Member postings for Andrew Johnston

The other problem with trading standards officers is that they will be completely clueless as to the technical issue you are complaining about. I tried to complain once about some castings from a ME supplier, but since the trading standards officer had absolutely no idea what a casting, or a machine tool, was I gave it up as a bad job. And just resolved never to buy from that supplier again.

 

Regards,

 

Andrew

There's always an exception to every rule. My imperial Harrison lathe has the cross slide dial calibrated in diameter reduction.

 

Regards,

 

Andrew

 

Blimey, my 200th post; and there was me thinking I'm an introvert, happier in the workshop than out partying!

Is it just me, or are the available smileys a bit limiting? Have a look at those available on 'Practical Machinist':

 

http://www.practicalmachinist.com/vb/misc.php?do=getsmilies&editorid=vB_Editor_001 

 

Much more scope to express the good, the bad and the ugly.

 

I wonder if anything can be done about this?

 

Regards,

 

Andrew

Hi Wolfie,

 

Correct, some, but by no means all, cast irons are soft. But then again copper is soft, but the recommended cutting speeds are only just above cast iron. The best advice for machining copper is to get somebody else to do it. It's 'orrible!

 

Once you get your book on hardening and tempering you'll be able to read all about the complex chemistry of iron and carbon, and why cast iron can have hard spots.

 

If you don't have one, I'd also recommend getting a copy of 'Machinery's Handbook'. Don't buy new, a secondhand one from decades ago is just as useful to model engineer.

 

Regards,

 

Andrew

Michael: All very sensible suggestions. Before we placed the order, we visited the machine shop in question. It's only a few miles away, which was one of the reasons for selecting them. We took one of the prototype heatsinks that I'd machined, so that they could look at it and ask questions. We also explained it's purpose and the finish required. It's may be slightly ironic that the finish on other parts of the heatsink is better than that where the IGBT module goes.

 

Still at least they're doing better than the sheet metal company we're dealing with; 'order, what order, we didn't know you'd placed an order'!

 

Regards,

 

Andrew

Hi Wolfie,

 

One thing to consider vis-a-vis machining the flywheel is turning speed. I'm not familiar with the Stuart flywheel, but I assume it's between 3 and 4 inches in diameter?

 

For a soft, as cast,  grey cast iron Machinery's handbook gives a cutting speed of 90 feet per minute with a HSS tool. If the flywheel is between 3 and 4 inches, then the circumference will be about a foot. So, you need to be revolving the flywheel at 90rpm. Given that the casting might have hard spots, and taking into account a smaller style lathe, a bit slower than 90rpm might be appropriate.

 

Regards,

 

Andrew

D.Eng:

 

That's a damn good question, and one to which I don't have an answer. Only they know that. Given that a lot of their work involves waveguides, and hence talking to electronics and radio types, I assume that they thought the same of me. Presumably they weren't expecting to be questioned about cutters, speeds, feeds and in what order they machined things, oh and why didn't they try this type of cutter?

 

Regards,

 

Andrew

Try this handbook, it'll tell you more than you ever wanted to know about O-rings and seals.

 

http://www.parker.com/literature/ORD%205700%20Parker_O-Ring_Handbook.pdf 

 

Regards,

 

Andrew

Here's the denouement.

 

We went to see the professional machine shop yesterday morning. They held their hand up and admitted a screw up on the wrong threads on the heatsink. That doesn't worry me, we've all been there, they've had the honesty to own up. There were also a few other issues to do with swarf in blind threaded holes and deburring techniques.

 

With regards to the finish on the pad where the IGBT goes, there was no arguement. Just a quick fingernail test and no question that I'd got a much better finish. I got the impression he was rather embarrassed. They have no means of measuring Ra or Rz. They're going to re-work one heatsink to see if it meets my requirements. Overall a positive and friendly meeting and we will certainly be working with them in the future.

 

Regards,

 

Andrew

I'm sure you'll find the book informative, although the physics/chemistry of iron/carbon alloys is pretty complex, and will probably require several readings!

 

Actually I think coming down Sutton Bank with a trailer was worse than going up. At least on the way up the only thing you're likely to lose is traction; a disadvantage of front wheel drive cars. On the way down, it's bottom gear, brakes, and 'oh #@*& we're sliding with the wheels locked'.

