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Thread: Crowning Bit

  1. #1
    Senior Member sholo's Avatar
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    Crowning Bit

    Seen this little 11 degree crowning tool on another forum, and was wondering if anyone here has ever used one? If so, how did it work for you - good, bad, not worth the time or money???
    m_Crowning bit.jpg

    Was thinking it might be handy for someone who doesn't own a lathe (like me), or even if you do have a lathe for that matter, to cut a uniform 11* crown. I assume you would still have to do some polishing to some extent (which is no big deal), only part that concerns me is the guide/centering rod on the front - would it not mess up your rifling a bit while it is turning?


    Todd
    Todd

    Don't sweat the petty things, and don't pet the sweatty things!

  2. #2
    Moderator rsterne's Avatar
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    The guide has to be a good fit in the bore (not tight, not loose) and they are designed to be turned by hand, so little danger of damaging the bore if used properly.... The end of the barrel MUST be 100% square to the bore before use or they will tend to run off center (especially if the pilot is loose) and cut more on one side than the other.... and therefore the crown won't be symettrical.... I am still of the opinion that the best crown is at 90* to the bore and recessed within a protective edge (or muzzle brake) to prevent damage....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  3. #3
    Administrator AirGunEric's Avatar
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    Quote Originally Posted by rsterne View Post
    I am still of the opinion that the best crown is at 90* to the bore and recessed within a protective edge (or muzzle brake) to prevent damage....
    Seems to be another message of "don't believe all the hype" that gets thrown about on the interweeb.



    I think the people machining the LW Match barrels for Crosman also agree with Bob- the crown on the barrels from their match pistols is quite straight in a small recess- no "specialty" angle to it.
    I'd say I care- but I'd probably be lying...


  4. #4
    Senior Member Doc Sharptail's Avatar
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    The 11 degree target crown pre-dates the inter-web, and it's hype by quite a bit. Green Mountain has turned out this crown on HB 10-22 target bbls likely by the millions by now. They are among the worlds most accurate bbls- of course turned out under tightly controlled factory tolerances.

    The Brownell's 11 degree crown cutter is a good unit. Just remember they are toleranced for .22 Long Rifle bores, which are a bit bigger than .22 airgun...

    Regards,

    Doc Sharptail
    "Ain't No Half Way"

    -S.R.V.

  5. #5
    Senior Member SeanMP's Avatar
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    Hmmmm...too much to say on this one.

    There are some excellent crown cutters out there but unfortunately the one in your pic is not one. The units that will give very good results feature an expanding mandrel and a separate cutter that spins on the mandrel shaft....and unfortunately they are worth a kings ransom. Like buy a Taig lathe expensive.

    The one in your pic is very much like the ones crosman and the Chinese are using now. The pilot spins inside the bore and guaranteed they will put a tooling mark on the rifling that we have all seen on the IB barrels. Chips are going to enter and then you've got an abrasive cutter spinning inside.....

    Bob is correct that the 90* crown is the absolute best you can do. The rest are the best we can achieve to provide some protection for the crown and the highest degree of accuracy. The 11* crown is simply the measured divergent angle of the escaping gases, so that was adopted as being the best possible compromise between impinging the gas and causing turbulence and protecting the crown. It's not a magical number

    The step crown is also a compromise but it has one problem. The barrel needs to be large enough that the step is not within the angle of divergent gas.

    The other problem is that this 11* divergent angle was measured on powder burners where the gases are moving at velocities that are much higher than the escaping air of an airgun. Less velocity means there is even less angle of divergence.

    There is one benefit to using the expanding mandrel type cutter and that is they by virtue align themselves with the bore. Thats a critical step in achieving a really good crown and on a lathe the setup to do that is most of the job.



    This is what the setup looks like when I do it and you also need something called a cathead to be able to manipulate the the axis of the bore.
    Sean

  6. #6
    Administrator AirGunEric's Avatar
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    Cathead?

    Sounds a bit perverted.

    People and their tools...
    I'd say I care- but I'd probably be lying...


  7. #7
    Moderator rsterne's Avatar
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    OK, Sean, I'll throw a question at you.... If you are cutting the crown in a lathe at 90* to the bore.... why do you have to make sure the bore is concentric?.... I realize that if the bore was going around like an egg, the tooling marks would be spiraling away from the edge of the bore.... but if the finish is good enough does it really matter?....

    In addition, in an airgun (or a powderburner, for that matter) aren't the escaping gasses going about the same velocity as the pellet/bullet?.... Therefore, if you have a stepped crown (ie 90* with an outer lip) wouldn't the bullet have to be pretty much the same distance from the muzzle as the distance from the edge of the bore to the lip before the air could be forced to change direction.... and if that's say 1/8", then the air would have to be reflected off the lip and then try and catch up with the bullet to disturb it.... That seems to me an impossible task....

    I humbly submit, therefore, that any 90* crown cut square to the bore on a lathe with even reasonable surface finish would be fine.... I would just remove any burrs with a polish with fine grit and a sphere.... eg. brass screw and bore bright.... If I'm wrong (and I may be), please enlighten me where my reasoning is flawed....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  8. #8
    Senior Member sholo's Avatar
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    Quote Originally Posted by AirGunEric View Post
    Cathead?

    Sounds a bit perverted.

    People and their tools...


    Alrighty then....stay away it is!

    Got home early today, started surfing some forums, seen that pic and thought I would ask your opinions on it. Glad I did!
    Todd

    Don't sweat the petty things, and don't pet the sweatty things!

  9. #9
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    Talk about perverted, I was just on your other forum and opened one of the pictures in your Condor post .http://airgunhome.com/agforum/viewtopic.php?t=7370 What the heck is going on there ?

  10. #10
    Senior Member SeanMP's Avatar
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    Bob

    In all the super slo mo footage I've seen for PB the gases are moving substantially faster than the bullet. Have a watch of this U tube I found on a quick search. Some of the others I've seen show the bullet emerging out of the gas plume.

    https://www.youtube.com/watch?v=otpFNL3yem4

    The reason for taking the time to align the bore axis is because they are never axially parallel to the OD. Well actually I have only seen one out of how ever many I've done. They are always a measurable amount angular to the barrel.
    Sean

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