Sunday, 13 April 2014

Clock gear spacing

After playing with the locations of the motors and gears I roughly sketched an assembly diagram with the location of the motors and shafts to allow me to estimate various part sizes.

My gears were described as "27 mm Diameter Plastic Cog Wheels for 4mm Motor Shaft, 40 Tooth Gears". So I needed to work out how far apart the centres would be when the gears were meshed together. Too far apart and the teeth won't meet, too close together and the gears won't fit. As an experiment I placed the gears next to each other and used a pencil to mark some paper, I measured the distance between these as 25mm.

Following the details on Roy Beardmore's gear calculation page I concluded that the Pitch Circle Diameter of my gears was 25.7mm and due to the gears being the same that also is the separation between each of the gear pairs. This is only slightly more than my approximated value.

Now I can accurately draw up the positions of the motors and shafts and start building the clock framework.

Thursday, 10 April 2014

Exploding E-Cigarettes

In the news this week was a report about a barmaid and an exploding electronic cigarette.

Apparently the device had been plugged into an IPad charger rated at 2A rather than the intended charger which was rated at 0.5A. A lot of people suggested this was user error but I got wondering if it is actually a design fault?

I had always assumed that a "USB Charger" was just a regulated power supply like a standard "wall wart". This would mean that the charging circuitry was actually implemented in the phone or tablet. A bit of investigation showed that this is definately the case and that most phones and tables have a battery charging chip such as the LTC4088. This chip provides the necessary current and thermal limiting that is needed to charge a battery. Also reading the standards for USB Battery Charging these define how the device should respond to different supplies from the USB cable and mean that a device can run with any valid USB power source.

Having seen a few electronic cigarettes their design is normally in the form of an external power adapter and a small docking station that connects the power cable to the cigarette. The fact that this device did not work with a different power adapter suggests that the current limiting was perhaps in the adapter rather than the cigarette socket.


Although it might specify in the manual that you should only use the provided charger that this is still a design fault not a user fault.
  • The E-Cigarette should not have built with a universal charger connector but then rely on a specific charger.
  • The charging circuitry should have been implemented in the cigarette holder rather than in the wall adapter. 
  • The device should have had suitable protection circuitry to handle over current and/or over temperature situations.
  • The battery could have been contained so that there was no fireball when the problem occurred.
My conclusion is based on the assumption that there were no actual technical faults with the device or charger which may not have been the case.  I am also assuming that there was nothing non-standard with the design of the IPad charger.

What do you think?


Tuesday, 8 April 2014

Making a Concrete Planter

After the bathroom was repaired we have a selection of hardiebacker concrete board pieces left over. Not wanting to chuck these away but also not really having space for them I decided to make a planter out of them.

The board was cut using an old handsaw that I'd previously used for sawing up thermalite blocks to make a hearth.

Note: This process really killed the teeth of the saw and I'll now need to either recycle it or sharpen the teeth.

I tried an experiment to see if the boards could be joined together using more concrete. This may be possible but my results were unsuccessful there was little bonding between the concrete and the board. So I decided to use up some of the zinc sheets I had inherited.

I chopped the sheets with snips, making about 20 in total. Rather than drill these individually, I built a simple wooden jig that allowed me to clamp these to the drill table as a set.

The sheets were folded in the vice. The concrete sheets were clamped in place and the metal was layed over the corners. The positions of the holes was marked and the metal removed.

Having been warned about the dust created from these sheets I doned some protective clothing and moved everything outside.

After the holes were drilled, I bolted the corner pieces onto the sheet using roofing bolts. These have a large head and a square nut.

I decided it was unnessary to have plating all the way down each side as the ends were sufficiently secure.

To finish it all off I gave the boards a coat of exterior paint. I was helped by my young daughter so it got a bit messy.

Tuesday, 1 April 2014

Scotland to switch to DC

Scottish Power have confirmed plans that following a "yes" vote for Scottish Independence they will be switching their electricity distribution network to DC.

"Tesla was right! says Scotland"

This change will allow them to drop their requirement to synchronise their generators with the English 50hz system. Keith Anderson explained that since the famous Edison vs Tesla Ac vs DC "War of Currents" back in the late 1880s there have been many technological improvements and DC fits with their renewable power and storage plans. "Many products are already DC" he said so eliminating the wasteful conversion could save Scottish consumers upto 11% on their on going electricity bills.

DC to AC conversion stations will be introduced at Moffat, Jedbourgh and a plant just to the north of Berwick on Tweed to allow Scotland to sell electricity to the rest of the UK.

The massive infrastructure project will bring jobs to Scotland and requires many houses to switch to a new design of plug and socket to avoid compatibility issues with older products.

