machines which I have made, am making, or intend to make, and some other stuff. If you find this site interesting, please leave a comment. I read every comment and respond to most.

Tag: Trevithic

Another Use for Magnets

I purchased some bronze disks for use in the model Trevithick dredger engine.  The disks 204mm diameter had been bandsawn off rod.  I had specified minimum thicknesses of 7mm and 12mm.  One disk was 9.2-9.7mm thick and the other was 12-15mm thick.

bronze blanks.JPG

The bandsawn blanks of LG2 bronze.

In preparing the disks for machining I filed off a few protrusions, and using a straight edge, identified the valleys and ridges.

The thicker disk was held in the 3 jaw chuck and both faces were turned flat with no problems except avoiding the needles which were thrown off in a wide arc around my lathe.  Final thickness 12.5mm.  A persisting divot should be able to be avoided in the final part.

The thinner disk needed to be packed out from the jaws of the chuck by 4-5mm in order that the lathe tool  cleared the jaws during machining.  In the past I have used machined packing pieces, but it is always a fiddle to hold the workpiece, the 3 packing pieces and the chuck key in only 2 hands.  Today I had a brainwave.

chuck magnets.JPG

I used rare earth magnets!

I tried to measure the thickness of the magnets, but they are so powerful that I was not confident that I was getting accurate readings.  So I just used them and measured the thickness of the finished workpiece.

I am sure that this idea is not original.  But it is to me.

bronze in 3jaw.JPG

Here is the thin workpiece held in the 3 jaw, and packed away from the chuck by rare earth magnets.  Of course the magnets are attracted only to the jaws, and not the bronze workpiece, which helps, but I will try this on steel later.  Should work for magnetic metals also.

After machining both faces I took various measurments of the workpiece thickness.  The measurements in mm were 8.73, 8.68, 8.69, 8.72, 8.70, 8.72.   Not perfect, but not too bad at all.   I wonder if I might have improved the measurements by surface grinding the magnets.  I wonder if the chuck and its jaws are contributing to the variation.  It was certainly an easy method.

If the workpiece had been thinner I could have increased the thickness of the packing by doubling up the magnets.

For those who are following the Trevithick Dedger Engine build, the bronze was not cheap.  The 12-15mm disk was $AUD90 and the 9mm disk was about $AUD80.  From George White P/L, Melbourne.  I will be nervously trying to not muck up the machining.

Trevithick Dredger Engine Flywheel

You must forgive me for rushing this blog.  I do feel rather pleased to have made a start on the Trevithick Dredger Engine.

The first part for the TDE was waterjet cut from 6mm mild steel.  I emailed the DXF drawing Tues, it was cut out Wed and picked up today.  Impressive service from Colin and Sandy at Waterjet Geelong.    340mm diameter.  The finish of the cut is so good that some light filing and rounding the sharp edges is all that will be required.  I will machine the bore hole after the shaft is made.  The magazine article is my workshop copy.  It will not remain this pristine for long.

flywheel flat.JPG

flywheel edge.JPG

The thinness of the flywheel is a Trevithick characteristic.

Buying Copper Pipe for Model Boilers

My next 2 projects require 150mm (6′) copper pipe for the boilers.  The Trevithick dredger engine will operate at only 30psi so the wall thickness needs to be only 2mm thick to comply with the AMBSC regulations.  But the other project, the vertical test boiler will operate at 100psi and the copper needs to be a minimum of 2.5mm thick, or preferably 3.0mm or 3.2mm.

I had purchased a 350mm long x 2.8mm wall thickness piece of 6″ copper pipe at an engineering club swapmeet, so I thought that would be OK for the test boiler, but when I examined it closely I noted some scribed lines from the previous owner’s intended project.  There were also some drilled holes, but they can  be used or filled.  The scribed lines were unacceptable, so thinking that they were not very deep I carefully skimmed the cylinder surface on the lathe.

Copper tube skimming 2.JPG

The headstock end is held lightly in the 3 jaw chuck.  The tailstock is centered in a piece of fitted Delrin.

