Turning the 'chamber' on an air rifle barrel blank
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    Default Turning the 'chamber' on an air rifle barrel blank

    I'm machining up an air rifle barrel blank and this particular barrel needs a taper cut at the breech end.

    The taper is just a tad under 5 degrees and goes from 6mm dia down to the bore size, 4.5mm.

    I'm using a tiny solid carbide boring bar.

    The taper is fine but when I get to the rifling, I am getting a burr where the steel is peeling over into the rifling grooves.

    At higher speeds (1200 rpm) this is worse. At low speeds (180rpm) it's better but still visible. I'm using cutting oil as a lube as the low speed.

    Not sure if I should try and clean up the finish by using abrasives or keep machining at the low speed and see if it eventually clears off the burrs.

    They don't occur on all the lands and they are only at the base of the land where it rises from the bore.

    20190304_115939.jpg

    Any advice?

    Cheers!

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    I think the burr is natural. Depending on the alloy, it could be greater or less, but in all cases, it needs to be manually addressed. I suggest using Swiss files first. In this particular case, the use of a carbide insert is the wrong tool because they will never be sharp enough. A well sharpened and honed HSS tool is superior is all respects for this job.

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    Quote Originally Posted by steve-l View Post
    I think the burr is natural. Depending on the alloy, it could be greater or less, but in all cases, it needs to be manually addressed. I suggest using Swiss files first. In this particular case, the use of a carbide insert is the wrong tool because they will never be sharp enough. A well sharpened and honed HSS tool is superior is all respects for this job.
    Hi Steve,

    The tool isn't a carbide insert. It's a solid carbide boring bar. It's very, very sharp!

    It's going to be tricking getting in there with a needle file and I want to avoid scratching up the other surfaces. The barrel steel is very soft (it's a 12ft/lb airgun).

    Might be able to get in there with the borescope in place and a long dental pic and run it down the grooves of the rifling. Obviously that burr is tiny (.177 cal rifle) but I'd prefer it not to be there!

    As the tool is actually a rod of carbide, I can adjust the rake, so was wondering if a more aggresive rake would be better or worse?

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    Quote Originally Posted by rich-at-home View Post
    Hi Steve,

    The tool isn't a carbide insert. It's a solid carbide boring bar. It's very, very sharp!

    It's going to be tricking getting in there with a needle file and I want to avoid scratching up the other surfaces. The barrel steel is very soft (it's a 12ft/lb airgun).

    Might be able to get in there with the borescope in place and a long dental pic and run it down the grooves of the rifling. Obviously that burr is tiny (.177 cal rifle) but I'd prefer it not to be there!

    As the tool is actually a rod of carbide, I can adjust the rake, so was wondering if a more aggresive rake would be better or worse?
    I think a boring bar of any description is the wrong process. All you need to do is kiss the barrel end with a tapered reamer and be done with the issue. Tapered reamers come in all flavors. I have needle files that would be ideal for the .177" bore. As far as the use of oil, don't. Try tapping fluid. It is much better for this task.

    When it comes to carbide, carbide requires high speed and high machine stiffness. Carbide uses reduced cutting clearances because it is brittle and these reduced clearances are necessary for tool durability. These reduced clearances increases tool drag and encourages burr generation. Further, I repeat myself, carbide will never be as sharp as HSS. How do I know this? Well, I've been doing this shit for 60 years and I am also an air gunner. I will also apologize up front if I'm wrong, but with 2 posts I'm guessing you are a beginner and doing this on a toy lathe. Toy lathes hate carbide.

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    Quote Originally Posted by steve-l View Post
    I think a boring bar of any description is the wrong process. All you need to do is kiss the barrel end with a tapered reamer and be done with the issue. Tapered reamers come in all flavors. I have needle files that would be ideal for the .177" bore. As far as the use of oil, don't. Try tapping fluid. It is much better for this task.

    When it comes to carbide, carbide requires high speed and high machine stiffness. Carbide uses reduced cutting clearances because it is brittle and these reduced clearances are necessary for tool durability. These reduced clearances increases tool drag and encourages burr generation. Further, I repeat myself, carbide will never be as sharp as HSS. How do I know this? Well, I've been doing this shit for 60 years and I am also an air gunner. I will also apologize up front if I'm wrong, but with 2 posts I'm guessing you are a beginner and doing this on a toy lathe. Toy lathes hate carbide.
    Hi Steve,

    I did look for a reamer to use but struggled to find one with the right angle of taper. 5 degrees. Steep for a reamer...

    The tool shown below. It has to be small to get clearance.

    I have tried high speed with flood coolant but found low speed with cutting oil (tufcut) was better.

    Regarding the lathe, it's a 650kg Harrison M300 toolmakers lathe, so no it's not a toy! (My other lathe is a toy...)

    If you can point me at a reamer that will cut from 4.5mm to 6mm over 9-10mm, I'd be greatful...

    (this is not my first rodeo but whatever this barrel is made of seems to be causing an issue.)

