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Carbon fibre pendulum rod

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Alan Crawley07/07/2020 10:29:05
19 forum posts

I have made the 30 day regulator clock from the book by Peter Heimann and have it running with a pendulum rod of 1/4" diameter EN3B mild steel, due to the cost of the Invar rod as described.

At the end of the book the author touches briefly on using carbon fibre rod.

He points out that it is impossible to thread so threaded ends are glued and pinned in place. The top end is easy to do, but I am wondering how to join the threaded section at the bottom and keep it all 1/4" / 6mm diameter. Has anyone turned this material, as I envisage boring a 4mm hole in the metal and turning the carbon fibre to fit and epoxy and a pin to secure it? I am lucky enough to have a very well equipped workshop, but no grinding machines..

Any suggestions welcomed.

Emgee07/07/2020 10:52:59
1980 forum posts
250 photos

You could use a carbon fibre tube and glue the ends in as mentioned, no machining required on the rod that way.


Pero07/07/2020 10:53:37
133 forum posts

Hi Alan

Yes, it can be turned but ideally you need very sharp carbide tools for the task. It's murder on HSS. You could use a brazed tip tool specially sharpened for the purpose or alternatively an indexable tip designed for aluminium.

It generates a nasty black abrasive dust but it shouldn't pose much of a clean-up issue with the small amount you would be doing.



Paul Lousick07/07/2020 10:56:24
1707 forum posts
627 photos

Can you reduce the end diameter of the rod and thread and use a thin wall sleeve to join them together. Metal ferrules in a number of sizes are available at fishing equipment suppliers for joining rods together.


Alan Crawley07/07/2020 11:05:42
19 forum posts

Thanks for the quick replies. They are all interesting, and I had wondered about tube. The only readily availableone I found is 6mm O/D with 1mm wall, and I have no experience of carbon fibre apart from fishing rods and wonder if it would flex too much.I like the idea of tube as it would be the easiest option to make.

Mike Crossfield07/07/2020 11:07:49
252 forum posts
36 photos

I used 6 mm carbon fibre rod for the pendulum in my regulator clock. I turned the end down to 4.5 mm using a carbide GT insert without any problem. Glued and pinned it to a length of threaded 6 mm steel for the 5 lb bob.


Alan Crawley07/07/2020 11:16:43
19 forum posts

Thanks, Mike, that was what I was wondering. I'm assuming you part threaded the steel rod and bored the plain bit, or did you use M6 studding? That would only leave about .5mm wall.

John Haine07/07/2020 13:21:12
3772 forum posts
219 photos

My "Arduinome" clock uses a 10mm dia CF tube as its rod. The top end is a light press fit in the lower suspension chop block with an M3 cross drill in which site a bolt to pin the joint. At the lower end the tube is plugged with an 8mm aluminium rod about 25mm long, which is epoxied in place. I used full fat slow set epoxy, mixed warm, applied to the plug (cleaned with meths first), then pushed into the end of the rod. The ali rod was a light push fit anyway so the epoxy did not have to bridge a big gap. The ali plug was first drilled and tapped M4 for a rating screw, but in the event the rating is done electronically in the Arduino that controls the clock.

The rod supports a 7 kg CI bob, which is drilled through 10mm to clear the rod from the top and 16 mm from the bottom, the bores meeting half way. The "nut" is a piece of BMS that is an easy fit in the hole and more or less fills it so there is the same mass above and below the centre of the bob. So far no sign that the CF / ali joint is failing.

Note that the CF rod is very light, so you will be losing quite a lot of mass above the bob compared with your steel rod. As mass above the bob speeds the pendulum up, you will find your clock runs slow if you use the same length. Be prepared to either calculate a new length or do some trimming.

Mike Crossfield07/07/2020 13:24:12
252 forum posts
36 photos


I bored the 6 mm steel rod to be a sliding fit on the 4.5 mm carbon fibre stub, then threaded the other end of the rod to suit the pendulum rating nut. From memory the stub was about 1/2 inch long to give plenty of bond area, and I used a 1/16 diameter pin. Similar fixing arrangement on the other end of the pendulum rod. No sign of anything having moved after several months.


Alan Crawley07/07/2020 13:44:15
19 forum posts

Thanks again Mike, you have confirmed that what I was hoping to do is achievable

With this design the top is easy as he rod goes into 1/2" square brass that clamps the suspension shim.It also means that a metre of carbon fibre is just right.

I can't remember what the difficult to machine material was that we had at work, but we found that a carbide inset just touched on a diamond wheel to bring it to a sharp corner worked well. I imagine it would help on carbon fibre.


old mart07/07/2020 14:28:38
2825 forum posts
178 photos

What are the relative coefficients of expension of the common pendulum rods?

Alan Crawley07/07/2020 14:42:42
19 forum posts

Steel is 11.7, Invar 1.5. Carbon fibre can be as low as 0

John Haine07/07/2020 15:01:04
3772 forum posts
219 photos

CF can be negative, but still very small.

old mart07/07/2020 16:28:09
2825 forum posts
178 photos

You could have an excellent timekeeper at that rate.

Bob Stevenson07/07/2020 17:35:37
489 forum posts
7 photos

Really interesting topic........

Does anyone know if CF absorbs moisture?

Former Member07/07/2020 17:54:53

[This posting has been removed]

Former Member07/07/2020 18:02:40

[This posting has been removed]

John Haine07/07/2020 20:52:38
3772 forum posts
219 photos

After mentioning my CF pendulum over on the group there was a bit of discussion about this which led me to do a bit of digging and this response.

"Carbon fibre has a near-zero coefficient of thermal expansion - there is some discussion over on the NAWCC discussion boards, and though the coeff is variable between materials it is generally better than invar. It is liable to absorb moisture slightly, which means its weight can vary, but what this does to timekeeping I have yet to work out. I do have a spreadsheet that takes into account all the material densities and shapes to calculate the period so it would be simple to do (in fact I ought to sooner rather than later). It's also MUCH cheaper than invar.

Edit: Now done the calculation. According to an NAWCC post the maximum weight gain of CF due to absorbed humidity is 0.2%. Plugging this as a density gain in my spreadsheet gives a change in pendulum period of 1.9 microseconds, which I think I'll forget about! The thing is, carbon fibre is so light anyway, especially compared to the bob, that its contribution to the MoI is minute - so changes to density also have a small effect."

Alan Crawley17/07/2020 09:33:14
19 forum posts

As a newcomer to clock making I read lots of information about it, and one thing i learned is that the weight of the pendulum is not important. With my current testing stage, I removed the bob and the clock started and ran without it, albeit quite fast! That surprised me!

The Model Engineer articles describe a glazed case for this clock, and I am somewhat confused about this aspect, too. It's all in Imperial dimensions, so I will use them. It is a thirty-day clock. The winding barrels have an effective diameter of 1-1/4" and sixteen turns, so 1.25 x pi x 16 =approx 63" The cable is doubled with pulley, so effective length is approx 31-1/2". When fully wound up the top of the clock to bottom of the weight is approx. 25", which gives an overall dimension of some 56-1/2" when weight is fully down, but the case is only 50-1/2" inside.Have I missed something?

Alan Crawley17/07/2020 09:34:49
19 forum posts

Edit- case is 51-1/2" inside

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