2 Man Tongs

by John

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…..