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Tag: Armstrong cannon model

Pre-teardown Model RML Cannon

Since New Year I have been slowly completing the model Armstrong 80Pr RML cannon which I am intending to keep for myself. The first example was given as a present to my daughter and son in law.

I expected that the 2nd example would be finished much more quickly than the first.

After all, most of the difficult design, casting, and machining decisions had been made first time round. And I had made extensive notes, diagrams, and photographs first time round.

However, there were a few obstacles to rapid completion…..

  1. I had made some design changes. Always risky. Always time consuming.
  2. I could not find some notes and photographs which I was sure I had carefully filed away.
  3. I could not remember how I had made some tricky small shapes, and had to reinvent some methods. In some cases that triggered a memory of the first method, and I realised that I had reinvented the first method.
  4. SWMBO had other ideas about the best method of using my time, and making model cannons did not enter her equations.
  5. I made some brand new mistakes, which had to be rectified.

But, here I am, very close to final teardown, and then for final assembly and completion.

With respect to final painting, finishes etc. SWMBO has made a strong pitch for the same finishes as model 1. My inclination was to aim for authenticity, and paint most of the model. SWMBO wins, as usual. “It has to look interesting and beautiful, not boring black.”

So here are some pre-teardown photos.

Hmm. That kitchen table needs re-polishing too!
The side platforms are an extra feature on model 2.
I redesigned the rear wheel brackets, and cast them in bronze. Looking at this photo reminded me to do some more finishing on the casting, and to make more authentic looking axles.

…and to remake the rear wooden platform. The screws are too big for the scale. And to make sights for both models.
….and to finish making the loader….and the riveting. My riveting technique has improved, no? Note the redesigned bracket for the recoil tube, and the redesigned elevation handle. (You probably don’t remember what these looked like in model 1.)

Almost finished the kitchen for SWMBO, so I should be free to finish my Armstrong 80pr RML in the next few weeks.

Meanwhile the computer which runs my CNC lathe went “bang” when I last turned it on, and it is dead. It is close to 20 years old, and it lived in an environment full of dust, swarf, mice, damp, and the odd tiger snake. Originally ran on Windows XP (some of you remember that one?). So I will install another oldish laptop to run the lathe, and will change from the parallel port interface to a “Smooth Stepper”, thus joining the 21st century. (I do hope that Stuart, my expert friend, is reading this.)

Painting a Brass Label

I tried a new technique for making a brass label. I hasten to add that the technique is new only to me.

The label after engraving on the CNC mill

This is my engraving setup. A 26,000 rpm 2kw head, clamped to the main spindle, and controlled independently. The Z and XY axes are controlled separately by Mach 3. I turn off the main spindle to avoid embarrassment.
Using V-Carve Pro to generate the G code, and Mach 3 to run the mill. 90º V bit, run at 18,000 rpm, 100mm/min. Not a bad result. Not perfectly centered. I will mill off 1/2 a mm on the right
Then a coat of gloss paint. That looks interesting, no? The paint does not adhere to the sharp edges. I might use that as another technique in the future.
The surface paint is removed with 600g wet and dry, leaving the paint in the engraving.

The contrast of the black on brass makes the wording easier to read.

BUT. I should have waited for the paint to dry completely before sanding it. Some of the dust has been embedded in the still wet paint, reducing the gloss and making it a bit dull and fuzzy.

Also, the surface needs to be finished with a finer grade of wet and dry. Then lacquered.

A (model) Cannon for Christmas

Christmas 2020 seemed to hold particular significance. Our children, their families, grandchildren all congregated and had a superb vegetarian meal with food preparation shared. Vegetarian, because a majority of our extended family are now vegetarian. One is a vegan, and some of us are inching our way towards that aim. Even the omnivores are mostly reducing their meat intake.

We enjoyed some lovely Australian wines, with Pavarotti in the background.

This was the first time that the whole family has been together for 9 months.

Everyone had a hand in food preparation. Two Hands Shiraz 2017.
Perfect weather. 25ºc. The grand-kids had a separate table outside. A very happy gathering. 3 daughters on the right. Sons in law and a family friend on the left. You know who at the end.

The grandchildren had been forbidden to get up for presents before 6am. And my son in law set their clock back an hour! So it was a leisurely start to the day. 7am.

