Member postings for Andrew Johnston

John S,

 

Makes sense, even dreaming spires need maintenance, unlike perspiring dreams.

 

Andrew

Can't say I've ever used a Drill Doctor, but the professionals don't seem to think much of them:

 

http://www.practicalmachinist.com/vb/cnc-machining/drill-sharpener-183618/ 

 

The cheap add-ons to bench grinders are useless. Other than that, I haven't tried much. Strange as it may seem I don't seem to need to sharpen drills that often. In the longer term I expect I'll either build some sort of jig for the surface grinder, or keep looking a Clarkson drill and tap grinding attachment.

 

Regards,

 

Andrew

Sid,

 

Ah, that explains a lot. I wondered why the power generators were so sanguine about the poor power factor of the bulbs, all is now clear. I got my first energy saving bulb free from the original Powergen, when it was an electricity generator rather than a supplier. They must have bought a cheap lot though, as it was rubbish and failed very quickly.

 

Are you in Canada or the good ol' US of A?

 

Many years ago I worked for a company that developed an electric vehicle drive system. At one end it had a large DC battery and at the other a three phase output to the motor. The system was bi-directional, so a three phase input on the motor terminals could be used to charge the battery. I can't say I was ever convinced about the large scale potential of electric vehicles, and this proved to be the case. In looking around for other applications for the system it was realised that if the three phase output was connected to the mains, and the battery was replaced by a large capacitor the system could be used as a power factor corrector, up to about 150kVA. The only losses would be due to switching losses in the inverter and ripple current in the capacitors. I thought this was an excellent use of the system, but sadly the company was closed before it had a chance to suceed.

 

Sam,

 

Thanks for the kind words, I'm glad at least one person enjoys my posts. I consider myself very fortunate that a peasant like me had the opportunity to go to King's and to do research at Cambridge. No thanks to my school though; it could all have been so different.

 

Regards,

 

Andrew

My experience of the new style lightbulbs is that they do reduce the electricity bill, but by no means as much as you might expect from comparing wattage ratings. The more expensive ones do seem to last longer, but I've had a number of the supermarket own brand ones fail very quickly. Also their power factor is appalling, ok for the consumer, but not so good for the electricity generator.

 

I'm also not entirely convinced about the overall energy savings, given that the bulbs are more expensive and complex to manufacture, and to dispose of afterwards. Which all takes energy!

 

Having said all that, I run them in all my most used lights, and as the other bulbs fail I replace them with energy saving ones.

 

Regards,

 

Andrew

Hi Guys,

 

Thanks for the information. Interestingly it pretty much mirrors that I've been given on other forums. The system is good, but it will not cope with heavy cuts. On the CNC mill I tend to run as close to the power limits as I can. Somehow I just can't see the Coventry system coping with that. The taper seems too short and too far away from the cutting forces to be stable under high cutting loads. I wonder if it is telling that the manufacturers own leaflets emphasis the ease of changing from centre drill to drill to chamfer tool, with 'nary a mention of milling?

 

While the time saved in tool changes would be welcome, I don't think the cost of the system is justified if I cannot also use it for heavy milling. Currently I use a Clarkson lookalike chuck with a rippa mill for roughing and use the TTS system for finish milling, drilling and tapping. I use a tension/compression head for tapping and hence use reverse spindle to extract the tap. Not too helpful if the tool holder unlocks!

 

The drawbar and washer that came with the Tormach were not of the best quality, so I suspect that making a new drawbar and washer out of hardened silver steel may be a relatively cheap way of increasing drawbar pull and thus easing the problem of the TTS holders pulling out.

 

Regards,

 

Andrew

I've been wondering about that too. At least I do have a profile; whether it imparts any useful information is another matter.

 

One thing to take into consideration, at least in Europe, is that everybody's light is a bit dim, now that the EU have banned the sale of incandescent light bulbs. It's probably not legal to hide it under a bushel either. After all it's not even a metric unit.

