johnsmachines

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. n.b. There is a list of my first 800 posts in my post of 17 June 2021, titled "800 Posts"

Tag: model Ottoman bombard

Ottoman Bombard Model. Bronze Pour. Fail.

This project was put aside when I broke some ribs unloading the melting furnace which I had borrowed. Each of the 2 halves of the bombard weighed about 8 tonnes in the original, and in my model will weigh about 8 kg each.

These 8 kg parts will be the biggest which I have attempted to cast.

I am using the lost PLA method, having 3D printed the parts in PLA.

Today I attached the PLA breech to a PLA pouring funnel (also 3D printed), and poured the investment medium around the part in a 5″ steel cylinder.

First I repaired the PLA part, where it was a bit ragged. Poured some melted wax where there was a deficiency due to unsupported overhang during the 3D printing process. I deliberately overfilled the area with wax. The bronze in that area will require some turning to get the eventual correct thickness. Melted the wax with a soldering iron.
Glued the breech to the 3D printed funnel, also using melted wax. All of the PLA will melt and burn out during the “burnout” process in the potters oven. That is a lot of PLA to burn out, so the windows will be open.

In order to minimise the possibility of air bubbles sticking to surfaces and corners, I painted the entire model with investment, before positioning it in the casting cylinder, and filling it with investment slurry. It will set overnight, and I will commence the burnout in the morning.

Fingers crossed for the pour late tomorrow afternoon.

It is now the next evening. I am despondent.

I woke early, and when I arrived at the workshop at 7:30am turned on the potters oven, and placed the cylinder containing the PLA model and investment medium inside. (Problem #1.) The cylinder was too big to sit vertically or horizontally, so I placed it diagonally. It was awkward, and I was concerned that the bore piece, being supported only at one end, might break free. It did. (Problem #2.) Started the burnout cycle at 250ºc, slowly increasing to 750ºc over 8 hours.

While that was happening I set up the melting furnace, gas cylinders (3 of them), tongs, bucket of water, face masks, gloves, aluminised apron, etc outside. It was going to be a warm day. Unfortunately it was also windy. Not ideal.

The furnace (centre), gas cylinders rear, dry sand tray front. Bronze ingots weigh 12kg. I had predicted that the casting would weigh 10kg.

Stuart arrived, and he checked his furnace. We lit it to pre warm the furnace and crucible. (Problem #3.) The crucible fitted in the furnace, with little space to spare. Just enough for the crucible with its tongs to fit. Stuart commented that it looked very big. It was, I answered “a 14kg crucible”. When the 12kg of bronze eventually melted it only half filled the crucible. It was not the size which I had ordered. It was too big, and restricted the gas flame, reducing its effectiveness. The melting phase required 3 hours! Much too long. (Problem #4.) (PPS. note added 23 Dec. I checked the dimensions of the crucible. It is a 30kg crucible!!! No wonder it was too big for the furnace! I had ordered and paid for a 14kg crucible. No wonder it was too big for the furnace. I should have checked before using it.)

The crucible has to sit on the furnace floor, reducing the heat exchange surface area, and narrow space on the sides restricting the flame volume.

Then it appeared that the flame was not as fierce as Stuart expected. The gas was piped from 2 cylinders, and one was not icing up as expected. It was close to full. Why was the gas not coming through? Could there be a ball valve somewhere in the system? Later we discovered that the pipe from that cylinder worked in only one direction because there was indeed a hidden one way valve. There was no direction arrow. (Problem #5.)

So, when we did get to the pour, and discovered the central core broken free (#1),

I inverted the now red hot cylinder to shake the core free. I calculated that the bore would fill with bronze and need drilling later. But would there be enough molten bronze to fill the cavity? I had allowed 1.5-2kg extra bronze to cope with unexpected contingencies but this would be cutting things fine.

So, we did the pour. There was a LOT of slag, possibly due to the slow melt. The molten bronze seemed to pour OK, and it filled the mold and the central bore. But it stopped about 3 cm from the top. Bummer!. Not enough bronze. Oh well. A learning experience.

I have washed and scraped off most of the investment. Oh Dear. A total failure. But, the threads were OK, so not a total failure. The worst area was the middle section which I had not painted with investment slurry prior to the the investment pour. I think that the PLA must not have been water tight, leading to the moth eaten appearance.

And worst of all….

It is only half the weight of the cannon, and it is just too bloody heavy!

I could fix the mistakes, reprint the part, and recast it.

But, you know what? I am not going to. The biggest issue is that even if I am able to fix all of the problems, and get a good result, it will be too heavy to move around. It will be too heavy to use even as a door stop. Hmm. Maybe I will clean up the failure and use it as a heavy door stop. Either that, or cut it up and reuse the bronze in the next casting projects, which will be much smaller!

This will be another failed, abandoned project to add to the list. (Chess pieces, etc). Oh well. Live and learn.

(it does cause me to appreciate the Ottoman cannon makers of 1465 who cast these parts with wood fires, where each component weighed over 8 tonnes!)

2 Man Tongs

I guess that title should read “2 Person Tongs” but I doubt that SWMBO will be volunteering.