We usually do a Kris Kringle for adult presents, but this year, we just decided to have no restrictions.

I had wondered (and to be honest, been slightly anxious) how the model Armstrong RML cannon would be received by my son in law (front) and youngest daughter (right front). Neither of them have any interest in weaponry or military history. My son in law grew up in Port Fairy where the original full size cannons are slowly rusting away. And my daughter took part in 2 casting sessions to see what casting was about. But neither had any idea that the model cannon was for them, and apart from the aluminium casting, neither had seen the cannon gradually being made.

Their reaction exceeded my most hopeful expectations. Both became teary, as did I.

The following video was made by my daughter. The daggy paper hats are part of our celebration. Totally unscripted. And I have had a few by this stage of the day.

rosy cheeks. Must be something in the water.

Wooden Base for Model Cannon

I have machined a wooden base and I will fasten the central column of the cannon chassis to the base. The reason is that people cannot resist swivelling the cannon around on its column and the the wheels tend to mark/scratch polished surfaces. Better to mark a wooden base than a polished mantelpiece. But how to finish the surface of the base? Any polish/paint will quickly develop marks from the wheels. I have decided against making steel railway lines for this model.

I have used an Australian hardwood (mountain ash, a very hard dense wood, reclaimed from a demolished building). I am thinking that I will just oil it. The colour of the wood will darken with age, but will never be as dark as the table, which I made decades ago from Australian Iron Wood. (note, not iron bark. Iron wood. The hardest, densest wood I have ever used. And yes, I have worked with lignum vitae, and Australian red gum. The marks in the surface of the table are only in the polish. The wood is almost impossible to scratch. My kids used to dance on this table 30 years ago.)

The burn mark on the end of the base is from the belt sander. I will remove it with hand sanding before oiling.

The machined finger grips on the ends were made on my vertical mill with a steel moulding cutter intended for metal machining. It worked well.

I discussed the finish which I wanted to achieve with my resident finishes expert. SWMBO. I wanted a slightly darker, low sheen finish, which would not get scraped off with the cannon wheels.

She recommended this stuff. It is a stained, penetrating oil. Smells very chemically.

It is actually a surface repairer, rather than an overall finish but I did what I was told.

OK. That looks good. The surface will be easy to touch up if required.

Then I read the rest of the label….


Not One of my Skills…Riveting

The cannon carriage, partly disassembled, ready for final riveting.

I am no expert at riveting, but I have had some good advice from an expert. He has had years of experience in the aircraft industry. Prior to this cannon project I would have inserted fewer than a dozen rivets. You cannot beat experience. And knowledge.

As you look at these photos, and grimace, bear my inexperience in mind. Actually, my results improved as the day wore on.

An eye bolt goes in the X hole.
Note the use of nuts on the rivets where access was difficult.
I give myself 5/10

I learned a few lessons as a result of this session of riveting.

  1. Riveting is a manual and knowledge based skill, which must be studied and practiced.
  2. Items which are riveted change their dimensions. Components which fitted perfectly when machined and bolted together develop gaps and warps after riveting. Not surprising, considering the hammering of relatively unsupported pieces.
  3. The tools must be perfectly designed for the job. The snaps must be the correct shape and size for the rivets.
  4. Rivets from different manufacturers differ in dimensions, even when supposedly the same.
  5. Soft components like aluminium can deform and break when riveted.
  6. Retired gynaecologists should not rivet. Stick to nuts and bolts.

I am hoping that the bruises and cracks and deformations which I have caused with the riveting will be camouflaged by the paint job.

Surprisingly, the carriage still sits flat on a surface plate. And the barrel sits squarely in the trunnions.

Model Cannon. Final Photos before Teardown

I have commenced the teardown of the model Armstrong cannon. But first I took some photographs, just in case the paint job is not the best. To explain, I do not have a great history of good paint jobs. SWMBO bans me from painting around the house (that is NOT a loss, believe me) because of runs, brush hairs in the paint, paint applied too thickly etc etc.

So here are the photos….taken with my iphone. Just to reiterate, this model cannon cannot be fired. It has no touch hole/vent.

In the firing position.

After the teardown I will complete the insertion of rivets, replacing many of the 8BA screws with rivets.