 

Regards,

 

Andrew

Neil, I don't know where you got the idea for air blast cooling from either! But the following is a quote from "Machinery's Handbook", so whatever else, you were not dreaming:

 

"Many high-speed steel tools are quenched in air, either in a stream of dry compressed air or in still air. Small sections harden satisfactorily in still air, but heavier sections should be subjected to air under pressure."

 

Regards,

 

Andrew

I have a Tormach CNC mill with an R8 spindle. Tormach have a proprietary tooling system (TTS) which uses 3/4" plain shank holders and a special 3/4" collet. The holders come with an integral collar that is pulled up against the spindle nose to give repeatable Z heights. The system is fine for drilling, machine tapping and light milling. However, I have had some issues with holders pulling out under heavy milling cuts.

 

As a replacement system I've been looking at the Coventry Easychange tooling system.

 

http://www.coventrytoolholders.co.uk/prods-easychange.htm#1

 

 The blurb says not suitable for heavy cuts. I have had varying viewpoints on professional machining forums as to whether the 1.5hp motor in the Tormach is capable of what would be regarded as a heavy cut. Given that the Coventry system is supposed to be for professional use I would have thought it would be fine with what is a fairly low power mill.

 

The system is not cheap though, and before I make a decision, I wonder if anybody here has any practical experience of this system, with either CNC or manual machines.

 

As a secondary consideration the Coventry system should be quicker for toolchanging. It can get real tedious when changing tools every few minutes on the CNC mill having to loosen the drawbar, change the tool and then retighten the drawbar each time.

 

Regards,

 

Andrew

Wot? The cheek of it! The grape vine supports will be a fully engineered product. Designed in 3D CAD, stressed to cope with the weight of grapes grown this year plus a safety factor, formed by guillotine and folder and TIG welded where required. And what about the project to design and make my own spark plug for the hit 'n' miss engine? Electrics or mechanics?

 

Regards,

 

Andrew

Hmmmm, where to start? There's always a danger that I'll get depressed if I write down all the things I haven't actually got around to doing yet, but here goes anyway.

 

Models:

Three quarters of the way through building an 'Economy' hit 'n' miss engine

 

Design, build and test a microprocessor controlled ignition system for the above engine

 

Not very far through building two 4" scale SCC Burrells from the LSM drawings, using the John Rex castings, most of which are still un-machined in the kitchen

 

Work:

 

CNC mill the prototype boxes for a battery management system

 

Design and weld up three stainless steel battery enclosures

 

Machine, or farm out to the professionals, all the other bits for said battery enclosures

 

Other Things I Got Talked Into:

 

Machine two wheel hub spacers for my neighbour's Moggy Traveller

 

Machine Tool Fixing:

 

Get the internal attachment running on my cylindrical grinder

 

Get the swivelling head attachment running on my cylindrical grinder

 

Weld up a wheel guard for the surface grinder

 

Fit a new coolant pump to the Bridgeport mill, to replace the two that have failed already

 

Design and make some bending rolls to make the wheel rims and 'T' pieces for the above traction engine

 

Design and make a cross slide guide for the hydraulic copy unit

 

Get around to fitting and using the Ainjest high speed threading unit

 

Wire up a new three phase switch and socket to the guillotine

 

Learn how to use the Clarkson tool and cutter grinder

 

Get the hardness tester cleaned and working, and work out what the hell the results mean

 

Finish repairing various Coventry dieheads

 

Other Stuff

 

Design, weld and fit some stainless steel brackets to the house, so that the grape vine will pass over the patio doors rather than draping everywhere, as at present

 

Drill and fit some stainless steel angle in the garden so I can get the Kiwi fruit plant out of the greenhouse

 

And so it goes on......................................