Taking off 0.05mm per pass, and using a sharp Diamond lathe tool (from Eccentric Engineering), when the marks finally were removed, the wall thickness was down to 2.45mm, just below the minimum thickness for the test boiler.  Bummer!  I can use that piece for the lower pressure Trevithic engine, but what to use for the test boiler?

So I contacted every Australian copper pipe supplier that I could see on the Internet, every model engineering vendor, and visited every plumbing supplies vendor in my region.  2mm thick pipe is available but nothing thicker.  Then to overseas suppliers.  Eventually I located some at MaccModels Engineering Supplies in the UK.  £7.67 per inch for the 3.2mm x 6″.  So my 12″ piece will be almost £95 ($AUD190) plus postage.  I took the opportunity to stock up a few other sizes which I will need for the 2 projects.  Postage came to $AUD170.   A bit painful.

So, to end this expensive story, my 3.2mm thick copper pipe is on its way.  I do feel a bit guilty about the Avgas being burnt to get it here, the kilogram x kilometers of air pollution and CO2.  And the annoyance that it is quite possible that the copper ore was mined in Australia, but I had to go to the other side of the world to buy a bit of the manufactured product.  Or is copper still mined in the UK?

Copper collection.JPG

Gradually accumulating the materials.  The copper sheets and disks are 3mm thick.  The square section rod is for the boiler foundation ring.  I don’t want to start cutting until I have all of the materials.


In the previous post I showed the boiler which I am intending to build.

In this post I will show you the steam engine which I also intend to build when I have AMBSC approval and I have accumulated the materials.  I have discussed the plans with the boiler inspector, and am in the process of redrawing the plans taking into account the required modifications so it can be run at public exhibitions.

It is the TREVITHICK DREDGER ENGINE, at 1 in 8 scale.

This was designed by Richard Trevithick, and made in 1806,   It was the first high pressure steam engine.  It also employed a steam blower, cylindrical boiler, safety valve, and many other innovations.  Incredibly, the engine worked in its first iteration.   Richard Trevithick was indeed a genius, although relatively unrecognised in his own lifetime, and for almost 2 centuries since.

Here is the earliest drawing which I could locate of the dredger engine.

It is from an encyclopedia which was published in 1818.




This is a drawing of a Trevithick dredger engine which was reconstructed in 1885, with some errors, including the bent connecting rods..   Of particular interest is the piston-cylinder assembly which is partly encased by the boiler to minimise heat loss. The large, thin, flywheel is a characteristically Trevithick feature.

Trevithic JD_SciMus1

The Trevithick engine as reconstructed, and as it is displayed in the London Science Museum today.  The blacksmiths who formed the domed boiler end, pentrated by the cylinder assembly must have been incredibly skilled.  (ps.  note added 10/4/19.  The domed end was a casting which included the cylindrical sides, and the flange at the chimney end.  It was not forged by blacksmiths.)

The Trevithick Dredger Engine was drawn at 1:16 scale by Tubal Cain in 1987, and modelled by him. His plans were published in “Model Engineer” magazine.

It was redrawn by Julius Dewaal at 1:8 scale and published on the Internet in 2016.  The plans are currently freely available on Google Images, as 9 pages of A3 plans.  The plans are beautifully drawn, but require some modification to comply with current AMBSC regulations.  I am currently redrawing the Dewaal plans with the necessary modifications.  When they are approved I will detail the modifications.  A 1:8 scale the flywheel is 340mm diameter (13.4″).


A schematic drawing of the engine, as it was reconstructed.

So, watch this space.  Getting the plans finalised and the boiler approved will take some time, as well as gathering the materials.  From my preliminary discussions with the boiler inspector I will need to learn how to braze in bronze, and I will document that process.  Finding a supplier of phosphor bronze in the required sizes is proving difficult, and I might have to settle for the nicer to machine, but weaker, LG2 bronze.  If any reader knows of a supplier of cold rolled phosphor bronze sheet 200mm wide and 6-8mm thick, I would be delighted to hear about it.