The commercially built boiler has been made and collected (13th Feb 2018) and the photo below shows it temporarily resting on the frames without any front boiler support so it looks at a peculiar angle.

I will not be doing anything with this for a while, this is just a blog record that it has arrived.



Boiler saddles

The two saddles are castings, the front one sits over a frame stretcher behind the crank axle and the rear one fits over the steam brake bracket under the wider section of the boiler.

The front saddle casting was definitly “wonky” not a parallel face anywhere. Also the drawing instructs the casting to be modified as it is too wide and does not allow room for the weighshaft connecting rod and I think drain cock rods have to pass it also. The modification instructs to machine away the end webs and silver solder in new ones at the correct width, all to be done before maching proper.

So without any paralle faces to clamp holding the casting was the first problem. In the end I used packing pieces in the machine vice to get a solid grip and aligned the casting as square as possible as I planned to machine the whole base whilst in this set up as trying to reset it up would be nigh impossible, and doing so would give me a square base from which to work and hold the saddle for subsequent operations.

Following the instruction to machine away the side webs I found they did not machine away completely to reach the correct width so I reasoned that I would leave them like that (hardly seen under the boiler anyway) and save myself some work.


Right hand side machined. Left hand side as cast

The base was then machined all round and flat leaving sufficient material above the base to machine the radius for the boiler.

It is worth noting at this point that the saddle radius to be machined is 1/16″ larger than the boiler radius. Looking at the sectioned side view of the Loco there appears to be a 1/16″ packer between the saddle and boiler. This is probably there because the drawing appears to show the boiler with a slight slope towards the smoke box and is not parallel to the frames. No doubt I will discover when I come to fit the boiler.

Anyway back to the saddle. The base has a 1/32″ deep slot that matches the ridge in the stretcher to which it bolts. However the bolt holes are half in the slot and half out, why? no idea. They are 5 BA and could have been moved either way to miss the transition line.

There was little chance of  easily drilling the holes once the slot was machined so I drilled and tapped the holes and  then machined the slot.


With the base done the saddle could be mounted in the machine vice for the saddle part to be machined 1 1/8″ wide,  then changing the clamping arrangement so the saddle radius could be machined. The vice with the saddle clamped in it was first centered in the Y direction and then the X direction was set to zero by a square vertical on the vice base against the base of the saddle. The X direction could then be moved 13/32″ , the dimension of the saddle base to the bottom of the saddle and set to zero again.


The saddle radius is 3 3/16″ and I chose to fly cut it using the boring head. To set the tool to the correct radius I first set the tool agains the vice jaw which is 1 5/8″ off the centre line. The tool could then be measured from this datum to get the 3 3/16″ radius.


My boiler construction is a butt and strap arrangement so I need a slot in the saddle to accomodate the strap.


The saddle in position

Fitting the saddle to the stretcher is a bit of a fiddly exercise to get the 5 BA set screws in. It really has to be done from underneath and a slim box spanner will just fit over the outer heads whilst an open ended spaner is a bit more fiddly to use.

The rear saddle casting was only a little better than the front so I followed the same process of machining the base to get a rectangular reference.  I cannot say I am impressed with the quality of these two castings.  As the casting was clearly very much oversize on it base depth I did mark out the bottom of the boiler radius point on the casting and then the underside of the base at 3/8″ (as dimensioned) from that so I had a line to machine down to. There are quite a few dimensions missing off the drawing but they can either be inferred from the steam brake bracket or scaled from the drawing. Whatever way I was going to machine it I was going to end up with thick and thin areas. It was never going to end up fully symetrical.

This saddle does not have the slot running through the base so drilling the fixing holes was not going to be a problem.

When fly cutting the 3 7/16″ radius it became more obvious that I was going to end up with thick side and an opposite thin side. Had I tried to centre up this radius to obviate the mismatch I would have ended up with very little end web on the base on one side and probably still have a thick and thin webs as the photo’s below show.  Just as well it will not be too noticeable under the boiler.


The finished rear saddle …note the web thickness are all quite even


Now look from the other side to see the difference!

It’s worth noting at this point that either side of the saddle are oil wells that provide oil to the rear axleboxes and they bolt to the frame and one of those bolts has its nut sitting between the frame and the saddle in the 1/16″ gap ! I have not found a drawing for these oil wells but they are shown on the general arrangement. They will certainly have to be fitted whilst the frame is accessible.

Update on the oil wells. I found the drawing for these items…. next to the coupling rods! Just a single elevation view but it shows the well with the fixing holes tapped so the fixings have to go from the inside of the frame which means before the saddle is fitted as one fixing hole is covered by the saddle. I could take the saddle off but have decided to fix with through holes in the oil well and nuts inside the frame.

Smoke box saddle

The smoke box saddle is a gun metal casting. The drawing gives alternate dimensions without identifying which loco the alternates are for. I measured my boiler and established that the boiler diameter was 6 5/16″ and the alternate wall thickness for the smoke box was either 10g or 13g. Well, 13g is 3/32″ so 6 5/16″ plus 3/16″ is 6 1/2″ which is one of the alternate measurements for the smoke box OD and thus the curvature of the saddle.

The saddle is stepped so it can fit over the K exhaust, the front step being used to fix the saddle to the frames by 3 bolts each side. The drawing alternate dimension for the 6 1/2″ smoke box shows 5/16″ between the smoke box radius and the bottom of the front step.

