Lapping aliminium.

[Fat ferret]

Hi all. Bit of an odd one but maybe someone can help. Not a usual topic on here but it's still metal working. Will try to explain clearly for those not familiar with rifle shooting.

I am trying to zero an air rifle. The pellets strike the target low and the scope has run out of "up" adjustment. I want to try adjusting the scope mount which looks like this.

https://www.google.co.uk/search?q=s...S7AOIQ_AUIBygC&dpr=1.55#imgrc=Hk_-YHpdOcSitM:

I know I could end up wrecking the mount but want to try anyway.

My idea was to get a piece of 1" round steel bar, same diameter as the rifle scope. Jig the bar so it is rubbing against the front cradle of the mount and apply some cutting paste and rub the bar back and forth over the scope mount thus wearing away some metal mostly from the front cradle and a little from the back cradle. This will make the scope look "lower" where the pellets are striking.

I know there are other answers like shimming the rear cradle, which I have tried but still not enough. Adjustable mounts are available but for various reasons I don't want to use them.

If you didn't understand don't worry, you can still help . I need to know what lapping paste to use on the aluminium to cut it slowly that will work in conjunction with the round steel bar.

Thanks.

 

[woodpig]

As far as I am aware what you suggest is exactly what some folks do in America on Full bore rifles. The only grinding paste I've seen commonly available is the stuff used for lapping inlet and exhaust valves for engines. This comes as a double sided tin with coarse paste one end and fine the other. This should be ok for the job I would have thought but it may take you a while. Halfords or other motor factors should still sell it, they used to. If not, I have some aluminium oxide blasting powder I could send you but you'll need to mix it with some grease or something. Not sure how well that would work.

 

[Harbo]

You could also wrap some coarse wet and dry paper around the steel rod.

Rod

 

[woodpig]

That would work Rod but the thickness of the paper means it could cut over size a little on the width unless you're careful.

 

[novocaine]

don't remove material, add it, by removing material you will induce other errors as you enlarge the clamp including bending the scope. use film as a shim on the rear mount, you can also us beer tins if you don't have film. if a shim isn't enough then the amount of material to removed is going to be the same and you'll have to take off a large amount. what happens if you turn the mount round?
as you've normally got about 18" of adjustment in a scope at 30 yards I'd suggest you need to look at what the issue is, as it's striking low it sounds like you are underpowered but it maybe that mount isn't sitting flush on the dovetail, hence turning it round. you can also shim under the mount on top of the dove tail if need be.

 

[Fat ferret]

Hi woodpig found the American sites but I note the special bar costs $50 and I would need to import one. I see your valve grinding paste on Ebay and it's around £5. Don't mind if it takes a while as I don't want to grind too much. Should work. Thanks very much.

Harbro I did think of that but unless I use an undersize bar it will be oversize. Thanks anyway.

Novocaine I am not underpowered the rifles running at 24ft/lb on ticket. Recoil is not bad but noticeably more than a 12ft/lb rifle. I can't turn around the mounts as they have an arrestor at the back that fits into the rifles dovetail stops. I can't buy an adjustable mount as I can't find one that fits my dovetail stops and want these to cope with the recoil. I have tried shimming the rear mount but still not enough. At 30yards I am just about have enough adjustment with some pellets I tried but far from ideal. If I shim any further I think I will run into problems with bending the tube. The problem is caused by the barrel pointing down but the lock up is solid. The barrel is not bent it's the breech causing the droop which is common enough even with top brands like hw.

I understand what your saying about other errors but have thought of this. My jig will mean the bar starts grinding at the front edge of the front cradle and I will slowly remove material until I have removed a small amount on the rear cradle and a little more of the front one. I only anticipate removing maybe 1mm from the front of the front cradle tapering to almost nothing from the rear of the rear cradle.
So the mounts should slope down. Then I can screw on the tops of the mounts. If the front one wont tighten up I may need to file it's feet very slightly. There is enough play in the screw holes that the top part wont bend the tube if I'm careful.

 

[woodpig]

Silver steel rod is accurately ground to size so you could use that. Shouldn't cost more than about £20 for a 1 inch diameter rod.

 

[Rorschach]

If you can't fix it by adding a 1mm shim at the back, then you won't fix it by filing away 1mm at the front.

