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Thread: The Webley Mercury pistol saga continued. Part 2.

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    ccdjg is offline Airgun Alchemist, Collector and Scribe
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    The Webley Mercury pistol saga continued. Part 2.

    In Part 1 of this series of threads on scratch-building a copy of the Webley Mercury pistol, I had got as far as making the basic frame, cylinder and barrel components:





    Since then I have been carrying on with the project (among other things), during lockdown and thought now would be good time for an update on progress for those of you who expressed an interest in the project.

    The next phase after building the basic carcase was to make the working parts of the pistol: the cocking links, piston, trigger and sear. But first I had to decipher the patent, as like most patents it was a bit short on detail. This can be the fun part of a project like this, trying to work out exactly what the drawings represent and how the mechanism actually works. Quite often there are things in the drawings that are not mentioned in the text, and just when you have decided that they are irrelevant and you leave them out of the build ,you find that they are important after all. Sometimes you can see important things in the drawings that are unclear, but there is no helpful description in the text, so you have to work things out for yourself. A good example in this case of this is the shortest of the three cocking links in this part of the drawing, marked as No 21, which the patent calls a “pawl”.(I looked it up and ‘pawl’ is as good a description as any).







    This has to press firmly in the notches in the piston during the two cocking strokes (items 22 and 23 in the drawing), but then has to be set free when the barrel is returned home. The patent describes it as “spring-loaded” but there is no indication of the design of the pawl or what sort of spring is used or how it is fitted into the pawl. In the end I took inspiration from the short cocking link used in Webley pistols, such as the Senior and Hurricane, and made it as follows:






    Once made and fettled a bit, it seemed to work well, so it must have been along the right lines. It was milled from carbon steel, so that it could then be heat hardened, which is important as it is going to have to take a lot of stress and wear. The long and short cocking links (items 19 and 20 in the above drawing) were also made from carbon steel for strength, using bar of 5mm thickness. This makes for harder work cutting and filing, but it is worth it in the long run for components that are subjected to a lot of stress. These links needed to be a sliding fit in the channel that had already been milled into the steel frame. Then all three components could be pinned together and the whole unit checked for free travel when attached to the barrel.
    I should point out that I am making the second version of the Mercury, not the first which is the one in the patent, and there are a few unknowns to deal with. I only have one photograph of the second version to work with (and a knowledge of its barrel length), and it was clear from the start that some of the external dimensions were slightly different from the patent version, and also that there would be some minor differences in the internal layout. This meant I could not just work straight from the patent drawings, and a lot more thought had to go into working out dimensions and the relative positioning of moving parts.
    This was particularly true in the case of the cocking link train and the piston. The cocking link train had to be an optimum length to maximise piston movement, and the positions of the two cocking notches and two sear-engaging notches on the piston also had to be precisely spaced for the whole cocking process to work. All this caused me a lot of head scratching but I finally came up with what I thought was a reasonable compromise.
    Now for a confession. It’s a sad fact that, whenever I make components from carbon steel for a project, after a lot of hard work and fettling I always end up having to make at least one of the components again, and on occasion even three times. Inevitably something unforeseen crops up later down the line which makes the part unusable – too short (never too long, which would be easy to sort out), pivot hole in the wrong place, wrong shape, you name it. I always try to think ahead and anticipate these snags, but never succeed totally. So when I am sweating away hacksawing or filing a very hard steel piece, putting a lot of thought, time and effort into it, it keeps coming into my mind that this could be the one that ends up in the scrap bin. Very off-putting! And the current project was no exception. When the assembled cocking link was tested with the piston in place, it lined up exactly as shown in the patent drawing shown above. The trouble was, when the gun was cocked and fired, the piston smashed into the spring-loaded pawl, which was protruding into the cylinder (it was being held there under the action of its spring). The patent diagram, which shows the pistol in the uncocked or fired state, was obviously incorrect. Just my luck. It should have shown the pawl sitting slightly ahead of the piston notch, so that it had ridden up against the end of the cylinder slot and had been lifted out of the cylinder cavity. That way, the gun could be fired without damaging the pawl and/or the piston head.
    So despite all my best intentions, the cocking link train I had so carefully made needed to be shortened by about 5mm, and as the short and long links had already been drilled and profiled, the only way I could do this without weakening the linkage was to make a new short link, 5mm shorter than the original. So it was back to the hacksaw and file So the cocking linkage pictured in the next group of pictures is actually my second attempt.




