Monday, July 23, 2007




Work has progressed - the boiler is now complete. The assembly sequence I used was as described in Alex Farmers book on boiler making - and, as it happens, his was also a GWR boiler, although bigger.

As the assembly gets bigger, it always surprises me just how much heat it needs. I work outside, so can only work on wind-free ( and preferably dry) days. Also, I find it essential to use refractory bricks- without them, I'd never get up to temperature.
Even so, it needed two good sized gas torches, and occasional oxy-propane, to complete the assembly. I couldn't do this alone, and gratefully thank George and Russell for their help.

Where ever possible (and that's most of the time), we worked with pre-placed solder - its much easier than applying a stick, as it tends to melt - or at least sag - if you're not quick. We put rings of silver solder under the heads of all the rivet - stays in the firebox. As the solder flowed, each was grasped on the outside with pliers, rotated and pulled into place. This (I hope) helped the solder to penetrate. Also - and very important - old texts always advised reamed holes and minimum fit. Alex Farmer advises precisely drilled and chamfered holes to allow clearance for the solder to flow. Easyflow 2 type solder will fill a gap of a few thou, but not more, and does need a gap for capillary action to work. The pre-placed solder resulted in an excess of solder around the boiler, which is waste, and gives away the amateur status of its maker! - a small price to pay.



I fitted the firebox doorplate and backhead after the stays had been fixed internally. This made it very easy to work on the stays which are pretty inaccessible if you follow the traditional sequence of construction), but I was worried that it might be more difficult to fit these plates at such a late stage. In face, it was perfectly straight foreword - my only slight problem was that the rear foundation ring needed to be nearer to 5/16 than to the 1/4 inch specified in the drawings, so a new piece of copper was machined up to fit.


I made up plugs to blank all the bushes - in some cases, my blanking pieces were in fact the part machined finished components - minus their through -holes.

The boiler was tested - at 160 psi - twice its working pressure - and certified by the club boiler inspectors. Up to this point, I'd left all the stay - rivets t their original length, as, If I'd needed to redo anything major, the extra length is important to get the heat in.

So now they've been cut to length with a Dremmel (and lots of cutting disks) but it doesn't distort the stays the way cutters would.
I plan to re-tset it at 1.5 x pressure, to prove that I havn't compromised the seal on any of the stays.

I've been working on the cladding and false backhead. Older practice was to leave a 3 1/2 in gauge boiler painted but unclad, as far as I can see, with small fittings screwed directly into the boiler. I didn't fancy this one bit, and all my fittings will be attached to the cladding only. Also, I'll put a bit of thermal insulation between the layers - if only to protect the cladding from the stay heads! I remember doing heat transfer calculations a long time ago, and finding that it's the existence of an air gap that is the major factor in preventing heat loss, not its size.
Having tried the boiler between the frames, its now starting to look like a loco. My list of things still to do is only half a page!




Tuesday, June 12, 2007

Boiler 3


Some of my boiler making tools.


I formed the taper barrel using a set of bending rolls. I decieded that the chances of cutting the sheet to exactly the right dimensions was pretty small - the copper stretches when rolled - so I left the sheet oversize, and planned to cut it after forming. I know this makes for less convenient cutting, but at least it wont be short.
Just as well; after annealing and rolling, the taper ended up at the opposite end to what I'd planned - it didn't matter one little bit. So once it was almost to shape, I could work out exactly where to cut it.
I rechecked the dimensions once I'd got rid of the overlap, and found there was still a little to file off.

I drilled the strap and riveted it to one side, pulled the barrel up tight with a strap, and drilled and riveted the other side. Before doing so, all the surfaces were finally cleaned and covered with flux.
I laid two strips of (high temperature) silver solder at the centre joint, and added some more flux. I heated it mainly from underneath - with the strap at the bottom and once the solder flowed,added some to the rivet heads. Then I rotated the boiler so the strap was at the top and added more solder, again including the rivets. A few minutes in the pickle bath left me with a clean barrel, which I marked off and trimmed to size - in typical GWR fashion, all the taper is at the top, with the base of the barrel horizontal.

