Friday, 20 January 2012

How to make an ashpan (1)

Sentinel 7109, when originally brought to Midsomer Norton Station in December 2004, came without a few items, one of which was its ashpan for mounting below the boiler.

I looked at the design in a previous article but have now decided that I want to have a go at doing the fabrication myself. Another article had a look at the ashpan from Sentinel 9622 at Teifi Valley Railway so there is a fair amount of information to go on for the basis of a design.

This drawing shows the basic idea:

Ashpan Plan View
A side view also helps:
Side elevation
I also scrutinised many photos of 7109 and other Sentinels for what their ashpan looked like. For example: (Click the photo to enlarge it).
7109's Ashpan Highlighted in the centre
(Photo cropped from original provided by John Hutchings, ILS)
The photo shows that, unlike the drawings, there are no cut-outs around the sides of the ashpan - which makes it easier to make!

Another ashpan from a Sentinel tractor emphasises the point:

100HP Boiler ashpan on Sentinel 6426 (Two-speed tractor)

So, to begin, some design decisions:

  • How big? The circular inside of the firebox is 34.5" in diameter so the ashpan needs to be slightly larger so that ash will fall into it rather than down the outside if not large enough. It must not be too large to prevent mounting brackets to be fitted.
Mounting at the base of the firebox (looking upwards)
The picture shows that about 1 to 2 inches in additional diameter will be fine. So it will be made 35.5" in diameter.
  • How deep?
    The drawing indicates 5.5" depth of side wall; however, 6" seems more like the picture of 7109's ashpan. So 6" will be the height of the sides.
  • What thickness of what material?
    Mild steel, 3mm. 9622's ashpan may have been this thick at one time but it certainly isn't now! Using non-stainless steel, there is a trade off between material thickness and weight. The difficulty of manoeuvring it below the firebox emphasises minimising the weight. 3mm mild steel is a good compromise for the base and side wall.
  • Any cut-outs to allow air to the fire?
    No cut-outs. By butting the top of the sides on to the bottom of the mountings, a gap is created for air anyway so no additional gap is required. Also, by using long studs from the mountings, some height and hence air-gap adjustment can be made for fine tuning the performance of the fire. The cut-outs are additional work but also makes the side wall more difficult to roll to the right curvature before fixing to the base.
  • How to fix the side wall to the base?
    Welded inside and outside butted corners of the side wall and base. Metal Inert Gas (MIG) is preferable because there are long continuous welds. Manual Metal Arc (MMA or 'stick') could be used but would not be so tidy.
  • How to get the ash out of the pan?
    The plan drawing shows a flap and this is present on the 9622 ashpan. The flap on 9622's is made from a single sheet of mild steel and bent at the edges to produce the triangular sides. 7109's flap will be made from the piece cut out of the main base with 2mm mild steel sides welded in place. Sentinel would have been making loads of ashpans and hence the flaps could have been made in quantity. 7109's is a one-off so this different approach wastes less material.
  • How long a 6" wide sheet is needed for the side wall?
    This is where we have to go back to school! The length is about the circumference of a 35.5" diameter sheet minus the bit cut out for the flap. So that's Pi x diameter = 3.14159262(etc) x 35.5 = 9' 3.5".

    So how big is the bit cut out for the flap? Now we have to go to big school for this one!
    At first, I thought this was about chord-lengths of circles but it's easier than that. We know that the flap is 19.5" wide from the drawing and we know the radius of the base (35.5/2 = 17.75"). So this isosceles triangle can be bisected to give two right-angled triangles for which we can find the angle subtended by the arcsine of the opposite over hypotenuse!
    This gives us an angle of 33.3 degrees which needs to be doubled to 66.6 degrees because there are two right-angled triangles in the isosceles.

    So this gives us 66.6/360 x circumference = 66.6/360 x 111.5 = 20.64".
    And thus the side wall length is the circumference minus the flap's bit which is 111.5 - 20.64 = 91".

    On a practical note, a 91" thing won't fit in my car so two 45.5" pieces it is!
I'd often wondered what trigonometry was actually for but now I know!
  • And how big a piece of 2mm sheet is needed to make the sides of the flap? It depends on the offcuts in stock! No trig needed here!
When the sheet steel arrives, it looks like this:
For scale, the part-feet showing are about 8" long!
To be continued!

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