How much runout is acceptable on recrowning tools?

[Methuselah]

I've been doing them by hand because I'm chincy and didn't want to pay for multiple shipping charges. Just found all of it in stock, yeh!

The 90 degree cutter appears OK (and runout is irrelevant anyway at 90?), the 11 degree about 3/1000".

That's the cutters.

The pilots, I only bought for .177 (.173, in steel) and .22 brass different story. In .22 the 90 degree is good (< 1/1000), the 11 degree with brass .22 about 3/1000" and the runout with the .177 pilot is almost 10/1000".

It seems I need a more accurate. 177 pilot, but, what about that 11 degree cutter? Is 3/1000 too much runout?

[subscriber]

Steve,

Runout on a piloted crowning tool makes one flute cut more that the others.  While ideally there should be no runout in the tool, runout does not make the barrel crown eccentric to the bore.

That said, I don't like piloted crowning tools because getting the pilot to fit sloplessly in the bore is very difficult, unless you have many replaceable pilots, or an expandable pilot that locks into the bore.  If the pilot is tight, then pushing it in is likely to put linear scratches on the tand tops.

Piloted crowning tools are great for production because they are fast to use.  But they often leave circular scars on the land tops from a few chips carried over from one barrel to the next.  Even worse, is when the pilot is driven by a drill press, and in lifting of the barrel the barrel is yanked sideway to free it from the pilot.

I assume that you would not do any of those actions, but if you are crowing with the barrel muzzle near vertical, and pull out the tool to check progress, chip must be carefully cleared from the tool flute, pilot and some depth into the barrel.  Else the chances are that a few (work hardened) chips lurk in or on the barrel or tool, to scratch the land tops.  Those scratches then act like file teeth to the pellet or slugs as it travels by and lead up the barrel.

If you must use a crowning tool, then a non-rotating pilot with a locking collet seems like the way to go.  The videos below demonstrate that:





https://www.brownells.com/tools-cleaning/bore-barrel-tools/reamers-cutters/precision-reamers-muzzle-crown-refacing-kit/?sku=513000038


Now, this fancy crowning tool is frightfully expensive and only justified if you crown many barrels.  So my thinking goes the other way:    Face the barrel muzzle flat, using a sharp acute cutting tool to cut from the bore to the barrel OD.  This is to minimize the burr created that would protrude into the bore.  Then use a round headed brass screw of the right diameter for the caliber to produce the actual crown, using a hand drill to drive the screw, with lapping compound, and an orbital motion to even out the lapping action.   Apply the lap with the barrel rotating in the lathe at minimum speed in case the barrel OD and bore are not very concentric.

Dead simple and no worries about concentricity of the crown to the bore.  No worries about defacing the lands.  The barrel OD edge can be broken in the lathe at 45 degrees, and or rounded with a hand file.

If you cut a crown on a lathe at and angle other than flat faced, then you need to dial out any cocenticity error between barrel OD and bore first.  Else you crown will be concentric with the barrel OD, and eccentric to the bore.

Keep it simple. Do not use dodgy tools just because you have them.

The images below are of a .25 Marauder muzzle showing a ding at 12 O-clock, as received from the factory.  Then after using a round brass headed screw and lapping compound to recrown as described above:

Screw used:  https://www.mcmaster.com/92453A535/








Now, I did go much deeper with my spherical crown than I am suggesting for you, after facing the barrel muzzle flat.  It did it because there were other dings in the muzzle that I wanted gone.  Functionall, you only need to clean up the lands and grooves, pluss a little perimeter around them.  Less, if your barrel is shrouded and not prone to being bumped or damaged.  More, if your barrel is bare muzzle and unshrouded, for some forgiveness, if the muzzle is bumped accidentally.

The degree of dishing depends on the size of the rounded screw head relative to caliber.  The larger the screw head diameter, the flatter the crown.  Here, I do not want the edge break at the lands to be "deeper" than 45 degrees.   The exact angle cam be determined easily in CAD.
 