I could now do a trial assembly on most of the boiler; I'll use a few copper rivets to keep the components in place during soldering - but for the time being, I use 8ba bolts in these holes.

The trickiest bit for me was to drill for the cross stays needed on a Belpaire firebox. The problems are that everything slopes, and the reference dimensions are on the firebox inner, so not visible. In the end, I marked and drilled pilot holes on the girder stays, and put in a rod ground to a point at each end. Then I assembled the firebox and used a wedge to drive the rod into the outer walls, giving me centre pops. I transfered these to the outside with large springbow calipers - and a lot of checking. I drilled through the end holes, then could mark out the intermediate holes more sensibly. After a lot of checking, I had all the stay holes in place - as pilot holes. A couple needed 'adjusting' as the drill wandered on the inner surfaces -probably because the surfaces are not all horizontal. I had to enlarge some of the holes in the girder stay to get everything in line. Once I'd got them all lined up, I made up a small 'scribe' which mounted on the pilot rods, so that I could get the holes to their final diameter in the correct position. I hope the photos show what I did. The final diameter of the girder stay holes was 3/8 inch - I couldn't use a drill as it would just have grabbed and damaged things, so it was a matter of grinding with the Dremmel and filing.

I've left the holes in the outer wrapper just as pilot holes for the moment - in case I burn them; I would rather not have drilled them at all at this stage, but I couldn't see how to do them later!
Now I cut both the inner and outer wrappers to size - except for where the backhead joins the outer wrapper.

Its now just a matter of soldering things up. Its often recommended to do this in very few heats. I find it more reassuring to do more smaller ones - a very real problem is that once the job is up to temperature, the flux doesn't last long, and at that stage all you can do is stop.

Sunday, May 27, 2007

Boiler 2

I decided that I didn't want to cut holes in any of the plates until I'd formed the firebox wrappers - so I started on those next. One way to form them is to make a wooden template and beat the annealed copper around them. Or you can form the sheet using bars of the appropriate diameter, which was the way I chose to go ( not being an enthusiastic woodworker !)




Once annealed, the copper is incredibly soft, and needs little more than hand pressure to start to form it. I used various formers, as the pictures show. This firebox has a combination of inside and outside curves (most do) which makes it arkward.
I just bent a bit at a time, checking against the flanged plates. A good fit is important, as silver solder has no gap filling properties.
When the copper stops bending easily, its time to re-anneal. It took me five or six heats to get the inner firebox wrapper to my liking - less for the outer.
Incidentally, I deliberately left the sheets oversize; putting lots of bends and folds in sheet is difficult enough without having to worry about having enough length - the first fold on my outer sheet ended up 1/4 inch from where I'd planned it, so I was glad of the excess. I won't trim the excess length for a long while yet.






Boiler 1

I know several model engineers who wouldn't touch boiler making - it's not for everyone. You need several sources of heat - a lot of it, somewhere safe to work, and you need to be aware that - unlike most aspects of model engineering, it is possible to get into a situation where all your work - and expensive materials - are irrecoverably scrapped.
I've got to say, I like the challenge, and satisfaction that its 'all my own work'. Then there's the lack of waiting time. Whilst its also true that there's a significant cost saving, this is definately not a good reason for doing it.

I bought a kit of materials from Reeves- this comprises all the copper sheet, bar and tubes. You really do need to have the correct materials, all in good condition, and, these days, of traceable quality.
Copper is interesting to work with. After annealing, its amazingly soft, and can easily be shaped - until it reaches a point where it work hardens. Its then time to stop and re-anneal before continuing. This is how the flanged plates are formed. It requires patience, and some very substantial formers . The formers are a lot of work to make unless you have a serious bandsaw, and are of course only required during the making of the components.