After firing, with the barrel depressed to 17º, after swabbing to remove persisting embers, ready for the 20lb of gunpowder in a silk bag, with a wooden rod in the middle to prevent the bag from collapsing while being rammed. Then the 80lb projectile with “plate like” copper gas check (to engage with the rifling) is lifted onto the loading cradle and pushed into the barrel with a wooden ram rod. The projectile had a rope quoit around the pointy end to jam it into the firing position, and stop it from inadvertently falling out while the carriage is rolled down to the firing position.

The firing rate for these 80lb cannons, with a trained crew, was about 1 round every 1.5 minutes.

There should be an OH&S sign saying, “best not to stand here during firing”.
Wooden side planks for the gun crew, and a short shelf near the front to rest the projectiles before loading, yet to be made.

So, wish me happy painting. Still haven’t finally decided on colours. But probably black for the interiors, silver for wheel assemblies, wheels barrel gears and brass components unpainted. Maybe a light grey-blue for the exterior of the carriage and the chassis.

Brake for a 5 ton Cannon

Well, actually, it is a 1:10 scale model of a 5 ton cannon. The model weighs around 5-10kg at a guess.

The brake is to control the descent of the barrel/ carriage down the 4º slope of the 5 meter long chassis.

On the original, the brake was a steel band on a steel drum which was attached to the big gear.

The drum is approx 600mm/2′ diameter.

This is how it appears on the scale model..

From the other direction. The stainless steel band winds around the drum, and is attached to a small lever which is operated through the shaft by a much larger lever on the outside of the chassis.

I was a bit apprehensive about this job. The lever is very close to 2 gears. the steel belt has to be properly tensioned, because the degree of movement of the control lever is quite restricted. And the width of the band has to be slightly less than the 5mm groove on the drum.

But, I found these…..

Stainless steel cable/plumbing ties. 4.75mm width, and in various lengths. And quite inexpensive.
The cable ties are sharp and springy. Feeding them into position was tricky. But after bending them around the pins on the shaft I was reasonably confident about silver soldering them with a loop at each end. The soldering was straight forward. Stainless steel silver solders well.
The handle is almost 1 meter long. 96mm brass on the model. It will be pinned to the shaft.
I will adjust the shaft length in the next workshop session.

So, for once, my apprehension was not warranted. The job was fiddly, but no major mistakes!

Typists Correction Fluid

WTF! I thought that this site was about model engineering, metal working etc.

Well. I just need to say that typists correction fluid is an essential tool in my workshop.

Not for typing, I hasten to add. But for silver soldering….

Today I needed to silver solder an extra 1mm thick disk to a tiny part, which already had 2 silver soldered joins. I had spent an entire workshop session designing and making the part, and I did not want it to fall apart when I added an extra component. Which I admit, was an afterthought.

And the central hole in the extra disk HAD to line up precisely with the threaded hole in the previously made part.

I had been advised by another GSMEE member that a metal surface painted with typists correction fluid WILL NOT accept silver solder. I have tried this method once before and it works. This is another demonstration.

In front of the correction fluid is the part, with the extra 1mm disk, silver soldered with the 2mm screw holding the parts together. And after soldering, the screw came out. It was not soldered into the assembly because it was coated with the correction fluid.
So annoying. WordPress used to enable rotating images. Not now. So these are the components to be silver soldered. Fluxed. And parts which I do NOT want soldered are coated in the correction fluid.
The work rests on brass blocks to function as heat sinks, to protect the existing soldered joins. This shot shows the workpiece after soldering. Has the correction fluid worked? Well, you have already seen the evidence. Amazingly, it does work.

This handle locks the elevation gears into position after the cannon barrel elevation has been set. Several more hours were required to file a central tapered ridge into the added material, and a corresponding groove where it rests. It all worked out OK.

Typists Correction Fluid. I hope that it never disappears from OfficeWorks.

Thanks again Frank Marrian GSMEE, and jimmymouse, for this great tip.

And Some More Bling on the Cannon

Attached the recoil tube yesterday. But I cheated. It is a gas strut.

The recoil tube from the front. On the original Armstrong RML’s it was filled with thick “Rangoon Oil”.

But, it is SO close to the dimensions that I required, that I decided.. what the hell. It is 18mm diameter (17.5mm required), and 200mm long (198mm required).

