moroeder Posted February 28, 2015 Report Share Posted February 28, 2015 I've recently discussed the M223 and the M308 one piece mount with a real Nikon engineer that knew what end of the scope to look through! I had been told the one piece 223 and the 308 mounts were the same except the labeling. He confirmed it and we discussed the virtues of the M308 scope which I am mounting on my 20" DPMS. Even though the box says mfg in the Phillipines he said the glass is Japanese. As you may know Burris started making their scopes in the Phillipines, not positive of all their scopes but the majority seem to be. Anyway, what I really wanted to convey to you is the conformal errors that are in the stock un-lapped mount. I lapped in the M223 XR (extreme range) 20 MOA mount and thought some of you would appreciate seeing how much out of round the mount is and why it's critical to lap it in. I measured the body diameter of the M-308 scope and made a gray cast iron lap to its dimension. The tube was pretty good and measured from .9995"-.9985" so I made the lap .9995" knowing the aluminum base would pick up some of the aluminum oxide 60 grit lapping compound. It did and by finishing the process the lap measured .9988" in diameter all the way down its 9" length. There is a photo attached of the mount on the lap as I used a lathe to rotate the lap and move the mount left to right. I then reversed the lathe rotation to protect myself from a bias imposed by doing the job by hand. I first lapped the caps singularly by hand to about 60% clean up same as the base, then put them together. Evenly tightening the caps as the lapping process proceeded rendered the final product as you can see in the pic. The attached photos will give you a pretty good idea what a new mount looks like as I progressed to a stopping point. Notice the last pic still shows some original annodizing left in the bore. The reasoning for stopping there was due to the fact the split line dimension across the arc (cord) was measured at 1.019". The bright areas measure right in at the target dimension, same as the scope tube. Haven't mounted it yet but I now know the original off the shelf mount could give me/ you some issues! Checking the slope rendered a 21.18 MOA after lapping, so that should work OK since the original mount that was once provided with the M308 scope was a 20 MOA. Quote Link to comment Share on other sites More sharing options...
survivalshop Posted February 28, 2015 Report Share Posted February 28, 2015 Nice job . Sure is nice having the tooling to do this kind of work . Thanks for sharing . A good hand lapping tool can be purchased from several companies & work very well , if used correctly .Having a lapping tool is good to check any scope ring set or one piece mount ring alignment . Some scope problems can be related to ring alignment & you would think that the billet one piece mounts would be perfect , but not always the case , could be who makes them . Individual ring sets can benefit greatly by checking their alignment & lapping , to true them, if needed ,as you have done to a one piece mount . Quote Link to comment Share on other sites More sharing options...
Toolndie7 Posted February 28, 2015 Report Share Posted February 28, 2015 Nice write up! It always amazes me how far out of round most mounts are. That's why I always lap my mounts. Quote Link to comment Share on other sites More sharing options...
planeflyer21 Posted February 28, 2015 Report Share Posted February 28, 2015 That surprised me about my GG&G 1-piece QD mount...only took a few passes and it was lapping totally evenly. Quote Link to comment Share on other sites More sharing options...
unforgiven Posted February 28, 2015 Report Share Posted February 28, 2015 Good info, I got the Nikon 1 pc.M.223 scope mount [was only $ 39.00] for the Nikon 1" tube 3-12X42 M233 scope and just tightened it up. <dontknow> Quote Link to comment Share on other sites More sharing options...
98Z5V Posted March 1, 2015 Report Share Posted March 1, 2015 ^^^ You squashed your tube. Well, it's not the first time you've squashed your tube, though. <lmao> Quote Link to comment Share on other sites More sharing options...