The firebox front and rear plates (Inner and outer), and the front tube plate were bought already flanged.
The boiler barrel is tapered - 3/8 inch difference in diameter over 10.5 inches, which works out as a 1 degree taper. As is typical of GWR practice, all the taper is all on the top of the boiler, with the underside lying horizontal. The drawings (and materials supplied ) require the taper section to be made up from flat plate. But there are alternatives - it could be made up from extruded tube, slightly streatched over most of its length. I've never done this, so can't comment on how difficult it is. I did seriously think about abandoning the taper altogether, and using seamless tubing of the right diameter. The small amount of taper could be built into the cladding, and no one would be any wiser - and I doubt if it would affect the operation or performance of the loco. (Can anyone comment?)

To make the tapered barrel, I needed some reference dimensions. I started with the front tube-plate, which I chucked on a 3 jaw chuck, from the inside. I also made use of a tailstock centre to stabilise it. I then took extremely light cuts - like 1 -2 thou. to create a smooth surface ready for silver soldering. Turning copper like this can be tricky - its material properties are such that it can grab, pull out of the chuck, and tear. This is mad much worse by the fact that it cannot be held very securely in the chuck without being damaged. My solution is to turn the chuck by hand. Also, to use a suitable cutting lubricant. Paraffin is suitable - and so is WD40! It makes a big difference to how freely the metal comes off. just take off enough material to produce a smooth, round surface.There was a significant hollow in the plate, so I removed it and tapped it to a better shape before continuing, as I wanted to remove a minimum of metal.



Thursday, May 17, 2007

Tender 1


The frames were already cut out, saving a bit of sawing. I checked that all dimensions were correct, and cut out the buffer beam and drag beam, from angle section. It never seems to be truely square, which shows up on the insides, when the angle fixings are mounted. So i lightly milled them all over, which got rid of the mill scale at the same time. Also the angle sections to lock it all together. I clamped the angle iron in place, and drilled through from the fames. Only then, I marked of the length of these pieces, taking the dimension from the frames rather than from the drawings - at least this way, they all fit accurately. I worked on a surface table, checking that all was square as I went along.

For the horn blocks, I milled the mounting surfaces to size, and riveted them to the frames, slightly overlapping the horn openings. then milled the horn gaps to size - if they were milled to size before fitting. This makes sure that the horn slots are accurate and truly square. If I'd milled them completely to size before fitting, the chances are that riveting would have introduced at least slight distortion.



I then had to make up the spring hanger brackets. Twelve of them amounts to quite a lot of work. With small components, holding them during the work can be tricky. It took me a long time to realise that, by far the easiest way is to make up the part as far as possible before cutting them off the stock. So I made them as a 'production run', setting up only once, and drilling all at once - with a cutting allowance between components. Rather than trying to mark out and centre punch the fixing holes (always difficult on such a small area, I set them up the milling vice a fixed length from the end, and drilled them in turn - I still needed to start the holes with a centre drill - without a centre pop, drills, especially small ones, will wander quite a lot.
The photos show progress. I took less than two hours from starting to having them all fitted to the frames.


Before fitting the wheels, I tried the axles and axle boxes in the frames - any small mis-alignment will cause the axles and / or the boxes to bind. A small amount of filing of the axlebox / horn surfaes was needed on one axle to get them to run freely. I also files a slight curve on the side faces of the axleboxes to allow a rocking movement, to allow for any irregularities in the track.

I then mounted the wheels. I'd been aiming for a slight press fit - some were ok, but others were more of a running fit! A spot of Loctite sorted that, with the assembly held in the lathe until set.

Also, I trimmed a 1/32 off the end faces of the wheels on the centre axle to provide a bit of end float -this will help the 6 wheels to negotiate tighter curves.

Monday, May 14, 2007

A panel saw


I've been trying to buy a panel saw (sheet saw?) for some time, but have been told that they havn't been made for many years. I was lent one a year or two ago, and found it much better for cutting large sheets of brass than the jigsaw I had been using.