To answer you question about how much runout is acceptable on a piloted crowning tool; I would be unhappy with anything more than 0.001" TIR.

[Motorhead]

I will NEVER use a piloted crowning tool !!!!

Make final lathe pass from Bore outward & touch remaining edge with a Cratex point under rotation both directions & done !!

[Motorhead]

Here a just done crown via lathe and cratex point.

[subscriber]

Scott,

I prefer a crowning method that does not contact the land tops near the muzzle, such as the round headed brass screw lap.   Cratex bits would be OK in my opinion, if they you do not allow them to enter the bore.  These bits are compliant, so holding your Dremel almost perpendicular to the bore would deburr the lands and grooves right at the muzzle, without risking the rubber bit entering the barrel - if you don't push too hard.  My evidence that the latter occurred with your method are the shiny land tops - image closeup below.

Even if cutting through the blueing has removed a tiny bit of steel, the risk of cutting a bell shape at the muzzle, and reducing pellet head support before the pellet is free from the muzzle, is greater than if the tool, bit, or brass screw cannot enter into the muzzle beyond the corner where the bore meets the muzzle face.

A good counter argument would be that your barrels win competitions and that no harm is done.  The next guy to push a Cratex bit into a barrel muzzle may push a bit harder and deeper, and think that more is better.   None of that is possible, if you keep any kind of compliant abrasive bit away from the muzzle.  The reason piloted crown cutters are "bad" is the potential for damaging the land tops.  My objection to Crated bit to do crowns is the same as for piloted crowning tools.

Cratex bits are great for smoothing chambers and removing abrupt steps.  Running such a bit into the lands at the breech end just makes for a conical lead; which is what you want, providing the taper does not continue so wide and so deep that the projectile is unsupported, and end up angular to the bore after loading.

[Methuselah]

Very helpful insight!

I didn't say, but I don't own a lathe or currently have local access to anyone that does.

My "takeaway" other than what components should be considered out of tolerance, is not to use my new tooling unless the barrel is in obvious need of recrowning (accuracy and physical condition), pilot fit is critical, and cutting is best accomplished with the muzzle pointed down?

Up to now I've had very good results with a drill and valve lapping paste. The only times I've recrowned has been where there is obvious damage and accuracy is very poor with all the pellets I have available.

After reading this great discussion, I can see it would be "unwise" to offer any recrowning services, paid or otherwise, based on using this tooling.

One further question. I've acquired 90 and 11 degree cutters, should I also purchase 45?

Of the three I've already recrowned, only one had damages farther into the barrel than I could deal with using only a drill and stone/screw method. So the cutters are my last resort (only).

Thanks!

THANK YOU VERY MUCH!!!!!

[Motorhead]

That is mostly Lighting error .. the barrel is a Stainless & 100% Silver color ( No bluing ) KARMA test barrel.
The Cratex point when used have been run against the belt sander making the working end @ 45*
I agree a long taper that enters bore would be questionable.

As to how well they shoot and winning championships .... yes I have personally won several national titles in FT, and over a dozen State FT championships with barrels personally prepped as shown.

So YES I get it and would not share it if the technique was not proven to work well.

PS, that is also a 400 grit Clover lapped bore and waxed ... never shot or mopped post waxing.  So bore is not an honest texture in picture to what it looks like once cleaned up. 


Another note to make is ... some barrels has DEEP rifling ( Differential between Land & Groove ) that require very substantial crowning bevels to clean up the groove depth, similar to one you had shared & different from picture I shared which is VERY shallow rifling requiring a much milder bevel to clean up.

Just FYI ...

[Methuselah]

Brownells issued me a return shipping label for the 11 degree cutter and .177 pilot. No idea what they're going to do with them, but now I have to make an unplanned trip to the post office...

Again, think I need a 45 degree cutter, or can 11 be used most of the time?

[Motorhead]

For an Air Rifle ... the angle is not that important, symmetry is however.

[Methuselah]

For an Air Rifle ... the angle is not that important, symmetry is however.

Thanks, appreciate all this great info!

