tips for opening up a rear sight base

[Whiterabbit]

hi guys,

I have a project coming to solder on a rear sight base. NECG's XL base isn't even close to the barrel diameter. They said just open it up. The way I've seen it is wrap the barrel or a dowel with sandpaper and have at it with the sight till it fits.

So, in the PAST I've done this with a Williams fire sight and a handi rifle. The fire sight was ALUMINUM, and I used wet-sandpaper with oil on it wrapped around a dowel. This worked, but it took a long, long long time.

This NECG base will be made out of steel. Furthermore I am worried that my hands will put uneven pressure on the sight and it'll be tilted rather than straight after I get it sized, if I can even size it!

I'd like some tips on how to make this job not take 2 weeks of sanding 6 hours a day to get the thing sized to barrel diameter. I will be turning an aluminum rod to diameter and using that as a blank to wrap an abrasive around to the perfect diameter. Any recommendations for the proper abrasive to use? proper grit so the base is properly prepared for soldering? tips to ensure the base isn't warped or uneven when finished?

In short, any tips? (besides wrap a dowel with whatever you got and go to town).

Thanks guys!

[andremajic]

Couple options:

Take it to someone with a milling machine to do it.

You could set it up in a 4 jaw chuck on a lathe, offset, with a spacer to hold the jaws apart with a boring bar taking off the material from the front, then reversing the part and doing the other side.

Or, you could use a half-round file and spotting compound like prussian blue or a black sharpie and slowly remove the material. A file would be MUCH faster than sandpaper. You can use a pair of calipers from either end as you remove material to ensure that you are taking off an even amount as you progress. If you have a piece of round stock that is the same diameter of the barrel, you can scratch with a scribe into the sight, using the round stock as a template.

There are a few more ways to skin this cat that I can't think of at the moment. Use your imagination and I'm sure you'll come up with more.

Andy

[Whiterabbit]

I was advised to buy a ball-end endmill and run it over the bottom of the sight. Not cheap @ 24mm (the barrel diameter at the sight base), but if I stick with HSS it isn't too bad. I only have to open one of these up.

[Sensai]

If you decide to do it by hand, you can use cerrosafe or another low melt point alloy to bed it in to prevent tilting the base.

[SODAPOPMG]

if you are only going to do 1 then just sand it use a coarse grit paper as the side that you will be sanding will not show and does not need to be pretty
clamp part in vise and use a guide for your rod to run along so it does not tilt side to side a little up and down will not hurt any
this should not take over 1 hour to sand to fit

[Whiterabbit]

any recommendation for an abrasive?

[MBTcustom]

Just my 2 cents, but when I have to do something like this, I mount the sight upside down in the vice.
Then I calculate Z and Y coordinates on an arch that matched what you want in the sight, minus the diameter of a smaller ball mill.
So you indicate center of the sightbase, and raise the table till you scratch cut right down the center of the original radius.
Then you lower the table (Z) .002 and then you cut say .010 plus and minus in the Y axis. Then you lower the table another .002 and offset the ball mill acordingly etc etc etc, till you have dressed a full radius into the base.
If you calculate it correctly, you can lay a perfect radius in the base (albeit with machining marks included)
Then take a piece of 400 grit sand paper, and lay it over the barrel right where the sight base is going to be located.
Put the sight on the paper exactly as it will be oriented on the firearm, and stroke it back and forth about 1/4".
Flip it over and check your work. If you did it right, you will see the machining marks being worked out of the profile.

At this point you can solder it on, or you can screw it on. Solder is better if you know how to do it, but be aware that any type of low heat solder has lousy filling characteristics, and the sight base had better match the barrel exactly if you expect it to hold. If you don't have a darn good fit on the sight base, then you are going to need to use either screws or a filling type of solder filled flux. If you use the solder flux you are going to need an oxygen/acetylene rig in order to get enough heat on there to make it work. Be careful to use just enough heat to get the job done.

[Whiterabbit]

Mr goodsteel,

I like the idea if nothing else, it means less of the workpiece is engaged with the endmill meaning less stress all around. A question for you.

What would be "better" for the purposes minimizing workpiece engagement and thus providing the cleanest cut that will require the least amount of rework with sandpaper and barrel: using a 3/8" ball end mill, 1/2" ball end mill, or a 3/4" ball end mill? (assuming I'm opening up to about an inch)

Logic tells me .75" will contour the face in a way that removes the most material so it's as close as possible to the proper curvature I am looking for. But I'm also left wondering if the 3/8" cutter will plow through the site with less stress, to the point where it would be worth it to either take more and lighter passes or bite the bullet and take more time by hand with the wetsandpaper to clean the tool marks.

I will be hard soldering the site on, whatever it takes to accomplish that deed. For now, my focus is simply on the prepwork.

