Ash Pan (April - June 2020)
The ash pan on the Allchin was seemingly an 'easy' assembly to make, however on execution the drawings were poor (lacking in detail and dimensions) and the assembly was incredibly complex and challenging to make!
The first step was to arrive at a working design. My model has errors in the boiler, errors in the dummy throat plate and error thus corresponding errors in the hornplates. All this equated to the original design not being suitable as it would not have fitted! Thus, a new design has to be created that was true to the original lines and fitted! The design was an iterative process using a cardboard template to offer up to the engine to identify where the key key features should be.
The initial stage of manufacture was the main body of the pan, this was flanged/bent from mild steel. A former was made from MDF to control the bend and locate the taper on the front end. Once bent, the sides could be finished to width to align with the inner faces of the hornplates, the lower part of the boiler was also reshaped with files to achieve a flat and square mating surface.
The ash pan on the Allchin was seemingly an 'easy' assembly to make, however on execution the drawings were poor (lacking in detail and dimensions) and the assembly was incredibly complex and challenging to make!
The first step was to arrive at a working design. My model has errors in the boiler, errors in the dummy throat plate and error thus corresponding errors in the hornplates. All this equated to the original design not being suitable as it would not have fitted! Thus, a new design has to be created that was true to the original lines and fitted! The design was an iterative process using a cardboard template to offer up to the engine to identify where the key key features should be.
The initial stage of manufacture was the main body of the pan, this was flanged/bent from mild steel. A former was made from MDF to control the bend and locate the taper on the front end. Once bent, the sides could be finished to width to align with the inner faces of the hornplates, the lower part of the boiler was also reshaped with files to achieve a flat and square mating surface.
The front corners of the pan slope up by 20degs forming the mouth of the damper, this was the first difficult part as to draw the rise up the 1/4" rad fillets on the body of the pan had to be brought up with it. To achieve this, cuts had to be made in the front corners of the pan so the steel had a space to be bent into then the joints silver soldered. The front cross stay with forms the top of the mouth and interface with the dummy throat plate was then made, fitted and silver soldered into position. Again this was different to the drawing to accommodate the errors in the throat plate.
The rear of the pan was next, this differs from the front as it slopes up at 45degs however the fillet of the body ends at the bend and transforms into a square edge. This was mega difficult to understand from the drawing and very difficult to achieve. Again sections of steel had to be removed to accommodate the bend and all gaps soldered and finished smooth.
The body of the pan was attached to the bottom of the boiler with 6 screws that were drilled and tapped into the foundation ring of the boiler - terrifying! The drawings state that Bronze screws should be used as there is a chance that the drilling will break into the wet side of the boiler. Again I changed the design here and ensured the drillings were sufficiently shallow that the pressure boundary of the boiler remained intact.
The body of the pan was attached to the bottom of the boiler with 6 screws that were drilled and tapped into the foundation ring of the boiler - terrifying! The drawings state that Bronze screws should be used as there is a chance that the drilling will break into the wet side of the boiler. Again I changed the design here and ensured the drillings were sufficiently shallow that the pressure boundary of the boiler remained intact.
Fire Grate and Trap Door
The fire grate and trap door was the next assembly to be constructed. Again the design had to be modified to suit the actual boiler opening, which was different to the drawing. Initially the grate was machined, this was bought from Reeves as a casting, as apposed to an assembly as requested the drawings. The iron casting was the hardest material I have had the misfortune to machine in a while, I burnt up two cutters on the first session trying to achieve some feeds and speeds without much success. I resorted to assembly a make shift autoclave heating the the grate to cheery red, holding it there for about 10 minutes and allowing it to slowly anneal. This process did marginally reduce the hardness however it was still a nightmare to machine. The one front corner of the casting had a huge hollow defect, which was only identified during the first machining processes. Practically this should have scrapped the casting, however as no replacements were commercially available, the defect was worked around and the design modified to suite.
The fire grate and trap door was the next assembly to be constructed. Again the design had to be modified to suit the actual boiler opening, which was different to the drawing. Initially the grate was machined, this was bought from Reeves as a casting, as apposed to an assembly as requested the drawings. The iron casting was the hardest material I have had the misfortune to machine in a while, I burnt up two cutters on the first session trying to achieve some feeds and speeds without much success. I resorted to assembly a make shift autoclave heating the the grate to cheery red, holding it there for about 10 minutes and allowing it to slowly anneal. This process did marginally reduce the hardness however it was still a nightmare to machine. The one front corner of the casting had a huge hollow defect, which was only identified during the first machining processes. Practically this should have scrapped the casting, however as no replacements were commercially available, the defect was worked around and the design modified to suite.
The grate is supported by two stirrups front and back that are in turn pinned to the grate via pins which were drilled and tapped into the grate. The stirrups align with a the outer gaps in the grate, which required the gaps to be enlarged with a small end mill, this was difficult with the hardness of the grate. The stirrups themselves were made up from steel plate and bent to size.
The trap door pivots from its front edge and allows the grate to pivots down out of the bottoms of the boiler, essentially to remove the fire from the engine. The door is then locked in position with a lever and catch mechanism which I made from stainless as this part will not be painted so it can be easily operated. The trap door itself was a simple bending from steel plate, however again required a former to be made to hold the corners in the correct location.
The trap door pivots from its front edge and allows the grate to pivots down out of the bottoms of the boiler, essentially to remove the fire from the engine. The door is then locked in position with a lever and catch mechanism which I made from stainless as this part will not be painted so it can be easily operated. The trap door itself was a simple bending from steel plate, however again required a former to be made to hold the corners in the correct location.
The hinge was constructed from a hollow cylinder silver soldered to a plate. The initial part was sized so that all three elements of the hinge could be cut from it. The challenge here was to achieve a deep ~50mm drilling though the cylinder at 1/16" dia, this was done slowly using the a sensitive drilling attachment on the lathe. Once soldered the three parts were milled to size creating the detail for interfacing elements and then finally cut apart and finished. The trap door and hinge assembly was then rivet together.
The aperture required in the main body could then is sized and located in the main body ensuring that the grate would land in correct location in the bottom of the boiler to allow it pivot path to clear. The hole was then milled out of the body and associated fixing holes drilled.
Damper Flap
The damper door assembly consists of a flat 'door' pivoted on a 1/16" plate turned round on its ends. The ends were then supported by plates fixed to the front corners of the pan mouth. I found that the drawings had to be changed again as they called for the plates to be vertical however in practice this would have reached over the curve on the bottom of the pan, so the plates were set at 45degs.
The damper door assembly consists of a flat 'door' pivoted on a 1/16" plate turned round on its ends. The ends were then supported by plates fixed to the front corners of the pan mouth. I found that the drawings had to be changed again as they called for the plates to be vertical however in practice this would have reached over the curve on the bottom of the pan, so the plates were set at 45degs.
The damper is controlled at its lower end with a complex tiny stirrup, a long control rod and a notched handle at its upper end. The lower stirrup was originally made from steel gauge plate and bent to suit, however experienced showed this to be in error at the material cracked when bending, so a second attempt was made machining all features from solid. This was when then riveted to the damper door.
The upper handle was CNC machined from stainless as I'm not going to paint it. The design was created in Fusion360 and machined in several steps.
Finally the trap door assembly and the latch/lever was riveted to the main body and the pan called done!