 

Regards,

 

Andrew

Strange but true, I did start building a three cylinder radial steam engine (Cygnet Royal, I think) during 'O' level metalwork. Of course I never finished it, but I do still have the bits somewhere. I also suspect that I was the only person in whole history of the school to use the shaping machine, while making parts for Cygnet. I had to use the shaper, as we didn't have any milling machines.

 

Of course any steam engine also deserves a microprocessor controlled steam whistle!

 

Regards,

 

Andrew

I hate centre drills! You've only got to look at the smaller ones and the centre bit goes ping. I use them only when I absolutely have to, in order to be able to use tailstock support on the lathe.

 

For general drilling I use Dormer 4 facet drills. On cold drawn, or machined, flat surfaces they do not need a centre drill to be used first. This applies both to manual and CNC mills. Can't say I've ever actually measured the errors involved. But if I drill and tap a set of M6 holes, and then drill 6.1mm matching clearance holes in the mating part they fit together with no problems.

 

On round, or rough, surfaces even the 4 facet drills will wander. In these cases I use 90° spot drills first. These are similar to stub drills. However, mine are carbide and are thus much stiffer than a normal drill.

 

By the way I do 99.9% of my drilling on the vertical mill; I almost never use the drilling machine.

 

Regards,

 

Andrew

It's clear; you need the dielectric liquid in the tank and the dialectic incantations outside the tank for the thing to work.

 

Regards,

 

Andrew

Hi Steve,

 

It may be a bit complicated, but I like to try and understand what I'm doing before building stuff. I have very little workshop time, and I don't want to waste time building stuff that will not work, where I could have deduced that by thinking about it in the first place. Just because coils and contact breakers have longevity doesn't mean I want to use them. They might work but they are not reliable. I remember being out there in the wet fiddling about with eroded contacts when the car wouldn't run properly! On my hit 'n' miss engine I am planning to use a magnet and hall effect device. Neither do I particularly want to buy an ignition system that uses a Darlington transistor and a few wire-ended resistors. A Darlington transistor is a bipolar device and therefore has both majority and minority carriers. The mobility of minority carriers in the semiconductor lattice is slower than the majority carriers, so bipolar devices are relatively slow to turn off. I'm using a MOSFET and I let the flyback voltage from the coil avalanche the drain-source junction diode. This occurs at about 800V. So, 800V on the primary times a 10:1 step-up gives several kV. Which is roughly what I see on the oscilloscope. Once I've got a microcontroller in the system, it becomes easy to implement things like speed related advance and retard, multiple sparks and other things I haven't thought of yet.

 

I did think about a CDI type ignition, but I didn't feel like designing the step up converter. It would probably have to be a flyback style converter, and it's a pain getting small quantities of gapped ferrite cores.

 

Finally I think it is a nice juxtaposition to have an old style hit 'n' miss engine together with a surface mount microprocessor ignition system.

 

Regards,

 

Andrew

Hi Hugh and Alan,

 

Good points, thanks! A further search on the internet found an approximation to the breakdown voltage that takes into account gas temperature. This brought the breakdown voltage for the previous parameters down to 12kV. However, as Alan has pointed out there are many other factors that fortunately conspire to lower the breakdown voltage.

 

So, I suspect the best we can say is that we need a few kV and a few mJ to produce a spark that will run a model engine. It is probably time to stop wielding the calculator and start building something. Bit of a relief actually, as the high voltage probe I bought for the oscilloscope is good for 15kV, so I don't need to find a higher voltage probe.

 

The first thing to do is knock up a small adjustable spark gap and see if my ignition coil setup will create a spark in air at STP.

 

Regards,

 

Andrew

Hi Martin,

 

Thanks very much for the link. The breakdown properties of air are much clearer to me now. I'm also an official idiot for not seeing what was in front of me! I didn't spot that the X-axis of the graph is a product, pressure times distance.

