Dynamic Balance of Bullets

[ThunderDownUnder]

I have been looking to make a device to test and measure the dynamic balance of the projectiles I use in F-Open.

I got interested in the effects after shooting 230gr, 30cal, Berger Hybrids in my 300WM. I could shoot consecutive X rings then have a bullet drop/move by a couple of MOA. Initially I thought maybe it was down to the shooter/wind, but I found others shooting this bullet experienced the same thing.

I wondered if the unexplained flyers were perhaps due to the bullets being unbalanced and the increase in yaw that it would bring about?

After experimenting for some time with methods of holding and spinning the bullets up to a speed suitable for test purposes I think I have made a device that can do the job.

I can spin the bullets up to 30,000rpm and use an accelerometer (small sensor) on a low mass aluminium plate which is secured to soft rubber vibration mounts. Using vibration analysis on my notebook pc to test for dynamic imbalance of the bullets.

I have only just come up with the solution so as yet no bullet data to present. I am interested in hearing from other forum members who have also pursued this line of inquiring about dynamic balance.

Anyone else made a machine for testing bullet balance?

Give me some time and I will provide some data on what I find out with my testing.

 

[ThunderDownUnder]

Just out of interest does anyone know if Berger or any other of the bullet manufacturers test the bullets for dynamic balance as part of their quality testing of their product?

Here is a link to a USA made test machine and if you scroll slightly down the page you will see a photo of a bullet undergoing balance testing.

http://www.balancemaster.com/

I tried this method of spinning bullets, (on two sets of bearings) and could not get it to work anywhere near well enough. The bullet boat tail and nose are both tapered and the bullet would move back and forth between the bearing supports.

The price of these machines is in the 10's of thousands of dollars!

 

[Titan]

I tried to build a copy of Harold Vaughn's static balance as detailed in his book Rifle Accuracy Facts in order to measure Center of Gravity offset of a bullet. While this isn't a dynamic balance it was easier to build with the material and tools I had available and I thought it would provide a similar result. It was an interesting experiment but the time needed to take measurements (6 per bullet) (approximately 2 minutes a bullet) and the observed effect on target made it seem a bit impractical for processing the number of bullet I would need for F-Class. I am interested in your solution if it provides a quicker way to take measurement of CG offset.

I posted some of my limited testing and results here post number 18. http://forum.accurateshooter.com/th...ity-offset-in-a-bullet.3949487/#post-37217418

 

[sdean]

Isn't that what a Junke machine does?

 

[geo.ulrich]

Isn't that what a Junke machine does?

NO

 

[dcali]

Isn't that what a Junke machine does?

No, the eddy current machines attempt to nondestructively measure the thickness of the jacket. That’s related to balance, but not the same thing. I’ve never used one, but have worked with them in industrial uses. And while not something I worked closely with, I do remember that they seemed a bit like black magic, and may or may not have required killing a goat. It’s been a while, though. Maybe they’re better now.

 

[dcali]

I have been looking to make a device to test and measure the dynamic balance of the projectiles I use in F-Open.

I got interested in the effects after shooting 230gr, 30cal, Berger Hybrids in my 300WM. I could shoot consecutive X rings then have a bullet drop/move by a couple of MOA. Initially I thought maybe it was down to the shooter/wind, but I found others shooting this bullet experienced the same thing.

I wondered if the unexplained flyers were perhaps due to the bullets being unbalanced and the increase in yaw that it would bring about?

After experimenting for some time with methods of holding and spinning the bullets up to a speed suitable for test purposes I think I have made a device that can do the job.

I can spin the bullets up to 30,000rpm and use an accelerometer (small sensor) on a low mass aluminium plate which is secured to soft rubber vibration mounts. Using vibration analysis on my notebook pc to test for dynamic imbalance of the bullets.

I have only just come up with the solution so as yet no bullet data to present. I am interested in hearing from other forum members who have also pursued this line of inquiring about dynamic balance.

Anyone else made a machine for testing bullet balance?

Give me some time and I will provide some data on what I find out with my testing.

Does it work? I would be concerned that the rotating inertia of the mechanism would cover up the signal you’re looking for. I’ve thought about it, but never gotten as far as building something.

 

[ThunderDownUnder]

Thanks Titan! Had a read of your interesting thread and also downloaded the free pdf book on ballistics by McCoy.

What do I expect to find by checking the dynamic balance of bullets, (if anything)?

1. I expect to find that copper jacketed, lead cored bullets with variable meplat point diameters and finish will vary the most.

2. I also expect to find the same bullets as mentioned above but with plastic, and aluminium tips will be next most variable group.

3. I expect bullets machined from solid stock to vary the least. Providing the machining is symmetrical around the bullets axis.