As I can't just buy one, I took an old wood saw, and ground off the cutting points. I then made up brackets to mount a fine-toothed 12 inch hacksaw blade, and drilled the saw blade to suit (A solid carbide drill goes straight through the saw steel)

It might look a bit odd - but it works. Its as easy as can be to cut a straight line right through a 4 ft by 2 ft brass sheet. I might get round to tidying the saw up - the real thing is styled like a tenon saw without the tenon. And there seems to be no need for a blade tensioner because of the way the saw works.

Valve gear 2


I initially assembled the linkage with all the joints unpinned, so I could check that it all fitted and could move freely without any obstructions.
Disaster!!!
The angles that some parts of the motion were moving through were clearly wrong. The pendulum levers - which support the expansion link, and the link hangers were both visibly wrong.
I checked all the components against the drawings - and discovered that the weighshaft -which supports the link hangers had been located in the wrong place on the frames- a dimension of 13/32 on the drawing had been set out as 13/16 in. (Actually, by the original maker of the frames -but I was supposed to have checked it). Everything else was correct - but -adding up the various dimensions on the drawing showed that the centres for the link hanger brackets were in fact wrong -by 1/8 in. Still, it made me read up and understand how Stephenson link gear is designed.

To make matters worse, I'd drilled all the mounting holes around both items,and the new holes would run into the existing ones. I thought about scrapping the frames -a lot of work - and of silver soldering fillers in, or getting them welded in. In either case, I think the distortion would have ruined the frames. As I had space to spare inside the frames, I made up doubler plates for the frames, using the existing holes as the attachment points, and fitted the brackets (shortened) to the inner plates only. You can only tell if you look inside the frames. I don't think the strength is in any way compromised, and the motion is in its original position. All the motion now looks right, including the operating angles.


I then set the expansion link to the mid position, (at only this point, there's no movement transmitted from the expansion link assembly to the valve gear). Then, with the intermediate joints and the hanger brackets at right angles, and drilled and pinned the joints. I used 1/16 taper pins - and was glad when I was finished and the taper pin reamer was still in one piece. (I held it in a pin chuck, and cut very slowly, with lots of cutting oil, and cleaned the reamer frequently.)


Then, with an airline connected to each cylinder in turn, and using very little pressure -only a few psi - and with the valve coupling links and the main connecting rods removed, I operated the piston valve, driving the piston from one end to the other (and through the cylinder end plates if you use too much pressure). Also, its seriously a time to keep fingers out of the way. I used a piece of plastic tubing to listen for air escaping from the cylinder drain cocks, to determine when the valve opened in each direction. I measured the valve crosshead position, and calculated the mid point. Then, without disturbing anything, I could measure the distance required between the valve connecting rod centres. By comparing that with the actual length of the valve connecting rod, all I had to do was to move the valve on the spindle by the required amount -and knowing that the thread on the valve rod was 40 psi, I knew haw many turns and flats to turn the valve lock-nuts by. It was quicker to do than to describe. I finally fitted the valve coupling rod.

Then, with the gear in full forward position, and the crank-axle at forward dead centre, I adjusted the forward eccentric until air could just be heard at the front cylinder drain cock. I locked the eccentric in position, and rotated the crank axle to back dead centre, and checked where the valve opened. It was a few degrees late, but I left it for the moment.

I then repeated the whole thing for the reverse eccentrics, with the reverser in full reverse.
Then repeat the lot for the other cylinder.

I then fitted the coupling rods, and again with only a few psi of air to both cylinders, rotated the crank axle wheels, and could feel that the valve events were about right.
So, standing well clear, and with the wheels clear of the bench, I increased the air pressure to 30 psi or so, and things started to happen! It initially needed about 60 psi to get the engine to run reliably. After only 10 minutes running, it would run on 30 psi, and eventually on less than 20 psi. Its surprising how much difference a few minutes running makes.