 

So, let's say we have a spark gap of 0.5mm. According to the Paschen curve that equates to a breakdown voltage of about 2.5kV at one bar (close to normal atmospheric pressure). Without bothering to measure it I assume that my hit 'n' miss engine has a relatively low compression ratio, let's say 6. If we start with air at 1bar and 20°C and assume that the compression process is isentropic, and take the heat capacity ratio of air as 1.4, then according to my calculations we end up with air at 12.29 bar and 324°C at top dead centre. If we now go back to the Paschen curve we come up with a breakdown voltage of 25kV for a spark gap of 0.5mm and a pressure of 12.29bar. According to my Bosch Automotive Handbook spark voltages are normally about 25-30kV. At least this is in the right ballpark.

 

The question is though, do our ignition systems really generate 25kV? And if they don't, how come they work? ETW talks about ignition voltages for model engines ranging from 2kV to 8kV. I suppose that what it all boils down to is that at some point one has to stop fiddling around with the maths and go and build something to see if it works!

 

Regards,

 

Andrew

Hey! The lathe is clean; I've added another photo to illustrate a dirty machine. The pile of aluminium swarf is over 4" deep. I knew the small shovel I made in 'O' level metal would eventually come in useful for something!

 

Hmmm, for some reason the system has decided that the emotions area is forbidden. Ah well, I'll just have to stick with plain English.

 

Regards,

 

Andrew

First, to answer a question posed a few posts ago. The best I can find on the internet as to the energy needed to ignite a air/fuel mixture is a few hundred micro-joules. However, most ignition systems generate many orders of magnitude more energy than this, often 100-200 milli-joules. I suspect some this is required for reliable ignition and some is marketing bo@~%ks, particularly in the retrofit automotive area.

 

I am in the slow process of designing my own microprocessor controlled ignition system for my hit 'n' miss engine. So far I've got 6kV out of a home made coil, with a 10:1 secondary: primary turns ratio. One of the goals of the design is to use coils that do not need the more normal 100:1 turns ratio.

 

Two other questions I have been unable to answer are; how is the breakdown voltage of air affected by the air/fuel mix, and how does the breakdown of air vary with increasing pressure. As air pressure decreases the breakdown voltage characteristics are well known; it's called Paschen's law. But I've found nothing about breakdown voltage versus increasing pressure. I do have some contacts in the professional IC engine reseach area, so I guess I'll have to fire up those contacts (pun intended) when I get back to working on my ignition system.

 

Regards,

Andrew

Here's my two pence worth, and a quick stir of the pot!

 

I don't think I'd part off completely using tailstock support. However, I do use tailstock support when partially parting off at a distance from the chuck. I've added a photo (my first upload!) to illustrate this. For reference the item is cast iron, 3.5" inches diameter, and is on a stub mandrel. I think the speed was about 350rpm.

 

The use of double sided tape, or wax, or resin, are well established techniques within professional machining for grinding, milling and, less commonly, turning. Of course the item is less firmly held than with clamps, but it is often the only way to hold some items. I have an item in my current work related design that will almost need the use of double sided tape. It is a plastic sheet 4mm thick, and 300mm by 400mm, that needs to be mostly milled away to a thickness of 1mm, leaving a series of 3mm projections. Doubled sided tape would seem to be an ideal way of holding it down and ensuring a consistent 1mm base thickness over the area.

 

It's telling that neither of the two posters who have objected to the said workholding have actually give any reasons as to why it is dangerous. I suppose that the item could fly out of the 4 jaw chuck, as it may only be held by friction with the chuck jaws. But this is not much different to milling a part parallel to the vice jaws.

 

I'm cynical enough to think that the fact both of them would have been rusticated/sacked for doing this says more about the intransigence of both unions and management of the time rather than possible dangers. In the 'good old days' you would probably have been sacked for changing a light bulb too; 'cause that's an electricians job, and that's a different union.

Regards,

 

Andrew

Thanks ady, I'll have to give that a go when time permits.

 

Regards,

 

Andrew