With all the dimensions extablished the first task was to get a flat surface as a datum from which to work. The obvious choice was the underside and placing the casting on the mill table upside down it appeared that it was nearly level so I chose to clamp the casting downs and machine the base to get a flat surface all round. The dimensions for the step from front or back of the saddle are not drawn so they were scaled from the drawing. There was only the slightest of cuts to get the step in the right place. The full depth of the step (5/16″) was not machined at this stage.


The next task was to establish a line on the casting where it would fit to the top of the frames . The web of the smokebox saddle is drawn at about 1/8″ thick so I mounted the casting on the mill table clamped so it was firm on the flat face just milled and scribed a line 1/8″ above the underside of the saddle web. This was the datum to which the saddle radius would eventually be machined. From this line I could measure and scribe a line along the side of the saddle where the top of the frame was going to fit and thus set the whole saddle to the correct height.


Having got the machining line for the frame top the saddle was firmly bolted to the mill table with a sacrificial base so the step could be machined. This was done using the set up shown in the photo.


The first cut was done on both sides to get a parallel edge so the overall width could be measured. This done the amount to be taken off both sides could be established. At this point the mind went into stupid mode and I totally miscalculated the depth of cut on the first side and took too much material off. Having returned to sanity mode the other side was cut to the required depth. To rectify the error I silver soldered a strip of brass along the step which brought the overall width to 5 thou below 4 1/8″ being the required width to fit between the frames.

A trial fit showed that there were three problems. The first was the step to clear the K exhaust. It fouled the front lip of the cylinder block so the step as drawn was in the wrong place. Secondly the rear  fixing hole is right up against the lip of the cylinder block which means the saddle hole is right on the very edge of the casting and time will tell if it can be drilled and tapped OK. The third problem was that the K exhaust outlet boss to the stack fouls the inner edge of the saddle so a small radius will have to be machined in it to clear the boss.

Having rectified the step position and the clearance for the K exhaust boss the saddle was trial fitted.


The three fixing holes have yet to be drilled and tapped.

Progress. I have now drilled the fixing holes and as feared the one on the right up against the step is right on the edge and not enough material to tap the thread. I shall end up with a smaller set screw through the hole with a filed down nut on the other side. The set screw will have to be purpose made so the head size matches the other two.

However before fixing the saddle I have to find a way of machining the curvature for the smoke box. There is no means for clamping in the vertical on an angle bracket mounted on the mill as the whole curvature has to machined in one go. It am considering putting a bar inbetween the sides and using the fixing holes just drilled to fix the bar in place and then the bar can be fixed to the angle bracket.

Before machining the curvature to 3 1/4″ radius to fit a 6 1/2″ smokebox  diameter I decided to check the alignment of the smokebox with the boiler bottom to verify the 5/16″ dimension on the drawing for the height of the bottom of the curvature to the base of the saddle.

I mounted the boiler on the frames and the first thing noticed was that the expansion link is proud of the top of the frames when in mid gear by 1/16″. This means the underside of the smoke box must clear this.


This then determined the underside dimension of the smoke box in the saddle and the saddle scribed accordingly . It did not match the as drawn 5/16″ dimension. There was very little to be machined off the front of the saddle and none off the rear to achieve a level saddle. The next thing noticed was that the boiler front sat too low for it to slide into a 3/32″ walled smoke box and this now explains why the elevation drawing  of the loco has packing pieces drawn on the front saddle to raise the boiler front up.

Having now understood the issues of aligning the boiler and smokebox I can turn my attention to maching the smoke box saddle curvature.

The saddle having a step on its underside was difficult to clamp securely to the angle bracket on the mill bed. To overcome this I fixed a parallel bar to each side  with the top of the bar firmly aginst the top of the rebate that the saddle uses for mounting in the frame and held them there with 4 BA set screws using two of the holes that are used for fixing the saddle to the frame. The third hole could not be used as it’s blind. With the two parallel bars fixed I checked everything was still level, i.e. it did not rock when placed on the mill bed. I then fixed a wide bar inbetween these two parallel bars and fixed it to the parallel bars with 4BA set screws  in tapped blind holes in the wide bar ends. Three 5/16″ fixing holes in the wide bar were then drilled on the slot centres of the angle plate. The photo below shows the set up. The middle fixing hole was not needed, it was firm enough with just two bolts. The spacer below the saddle is just to give clearance to the boring head set up and was removed for cutting.


The saddle was checked to be square on the angle plate before finally tightening the fixing bolts.

The set up on the mill was achieved by first setting the face plate surface on the X table on the centre line of the quill and setting the DRO to zero. Next the Y table was centered around the saddle by a centre finder on each edge of the two parallel bars and dividing the measurement by two to give the Y centre line of the quill and the DRO set to zero.

The boring hear arm was set to 3 1/4″ by moving the angle plate surface back in the X direction to 3/14″ from the quill centre line and setting the tool tip aginst the angle plate surface.

Finally before starting any cutting I moved the saddle to the X position so the tool was at the final cut position as scribed on the saddle and hand moved the tool to check the swing and where the edges of the saddle would be cut to see if they were reasonably equal …… they were, so the set up was a success.

The cutting process was taken in 15 thou steps to avoid too much “hammer” on the saddle and with only 3″ of quill movement I machined down to the limit of the final cut and then moved the mill table up and finished up the last 1″ of the saddle machining in steps as before. The final cut did not quite clean up the last 1/2″ of the saddle curvature in the central part of the curve. As its under the smoke box it will not be noticed.