25 PDR Breech assembly

[Francis Samish]

More of some comments as to 'how not to do it', but hopefully of some help to other builders. I started assembly of my 25 Pdr with the breech, as it seemed a smallish assembly to work upon. The two cheek pieces are secured by countersunk screws. You can countersink these about 3 or 4mm deeper to give the screws more purchase in the block proper. I spent some time scraping the ways on both the cheeks and the breech, so that the breech block moves freely. The outside of the cheek pieces are also scraped down level with the breech, and all the holes and join lines will be filled and sanded down prior to final paint, so that the whole unit looks as though it is one piece. I managed to shear the thread off the end of the cam lever unit that raises the breech block, so had to drill out the shaft from the back, and insert a 6 BA stud so that I could tighten down the operating lever. I also added an M2 grubscrew bearing on a flat files on the cam lever shaft, plus a washer to hold the lever clear of the surface of the side of the breech, so it would not mark the paint when it was operated later. Of course in the final stages of fettling the unit, I managed to drop it and this tapped one of the cheek pieces out of alignment - hence the four little holes in the back of the breech cheeks, which have loctited-in 2mm studs inserted, just in case I am hamifisted enough to have it happen to me again.

[Francis Samish]

Rear view of breech

[Francis Samish]

Breech from underneath. I still need to add a detent to hold the block in the closed position, and stop the breech block dropping the bottom when opened.

[Francis Samish]

Detent for breech, components from left. Armortek falling breech, with dimples for detent. Detent plunger, made from brass bar turned to 5mm OD and then domed at one end. Cut down biro spring. Screwed adjuster plug - actually an offcut from an M6 bolt with a slot cut in the rear with a junior hacksaw for screwdriver. Visible behind in the breech block is the 5mm hole drilled through from the rear of the breech to take the assembly. Only the rear 10mm is tapped M6, so as to leave the bore smooth for the detent plunger and spring. To get the correct positioning of the dimples in the falling breech, I spotted the raised and lowered positions through from the rear of the breech with a 5mm transfer punch inserted in the detent bore. After centre-punching, a 2.4mm drill was used to create the pilot shallow dimple in the falling breech, followed with a 6mm drill to enlarge the recess so that the detent would ride in and out easily. Adjustment for pressure to allow reliable retention of the falling breech, yet still allow easy operation of the lever, is achieved by screwing the adjuster plug in or out from the rear. There is no need to use loctite to secure the plug as the spring tension will keep it in position.

[Christian Steinhauer]

Hello Francis,

the spring pin ist a fantastic idea.

[Carleton Brown]

Looking good Francis, will reply to your email now.

Carleton

[Francis Samish]

Gun sight now completed. Could not resist making the side to side tilt mechanism for the sight operative, so filed, drilled and tapped new brass block to take handwheel screw, then added collar with M2 grub screw to allow side to side movement. Pivot pin to secure two arms of sight mechanism replaced with screw to allow disassembly later for painting. Brass sight bored out from front and back to allow sighting through, and possible later fitment of plastic lenses and cross hairs.

[Francis Samish]

Gun sight assemblies. Sighting cone is secured into casting using handwheel. Small brass plug at bottom of picture is wedge that is locked in place by turning up handwheel. Ranging cone and sight can therefore be set exactly to angle required when barrel, breech and recoil slide are finally assembled. Also shown are turned brass spacers that fit around the M2.5 sight clamping screws, which provide a flat surface to clamp up against so as to stop any tendancy of the smaller (RH) gun sight plate to pivot around the brass sight tube when tightening everything up. Grooves in the sight and also the top recess in the ranging cone assembly need to be deepened to suit.

[Francis Samish]

Sight from front underneath. Shows pivot pin upon which complete assembly is hung, which goes into the LH support plate. Rod with clevis will connect to crank fixed to LH gun trunnion, to allow sight to move in concert with gun as barrell is elevated.

[Francis Samish]

OK, so it has been a while since I last posted on progress of my 25 Pdr. Around six months.... But as I had promised that I would have something more or less presentable for Tankmod, I managed to get a round tuit, and thought I would post some pictures which might be a help to those builders who did not manage to get to Bovingdon this year. The gun is unfortunately incomplete, but it was only 10:30 on the Friday before Tankmod that the wheels went on. For those who care about these sorts of things, I appear to have logged about 90 hours to get to this point, a goodly proportion of which was spent 'mantling' and dis-mantling the various subassemblies to each other so as to get the operating parts of the gun working as free as I felt they should. The slew gear still needs a bit of work - another 1mm sliced off the left shoe mount should do it, I think... - but that's a job for next week.

[Francis Samish]

Here's the the rear of the trail, assembled very much as per the kit's instructions, apart from the split pin in the towing eye retaining nut. Which, I freely admit, is something of a touch of vanity...

[Francis Samish]

I have to admit that this the first time I have photographed something this big - most of my work to date has been in the 'braille scales' of 1/76th, 1/43rd with a dabble or two in 1/35th. And no, your eyes deceive you not. The 25 Pdr IS sitting level. It is only the hedge which is running downhill at 10 degrees (that reminds me: I really must get the hedgetrimmer on the tractor and give the blackthorn a haircut...).

[Francis Samish]

Shield segment open

Well, its a week on from Tankmod (sort of), and I've been beavering away to finish the field piece. Barring some rivets on the outside of the trail, everything according to the kit instructions is now done - except for the various equipment bags, tools and wooden cases. All of which can only be fitted after painting.

I have deviated from the instructions and added a third curved guide for the sight shield segment to move in, to keep this part clear of the inside of the shield after it has been painted. A little brass stub was turned up to act as a handle and rivetted into a hole drilled into the segment. The topmost button head screw on the curved guide was replaced with a countersunk screw, as I found that otherwise the head would prevent the top shield plate from sitting fully horizontal.

[Francis Samish]

And here's the segment closed. The whole segment can be flicked back and forth with one finger.

[Francis Samish]

Couple more general shots of the shield now. The hinges needed a quick ream through with a drill to clear them for the pins, and then were loosely assembled onto the drop down section and the main shield plate. Only then were all the button headed bolts tightened hard up, to allow the hinges to 'find their own centre' as it were. I still needed to draw a couple of the holes with the needle file, which is where the time goes on building these things, if you want to get it 'just so'.

I was however pleased to find that when I traversed the gun fully, nothing touched either side, though at the extreme elevation, the barrel clamp just kisses the top of the shield opening. Perhaps I'll take a file to it when it all has to come off again for painting.....