I degassed the strut by drilling a 1mm hole, and the gas came out under considerable pressure. I had to do that, because the strut was too strong for the cannon. Even degassed, the strut has enough shock absorbing action to be useful and realistic. I made some brass brackets and a cap, for visual consistency.

Today I made the lever which locks the elevation gears.

Not much to show for an entire day in the workshop, but it did involve a lot of planning, a bit of CNC cutting, and silver soldering. Still some small details to add.
The Port Fairy original. Some bits are missing.

Happily Using Technology

Yep. To make my models I use a computer for drawing, making lists, ordering fasteners and materials and tools on Ebay and from suppliers, driving CNC 3D printer, driving CNC machining tools. And laser marking parts. I also do a lot of traditional machining, hand filing and sanding.

This is 2mm thick brass sheet. I asked Stuart T, who has a 30w fibre laser, to mark the elevation scale protractor for my Armstrong 1:10 RML cannon. Not sure of the outcome, I supplied some blanks (LHS), and cut out parts (RHS), and a CAD drawing of the part and the text.

The cannon barrel will elevate to 30º, and depress to 5º for firing, although in practice deviations from a degree or two from 0º were rare. Plus there is an extra mark for 17º depression, which was the reloading angle. The numbers which the laser marked are only 0.6mm high. Hard to see with the naked eye. But in scale.

Lasering the tiny marks and numbers took 2 seconds per pass. That is, 2 seconds to make all of the marks and all of the numbers. After some experimenting, we settled on 50 passes. Which was still less than 2 minutes per part.

Click on the following frame to see the video…

How fantastic is that????

So quick, precise and clear. Yep. I am quite happy to employ any new technology which is available to me.

Then today I made the fittings to secure the recoil tube, and drilled and tapped the 8BA fasteners.

The carriage, chassis and bling is really coming together. Just a few more bits to make and install, then the final riveting and painting. Ready, I hope, for Xmas.

The recoil tube is a commercially available gas strut. It was so close to my 1:10 scale dimensions that I decided to cheat, and use it.

I drilled a 1mm hole in the gas tube to release the gas and oil, turned and re-threaded the front end of the piston rod to 6mm, and made the brass supports and end cap. I released the gas because it was too stiff for the model. Even without the compressed gas the strut has some “shock absorbing” activity, and I am quite happy with the decision.

More Small Cannon Parts

I suspect that this post will not be of much interest.

But the parts represent 2 whole days in the workshop, so I am writing these notes for my own diarising more than your entertainment.

The chassis of the model Armstrong RML cannon has a cross bar, which is bolted to the longitudinal bar, and is attached to the side girders with some small, shaped clamps.

On the original Port Fairy cannon. There is a 4º difference between the girders and the central bar.

The crossbar is under the big gear. Still some shaping required to improve the appearance. When I have finished the gaps will disappear. The difference between the original and the model cross bar relates to pragmatics of shaping miniatures. Compromises inevitable. And if you noticed, the fasteners on the left are BA8, and 2mm cap screws on the right. I need to buy more BA8’s.

The cross bar doesn’t look much, but it has 4 bends and a twist. The space was too tight for me to measure the angles, so I bent the cold bar by estimating the degrees by eye. Same with the twist, except that the twist had to be confined to the section not attached to the girder or the central longitudinal bar. So I heated that to red heat with oxypropane. The twist was 4º. But I eye balled that too.

8BA bolts x6 in a hex pattern join the cross bar to the longitudinal bar.

The clamps required some planning. I considered machining them from solid bar, but work-holding was going to be problematic.

So I silver soldered 2 strips together, cut off the pieces, then sanded, filed, and manually bent the angles.

The silver soldered join overlaps by only 2mm, but it survived some aggressive bending. 100mm long. The parts were sawn off, then further sanding, filing, and hole drilling. Workshop dirty fingers with swollen arthritic joints.

p.s. Another day later, more of the same…

This is the underside of the model Armstrong cannon carriage. I have bolted on the 4 cast bronze fittings which hold the carriage onto the chassis. Later those fittings will have wooden/steel disks sandwiched and bolted onto them to become bumpers at the extremes of travel of the carriage on the chassis. All of the cap screws will be replaced later by hex head BA bolts.
this is a view of the underside of the chassis, with the carriage secured above. An unusual view.