I am still planning to pour a model bronze Ottoman bombard.

Wooden version. 520mm long, 107 diameter. I visited the 5.2 meter original near Portsmouth UK after I made this model, and I have refined the design of the new model.

The plastic model has been 3D printed, the flasks for the investment powder mould are ready, and I have the potter’s oven ready to dry, burnout, bake, and heat the moulds.

I have borrowed a melting furnace from Stuart Tankard, which is large enough to fit the crucible.  The crucible has 14kg capacity.  The crucible itself weighs 4kg.  Unloading the furnace from my Toyota Landcruiser cost me a couple of broken ribs, which set back the project a few weeks. 

Then I wondered about tongs to insert the crucible into the furnace, and, more importantly, how to lift the crucible full of molten bronze out of the furnace and pour the bronze into the moulds.  The weight to lift and pour I estimate to be: bronze 10kg, crucible 4kg, plus tongs say 4kg = 18kg.   The crucible with its bronze load will be at approx. 1100ºc / 2000ºf so some distance will be required for the gloved hands from the red-hot load.

3kg graphite crucible with tongs on the left, then ~5kg, and 2 x ~6kg. My new 14kg crucible at rear.

I have several pairs of tongs for smaller crucibles, but nothing approaching a 14kg crucible.  So I asked Stuart T for his thoughts on the matter.  He recalled seeing a video by an MSMEE member and suggested that I check it out.

John M’s tongs looked like they had been designed by an engineer, which was actually the case.  I contacted him (by email because Melbourne is in Covid lockdown), and he generously offered to send photos, a video and a drawing.

I copied his design, with a few modifications based on the materials which I had on hand, and also to enable a 2 man lift and pour.  In retrospect, I could have fabricated a one man pouring apparatus, using a swivel on a frame, but to be honest I would prefer someone else present for safety reasons. 

8″/204mm ID pipe. I need to reduce the ID to 185mm
To reduce the diameter from 204 to 185mm diameter I cut out 19 x 3.2 = 60mm plus a bit extra, then rolled the new diameter to 185mm ID. (shop made rolling machine)
Yep, it fits the crucible.

The remainder of the tongs construction was basic cutting, welding, and drilling.

I cut more of the tongs away to reduce the opening diameter, to still fit Stuart’s furnace. The overshot bent handle closure requires a positive action to open the tongs- a safety feature.
With the second man handle slotted in place
Testing the crucible and tongs in Staurt’s furnace. Cold.
The open position

I used to be a half reasonable amateur welder, but lack of ongoing practice lately, and dodgy eyesight is my excuse for the lumpy welds and essential use of an angle grinder.

Next steps….   I need some dry, non windy weather, and availability of assistance for the pour.  I will make the first mould, of the breech since it is shorter than the barrel, dry it, burn out the PLA, and bake it at 750ºc.  That will take most of a day.  While the baking is in progress (about 4 hours), I will start the melting of the bronze ingot.  Stuart says that I will require 2 full 20kg cylinders of propane.

Then the pour.  Then after some cooling with fingers crossed. Camera running…..

3D Printer Enclosure

It has been cold here during the current lockdown. And I mean temperatures. Not by American midwest standards by any means, but since we are confined to our homes except for limited predefined purposes, some days and nights are chilly. Down to 5-8ºc here.

I have been spending a lot of lockdown time doing 3D prints. And really struggling to get decent results.

Some of my GSMEE colleagues have been urging me to make an enclosure for my 3D printer. To be honest, Stuart T had urged me originally to buy a printer with an enclosure, but I pressed ahead and purchased an open structure model because I wanted the extra print size it offered. The Creality CR10s can print up to 300x300x400mm which I have fully used for my Ottoman bombard prints.

But in recent weeks, with the onset of the cold weather, I have noticed a distinct deterioration in print quality, particularly with poor layer adhesion when printing overnight, when the house heating is turned down or off.

So I decided to make an enclosure!

But, I did not have the materials on hand, and visiting hardware stores is verbotten with lockdown rules.

So, don’t laugh. This is what I cobbled together……

A couple of cardboard cartons, an artist’s A0 paper case (SWMBO hasn’t noticed it missing yet), and a blanket.

The heated printer bed is the heat source, at 50ºc. And I was surprised at the temperatures reached inside the rickety construction.

The steep temperature rise on the left is inside the enclosure after printing started. As you can see, the temperature rose from about 18ºc (room temp), fairly quickly to over 30ºc. After midnight, when the house heating was turned off there was a slow drop to 25ºc, and then a further drop to 18ºc when the printing finished and the bed self turned off.

The temperatures were measured with this gadget. A temperature/humidity logger.

Inkbird Temperature/humidity logger.

And the printing result??

The printer inside its enclosure, on the dining table.

This is the best quality print which I have had since the onset of winter weather. It is solid, water tight, and a reasonable finish. 0.2mm layer height. It is a molten metal pouring funnel, so I was not trying to get a super smooth finish, just an intact water tight object.

As soon as I can get access to Bunnings, I will make a more purposeful enclosure. Meanwhile, the cartons and blankets can remain in use.

Getting close to the first big bronze pour!