Once the engines freed up, I then repeated the valve setting procedure, making minor adjustments. I thought that the valve positions weren't exactly right, as the valve events weren't quite equal for the two halves of the cycle. I was able to rotate the valve rod which is threaded onto its clevis position, to get the valve position just right. (Again, a temporary measure, but easy to do and get it right. Then I found that, as I reduced the pressure, the engine would always stop at the same position - again, small adjustments of the eccentrics and the valve position helped. Once I was satisfied that I'd got it as right as I could, I dismantled the valves, and tightened up the clevis pins -four flats -which I was then able to correct for on the actual valve adjusters.

Now it was running properly, I could lock up the eccentrics permanently - I removed each grubscrew in turn from the eccentrics, and replaced them with one which I'd drilled right through in the lathe. I used this as a guide, and drilled into the main crank axle. I then removed the drilled out grubscrew, enlarged the hole in the crank axle, and fitted new screws, with a protrusion turned to fit the holes I'd just drilled.

Valve Gear

The Hall uses Stephenson link gear mounted internally between the frames, and the cylinders have piston valves.
Most of the valve gear components are straightforeward to make, although the eccentric strap might be worth a comment.
The first step was to machine up a bit of bar to 1 1/8 diameter, with a lead that was a few thou less - as a go/ no-go gauge, and also for use as a jig.
To machine up the eccentric,(from a casting), I sawed it in two, to form the two halves of the strap, then milled the surfaces square. I used a fine hacksaw blade, and was careful to mill off the least possible metal. Then I silver soldered the cut halves together again, and set up in the 4 jaw to bore the eccentric. (To set up work in a 4 jaw chuck, I use a dial gauge mounted on a tool holder, and TWO chuck keys on opposing jaws. This let me move the work together with the clamped up pair of jaws - much quicker and easier.
A simple jig made sure that all four eccentric links were the same length.

Monday, April 30, 2007

Coupling Rods



The spacing of the coupling rod centres is critical to the free running of the loco. I measure each side separately and comparing it with the drawing. Any inaccuracies in making up the axle-boxes or in setting the crank-pins will show up here.

I started by marking out the centres and outline on the rod blank, then drilled the centres, starting with a small centre drill, then drilling out at (say) 2BA clearance at one end, and 2BA tapping at the other.

The rods tapered from 3/8 to 1/4 in. - i.e a taper of 1/16 in. on each side.
So I marked out the centre line on the support bar, and another, offset by 1/16 in. I drilled and tapped a fixing for one end of the bar on the centre line, and attached the rod blank. Then I lined up the other centre on the 1/16 in offset line, clamped and drilled through 2BA tapping. Now I removed the rod blank, and opened the 2BA tapping hole on the rod blank to 2BA clearance. Also I tapped 2BA threads in the support bar and bolted the two together.

I clamped the support bar in the milling vice, parallel with the main axis of the milling table. I then cut away one side of the taper, turned the blank over (i.e. upside down) and cut away the second side.

I then rotated the entire assembly of support bar and blank through 90 degrees in the milling vise, and milled the back to profile. If, as in my case, there was a slight taper on the back of the rod, I set this up by making a spacer washer equal to the taper, and placed it under the appropriate 2BA fixing bolt.

I then opened up the bearing holes to their running diameters - in my case, 1/4 and 3/8 in.



In the case of the trailing coupling rod, I decieded that the most accurate way to measure it was to mark it out directly from the job. I made up a piece of rod as a scribe - the photo says all!
Once all the coupling rods had been made up to this point, I was able to try them on the chassis, and check that they ran freely. The chances are that they will stick at some point and it may be necessary to 'modify some of the hole positions - the drawings show the driving axle bearings as 'reamed', whilst the driven bearings as shown as 'drilled' indicating that some clearance is to be allowed. Once I was satisfied with the hole positions, I enlarged the holes to accept the bushes, which were pressed in, and finally re-reamed. I actually reamed them all, and, had it been necessary, would have opened up the driven bearings with an expanding reamer.