 

John: Generally modern glider trailers tow pretty well. Older ones are much more of a mixed bag. Weight balance and distribution is critical. I find I need about 50-70lbs of downforce on the hitch, plus weight in the front of the car to retain steering. Even if the balance is correct, weight in the back of the trailer can lead to instability. Ultimately the car is the most important factor; it needs to be heavy for stability. I used to drive Citroens, which had self-leveling suspension, a real joy for towing. Now I have a Peugeot, and while it's a good tow car I need to pay more attention to weight and balance. Set up properly, the biggest danger is forgetting you've got the trailer on the back! In the UK we have a speed limit of 60mph for braked trailers. I normally stick to 55-60, as faster really hammers the fuel consumption. I've towed through quite a lot of towns and cities, before bypasses were built. The main issues are that it is more difficult to change lane, because of the length, and you need to be careful about the back of the trailer swinging out when turning at junctions. I've even been through the centre of Edinburgh with a trailer, due to a slight navigational error coming off the Forth road bridge. I don't know what your steam launch weighs. I estimate my single seat glider and trailer weigh about 1400lbs in total. My new two seat glider and trailer is well over a ton. There is a myth (?) that glider trailers are longer than permitted by UK law, but a 'blind eye' is turned as they are classed as an indivisible load. I've added a picture of your scribe picking up a mate who had made a navigational error in Scotland and was consequently in the middle of nowhere. This particular trailer is about 13 metres long. Real pain to turn round on narrow country lanes. We entertained a whole restaurant full of pensioners while turning it round in a hotel carpark, having missed the turn off to 6 miles of single track road up to where the glider was.

 

Regards,

 

Andrew

First, the speed lever. And as a rider, I'm typing this after a couple of pints of beer, so don't blame me if it's nonsense! I think the speed lever (48), the speed screw (51) and speed spring (52) form a spring loaded stop against which the striker arm (44) operates. The nearer to the cylinder head the speed lever is, the harder the striker arm has to push against the spring in order to hold the exhaust valve open. To generate the higher forces the governer has to run faster. Does that make sense?

 

Charles: Looking at your pictures, is your rocker arm an iron casting? Mine was a crappy light alloy casting that is too small to start with! I'm still deciding what to do about it.

 

Regards,

 

Andrew

I think Alibre do some online tutorials. Alibre is a 3D parametric package, but personally I wouldn't ever go back to 2D CAD systems. I design everything (professional and hobby) in 3D. The 3D packages normally automatically generate 2D drawings. You usually have to place dimensions and notes to suit. The dimensions are sized automatically, all you do is click on the two features you want to dimension. However, everything is associative; change the 3D model and the next time you open the 2D drawing the dimensions update automatically.

 

Alibre used to do a free low-end version, but they seem to have stopped this. They now offer a low-end package for $99.

 

www.alibre.com

 

Regards,

 

Andrew

Sorry, dumb use of abbreviations on my part.

 

WPS1 = Workshop Practice Series Book1 - Hardening, Tempering & Heat Treatment

 

Regards,

 

Andrew

 

PS: on a note from another thread; I do know Sutton Bank, been there a few times, but never flown there due to the rotten weather. I did get as far as getting in a glider, but then the tug pilot wimped out 'cause the cloudbase was only 300 feet. It's a bit of challenge towing a glider trailer up Sutton Bank itself. Real bummer if the car in front stops!

Hi Wolfie,

 

As always in engineering the answer is yes, no, but!

 

For high carbon steels, to harden parts you heat them above the critical temperature and then cool rapidly by quenching in water, brine or oil. As you correctly say, that makes the material hard, but brittle. Tempering is the process of heating the part to a much lower temperature and then quenching. This trades off hardness against toughness. So somehing like a cold chisel will be tempered at a higher temperature than a hand scraper, as the chisel needs to withstand shock loadings, ie, less hard and tougher.

 

I recommend the book WPS1 by Tubal Cain, which has a good explanation of the complex processes that are happening when hardening and tempering steels.

Materials other than carbon steel harden by other processes. Materials like copper and some stainless steels harden by being mechanically strained, which is a PITA when you are trying to cut them. Other alloys can be precipitation, or age, hardened. This involves heating parts to moderate temperatures and letting them soak, allowing some of the alloying elements to migrate within the lattice structure and harden the material. An example of precipitation hardening is aluminium alloy rivets for aircraft. Before use these are heat treated and then have to be kept cool and used within a certain time. After fitting and forming they age harden over time.

 

Regards,

 

Andrew

 

 

 

I assume that the use of roughness parameters that mirror AC electrical signals is no accident, but was developed because the early measurement units used analogue electronics. Hence, parameters were devised that were simple to measure with analogue filtering and processing. The measurements are also altered according to how long a measuring stroke you use, how quickly the probe moves, and the radius on the tip of the probe.

 

All in all it's more of an art than a science, unless you have an electron microscope to hand and can 'see' the surface directly.

Chris: I'm well aware that coolant temperatures, and cylinder heads, are normally above 100°C. However, this is not a conventional vehicle and we had a gaurantee from the overall systems developer that we would get coolant at an agreed temperature below 100°C.

 

Alan: It's all a bit more complicated than just bolting the module down! The module is fastened with a number of high tensile bolts that have to be tightened in the correct sequence and to an exact torque. This actually pulls the parts of the module into place, forcing the internal contacts together, and forcing the alumina substrates into contact with the heatsink. The manufacturer's quoted force per alumina substrate on the heatsink is over 3000N. So for the whole module, with three substrates, that's over 9000N. At these sort of forces we assume that the thermal paste layer is going to be pretty thin, and is really just filling the voids. I can't remember exactly what thickness of paste mask we went for, but I think it was 4 thou. For each substrate the paste was masked as a series of small squares, separated by gaps.