I would prefer to use 2mm metric bolts rather than 8BA, which is a similar diameter and pitch, but unfortunately I have been unable to find a supplier of 2mm bolts with hex heads. BA bolts are several times more expensive per piece than metric, and it adds up when using hundreds per cannon.

CNC Machining a Small Part

The part measures 20x12x7mm.  And it has some tiny details.

Not quite finished here. Still needs a shaft hole drilled and reamed, and the top holes to be threaded.

The design is simply and quickly drawn on V-Carve. A rectangle with rounded corners for the base, and a rectangle with 2 arcs on each corner of the column. Circles added for fastener holes.

This is where it ended up….

The part is a bracket for the shaft. It locates the shaft in 3 dimensions, so the height of hole above its base is exact.

There are many ways to approach the machining of the part, and this is the technique which I used……

The part is machined in the end of a piece of material which can be held in a vice for milling, and later held in a lathe chuck for parting off. The hole for the shaft was made after parting off. The parallel end faces permitted it to be held in a vice. The shaft hole could also have been made by holding the brass rod in a vice or chuck before parting.
Not quite finished. When the bracket comes off next time, it will spend some time in the gemstone tumbler to take off the sharp edges and improve the surface finish.

The control wheel for the elevating gears was found in my rejects box. It was made for the triple expansion engine. It looks pretty good? Cant remember why I rejected it for the triple. Maybe my standards are lower these days.

There are not many photos of these cannons on the net, and none of them show this wheel. Or was it a simple handle? The shaft has a squared end for a wheel/handle of some sort. So this wheel is my best guess as to what would have or could have been used. Virtually all of the cannons remaining of this type have had the small parts removed/souvenired/stolen which is sad. Some old photographs of bigger Armstrong RML’s show wheels of this type, so I feel justified in making this design assumption.

P.S. And after making that comment above, I rediscovered this photo a few days later. I think that it is the Armstrong RML at Portland, Victoria. Note the hand-wheel at the front, which will be for barrel elevation. This is a different setup from the gun which I am modelling, with the gears within the carriage, but the hand-wheel is similar to what I came up with.

Another design consideration. SWMBO likes the cannon without the chassis, as in the above photo.

But this is how it looks on the chassis.

…and there are many hours of effort in making the chassis, and movement gears/brake/big wheel/riveting etc. and still more to be added, such as the projectile loader, gunner platforms, etc.

The gun and its carriage have brackets which make separation from the chassis very difficult/almost impossible. So I am considering a design modification which would permit a choice of with or without chassis. What do you think?

(please note. this is a MODEL cannon, has no touch-hole/vent and is therefore not capable of being fired.)

Fitting the Barrel Elevating Gears.

Firstly the right hand carriage side was removed from the carriage.

This is a side of the model Armstrong RML carriage.

I turned a disk with a small hole to locate one arm of the dividers at the centre of the trunnion, and positioned the quadrant gear. Then super glued it, and its pinion, into position. Marked the locations. The super glue will be removed later.

Then drilled and reamed the pinion hole.
The location of the barrel fitting was determined after reassembly of the carriage, with the quadrant gear still glued into position. Very tentatively drilled and tapped the holes for the bronze fitting into the barrel. That cap screw will be replaced by a shop made countersunk screw.

The bevel gear case was located through the pinion gear hole, and keeping the control handle shaft level. The case was drilled and bolted into position. The control handle shaft will be replaced by one of smaller diameter, in keeping with the 1:10 scale.

starting to look like the real thing….
That cap screw is temporary.

A couple of days in the workshop working out how to position those parts and drilling, tapping, and reaming. The other cannon will take less time.

Barrel Elevation Gear

This photo is the original Port Fairy cannon, and the 3D printed 1:10 model size copy. The original is ferrous and the guide is bronze or gunmetal. I decided to make my scale model versions from brass, for ease of construction, and to avoid rusting because these parts will not be painted.
I did not have a piece of brass big enough and thick enough to cut a 360º disk. It would have been 182mm diameter and 2.5mm thick. So, I made this fixture, and attached an aluminium plate.
Bolted on some bits of brass bar the correct thickness, using holes outside the gear, plus one which will be incorporated into the gear.
Then cut the teeth, using my CNC rotary table.
Then used the same fixture held in the milling vice, and cut the interior contour, and a rebate.
The ends still need to be shaped.
Showing the curved guides, rebates, and the bronze castings which secure the gears to the barrels

Making brass fittings is always a nice and enjoyable part of a modelling job. Making “bling” as my GSMEE friend John B characterises it.