To create the radius around the big-end eye - I cut away as much surplus as possible. With the big-end located on a 3/8 round bar in a machine vice. I clamped this to the table of my bench drill, and presented this to a grinding wheel in the drilling chuck. I held the other end of the coupling rod by hand, and slowly rotated it against the grinding wheel. The radius was soon established.

I've seen a similar arrangement suggested using a milling cutter in a vertical mill. I did try it once, but never again. I thought it dangerous and would strongly advise against it.

Friday, March 30, 2007

Just for a change

Just by way of a change, some pics of my finished locos - not me driving! This is a 5 in gauge B1.















And this is my A3 Pacific, again in 5 in gauge.

Cylinders again


I then turned, bored and reamed the valve chest (sorry, no pics of this) and cut the two recessed passages where the ports were to go, using a parting of tool. Then the ports were drilled and filed all the way through. Accuracy is very important here. The valve chest is to be a light press fit into the cylinder - the end parts of the valve chest stick out of the cylinder, and their o.d. is not critical to the thou.

So I turned this part down until it would just pass into its bore in the main casting - then withdrew the tool by a thou (diameter) and cut a bit more. I tried it against the cylinder casting, and checked that it wouldn't enter the bore. I actually went over the surface with some very fine abrasive nylon (like panscrubs) to keep the interference fit to a minimum. The problem is that there is very little metal at the valve area, and it can easily collapse if the press fit is too great.

(I've heard it suggested that a sliding fit and loctite might be better - as retains its strength to over 300 deg C, and the steam temp is unlikely to be more than 150 deg C, then this should be ok. I've also heard it suggested that to make the sleeve a sliding fit - then both surfaces are tinned, and the sleeve is sweated in place. )

I was now ready to assemble the valve chambers into the cylinders.
I actually used a press fit for the first cylinder, and was concerned about collapsing the ports (I put .125 in shims in the ports to keep the dimensions right, and had difficulty extracting them. So I used the soft solder method for the second side. On balance, that's worse. The reality is that, once the surfaces have been tinned, the sleeve - which was a sliding fit - can only be inserted once the solder is molten. Not good for the fingers !!

The next step was to drill the internal steam passages right into the valve chambers, and to fit plugs at the access holes. All easily done using the vertical mill as a drill, with the casting set at the appropriate angle; without the facilities of the mill, it would have been much more tricky to get the angles right.
Then the cylinder end caps were machined up, and the fixing holes set out. I use a dividing head; I've made up a mounting plate which attaches to the lathe cross slide, and sets the dividing head at lathe centre height. It wasn't a lot of work to make, and it makes setting up the dividing head extremely easy. I just put it on loose and fit a length of bar in its 3-jaw chuck. The carriage was advanced until the other end of the bar entered the lathe's 3-jaw chuck, which was then tightened. Finally the dividing head table was locked in place, and the dials / dro were zeroed. ( Its quicker to do than to describe).
I always drill fixing holes at tapping size, then clamp the component in place. Its then trivial to drill through into the fixing surface, at tapping size. When I can, once I've drilled the first hole through, I'll put a tap through both surfaces, followed by a bolt. I do this a second time on an opposing hole, then the location is properly defined, and all the holes can be drilled.


The end caps were removed, and drilled through clearance size.
The back cover was fitted, complete with gasket. Also, I measured the crossheads to determine the exact distance between the slippers and how central - or otherwise - was the piston rod.
With this knowledge, the assembled cylinder was set up in the mill, and the guide bar facings were machined.