    You can see from the next pic how the linkage looks when fixed to the barrel and cylinder. When the cylinder is eventually fixed to the grip frame, the linkage will sit in a channel in the frame and the spring-load pawl will be pressed flat against the cylinder.




    With the cocking linkage made, attention could then be turned to the piston. I have found that the simplest way to make a piston is to use steel tubing that is a loose sliding fit in the cylinder and then to block one end with a steel plug. This requires a lot less effort than boring out a steel round bar, especially if tubing of the right diameter can be bought. In this case my cylinder had an internal diameter of 25 mm, and 25m tubing suitable for the piston was readily available on the net. The plug for the piston head was turned down from steel bar until it was slightly oversize to fit inside the tubing. The tube was heated to a dull red heat and the plug tapped in place. As a belt-and -braces way of further securing the head in place, three holes were drilled into the head from the side, tight fitting steel pins hammered in, and then the excess cut off and the pins filed flush . The head was then drilled and tapped to allow a piston seal to be fitted.

    As a first stab at a piston seal, I decided to use PTFE rod, and to turn one down on the lathe. I have never used a PTFE seal in my repro’ pistols before,as leather was always more appropriate. Anyone any thoughts on PTFE versus leather? Anyway, I will give it a try and if I am not happy with it, I can always go back to leather.

    The final parachute-style seal was fitted to the piston with a countersunk 2BA screw and washer. The final piston was a close sliding fit in the cylinder, and when pushed down hard into the cylinder, it held the compressed air well. The sequence of events in making the complete piston is summarised in the following pics. (Note that the actual thickness of the plug used to form the piston head was about one half of that shown in the first frame).







    The piston once made then had to be cut with four notches: two to receive the spring-loaded cocking pawl, one for each cocking stroke, and two for catching on the sear after each cocking stroke. It is interesting to see how the double cocking action works, and you can understand this from the following drawings.






    The piston notches shown in red and labelled as A1 and A2 are the ones that the cocking pawl locks into. The notches shown in blue, labelled B1 and B2 are those intercepted by the sear.
    (1) shows the gun before cocking. (2) shows the first cocking stroke, where the cocking pawl has connected with notch A1 and pushed the piston back until it is held by the sear in notch B1. The barrel is then returned home to give situation (3). The piston is now cocked in the first stage, and if wanted, the gun could be fired for a low-power shot. When the barrel is pulled back again for the second stroke, the pawl engages with notch A2, as shown in (4) and the piston is pushed back again until the sear engages with notch B2, giving situation (5). The barrel is then returned home, as in (6) and the gun is now ready to fire on full power.



    As you might imagine, the relative positioning of these notches is critical for the gun to cock properly, and also to take full advantage of the available swing of the barrel to ensure a maximum swept volume. Also the angle of the notches was important to ensure that no slippage might occur during cocking. Needless to say, this stage caused a lot of puzzling out and modelling with cardboard cut-outs before committing the piston to notching. When I was reasonably confident that I had the layout correct, the piston notches were then milled in and it was with huge relief that a trial run showed that all was OK.
    The piston also needed a slot milling along its top, exactly opposite the train of notches, so that a screw protruding through the cylinder wall would ride in the slot and so would keep the piston notches always in line with the cocking pawl and the sear. If this feature was omitted, the piston could rotate out of line with continuous use of the gun, and the gun would then fail to cock. The only recourse would then be to disassemble everything and realign the piston, not something you would want to do after every dozen shots or so. The patent makes no mention of this feature in the text, but once I realised it had to be there, I was able to find it the patent drawings, with its guiding screw. The lower picture shows the guide slot.