More Bronze Pour Problems and Cheap Spanners


I had 3D printed another tree with 4 cannon parts. Brackets. The complete tree fitted into a steel flask 100mm diameter and 120mm high. So I repeated the steps of the last successful pour, and painted the tree with investment slurry, mixed the main investment, degassed it, poured it, and degassed the entire flask, investment and all. That method had worked well before, so I repeated it.

But I was a bit concerned because the investment was only a few mm thick at the bottom of the flask. Would it hold up? Read on.

So then commenced the drying, burnout, and baking cycles in the potters oven. Normally it is about an 8 hour process, and I did not get to start until 12 midday. So I was in for a long day.

But then the oven started to play up. It would suddenly switch off. The temperature gauge would swing wildly. And would not heat above 400ºc and it needed to reach 710ºc.

I did not know the source of the problem. Thermostat? Wiring? Controller? Power supply? I did know that the thermostat wire was not rated for temperatures above 600ºc, but it had worked OK previously. So I turned everything off, and removed the electronics compartment. Changed the thermostat wire to the proper grade (thanks Stuart!), then found a loose main heater element join, so fixed that too. It all took another 1-2 hours.

Started up the oven again. The temperature had dropped to about 200ºc, but the the temperature started rising slowly, so maybe the problem had been fixed? By this time it was 4pm, and there were still 7 hours of heating required, so it WAS going to be a late night in the workshop. Made my peace with SWMBO. She was happily watching the footy, and not too worried about about my travails. (and our team won convincingly!)

To finish this story, I eventually poured the bronze, and my earlier concerns about the thin layer of investment at the bottom of the flask were realised. The bottom fell out, and molten bronze poured out through the breach. I normally rest the flask in a tray of sand when pouring, and fortunately, the bronze seemed to harden when it hit the sand, and the outflow ceased.

This was the result….

Amazingly, the parts seem fully formed, with no voids or bubbles. The ugly lump underneath is the bronze leak through the bottom. Note the length of feeder sprue. And the funnel. If you zoom into the photo you will see that the fine detail of the 3D printing has been reproduced. I will cut the parts off and finish them tomorrow. I got home about midnight. I needed that shot of single malt.
The bronze brackets, after sawing them from the tree. I will add a photo after another session of machining and finishing them.
After some more tidying. The investment powder can be persistent.


I have several machines which use 40ER collets. I have enough collet spanners, but only one locking spanner for the chucks, and it is always on the wrong machine. So I decided to get some more locking spanners, and I sent my drawing to the laser cutting company. I picked up 4 spanners from them a few days later. Cost $AUD55. (cheap!)

My Colchester, with ER40 chuck. And one of the new stainless steel locking spanners. Drawn up as a dxf file, which was emailed to the laser cutter. The square hole is to lock the carriage to the bed. A few moments on the belt sander removed the sharp edges. Not elegant, but works perfectly.
2 spanners are required to tighten the ER collet. Here I am making a jig which will be used to cut the quadrant gear which elevates the cannon barrel. More about that in a day or 2.

Cannon Update

Not much happening to show visual impressions, so fewer posts, but lots of hours making bits function.

The three main gear shafts now have brass end caps. They will have oil cups drilled into the 12 0’clock positions next time the caps come off.

The adjustable parallels do not get a lot of use, but they are very handy to align parts in horizontal positions, like the holes in the end caps above. Especially when the girder is at an odd (4º) angle, and even the bottom of the girder is at 1º.

Currently I am planning the making and installation of the barrel elevation gears. Here is a PLA version, paper clipped into position.

Very handy having plastic versions to decide drilling positions etc. In the above photo is a plastic version of the main elevation gears, printed at the correct centres. The little bronze bit is the casting which is screwed to the barrel.

And just to demonstrate the current appearance of the cannons….. Lots of bits yet to be added, but it is exciting to see the size and form of the models.

Tension Drilling.