The guide bars are gauge plate - (I find that J&L (http://www.jlindustrial.com/index.jsp) offer an excellent range of sizes )
These needed tapping 8 BA for the motion plate fixings. Gauge plate is in many ways lovely to work with, but it is tough, and I was beginning to doubt if the tap would survive. So I put in a new tap, and plenty of cutting oil. It picked up the original thread, and very quickly, to my amazement, the new tap was right through, with practically no effort - and, more importantly, no chance of breaking the tap. I tried an 8BA bolt, only to discover that it wouldn't go in. I checked - I had used an 8 BA tap. I then ran through with one of my older taps. Again, it went through easily, although it was cutting slightly. The end result was an accurate thread in tough material with no risk of breaking a tap. I can only guess that my new tap (again J&L) was slightly undersize - by accident or design, but it made the job just so easy. I've seen serial taps in metric sizes, but not in BA before. The difference was amazing.

Tuesday, February 20, 2007

Cylinders

The cylinders on this loco are gunmetal. Simply because that's what came with the bits I bought! I much prefer gunmetal anyway. I don't run my locos a lot, and worry that cast iron cylinders will rust. The other side of the coin is that you can use conventional piston rings with CI cylinders.


The first thing I did was to make up the boring bars. Mine were cross drilled bar, of as big a diameter as could pass through the cast bore - within reason. I have quite a few carbide drills intended for drilling glass fibre printed circuit boards. They do have their uses -but are very fragile. They all have a 3mm shank, which is ideal for making small cutters. They're easily ground using a diamond wheel (but nothing else will do !) The tip is held in the boring bar by one grubscrew which presses on the shank of the cutter, and another grubscrew behind it, which is also used to control the advance of the cutter. I needed to make more than one cutter with different lengths, or a spacing piece to pop in, as the range of adjustment of the cutter is small.



So onto machining: I cleaned up the castings and to establish some working surfaces and datums. Only then could any meaningful measurements be taken. I've seen it recommended to plug the cylinder and valve chest bores with wood so that centres can be established. Personally, I prefer to work from the datums which I've already established, and anything else is just a check that it looks right (which sort of validates the calculations. All I did was to put broad masking tape across the bores, so the centres can be marked -its only for a visual check.

You'll see the datum milled on the outside front of the cylinder casting. Also, the machined surface of the bolting face.



I fitted a 'boring table' to the lathe cross slide, and mounted an angle plate on that. My angle plate is actually a cube, which is much more rigid - and versatile. The cube was set parallel to the lathe axis, by mounting a test bar between centres and running the cube up against the test bar, and locking in place. I now had a surface which was parallel to the lathe axis, at a known distance (the radius of the test bar) from the centreline. Also, by measuring from the surface of the boring table to the bottom of the test bar ( using gauge blocks) I now kånew the vertical distance from the lathe centre to the boring table.
With that information, I now knew how much packing was needed to set the cylinder at the correct height to bore the piston chamber. Also, by moving the cross slide back using the dial scale, I moved the casting to the right position in the 'Y' direction.
I locked the cross slide, then re-checked calculations and positioning. By marking the bore centre with masking tape, and with a centre in the tailstock, I was able to check that the casting was correctly in position. (just a rough check, but it avoided any major errors.

These particular cylinders - typical GWR - had a very pronounced offset between the bores, making them difficult to pack. You'll see from the photo that I used the second cylinder as a packing piece to avoid packing at extreme angles.

Now I fitted the boring bar with a tailstock live centre to provide support. Again, the fact that it fitted was an indication that the casting was correctly located. The cylinder was now bored - a few thou at a time. By making the cutter from carbide, it wasn't significantly damaged by the remnants of casting sand in the cast bore.




Before the first cut, I rotated the chuck by hand, and wound the carriage (and casting) past the cutter to check for any drastic irregularities on the casting bore - the cutter can be damaged by intermittent cuts and severe shocks. Once all was well, the self act moved the casting past the cutter and out the other end. Once complete the carriage was wound back and the cutter advanced by 5 or 10 thou. and take another cut. It took a few cuts before the cut was continuous through the bore.

One of the difficult things is to measure the bore. Its at this stage that I realised that, before I started, I should have made a go / no-go plug gauge or two, so that I would have known when I was reaching the final diameter. So, do that before you start setting up !