Monday, 31 March 2014

Breaking the mishap chain

Human Factors Lessons Learned From Aerospace Accidents and Incidents in Research, Flight Test, and Development
By Peter W. Merlin, Gregg A. Bendrick, and Dwight A. Holland

This book is a series of case studies looking at mishaps with aircraft and space vehicles. It examines what leads up to the problem that occurred and how such accidents can be avoided in future.

Test flights are always thought of as being dangerous but this does not have case, what these reports examine is that the cause of failure is nearly always the accumulation of several errors in the system, design or testing process. Sadly in these cases the end result is often loss of life or serious injury. As the title might suggest the studies focus on the human aspects rather than technical defects, a lot of the focus is on the test pilot but it soon becomes apparent from reading these just how many other people are involved in a flight.

The studies are divided into three sections, Design Factors, Physicological Factors and Organizationl Factors each section looks at 3 different craft and situations. The design factors looks at the aircraft user interface, re-use of user inputs and pilot induced oscillation. The physcialogical factors focus again on the pilot and their ability to physically and mentally handle the stresses of flight. One of the cases inspired the film and series "The Six Million Dollar Man" and shows that even experienced test pilots can become distracted if given too much to do. Finally the organization factors looks at how the procedures and origanisation can let down the pilot. There is an analogy of layers of swiss cheese with holes all lining up to show a hole all the way through, a problem that could have been caught and corrected at many points is let slip through the net. One particular disaster was the side effect of trying to combine a test flight with a photo oppertunity, something that is strongly advised against.

The photos and diagrams in the studies are not the most detailed but do add and explain the text. There is quite a lot of aerospace terminology in the studies but it is explained along the way. I did not need to use any additional research to understand acronyms and terms but I did find it necessary to cross reference earlier parts of chapters. Those with a basic technical background should have no issues with understanding the studies.

Even if you are not in the business of making safety critical systems I feel that understanding how these errors accumulate and interact can help with designing and testing of any system.

Monday, 24 March 2014

Richard Deacon at Tate Britain

When I saw the posters for this exhibit I knew I had to go and see it.

Like the fliers these posters feature a work called "After" in wood and metal which fills one of the later rooms of the exhibit. The posters don't really do it justice and you have to walk around it and see the detailing of the joints and stainless steel combined with the wood to fully appreciate it.

There are 6 rooms in the exhibit with a handful of sculptures in each room. Some of the first works such as "Untitled 1981" are in laminated wood, I thought the construction was quite rough with the glue oozing out of the seams. However if you take a few steps back then the piece shows off it's elegant curves. All the works are like this having different views and detail from close up or far away. I also noticed an obsession with self tapping screws which were used in a lot of the sculptures.

My favourites were in the last room. Out of order is steam bent and blacken wood with metal joining plates. I like the fact that the blackening was not intentional but a side effect of the experimental process of creation.

At £11 it's quite expensive for a small exhibit but as it had been a long time since I'd been to Tate Britain I took the opportunity to see some of the other sculptures such as the Henry Moore, Barbara Hepworth and Anthony Caro. This meant that I felt like I got better value for money. If you can't afford a trip then a you can see a lot of the sculptures online but as mentioned above you will get more from being able to see these from all angles and perspectives.

Richard Deacon runs until 27 April and Tate Britain

Tate interview with Richard Deacon

Thursday, 20 March 2014

Box lid an afternoon woodworking project

Not so long ago we bought a box to put by the front door and put our shoes in, the old portis shoe rack now lives by the backdoor. The box came with a rather strange lid, a sheet of ply with missing corners that just sat on the top in some grooves. I'd never really liked the lid, it was easy for it to fall into the box and the edges were starting to splinter which was making it dangerous to use. So I decided to make a hinged one.

I had some 18mm ply left over from the shower room refit which was a good size for a lid so I purchased some pine strips for edging and four small hinges.

I wanted the lid to open with the box against the wall. So rather than fixing the hinges to the box I used a strip a suitable offcut to act as a support to attach the hinges to.

I cut the support to length and the ply to size. Some fixing holes were drilled and countersunk in the support.

I marked out then chiselled recesses for the hinges in the support and then matching ones in the ply lid.

The edging I cut roughly to length and planed roughly to width.

I then marked and cut the mitres with a tenon saw using sandpaper to get them a good fit. These were glued and pinned in place. Once each one was attached I planed it flush with the surface taking care not to take off the top layer from the ply.

The hinges were screwed into the support and then I clamped the support to the lid with a spacer to allow me to fix the hinges to the lid.

Finally the support was screwed onto the box with some long screws. We now have a easy to use lid for our box.

Additional reference
Tips for fitting hinges
Shortcuts to fitting hinges
Mitre joints

Workshop Practice Series