Ben Diss Posted March 1, 2015 Report Share Posted March 1, 2015 Never thought about using the lathe to lap rings, but I see how it would work well for a one piece mount. Can you tell us how you protected the lathe from the lapping compound. I'd be concerned about the abrasive getting on the ways, but it seems like laying down a cloth over the ways wouldn't be safe. Quote Link to comment Share on other sites More sharing options...
unforgiven Posted March 1, 2015 Report Share Posted March 1, 2015 ^^^ You squashed your tube. Well, it's not the first time you've squashed your tube, though. <lmao> Probbably shouldn't used that 2' cheater.......but it feels so good when I let go. :unsure: Quote Link to comment Share on other sites More sharing options...
moroeder Posted March 3, 2015 Author Report Share Posted March 3, 2015 Never thought about using the lathe to lap rings, but I see how it would work well for a one piece mount. Can you tell us how you protected the lathe from the lapping compound. I'd be concerned about the abrasive getting on the ways, but it seems like laying down a cloth over the ways wouldn't be safe. The lap was charged, charging is forcing the grit to get stuck in the soft material using a hard steel roller and rotated at 15 rpm so there was no slurry to drip from the process. The lapping compound was the typical Clover valve lapping paste so it held tight to the lap. Notice the spiral grooves in the lap would hold extra paste and the swarf bi-product. After a few passes I would wipe the lap clean and reapply and recharge the lap. Frequent clearing of spent abrasive and aluminum is a must because the aluminum will take on some of the lapping compound because it is softer than the grey iron. The ideal lap would have been a softer material like lead, or sand mold cast aluminum. The mount was made from pressure die cast aluminum assuming a 6061 alloy with a T6 heat treat which makes the aluminum structure stronger and no porosity, A sand cast soft 300 series alloy aluminum round bar turned to the correct dimention would have even worked better. As far as the lapping compound around a lathe, just be careful and don't get the compound where it doesn't belong. Another reason to use a lapping paste and not the common slurry sometime ised in production lapping. Quote Link to comment Share on other sites More sharing options...
SteelonSteel Posted March 7, 2015 Report Share Posted March 7, 2015 Great write up moroeder, appreciate your time! About to lap my 1 pc mount (a high 30mm from Armalite). Question: Appears you lapped with the mount not attached to the upper, so when you do secure the mount to the upper do you think the mount might bend or deflect slightly? The thought process behind my question is that the 1 pc scope mount is soft material and has a thin cross section relative to the mass of the upper. When mounted and tightened to the upper it may deflect/conform to the upper, then throwing out any alignment achieved as a result of the lapping process. I am just trying to decide if I should lap the mount attached or not attached to the upper. Any thoughts from anyone? Quote Link to comment Share on other sites More sharing options...
Ben Diss Posted March 7, 2015 Report Share Posted March 7, 2015 Great write up moroeder, appreciate your time! About to lap my 1 pc mount (a high 30mm from Armalite). Question: Appears you lapped with the mount not attached to the upper, so when you do secure the mount to the upper do you think the mount might bend or deflect slightly? The thought process behind my question is that the 1 pc scope mount is soft material and has a thin cross section relative to the mass of the upper. When mounted and tightened to the upper it may deflect/conform to the upper, then throwing out any alignment achieved as a result of the lapping process. I am just trying to decide if I should lap the mount attached or not attached to the upper. Any thoughts from anyone? Perfect. Thank you! Quote Link to comment Share on other sites More sharing options...
SteelonSteel Posted March 9, 2015 Report Share Posted March 9, 2015 Went to Walmart this morning (Sunday) at 7am to grocery shop (best time to go, no crowds). Always drop by the ammo case, just to check on 22 LR, no luck. But, did find some very economical Weaver rings so I bought 2 sets, put one on the old Glenfield 60C I've had since high school (30+ years ago) and lapped the rings. Just wanted to practice technique with the lapping kit. If I ruined the rings, no big deal. Went really well, will lap the next set on a Ruger 10/22, then will work on the AR and the Ruger 270. Lapping is not difficult at all, will try to send pics after I do the 1 pc mount for the AR. Think I will mount it to the upper prior to lapping. Quote Link to comment Share on other sites More sharing options...
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