Also, keep an eye on the boring bar. The cutter is roughly in the middle of the bar -its weakest position. So the bar will deflect with the cutting forces. After two or three cuts, do a second pass without advancing the cutter. Whatever cut you then achieve is as a result of the deflection of the boring bar. Keep doing this until no metal is removed during the run. Check the bore diameter before and after this procedure - You'll want to do this for the final few cuts -by doing it earlier, you'll get a good ides of how much to allow for the final cuts.

Once the bore was to my liking (parallelism is more important than getting the bore to an exact diameter) I removed the boring bar, and moved the packing to set the cylinder to the valve chest height One of the packing pieces was the same dimension as the offset), and I adjusted in the 'Y' direction with the lathe saddle index. I repeated the boring as before.

Once complete the boring bar was removed. This was a good time to face the end surface of the cylinder, as it had to be exactly at right angles to the bore in both planes. So, I cut it using a huge endmill held in the lathe chuck. I needed need to be very careful doing this, as the cutting forces could easily have pulled the cutter out of the chuck, resulting in an inaccurate surface at the very least. So I took very light cuts, and locked up all slides not in use, and upmilled only.

<I then drilled the transfer passages between the valve chest bore and the cylinder. The mill was set up at some wild angles to achieve this; it took a lot longer to set it up than it did to do it. What I actually used to make the cuts was a small slot drill - much more rigid than a normal drill, but I had to hold it in a drill chuck as the collet chuck got in the way.

Then I had to drill and tap for the bolts mounting the cylinders to the frames. The drawing showed bolts, but they're going into shallow, blind holes. If you get the bolt length wrong, the threads would be damaged. So, instead, I fitted studs, retained with Loktite threadlok; I think this makes a better job.
So my next session will be to assemble the cylinders and related bits.

Saturday, February 3, 2007

Frames



I started with a set of frames which had already been made. I was going to say - which saved a lot of sawing - but in fact I would probably have cut them in the mill.
The important thing is that the two blanks are riveted together and treat as a pair until complete.
As I had no idea how mine had been done, I started with dimensional checks. I soon found that the axle box spacing was marginally different (of the order of 20 thou) from one side to the other. And the horn slots had been finished individually and were different (hence the axle box spacing). Apart from that, they seemed ok.
I set the frames on parallels standing on a surface table.


The structure was not truly parallel, with a rock of at least 1/16 in in one corner. I slackened fixings and kept rechecking, until I found that the central cross braces were the cause of the problem - one was a few thou out of parallel. I milled a few thou off to make it truly parallel, and made up a shim (only 5 thou) to keep the spacing correct. I doubt if it was worth it - but at least its accurate. I kept rechecking as I re tightened fixings, and eventually it was dome.

The next thing was to put in the axleboxes and check the axle centres. For the completed wheel sets, there was only one way to fit them because of the differences in hornblock and axlebox dimensions. I actually checked these by mounting the wheel sets between centres on the lathe, and measuring the offsets from the axle to the hornblock slots.



Again, all were different. I guess that the frames had been set up as a pair, with the blank axleboxes fitted, and then the axlebox bearings had been drilled and reamed in situ. So the wheel centres do end up the same on both sides of the frame.

If I was starting from scratch, I'd have done it differently. Like, I'd have kept the frames as a pair until after the hornblocks ha been milled.

For the axleboxes, I's have squared up the axlebox material, and drilled and reamed - or bored - the axle bearing. Then I'd set it up in the mill, with a stub axle through the journal, and resting on the jaws of the milling vice, and the upper surface horizontal. Then I'd have milled one of the recessed for the horn block.
Having done that and set a stop on the milling machine quill, I'd have rotated the axlebox through 180 degrees,(a small parallel and a couple of jackscrews will do the job) and milled the other horn block bearing. That way, they all end up identical, and with the axles central.