[subscriber]

Steve,

Your questions should be considered in context.  If you are recrowning only airgun barrels, many of them may be as soft as mild steel.  The softer the metal, the more of a burr tends to be raised on the trailing edges of the rifling lands.  With firearm barrels, the steel tends to be harder, so the burrs are smaller.  The remnant burss are "shot off" in practise by more substantial bullets than airgun pellets or slugs.

Where I am going is that while the Brownells cutters are great for setting up the general shape of the crown on an airgun barrel, the actual functional crown is best produced at around 45 degrees as a small edge break, using a rounded brass screw head as am orbital lap.  You could do that with a flat crown, 11 degree and 45 degree.  However, I think that if looks are important 45 degrees might look odd with a polished section at the bore edge.

What both the 11 and 45 degree crowns offer is protection of the bore edge from damage, with the 45 degree crown being deeper than the 11, if both are done to the extent where the OD of the crown almost meets the OD of the barrel.

The flat cutter is more forgiving, if the barrel OD and bore are obviously eccentric.  Then you break the edge at the barrel OD with a file, while rolling the barrel on a flat table.

Cutting the crown upside down would be best for not allowing chips to fall into the bore, but the cutter would be trying to fall out all the time, making the job harder.  Having the barrel horizontal might be better, except for the risk that weight of the cutter is trying to lean the cutter on the pilot at an angle.  Hence you see most people use those cutters with the barrel muzzle up. 

Using grease for cutting lube might keep chip from falling in.  Whatever method is used you need to be aware of where the chips go, when you remove the cutter to inspect the crown, before adding a few more revs to clean it up.   That is when chips can get on the pilot, to scarr the land tops as you cut a little deeper.  In other words, you should clean the bore, cutter flutes and pilot before cutting some more.

I stuff a bit of oil soaked tissue paper in the muzzle end of the bore about an inch below where the crowning action (or pilot in your case) would operate.  Then use compressed air to blow the "patch" out, so it exits from the muzzle; before running more oily and dry tissue or a patch through.  Before inserting the pilot again, I want to have seen a chip free bore and cutting face.  Then add another oily tissue plug before cutting further.

[Methuselah]

@subscriber

Makes perfect sense after a few reads 

I'd been doing some by hand and have been stuffing the end to keep debris out, but not compressed air to chase.

Very good synopsis on doing it safer. All I've worked so far were obviously damaged where I had nothing to loose trying...

I think experience plays in pretty heavily before recrowning a higher quality barrel on a high-priced gun, at least with any tooling setup short of a lathe 

[subscriber]

Yes, Steve; if it isn't broken, it does not need fixing.  If it is costly, be more careful. 

How broken is your barrel and what simple means can be used to improve it:
https://www.gatewaytoairguns.org/GTA/index.php?topic=130555.0


I like your signature line; except the guy in my mirror seems ten years older than me...

[Methuselah]

Yes, Steve; if it isn't broken, it does not need fixing.  If it is costly, be more careful. 

How broken is your barrel and what simple means can be used to improve it:
https://www.gatewaytoairguns.org/GTA/index.php?topic=130555.0


I like your signature line; except the guy in my mirror seems ten years older than me...

wow, that's a nice write up!

I seems to be saying most or all barrels can be improved. I'm thinking more in line with Malcolm's philosophy in Jerasic Park though. Same as you I reckon  ... I  ain't fixing it if it ain't broke.

It IS  a great write up. I plan to re-read it.

The mirror,  yeah 

[JPSAXNC]

Hi Steve, If you don't have lathe, tool runout is probably not an issue because trying get an exact aliment holding the parts in a jig and rotating the tool by hand. Is not worth worrying about runout. I would use the brass screw idea and be done with it. HTH

[rkr]

I like piloted crowning tools, just use brass pilots and keep barrel vertical so chips won't get in to barrel. I make my own pilots so they are perfect fit to barrel. Results have been good, my longer range guns shoot well under half MOA at 100M. The best combo for me is to do the barrel at 90deg in a lathe and then finish the rifling and half a mil outside with 11 deg tool.