the difference.
Regards,
Alan.
Thread: Filing Machine
19/09/2010 12:12:18
 Axel
The Proxxon MBS240 mini bandsaw has a diamond blade option -needs a water drip tank for the coated blade. Not cheap but useful for miniature work.
Regards,
Alan.
Thread: Ignition coils for small engines
16/09/2010 00:40:36
Andrew,
The spark gap voltmeter is easy to build and a good rough and ready indicator of what an ignition system is doing. If you use balls from a race that are not too chewed you will get around 2.5KV/mm for balls around 5mm. The advantage of balls is that they are more reproducable than points and less subject to wear. ( you get field emmission with a few volts and a sharp enough needle) Some people put the gap inside a glass tube to  a. jazz it up  b. improve the stability  c.pretend that they are Dr Frankenstein.
 
I would not use a scope probe unless you run into a gap to avoid the risk of flashing the series resistor of 1000M ohms ( if it's a 1000:1 probe). Most  "Car Ignition Scopes" use a coupling capacitor of a few pF ( turn or so of well insulated wire around EHT lead or a fancy clip) into a few M of coax with -say- a reliable 2.2nF capacitor across the coax to make a 1000:1 capacitative divider. You don't use a 1000M resistor because there are no low frequencies to see, the -3dB point being around 70Hz.  You calibrate the thing with the scope cal sig or an audio oscillator  or anything else handy- eg a multivibrator hooked up to the EHT lead ( coil disconnected) then adjust for 1000:1 division or whatever.
 
Normally you see a fair overshoot spike before the gap fires, you get to see what the mixture sees. The overshoot before ignition will jitter, it's the random delay before the channel in the gap ionises so the bigger the spike the less delay and the smoother she runs. Usually people trig the scope from the points/ Hall device. For more than one plug you bug the coil lead so you can see the different plug breakdowns sequentially( with a distributor). For seperate coils you run seperate taps to the coax inner.
 
It is a nice idea to always use DC scope coupling with a hv probe. If you go AC the series cap can charge toward   the  full direct voltage level ( 25KV on a CRT display) before it ionises and takes out the input stage of the scope.
 
regards
Alan.
15/09/2010 22:05:20
The Paschen curves are not really applicable to a spark plug because of the geometry of the gap, they are for large parallel and smooth plates in Dry gas. Dielectrics ( fuel-air mix in this case) break down in a complex way. For example UV will cause the gap to fire at a much lower potential as will ionising radiation ( eg geiger tube) or carbon particles from previous cycles. Water vapor will also lower the breakdown voltage as will any  combustion products.
 
 For a spark plug the electrodes have relativly sharp edges and accumulated very small carbon particles so field emission will set in long before simple breakdown as the voltage rises accross the gap. The ionisation  will then avalanche the channel accross the gap.
So you don't need so many volts but rough points help a lot. That is one way in which abrading the points really helps.
 
Regards,
Alan.

Thread: gundrilling, and old ME articles sought
14/09/2010 23:45:57
Thanks for the link Andrew works fine.  Spent a few moments trying to work out how it cuts, think I see it now. The clearance Vee takes the chips and oil away, one edge of the vee forms a cutting edge to sweep the outer part of the bore. The oil hole sweeps over the centre- it is way off centre so it removes metal left by the vee edge- it is the edge of the oil hole that is the inner cutting edge. ( like the cone-with-a-hole de-burring/ countersinking cutter) Is that right? It would have to start cutting in a bore as it is initially unguided by a point or chisel.
 
Regards,
Alan
Thread: Message service
14/09/2010 22:50:55
Thanks Jason and Neil
Fortunatly there was a nice number when I went looking for it! so thank you both  for pointing that out. But no message no number so you don't know  without being told!  Perhaps a quick user guide to the facilities would be useful - it's probably right under my nose at this very moment?
 
 
 
Thanks,
Alan.
13/09/2010 23:42:43
David
Would it be possible for members to get an indication of new mail messages when they login?
You have to go and look for new messages in the inbox- there may be quite a few people who have messages waiting who do not know about the service.
 
regards, 
 
Alan.
Thread: Chester's CQ2365/ 920
11/09/2010 21:55:46
Is there a crack in the headstock casing?if it has cracked then the bearings would move apart slightly under the preload. There may be some adjustment to preload the roller bearings, might be a selected distance piece on the spindle. Manual needed!
 