    20190305_130330-medium-.jpg

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    Rich,
    There are 2 things that I have found to help with the burr that you are getting. One is feed from inside to outside (away from the headstock) and the other is to push a tight fitting bronze brush thru the bore so that it exits at the end you are machining. This helps to stand up any burrs and then you make another pass with minimum or no infeed. Another thing that you can try is varying the feed rate if you are using power feed. I also agree that a honed HSS tool is probably better for what you are doing here. You may need to try different tip radius geometry and use lots of clearance.

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    Quote Originally Posted by pfiori View Post
    Rich,
    There are 2 things that I have found to help with the burr that you are getting. One is feed from inside to outside (away from the headstock) and the other is to push a tight fitting bronze brush thru the bore so that it exits at the end you are machining. This helps to stand up any burrs and then you make another pass with minimum or no infeed. Another thing that you can try is varying the feed rate if you are using power feed. I also agree that a honed HSS tool is probably better for what you are doing here. You may need to try different tip radius geometry and use lots of clearance.
    Hi pfiori,

    Feeding from the inside out sounds like a good idea, not quite sure how I'd do it as I can't see the tip of the tool once it's in a few mm. I think I should be able to estimate when it's clear of the rifling when fully inserted and then bring it out a touch for a pass...

    I'm feeding on the compound slide so manual feed.

    I tried the bronze brush! It certainly helped when there was a big burr but it's not picking these tiny ones up.

    I do have some length to play with, so I can face off and start fresh if needs be.

    What I really need is a rifling button! But there's a choke on the muzzle so that wouldn't work either. Not sure if I cast a lap in the bore, if it would pick those burrs up...

    When I was looking into tooling for this, I had hoped to find something like a chamber reamer or cone shaped cutter that I could use, but I couldn't find anything, so I decided to try a tiny boring bar.

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    Air gun barrels are soft steel so a burr is always going to be present. I would machine a soft copper or brass rod to a taper somewhat less than the one existing and tap it into the throat of the taper so that it contracts the lands at the burr. Burrs are thinnest at the attachment point to the base metal, so the copper will grab the burr and drag it off the land corner.


    RWO

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    Quote Originally Posted by RWO View Post
    Air gun barrels are soft steel so a burr is always going to be present. I would machine a soft copper or brass rod to a taper somewhat less than the one existing and tap it into the throat of the taper so that it contracts the lands at the burr. Burrs are thinnest at the attachment point to the base metal, so the copper will grab the burr and drag it off the land corner.


    RWO
    Hi RWO,

    Interesting idea! I will give that a try.

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    Feed from inside out. Draw it out and determine starting point. Or make a reamer, either multi flute or D reamer.

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    What I do on rimfire barrels after chambering is to take an old bore brush and wrap an oiled patch with jb abrasive on it and scrub the throat with it to remove these burrs since soft 22 Ammo will not wear them off. It doesn’t take many strokes either.

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    I think I'd turn up a couple laps and knock the burrs off that way. And smooth up the leade while I'm at it.

    I also JB the hell out of rimfire chambers. Centerfire, I just go and shoot any burrs and tool marks out of them. Takes very few rds.

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    Quote Originally Posted by wesg View Post
    I think I'd turn up a couple laps and knock the burrs off that way. And smooth up the leade while I'm at it.

    I also JB the hell out of rimfire chambers. Centerfire, I just go and shoot any burrs and tool marks out of them. Takes very few rds.
    What material would you use for the laps?

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    Quote Originally Posted by rich-at-home View Post
    Hi Steve,

    I did look for a reamer to use but struggled to find one with the right angle of taper. 5 degrees. Steep for a reamer...

    The tool shown below. It has to be small to get clearance.

    I have tried high speed with flood coolant but found low speed with cutting oil (tufcut) was better.

    Regarding the lathe, it's a 650kg Harrison M300 toolmakers lathe, so no it's not a toy! (My other lathe is a toy...)

    If you can point me at a reamer that will cut from 4.5mm to 6mm over 9-10mm, I'd be greatful...

    (this is not my first rodeo but whatever this barrel is made of seems to be causing an issue.)

    20190305_130330-medium-.jpg

    I’ve used Drill Services Horley for specially ground drills and reamers.


    Reamers & Reaming Sets (Carbide, HSCo & HSS) | Drill Service

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    Quote Originally Posted by lionelt View Post
    I’ve used Drill Services Horley for specially ground drills and reamers.


    Reamers & Reaming Sets (Carbide, HSCo & HSS) | Drill Service
    I will give them a go! One of my mates suggested them too. Thanks.

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    Quote Originally Posted by rich-at-home View Post
    What material would you use for the laps?
    Copper or Brass, preferably.

    Bear in mind, proper lapping requires a figure 8, or other random motion, to create an even surface. In theory, it won't work for a cone, as you can't use anything but a radial motion which will leave rings anywhere there's a scratch or junk imbedded in the lap.

    But if you've got suitable lapping compounds already, it might be worth a try. You'd want to do very little before replacing the lap, or cleaning it up in the lathe.

    And you may not be able to take enough out to break the burr to begin with. But I think worth giving it a try before buying a throating reamer.

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    Some success at last!

    I turned up a copper rod with a couple of bands that are the same diameter as the grooves in the bore and tapped it into the barrel.

    It pushed off all the burrs!

    burr-cleaner.jpg


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