Do you know what tension drilling is? Well, read on.

Having made the gears which position the carriage on the chassis of the Armstrong RML model cannon (I assume that regular readers will know by now that RML stands for “rifled muzzle loader”), I had to drill the chassis for the gear shafts.

There are 3 shafts, 8mm, 6mm and 5mm diameter. I knew the theoretical distances between the shaft centres by applying formulae taking into account module and tooth numbers. And also by using “Gearotic” software.

(I tripled checked with a lash up and direct measurement.)

But! I did not know the distance between the big gear and the rack gear. Because, the rack is attached to the base of the carriage, and the big gear is attached to the chassis. Considerations such as trolley wheel diameters, rectangularity of chassis and carriage, and position of the trolley wheels on the carriage all come into play. I will not bore you with details, but determining that measurement involved a lengthy, tricky, and complex setup using a surface plate, height gauge, adjustable parallels, straight edges, and averaging the errors. Amazingly, it turned out OK.

Then came a decision. To drill and ream straight through both girders at once, or to measure and drill/ream them individually. Luckily for me, I had a visit from GSMEE member Swen, (to borrow a tool), who is a retired ex-army Warrant Officer artillery fitter/turner. When I explained my dilemma, he was in no doubt. Measure them and drill them independently, he advised. So I did just that.

But, having invested many, many hours to date in making the chassis’, drilling a big (relatively) hole in the chassis girder was a very tense moment. (hence “tension drilling”).

Before drilling any more of the 6 holes required, I tested the fit between the rack and the big gear. Amazingly, it seemed pretty good. Maybe a little bit tight, but not too bad. So, I drilled and reamed the remaining holes.

Collars, splines, pins, bronze bushes and brake fittings yet to be made.

That photo represented a very long day in the workshop. I think that I arrived home about 9pm.

And there was a problem.

The big gear and its partner would rotate freely in one direction, but were catching and lumpy in the other direction.

Closer examination revealed that the teeth of the pinion appeared to be bent, allowing free movement in one direction only. Hmm…. how could that have happened? And how to fix it?

Root cause analysis of the issue concluded that the mill Z axis must have been bumped when I cut the teeth on that gear, causing them to be slightly off centre, producing the “bent” appearance. (the top photo shows the faulty gear. Can you make out the distortion?)

Solutions? Make a new gear. Or fix the distorted one. I decided to try the second option. I was not wanting to make another ratchet. So, I filed and tried, filed and tried, filed and tried…. you get the picture. And gradually the lumpiness disappeared. Several hours later, with blisters appearing, it seemed quite good, and will not be visible to casual inspection. You, dear readers, will be the only ones to ever know.

Yesterday I drilled the second chassis. I completed the task in only 2-3 hours. A fraction of time compared with the first one.

Ducks in a Row and Bevel Gears.

Another small bronze pour yesterday, and it was my best one yet.  No bubbles.  No voids.  And excellent surface definition.  What did I do that was different?

First, the 3D parts were printed already attached to the tree.  So the trunk and branches were 3D printed with the parts attached.  That meant that I could determine more accurately the bronze flow, the gaps, the spaces.  The only “failure” was that I added some wax air vent sprues as an afterthought.  And those wax parts were the only part of the pour which failed.  Fortunately, the absence of the gas vents did not seem to matter.

The 3D printed tree. There are 9 PLA brackets ready to be replaced by bronze. I increased the height of the trunk for extra melt pressure. The air vents failed, and were not needed anyway.

Next, I painted the tree with a slurry of investment. The slurry was much more watery than the normal investment, but it was thick enough to leave a thin layer of investment on the surfaces, paying particular attention to the corners and internal edges.

Then I used my new, 1 hp vacuum pump to degas the investment mixture. It took about 15 seconds to reach maximum negative pressure, compared with about 1-2 minutes which the 1/4 hp unit was taking.

Then, after pouring the investment, I placed the full flask containing the tree and investment, and degassed the entire unit. I was shocked at how much extra air bubbled out.

The rest of the process was as usual, drying for 4 hours (except that this time it started at 6am, having put the process on an automatic start timer), burnout 2 hours, and baking 3-4 hours.