    The next step was to make the trigger and sear pieces. I used the same 5mm carbon steel plate that I used for the cocking links. Although this made hacksawing and filing harder than if I had used mild steel, it did mean I could heat harden them once finished and they would stand up much better to wear and tear. This was particularly important for the sear. This is the sequence in making them. They were first made oversize and only roughly shaped, and with the holes for the pivot pins also drilled, they could then be fitted onto the frame with the pins and then sized and shaped more accurately. The long arm of the sear lever was also drilled with a hole for attaching a spring. The last picture in the sequence shows the trigger and sear in place, with the sear spring fitted.





    Now was the moment of truth, as I had all the necessary bits to assemble the working parts of the gun and to see if the gun actually cocked and fired. So using a weak main spring (as I did not want to put the load bearing parts under much stress until they had been hardened) the gun was assembled:




    With bated breath I broke the barrel and cocked the gun. Yes, the pawl picked up the first piston notch OK and pushed the piston back until there was the reassuring click of the sear engaging. The gun had successfully achieved the first cocking stage! I then returned the barrel home and re-cocked the gun. Yes, the pawl picked up the second notch and pushed the piston further, but disaster! Everything came to a dead stop and there was no way I could get the sear to engage with the second notch. So the gun was taken apart and checked. The notch distances seemed OK, there was nothing obstructing the cocking links in their channel, and nothing blocking the trigger or sear movement in the second stroke. What the hell could it be? After reassembling and hitting the same problem again, I agonised over the problem, untiI I realised what an idiot I was. When I made the piston, I deliberately made it a bit longer than necessary so that I could trim it to an optimum length later on. I had forgotten this. It turned out that it was just that bit too long and was hitting the end of the cylinder in the second cocking stroke, before the notch could engage with the sear. So out came the hacksaw and with 5 mm trimmed off the end of the piston, the gun worked perfectly, at least under very light spring load. The sear now engaged after both cocking strokes, and the trigger worked fine so that the gun could be fired after the first or second cocking stroke. Testing the action proper with powerful springs would have to wait until the rest of the gun was finished and all the relevant parts heat hardened. This would include case hardening the piston, as it was made of mild steel and the notches would become rounded, and pretty soon useless, if some protective measure wasn’t put in place.


    The next aspect of the build I wanted to tackle was the “cladding” that was to go around the grip frame and help make the pistol begin to resemble the original prototype more closely. I had decided that the cladding would consist of two aluminium panels screwed to each side of the frame. Aluminium was chosen, as it would not add too much extra weight to what was already promising to be a heavy pistol, and also it would go some way to representing the aluminium alloy casting that the prototype was (presumed to be) made of.

    The following pictures show the processes involved in making and fitting the side plates. Each plate was profiled to give a snug fit against the curvature of the cylinder, but their inner surfaces were milled out in steps so that their weight was reduced as much as possible. The plates were then drilled and screwed into tapped holes in the frame.



    So that is where I am up to right now. There is still quite a way to go, including making the grip plates, trigger adjuster, safety catch, and front sight. Fortunately the rear sight used on the original prototype Mercury (second version) was the same as the Webley sight used on the Hurricane pistol and other Webley airguns of the period, and as I happen to have one of these (which I snapped up on this forum recently) that will help save me some time.
    Once all the parts have been made, fitted and polished. I will then have to etch the lettering on the gun, blue the steel parts, and anodise and black the aluminium cladding. Only then will I put the gun through its paces, experimenting with different strength springs (the prototype used two concentric springs), piston seals, transfer ports etc. It will be interesting to see how the gun performs and how effective the double cocking action is. All will be reported in Part 3 of this mini-marathon, unless of course the gun completely disintegrates on its first serious outing.
    Last edited by ccdjg; 10-08-2020 at 06:58 PM.

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