March 2025

Much of the previous working party was taken up with planning how to carry out machining of the crossheads, and not a lot of actual machining took place. However, we did order the materials we knew we would need, which meant we were all set to start cutting metal this time.

The new drawbar has been ordered from the profile cutters, but has not arrived yet (Breaking news; it arrived the day after the working party!). In preparation for it, Chris found a length of appropriately sized square section steel bar, and cut a couple of lengths to make brackets for mounting the yaw damper. He used the Colchester lathe with a four-jaw chuck to face off the ends and to drill holes through the two bars. Each hole needed to be a defined dimension from the edge that will be welded to the drawbar, and hence is not central through the bar; easy to do using a four-jaw chuck.

To complete the yaw damper brackets, Chris removed the mill scale from the surfaces of the bars using an angle grinder and flap wheel. They are now ready to be welded to the drawbar.

Zoe brought the final slidebar packing piece, which she kindly made at home for us, to the pub one evening to give to Dave. This and the corresponding packing piece, which she made previously, require milling to the exact thickness, but that will not take long. This means that we are very nearly ready to fit the slidebars.

The slidebars are fitted to the motion brackets using fitted bolts; one each side. Since the fitted bolts are matched exactly to the holes in the slidebars, they are slightly different. We normally order fitted bolts from CNC machining companies, but this is not efficient for one-offs because the majority of the cost is incurred by setting up the CNC machine. For one-offs it makes more sense to make the bolts ourselves.

Ideally we would start from a hexagonal section bar. We found a suitable sized hexagonal bar on the steel rack, but we weren't sure what grade of steel it was. Furthermore, when Dave 1 started to clean it up he discovered it to be very heavily pitted with corrosion, so it didn't seem such a good idea to use it. A quick look around the C2 Shed located a length of round section bolt grade steel bar which was large enough to machine the slidebar fitted bolts from. The bolt heads will need to be machined, but at least we know it is an appropriate grade of steel.

It didn't take Dave too long to turn the first bolt to size on one of the big DSG lathes. The head is 41mm across corners, the fitted section 25.04mm in diameter, and the threaded section 24mm in diameter. We have an M24x3 die, so Dave just had to run that along the threaded portion and, apart from the hexagonal head, the bolt would be finished. Well that was the theory.

An M24 thread is quite chunky, and Dave rapidly discovered that cutting it by hand requires more force than he could exert. Merik was working nearby, saw Dave struggling, and came over to assist. Merik's view was that the only practical way to cut an M24 thread is on a lathe. Dave did learn screw thread cutting on a lathe back at college, but that was 40 years ago and the only thing he could recall about it was that it was rather difficult!

Keith kindly swapped lathes with Dave, so that we could use the Elliot lathe which is better suited to cutting metric threads. Merik then showed Dave how to set the lathe up, and showed him the sequence of operations required. There are a lot of things to consider, but it's all quite logical when you think about it. Merik started turning the thread, with Dave watching intently. After about a dozen passes, Dave felt confident enough to take over and finished turning it. A run over with the M24 die gave the thread its finished form. Dave learned a great deal that afternoon!

The second slidebar fitted bolt is almost identical except that the fitted section is 24.87mm diameter. Dave again used the DSG lathe to turn it to size, then transferred to the Elliot lathe for the thread cutting. Dave felt confident enough to start the thread himself this time. After Merik checked his first pass and confirmed he had remembered all the instructions correctly, Dave completed the threaded section of the second bolt.

This just leaves the hexagonal bolt heads to machine at the next working party. We will again ask the advice of the Works' staff, as they will definitely know the best way to do it. A huge "Thank you" to Merik for taking the time to teach Dave a very useful skill.

Andrew had requested that the taper reamer, for machining the gudgeon pin locations in the crosshead, be made available to us at the working party, and the Works' staff duly obliged. With the Asquith radial arm drill running as slow as it would go, and with lots of machining fluid, Andrew very carefully reamed out the holes in the crossheads. This went well, and he got an excellent finish in the tapered holes.

To locate properly, the taper on the gudgeon pins has to match that of the holes perfectly. In order to make this achievable, the taper reamer is accompanied by a matching former (a tapered bar of steel). We knew that the former could be used to turn a corresponding taper onto the new gudgeon pins, but how?

Most of the lathes at Boston Lodge have a "follower" mechanism located behind the saddle, and Dave suspected that it could be used to reproduce the taper. However, none of us had ever seen it used. Andrew therefore asked Dewi for instruction. Dewi showed Andrew how to set up the follower on one of the DSG lathes, and how to use it. Again, it's quite logical one you understand how it works.

Using our dial test indicator (DTI), Andrew set the follower to the same angle as the taper former. Replacing the follower with a steel bar then allows the taper to be machined on the new component.

Andrew started by making an extended gudgeon pin, with matching parallel sections each side of the taper section. When fitted to a crosshead, the parallel sections can be held in V-blocks on the table of a milling machine to ensure that the crosshead is perpendicular to the table. After machining, the gudgeon pin was offered up to one of the crosshead. Disaster! The taper on the pin didn't exactly match that in the crosshead. How could that be?

It turns out that machining tapers needs to be a lot more accurate than we expected. For a start, the three-jaw chuck in the lathe wasn't perfectly centred. It may only be a fraction of a thou out, but that's enough to mess things up. And our DTI is getting a bit long in the tooth, which also contributed to the problem. The first issue was overcome by Andrew machining a centre point. Even held in an off-centre chuck, the newly machined centre point will be perfectly centred, provided it is not removed from the chuck. The workpiece can then be held between centres, eliminating some of the possible errors. Keith lent Andrew his DTI, which is accurate to half a thou, and this helped to remove more of the error. It was also noticed that there was a very tiny change in angle of the taper a few millimetres into the cut. This is due to backlash in the system being taken up. The way to overcome this is to ensure that the taper is long enough that the erroneous section is not required to locate in the hole. Even the engineer's blue which we were using to test the fit in the crossheads is a bit thick, and Keith provided us with a thinner compound which gives more accurate indications.

After finding another piece of steel, and with all the advice taken on board, Andrew tried again. This time; success! The new taper fits the crossheads absolutely perfectly. Another huge "Thank you", this time to Dewi, Keith and Tom, for teaching Andrew how to use the follower system and helping him to overcome the challenges of high precision machining.

While the lathe was set up, Andrew machined a couple more tapered bars. The original drawings of the gudgeon pins suggest they cannot be made from a single tapered section, but we know that we are modifying the crossheads (the holes have been reamed to a larger diameter and the side faces will be machined back to accept wider little end bearings) which may make it possible. We may not be able to use these additional parts, but if we can then we've saved a great deal of machine set-up time.

The brass for the crosshead slippers was delivered in time for the working party. It arrived as a single, rather heavy, bar. Chris therefore cut it into four, and used the Wanderer mill to face the ends off. We could only buy brass bar which was a little over width, so, again using the Wanderer, Chris started to reduce the width to something closer to what we want. Since we want the slippers to locate precisely in the crossheads, we are leaving them slightly oversize at present and will machine them to fit in due course.

Mike offered his services to us during a quiet spell in his duties at Boston Lodge. The safety chains between engine and tender had been removed for cleaning and painting last year, and were ready to be refitted. Mike sorted through the parts to find the best combinations of eyebolts, rubber pads, shackles, chains and hooks at each location. Having fitted them, he then found some lock nuts and split pins to ensure that everything stays in place.

After three days of work we were very satisfied with the progress made. We had learned new skills, made several components which will soon be needed, and we had made a lot of swarf! We can't wait to get back to continue the work.