The cast tree was looking hopeful. And not much surface oxidation to see. (I had given the 15% phosphor copper a full 2-3 minutes to work this time.)
….and there are my brackets. 9 ducks in a row. They need a bit of filing, and some time in the gemstone tumbler.


Top is a bevel pinion as it arrived, and a mandrel which I made. Middle row is an unmodified bevel gear which is too big for the case. Bottom row is a machined bevel gear which now fits into the case, and a pinion on shaft, which also fits into the case.

While the investment flask was cooking, I experimented with the bevel gears which move the cannon barrel elevation. I had cast some bronze gears, teeth and all, some weeks (or was it months?) ago, but was not happy with the result. So, I had bought some bevel gears on Ebay. They are spare parts for an RC model car. Not quite the correct size, but close. The metal is HARD. Sintered? But, machinable with carbide cutters. (ps. added weeks later. Even carbide cutters struggled with machining these gears, so for the second set I used a tool post grinder on the lathe. That worked well, and produced a better finish.)

It all now fits.

Now before you all start shouting at me to make the bevel gears from scratch, let me just say that I might do just that. Not yet decided.

Rack Off

As in, not yet installed.

I spent few hours finishing the racks today. But not yet installed. Some photos.

Firstly the racks were surfaced…
… then drilled, then given an outline using CNC.
….then tested against their corresponding circular gears. But not yet installed.

For once, a job proceeded without a mistake. Hooray. Hmmm. Look at that big gear. Thinks… “I quite like that blackened inner area with the polished bronze hub and teeth”.

(p.s. For non-Australian readers, “rack off” is an expression sometimes used in Oz, when telling someone to leave or desist, in a forcible, but not quite foul manner. Used in the post heading in a hopefully, mildly humorous effort to be eye catching.)

On the RACK

The final gear in the cannon carriage positioning train is a rack gear. It is 198mm long, 7.2mm wide plus tabs for bolting it to the carriage of the model Armstrong cannon.

A rack gear is a flat gear, and it is cut with the same cutter (number 8) which makes a circular gear of 135 teeth or more.

The teeth of a circular gear are cut by dividing 360 degrees by the number of teeth. But the pitch of the teeth of a rack gear is determined by a formula found in Machinery’s Handbook. rack gear pitch = module x 3.1416. Which for my module 1.25 = 3.927mm. Hmmmm. 3.1416. That is a familiar number. Light bulb in brain switches on! A rack is just part of a circular gear of very large diameter.

At first I thought that I would use the same mill arbor which I had been using for the circular gears, but as soon as I started to set it up I realised that the stick out of the arbor would be ridiculously excessive. So, reluctantly, I set up the horizontal milling attachment of the mill. Reluctant, because the attachment is heavy, fiddly, and time consuming. Luckily, I had a 22mm shaft for the attachment, the correct diameter for the cutter. I had never used this shaft before, and it was missing the nut, and bronze bush. Bought it on Ebay years ago. Found a suitable nut and made a bush.

This is the setup. The 350mm shaft was not long enough to make the rack in one setup, and it took a bit of trial and error to work out the best compromise.

The horizontal attachment on the vertical mill. Setting it up takes me a couple of hours.
No CNC here. Just lots of calculations using 3.927.

I need 2 racks, so I will split this one down the middle.

Did not have a piece of bar stock big enough for this job.

So I joined 2 pieces end to end with silver solder. Will that be strong enough? My friend Stuart T insists that a well made silver solder join is stronger than the parent metal, so we will see.
It meshes nicely. The mounting holes (which I did not use) will disappear when the outsides of the 2 racks are shaped.

A bit of a story about that heavy horizontal mill attachment. When I bought it some years ago, I put it in the rear compartment of my SUV. But on the way home my SUV was T-boned by an idiot at an intersection. No injuries, but a big expensive dent to the passenger side of my SUV. Air bags activated. And the rear window was smashed. I could not figure out why the rear window was broken. After the police and fire engines had finished, and the tow trucks arrived (my car was out of action for 2 months, the other vehicle was a write off), a by-stander approached me with a familiar object which he had found in the gutter on the OTHER side of the 8 lane highway. Yep! It was the horizontal mill attachment. Been flung through the rear window by the violent impact of the collision, and across 8 lanes of the road. It was scratched, but otherwise intact. And thank goodness, it had not hit me or anyone else in its trajectory!