Thanks :)

[MBTcustom]

Use a 3/8" solid carbide 3 flute ball mill, running fast (bout 1500-1800rpms) so that you can get a good finish. Make sure your steps are about .010 apart.
When sanding, don't bother wetting the sandpaper. It breaks down too quick to make a difference, and sandpaper is much cheaper than time is. Just cut it into 1" strips and keep it moving. You'll be where you need to be in no time.

[Whiterabbit]

big thanks

[Whiterabbit]

My local machineshop supplier had a 3 flute, but no coating and the cutting surface is larger than I wanted (1" cutting depth rather than .5). I have some practice bits of mild steel, and an alloy steel. Hopefully I can get a sense of cut methods with these then go for it. the base is ordered and on the way.

[MBTcustom]

Use dark cutting oil and have at it! Nothing wrong with bright HSS, you just have to go a little slower on the speeds and feeds.

[Whiterabbit]

its solid carbide, as recommended. I'm not at a skill level yet to where I can really deviate from recommended practices. Just not coated. They had coated carbide, but only in a 5 flute, we both figured I didn't have enough machine speed to run it.

[Whiterabbit]

Just my 2 cents, but when I have to do something like this, I mount the sight upside down in the vice.
Then I calculate Z and Y coordinates on an arch that matched what you want in the sight, minus the diameter of a smaller ball mill.
So you indicate center of the sightbase, and raise the table till you scratch cut right down the center of the original radius.
Then you lower the table (Z) .002 and then you cut say .010 plus and minus in the Y axis. Then you lower the table another .002 and offset the ball mill acordingly etc etc etc, till you have dressed a full radius into the base.
If you calculate it correctly, you can lay a perfect radius in the base (albeit with machining marks included)
Then take a piece of 400 grit sand paper, and lay it over the barrel right where the sight base is going to be located.
Put the sight on the paper exactly as it will be oriented on the firearm, and stroke it back and forth about 1/4".
Flip it over and check your work. If you did it right, you will see the machining marks being worked out of the profile.

At this point you can solder it on, or you can screw it on. Solder is better if you know how to do it, but be aware that any type of low heat solder has lousy filling characteristics, and the sight base had better match the barrel exactly if you expect it to hold. If you don't have a darn good fit on the sight base, then you are going to need to use either screws or a filling type of solder filled flux. If you use the solder flux you are going to need an oxygen/acetylene rig in order to get enough heat on there to make it work. Be careful to use just enough heat to get the job done.

Worked like a champ. I ordered my sight base yesterday so I don't have it yet, but I practiced on unknown but alloy steel yesterday. Since it was square, I did top down rather than bottom up, and took .01 passes (which took a long time, but that's OK, this is just a test cut) for the full width of that chord, then lowered the bit and made another series of passes.

Since my Z is .0005 not .001 per mark I made two goofs, but given .01 resolution, they were meaningless. I mocked up a barrel out of aluminum to the same diameter (and for fun I bored it .5" and crowned it), and I barely see light through it when I hold them together. 5 minutes with some sandpaper (ok, maybe 30 minutes) and this thing is gonna be dead nuts perfect.

Great advice.

http://castboolits.gunloads.com/attachment.php?attachmentid=86135&d=1383374306

(nevermind the burrs. I'm an amateur.)

[MBTcustom]

Bingo, you got it.

[Whiterabbit]

And that, kind sirs, is the best I can manage on a Chinese mill with no skill or education as a machinist.

http://castboolits.gunloads.com/attachment.php?attachmentid=87116&d=1384107980

(this is before deburr and before sanding on the barrel)

When I go to sand it, shall I, instead of laying sandpaper over my .945 barrel, measure the thickness of the sandpaper and turn my blank down that much so that my blank WITH sandpaper on it is exactly .945? Is that the best way to get a "solder-ready" surface close coupled to a .945 diameter?

[MBTcustom]

It would be better certainly, but .010 bigger on the radius is going to make such a minute difference to such a small section of the diameter, that you will barely be able to measure it. If you just go to lapping it on the barrel, and keep your paper tight, that will get you scary close to perfect.
Still, you've got a lathe, so why not use it?:twisted:

[Whiterabbit]

That's kinda what I'm thinking!

Have plans for a barrel channel opener tool, too. Thought I could turn a 6-9" piece of aluminum to .945, then drill and tap the sides at either end for some 3/8" pegs for a good grip to really plane that guy down nicely.

I have to say, my mill is kinda on loan (paid $0 for it) and its nice to have, but I'm REALLY glad I outright own a lathe. What an unbelievably useful tool.

[MBTcustom]

Check out the barrel bedding tools from gunline.
http://www.brownells.com/gunsmith-tools-supplies/stock-work-finishing/stock-making-hand-tools/barrel-channel-cutting-tools/barrel-bedding-tool-prod6796.aspx
It's literally just a stack of washers the correct diameter, sandwiched between rubber O-rings and held in place with roll pins. You can't make it for what they sell them for, but they work amazingly well, and I have made many custom sizes for he ones I have bought.