Would try mounting a dial by the toolholder  and then putting the plunger onto the face of a chuck, apply hands to the chuck and see what the deflection is when you apply force to the axis of the spindle.  Would then try same at 90 degrees on the side of the chuck and then on top to get an idea of  what movement might be there. Similar variations on the three axis would suggest no preload on the taper bearing. But don't use that hammer yet!
 
regards,
Alan.

Thread: help with seeing
11/09/2010 21:28:25

The earlier post  about mentioning points of light and dark lines is that it might help someone who reads this post. 

If you get a lathe then you can get the manufacturer's handbook and read lots of books about turning. However the issue of a pair of eyes is not associated with an owner's manual. You normally find out about bad things too late. Hence the comments which can be signs of serious retinal problems that can be contained if treated early.  It happened to someone close recently, we are glad we went to Moorfields in London.
 
Trivial detail... Benjamin Franklin is credited with having two lenses of different focal lenths cut in half then cemented to produce the first Bifocals.
 
The varifocals avoid Franklins join and don't tell everyone that you are wearing them! But as said it's what you can adapt to. Specsavers seem to have a very good policy on helping those who find the adaptation not so easy, after trying the others that's where we go now.
 Regards,
Alan.

09/09/2010 13:10:58
Peter
Perhaps you need medical advice on this one. Vari and bi focals do take a bit of getting used to. Some people adapt immediatly ( my SHMBO did), others have great problems looking down when walking. When you consider how the mind needs to adapt so that you -without thinking- 'look and see' what you want but you have different focal lenth lenses in  the way the adaptation is remarkable. But it may come after a period of strain during the adaptation to the optical changes.
 
Gordon's experience with glass reflects mine. I now tend to wear a visor type face shield when doing dusty or chipping work. I have a Trend visor with a fan and filter which is great for dusty work or for avoiding coolent aerosols. Some of the marks that can be found on visors well justify their use. It is only too easy to wipe spec lenses with a  cloth that may have some grit or swarf that you can't see because of dirty glasses!
 
Incidentally, floaters, hairy string and other unidentified floating objects are perfectly normal with aging. They are blood cells that have clumped together floating in the fluid near the retena. If however you see bright spots of light and perhaps a dark line across the visual filed then go to an opthalmic hospital as soon as possible. The spots are not the same as the jazzy lines during migrane.
 
regards,
Alan
08/09/2010 21:41:28
Have worn Varifocals for about 8 years now as my near point wondered away. There are quite a few different  permutations of focal lenths possible. Last time I went for a pair  with a wider reading region which gives the reading part much better focus. Although it cost a bit more the better focus is a great improvement , wished I had changed sooner.
 
I used to have polycarbonate lenses rather than glass however the polycarbonate lens are much more prone to scratching than the "crystal" resins now used so have gone resin lenses. Don't like the thought of glass windows. I use illuminated magnifiers and sterio microscopes a lot as most of my work is small. 

On the headaches front, CRT displays with low refresh rates are very bad news for some. Years ago I switched over all the displays at work to LCD, many office people noticed a great improvement in end of day tiredness.
 
If you still like or use CRT's then check that the refresh rate ( in display settings)  is set as high as possible- equal or above 75Hz. The efects of the high frequency flicker are not directly felt but do cause longer term tiredness and headaches. Some even have problems with the 100Hz flicker from ccfl and conventional overhead lighting. Mixing in some tungsten or DC led lighting can help.
 
Regards,
Alan.
Thread: BMS angle not 90 deg
08/09/2010 00:24:11
  Was told some years back that  BMS is hot rolled that has been pickled ( acid dipped) , washed,  then cold rolled to "clean it up". Light oil stops it rusting too much before delivery.
Perhaps it is the cold rolling that puts the stresses in that make it tricky. 
 
Another machining nasty is hard drawn brass which will take the tip off small drills. The HD brass was some 5mm from a model shop that was open Saturday morning when I had a big rush job needing 100 1.0m holes straight through. I demolished many decent HSS drills and some very nice solid carbide drills before I realised the cause. Some CZ121 brass then drilled just as you would expect.  
 
Regards,
Alan.
Thread: What is the ultimate lathe for model engineering
07/09/2010 17:10:15
One of the ways of accessing better equipment is to have a small business -say making a simple tool or a model kit. Advertise via a web site, do a few local shows where the costs are minimal. With a bit of luck you can afford whatever your heart's desire may be as a perfectly legit cost paid for by sales. Works for a lot of people and can be a lot of fun.
 
regards,
Alan.
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