Crowning Bit

[sholo]

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

[rsterne]

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

[AirGunEric]

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.

[Doc Sharptail]

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

[rsterne]

Obviously NOT an engineer

and proud of it !!!

Bob

[SeanMP]

... but just how fussy do we have to be, really?..

Obviously NOT an engineer


FWB was kind enough to provide this example.....the last point of contact is more important than anything else

[rsterne]

That was a pistol, the classic example of a "dump shot".... way more gasses produced by the powder than can be used to accelerate the bullet.... I'm not saying a similar photo of an airgun wouldn't show the same thing, but considering the redisual pressures are so much less I'm thinking the "cork from a bottle" gas escape would be a lot closer to bullet velocity....

I agree, if the bore is axially not parallel to the outside of the barrel, then 90* to the barrel OD would not be 90* to the bore.... If the last 2" of the bore was out of parallel by 0.010", then the side to side error of the crown would be 0.001" (on a .20 cal).... Assuming the bore was straight, however, that would require it to be off center 1/8" over a 24" barrel.... Since I've never seen a barrel out that far, the logical conclusion is that the side to side error on the crown caused by a non-parallel bore would be less than 0.001".... I'm betting hardly a bullet in the world has a base square within 0.001", and if it does, how about the same tolerance over a box of bullets.... or a tin of 500 pellets?....

I'm not saying a good crown isn't important.... The more variables you can remove the smaller the potential group size.... but just how fussy do we have to be, really?....

Bob

[SeanMP]

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.

[AirGunEric]

Cathead?

Sounds a bit perverted.

People and their tools...

[sholo]

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!