This new thread chronicles the revised build of my experimental clock, and this, the MkIV, is intended to incorporate lessons learned and suggestions received. As always, comments and questions welcome.

I decided to stay with the same basic format, that is a tripod frame inside a drainpipe that can be evacuated with a vacuum pump, impulsed with a side-mounted electromagnet, and using the existing software. So mainly about improving the mechanical clock.

• A heavy cast-iron base with 3 built-in levelling screws, wide-spaced
• Control wires and vacuum connection to enter at the base rather than on top.
• 114mm soil pipe rather than 67mm to provide more room inside and allow the internal tripod to have a larger footprint. The top can be sealed with a commercial end-cap. New soil pipe preferred, but I'm mean enough to consider second-hand.
• A revised suspension:
  • More accurately made
  • Screw adjustable for height and able to adjust the angle of suspension spring with the electromagnet
  • Improved clamps using shims
• Revised sensor and magnet holder, adjustable relative to bob
• Tubes and slit to narrow the Infra-red beam.

Plan A was to buy a section of cast-iron round from Noggins End:

but I realised I already own a block of cast-iron with a handy cave underneath that can be used to hide the microcontroller:

Saves money too, all I had to do was cut it. Alas my dear old angle-grinder wasn't up to the job. Belched smoke much more than in this photo, and it is now a late grinder, pushing up the daisies, kicked the bucket etc.

First expense: replacement grinder with a bigger motor, £70, roughly twice what I would have paid Noggin's end for a blank.

Anyway, the initial design:

Cutting the base was hard work because a 115mm angle grinder doesn't cut deep enough, and I had to finish with a hacksaw, also on the small size. After what seemed like an hour's sawing I had to stop for tea; I'd actually lasted less than 13 minutes. Not as fit as I used to be!

Had just enough time to plonk the block on my mill and skim the lower side. Horrible job, nasty hard skin and black dust everywhere. (Halfway through I realised I'd forgotten to put down paper.)

Out bashing relatives tomorrow but I may have an hour to make a start on the new suspension:

Now I'm filthy, exhausted and bankrupt. I can only hope the effort is worth it!

Dave

The jury is still out. John Haine made the same point, but Michael G raised the merits of kinematic design in which 'mechanisms can be defined as assembly of rigid members connected to each other through joints'. It observes that objects have 6 degrees of freedom, and that total rigidity is provided by locking all of them and no more. It suggests the designer shouldn't add structure unless it locks a degree of freedom that's still free. Adding structure that doesn't reduce a degree of freedom is just more weight and complexity, that might reduce a structure's rigidity and strength.

Looking at a single column as an example, resting it on a flat base only constrains one degree of freedom – it can't be pushed through the base! But it can be slid sideways or lifted off, ie unconstrained in 5 of 6 directions. Fitting the column into a pocket cut into the base is a considerable improvement. Once located inside the hole, the column can't be moved sideways, it can only be lifted up. All six degrees of freedom are constrained if the column is bolted into the pocket.

Or are they? If enough force is applied to the top of the column, it will bend. This can be constrained somewhat, in one direction only, by bolting a crosspiece to a second column:

The structure is still weak if the beam is pushed backwards, hence good to add at least one more column and connect all three, forming a tripod, all joints being rigidly fixed together:

I don't believe four columns are any more rigid than three. Four columns are advantageous for reasons I don't need: they open up a rectangular space inside, and – potentially important – four columns can take more weight than three. But my lightweight bob doesn't put much force on the structure.

Diagonals would stiffen the structure by preventing the top from rotating relative to the base. This is desirable, but are difficult to build and reduce access to the interior, which is needed to hang the pendulum and adjust the innards. I hope to get away without diagonals.

Although my tripod is plenty strong enough, I am worried that it might sway at a resonant frequency. Really bad if my wonderful new design vibrated like the Tacoma Bridge or a bass guitar. I've no idea how to establish the resonant frequency of a tripod, or any other structure. Anyone know?

Twanging the end of a steel rule flat with one end held flat on a table shows resonance can be a serious problem in strong structures; I reckon the breaking strain of a steel rule is about 2 tons, yet a gentle push vibrates it enthusiastically.

Dave

I should be doing urgent housework before eating but I couldn't resist seeing what Solid Edge can do. It supports vibrational analysis, with the following result:

So SE thinks the assembly will vibrate naturally at 13.58Hz, and I expect the pendulum to vibrate at 1.2Hz I guess that's OK, but yet again I don't know!

Before jumping to conclusions, I need to add all the components, in case they make a difference, and then try and understand it. Could take a few years!

Dave