 

[woodpig]

Lapping has been an excepted way of doing it for many years and won't risk bending the scope if done correctly, especially on a one piece mount. Adding shim material on the other hand is not a good method and can result in a bend if you're not lucky. The alternative is an adjustable mount.

 

[Fat ferret]

Agreed, I have already shimmed as much as I dare and it's still low. 1mm is too much to shim whereas I could lap as much as 2mm off and not compromise the mount or scope.

As woodpig stated the alternative is an adjustable mount but I can't get one that fits my rifle properly and don't trust it not to adjust when I don't want it to.

 

[monkeybiter]

Have you tried different diameter pellets, you can get [as you probably know] different sizes within a nominal calibre, the difference in performance can be quite stunning.
Another thing to consider is pellet weight. Lighter pellets will get to the target quicker, before they've had chance to fall as far from the aim point.
Many years ago I had an unexpected drop from aim which turned out to be the scope drifting rearwards due to recoil and causing deflection of the rear mount [solved by an arrestor] perhaps your rear mount/arrestor could do with a check?

If you do end up altering your mounts, I would suggest doing a calculation first based on the discrepancy in aim/impact with the scope centrally adjusted to give you an idea of the amount to be removed, you could then remove the majority with small files before lapping in.

 

[Fat ferret]

Hi Monkeybiter, using a one piece mount with arrestor built in and not shifting.

Yes I'm working my way through all the common pellets. Can't shoot lightweights but middleweight JSB's are the best so far with 1 1/2" 10 shot groups at 30 yards which isn't too hot but hoping this will improve as the rifle beds in. With these pellets I am just able to aim dead on but right at the edge of the scopes adjustment which isn't good.

I know I could wreck the mounts but don't mind taking a chance. Will buy some paste and give it a go. Thanks all.

 

[Inspector]

My very limited experience with lapping has the compound imbedding in the softer material and wearing the harder. I would think the aluminum will pick up the abrasive and wear the steel bar unless the bar is softer. Am I out to lunch?

Pete

 

[woodpig]

Some useful information here on lapping Alloy scope rings.

http://www.24hourcampfire.com/ubbthread ... /3645701/2

I've done it as well and it does work.

 

[novocaine]

Well as you are set on doing it (not trying to be insulting) then use a wooden dowl and put the top clamps on. You want the same angle on those as the lower. Lapping paste, id use 220 grit to remove material then 600 to finish. bore lapping paste will be fine. Not a fun job and one ive only had to do once because of a bent barrel (note, never let a friend borrow anything). You are right not to shim more than you have although id be tempted to do both to save on work required. nothing wrong with film shims, its how we were taught to do it.

 

[Walney Col]

That would work Rod but the thickness of the paper means it could cut over size a little on the width unless you're careful.

if you want to lower your front mount (say 0.02") then take your 1" bar and turn or emery it down to

1" - 0.02" - the thickness of the wet and dry you're going to lap it

with and you'll still have a 1" diameter mount when you've finished.

Col.

 

[Eric The Viking]

I'm amazed: If you've already shimmed it and it's still off, something else is (seriously) wrong, especially at that power -- the trajectory should be relatively flat. Have you got any idea why it's so badly off?

E.

 

[Fat ferret]

Inspector, you may be right. If I use a wooden dowel it should work as the dowel would be softer.

Novocaine, your not insulting. I'm set on doing it because I can't think of anything else to do. I can't shim any further. I could buy an adjustable mount for £30 but it wouldn't fit my arrestor slots so would need a separate arrestor block. Would rather try modifying what I have. Don't think you will damage the scope unless you over tighten.

On another note a friend had a long barrelled HW35E which was bent down. I solved the problem for him by bending it up. It was a 15mm barrel in 22 so not a great amount of metal and I just took it off the gun and bent it over a padded sawhorse. It was still very accurate.

Anyway lots to think about. Thanks all.

 

[Fat ferret]

Wayney coll, could do that. Might work better with a wooden dowel which is the option I'm leaning towards.

Viking, its a break barrel. The barrel points down in relation to the cylinder where the scope is mounted. Not by much but it's noticeable and makes all the difference at longer ranges. The breech lock up is solid just points the barrel down slightly.

 

[woodpig]

Get yourself a nice PCP. :lol: