Modski66
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Post by Modski66 on Mar 20, 2022 20:19:19 GMT
I have been trying to get around to this post for a while, but our new kitchen is taking up much of my spare time, however I tested positive for Covid-19 this morning, so have now been banished from most of the house which gives me a bit of peace and quiet to catch up on a few things. Anyway, back to the subject. A few weeks ago, a broken Desert Eagle came up for sale and James gave everyone a heads up as is was listed as spares or repair for £45. It was not far from me, and I offered £40 collected, and took on the challenge of resurrecting the box of bits. The guy had been very thorough in stripping it down, with all the bits grouped and bagged, so it didn't take long to work out that the problem was the central part of the valve had snapped where the two ports direct co2 forwards down the centre and to the back of the pellet.  As this pistol has not been made for around 20 years, finding a spare was highly unlikely, so as I decided to see how much I could push boundaries of what my hobby lathe could do. More to follow... |
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Modski66
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Post by Modski66 on Mar 20, 2022 20:32:43 GMT
First things first, I needed some material to make the replacement with, and decided to go with stainless steel as it is relatively hard wearing, and trying to do any machining and hardening of carbon steel is really beyond the process capability of my humble shed. A2 Stainless steel bolts are readily available on ebay, so a couple were procured use as a cheap source of bar-stock.  After sawing the bolt-head off, it was into the lathe chuck to machine the thread away and rough-out the biggest diameter of the valve core, which was 8mm.  |
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Modski66
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Post by Modski66 on Mar 20, 2022 20:53:19 GMT
Once I was close to the 8mm outside diameter, I began boring out the main central gas way while the bulk of the material was still in place to soak up some of the heat, and maintain structural integrity during drilling.  Needed to be very gentle here, and quite accurate with the depth to make sure it would meet up with the two angled ports when they were drilled. Once completed, I moved back onto the outside form.    At this point I am beginning to form the shape of the back end of the valve core, which means the part needs to come out of the chuck and be flipped round to continue the back end features. This is quite tricky, particularly on a cheap Chinese lathe, as trying to get the reversed part concentric with itself is very important, and any misalignment is particularly noticeable on small parts. More to follow... |
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Post by flatrajectory on Mar 20, 2022 22:30:20 GMT
Nice work so far Mark. But.... if you had asked me first.... I may have a valve.. Not saying I have, just may have. I have a couple of valves knocking about, but not sure which umarex pistol they will fit. If your new build goes tits up , give me a shout.
Pete.
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Doyley83
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Post by Doyley83 on Mar 20, 2022 23:06:53 GMT
Superb work so far mate, what a talent. Can't wait to see the results
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Modski66
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Post by Modski66 on Mar 21, 2022 9:51:12 GMT
The next step is to part off the lump that was in the chuck and begin forming the seal edge.  For this I used some tool steel and ground a cutter to make an undercut that creates the edge that presses against the o-ring. Now comes the tricky bit, drilling the transfer holes, they are at an awkward angle, and I don't have a milling machine, so it's out with the Dremel and a steady hand. The original broken part is on the left for me to use as a visual reference.  More to follow... |
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Modski66
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Post by Modski66 on Mar 21, 2022 14:45:25 GMT
The drilling didn't go so well, I broke four drills and wore out three diamond coated Dremel tools producing these holes, my grip wasn't firm enough either and I buggered up the seal edge in the process. 😭  But in the end, I finally defeated the stainless steel and was able to re-cut the seal edge and file the flat onto the rear spigot. Again, no milling machine, but I like to think that I am reasonably good with the needle files.  My guess is that the flat is to allow some gas backwards to operate the blowback. Here are the old and new parts, the largest outside diameter is 8mm,the two co2 holes are 2mm.  After a bit of head scratching, I was able to get the pistol back together, new seals in the valve, and the moment of truth - tightening the plug to pierce the co2 capsule... More to follow... |
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Modski66
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Post by Modski66 on Mar 21, 2022 15:16:31 GMT
Hoorah! Not the slightest leak. 😁 Now to pull the trigger. Not so good as the slide goes into full automatic! 😱 After some advice from Mr HH, it turned out to be a worn feature on the slide that is supposed to disengage the trigger bar after a shot is taken, it just needed a 0.5mm packer on the bar, and a tweak with a file to the end of the bar that engages with the hammer. So £40, plus the cost of a stainless steel bolt, a few drills and some time in the shed and I have got myself an iconic replica pistol. 😎 Okay, it's a plastic fantastic revolver-matric, it is a bit rough here and there, but it's a hoot to shoot, and over the short distance I have currently been able to try it out, it seems quite accurate. Now where's my leather suit....  |
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CaptDAR
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Post by CaptDAR on Mar 21, 2022 15:40:32 GMT
Very well done Mark. A great read as well sitting here in Costco waiting for new tyres to be fitted. Love the suit.
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Ade C
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Post by Ade C on Mar 21, 2022 16:11:40 GMT
Excellent job!
The one I bought off Headhunter seems to be suffering from the full auto issue. Can you provide more information on the remedy?
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TalinMoracius
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Post by TalinMoracius on Mar 21, 2022 18:02:03 GMT
What a superb read! Well done, I think that’s a very impressive repair
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Modski66
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Post by Modski66 on Mar 21, 2022 19:09:36 GMT
Excellent job! The one I bought off Headhunter seems to be suffering from the full auto issue. Can you provide more information on the remedy? I will take some pictures for you when I am released from confinement, I can't really describe without some imagery! |
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Modski66
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Post by Modski66 on Mar 31, 2022 21:03:54 GMT
I am free! So here is the area in question that is the root of the full-auto slide effect.  In this image the slide is removed and the mechanism partially pulled out from the grip. There is a a short bar with a lug in the middle, and a longer link bar between the trigger and the hammer. The principle of operation is as follows. When the trigger is pulled and the hammer strikes the valve, the slide is driven back very quickly, at this point the shooter will still have the trigger fully back, because the mechanism is much faster than the finger. To stop the full-auto effect, the long link bar needs to be momentarily disengaged from the sear. This is where the short bar with the lug comes into play. On the inside of the slide is a 'liner cam' that pushes the lug down as it moves backwards.  Hopefully the picture makes sense, there is an edge inside the slide with a dog-leg towards the right end, as viewed in the image. The short bar is pushed down against the long link bar, disengaging it from the sear when the slide is at its rearmost position, keeping the hammer cocked. Over time the alloy lug on the short bar gets worn away by the slide, so it doesn't press the long link bar down far enough to disengage from the sear. Here is a closer look at the worn lug.  I decided that trying to build up anything robust enough on the lug was beyond my capabilities, so decided to give the long link some pre-engagement by adding a 0.5mm packer to the top edge.  In this picture you should be able to make out a black line on the top central portion of the long link. This is a piece of plastic card, superglued in place, scoring the alloy first to give it a key. This has worked for me for now, once the alloy wears some more, I will have to revisit the problem! Hope this helps, any questions, just ask. 😁 |
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Post by HeadHunter on Apr 1, 2022 7:31:15 GMT
There's always more than one way to skin a cat..... When I fixed mine I built up the worn lug by adding some carbon fibre (as it's hard wearing, light and strong) to the top of it.  The red arrow points to where I built it up, that's where the wear was so I decided on a direct replacement there rather than adjusting the geometry of the arm's movement. The original design is pretty good but it should have had a hardened steel sear there instead of using a metal with the same mechanical properties as 'chewing gum', even if the parts had simply been made from steel in the first place the problem would never have arisen at all. |
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Modski66
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Post by Modski66 on Apr 1, 2022 14:27:23 GMT
On mine I think the wear on the curved lug was down to the abrasive nature of the polymer slide. My suspicion is that it is ABS with a chopped glass-fibre fill to add strength, but if not lubricated it will wear away anything it rubs against.
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