 

Regards,

 

Andrew

So, is decimalisation a necessary, but not sufficient, condition for metrication?

 

Regards,

 

Andrew

 

Hi Martin,

 

First, thanks very much for the links to the Ra versus Rz notes. They have been very useful. It would seem that our machine shop may or may not have met the specification, according to how we interpret the data.

 

On the other point, no it's not a typo! The whole system consists of a diesel engine driving a three phase generator into our AC-DC inverter, which then produces a high voltage DC bus. The ambient temperature in the engine bay, where our inverter is located, can reach 100°C. We're also about six inches from the exhaust pipe; nice!

 

Our system utilises the normal engine coolant for cooling the power electronics. There are two points to note. One, we're first in line after the radiator, so the inlet coolant will be a lot cooler than 100°C, and second, the heatsink is pressurised, so it shouldn't boil at 100°C. You wouldn't believe how many screws there are to hold the lid down and make sure the O ring seals work. You also wouldn't believe how small a piece of swarf is needed to cause a dribble of water past the seals.

 

Don't worry about 'not helping'; I love a good technical discussion.

 

Regards,

 

Andrew

Not all D-type connectors are 4-40UNC, you can get the fittings in M3, I think, but the vast majority are still 4-40UNC. Only last week I hand tapped a load of 4-40UNC holes for some prototype boxes that use D-type connectors. Actually the screw fittings are a PITA. They're usually nickel plated brass. As you tighten it up, just a little more and it'll be snug; then ping, the damn thing breaks at the base of the thread.

 

All this mention of sunsets brings us on to another point. What is time if it isn't an imperial measure? Sixty seconds in a minute, sixty minutes in an hour, 24 hours in a day, anything from 28 to 31 days in a month, 365.25 days in a year for goodness sake! It's just crying out to be metricated.

 

Regards,

 

Andrew

An addendum:

 

For the idly curious I've added a photo of my surface roughness measurement unit. Given the relatively small differences in Ra values between the two heatsinks I find it amazing that a quick fingernail test can easily differentiate between them.

 

I've just had a proper look at the standard I got with the measurement unit. It says Ra is 2.97µm and Rz is 11,38µm. So it looks like the machine shop did meet the specification, but I'm still going to gently raise it as a issue. We're about to give them a whole load more work, so they need to do what I want, or at least tell me why I can't have it!

 

Regards,

 

Andrew

Thanks for all the information; plenty for me to read. Don't know why I didn't think of looking on the Taylor Hobson website; after all they made my surface roughness gauge, doooh!

 

Right, now to try and answer a few points.

 

Ian: I've posted a picture of my Rubert set; did the one you see look like this? Rubert, and others, also made sets for specific operations. I have one with six squares on it aimed at shapers.

 

Sam: Sorry, I tend to forget that not everyone knows the esoteric abbreviations. In simple terms IGBTs look like a bipolar device at the output, but the input looks like a MOSFET, so it's easier, but not simple, to drive. I won't bore everyone with the details unless people are interested.

 

Chris: I did get one (well used) standard when I bought my roughness gauge, but some new ones would be good!

 

I've added a photo showing the bottom of the IGBT module. For reference it's about the size of a paperback book. Each half bridge has the IGBT and diode dies mounted directly on an alumina substrate, which is what the gold rectangles are. Why you might ask are there ICs outlined on the bottom? We need to use a thermal paste specified by the manufacturer between the device and the heatsink. They recommend a thickness of 35µm and the use of screenprinting. Screenprinting, through a mesh, seems a little crude, so we had the idea of using a stainless steel paste mask, as used in the automated assembly of PCBs. The picture shows a test done at our assemblers using a gash paste mask, before we went to the expense of designing and having made a custom mask that covered only the alumina substrates.

 

In essence the IGBT-metal-coolant interface is a damn nuisance, but we have to have the metal in the way for other reasons. We just need to minimise the thermal resistance.

 

Right, back to the surface roughness measurement. The Ra measurement is essentially an arithmetic mean, easy to do with analogue electronics. The Rz measurement seems to be an average of the peak values only. This would be a bit tricker with analogue, but easy with digital measurement. The use of Rz would seem to be better intuitively, as it gives a measure of the total depth of the roughness, rather than just an average. So, in electrical terms it takes into account the crest factor of the surface. So, as far as I can see there is no simple way to relate Ra and Rz?

 

It would seem that we cannot really say whether the professional machine shop has or hasn't meet the specification, unless they can measure Rz directly. However, we know that the heatsinks I made work at full power and high ambient temperature (100°C). Personally I think they ought to be able to match, or better, what the peasant writing this can achieve in his garage.

 

Should you be curious about the use of this heatsink look at www.provector.co.uk and look under 'News' and '610 Series'. It will tell you a bit more about the unit, who the customer was, and where it might be used.

 

Regards,

 

Andrew