I don't think I'll find a lot of badly unbalanced bullets as modern production techniques have provided very uniform and well matched bullets for me so far. The big 30 cal, 230 grainers have the most mass of any bullet I shoot and I expect the heaver larger diameter bullets will be the group of interest for most. Having said that, I have in place a method for testing .22 all the way up to 30 cal and will test a variety of calibres as used in F-Open.

I have found that other competent F-Open shooters in my area have had similar experiences to me with this bullet so I'm keen to start testing.

Please don't fill this thread with comments on shooter error and environmental conditions being the real reason for the observed flyers. It could very well be the case, but this thread is simply about testing for dynamic imbalance in bullets.

 

[geo.ulrich]

No, the eddy current machines attempt to nondestructively measure the thickness of the jacket. That’s related to balance, but not the same thing. I’ve never used one, but have worked with them in industrial uses. And while not something I worked closely with, I do remember that they seemed a bit like black magic, and may or may not have required killing a goat. It’s been a while, though. Maybe they’re better now.

Take the specific gravity for lead then for copper figure difference then calculate the wall thickness variations which are held pretty well in .0001 ths this isn't where the problems are starting from.... I might get attacked for this but I have played with the juenke machine and to be honest I don't know what it measures . you can check a good jacket and have all kinds of deviations or check a crappy jacket and get great numbers with seated cores or take a good jacket and get good numbers and a bad one and get bad numbers nothing was ever consistant.

 

[dcali]

Take the specific gravity for lead then for copper figure difference then calculate the wall thickness variations which are held pretty well in .0001 ths this isn't where the problems are starting from.... I might get attacked for this but I have played with the juenke machine and to be honest I don't know what it measures . you can check a good jacket and have all kinds of deviations or check a crappy jacket and get great numbers with seated cores or take a good jacket and get good numbers and a bad one and get bad numbers nothing was ever consistant.

That's not unlike my experience with similar systems at work years ago. What they were supposed to do and what they actually did only sometimes lined up. It didn't give me a lot of confidence, but who knows.

 

[normmatzen]

I have looked into the Juenke machine and found that the theory of operation is valid but the implementation is old school.
What this machine does is effectively measure the thickness of the bullet cladding and also if there is any internal voids between the cladding and the lead core. This is done by selecting a frequency of operation that will be sensitive to the brass and to a lesser extent to lead. The original Juenke device was built using some really old transistor circuits and had to be very carefully adjusted to be sensitive to the desired perturbations within the bullet. Carefully set up and carefully used, one could pick out those bullets with thickness abnormalities and air bubbles between cladding and core. Due to the difficulty in setting up the Juenke machine they were VERY expensive as are the ones made today copying the original Juenke device.
I am aware of some folks trying to build a similar machine with digital circuits. Most of these efforts ignore the simplicity of using an LC oscillator where the inductor couples signal into the bullet and measures the drop in amplitude of the oscillator. If the oscillator inductor has some signal induced into the bullet, this difference can be easily measured by modern analog circuitry to a great accuracy. Most of the digital designs use a fixed oscillator which is isolated from the bullet load and the induced signal is measured to detect decreases. The problem is, the directly driven analog oscillator is way more sensitive to perturbations in the inductive losses. These losses show differences in oscillator frequency, amplitude and distortion
which are way easier to measure than the very small amplitude drop with an inductive sensor driven by a low impedance output oscillator in the digital versions.
I have done some preliminary designs of a device to do this function, but building things are not as much fun as when I was younger!
I believe that the use of modern high gain integrated amplifiers would make this design easy to build and to use.
I think there are a few folks using Juenge machines that have found a lot of success in grading out duff bullets!

 

[ThunderDownUnder]

Here are a couple of videos of initial bullet spin testing.

I have completely changed the method for holding the bullets under test. Tried all sorts of bearing sizes and types but none worked well enough. These videos are simply of earlier attempts where I tried the method used by the commercial machine as used in the link posted previously,

It became apparent to me that the electronics for the vibration analysis was going to be easier than spinning the bullets up to 30k rpm or above. So initially I concentrated on a mechanism for spinning the bullets!

 

[ThunderDownUnder]

Here is an example of a machine used for testing dynamic balance for engineering students. Illustrates the use of soft mounts on the test bed, hard mounts also work.

It clearly shows that if the item under test is statically balanced that it does not necessarily mean it is also dynamically balanced.

Here is another interesting video which uses modified low velocity 22 bullets to illustrate imbalance. Its amazing to watch that one bullet with symmetrical cut outs actually flies pretty straight. I'm sure there is a lot of other forces going on with these modded bullets but its still an interesting vid.

 

[geo.ulrich]

I have looked into the Juenke machine and found that the theory of operation is valid but the implementation is old school.
What this machine does is effectively measure the thickness of the bullet cladding and also if there is any internal voids between the cladding and the lead core. This is done by selecting a frequency of operation that will be sensitive to the brass and to a lesser extent to lead. The original Juenke device was built using some really old transistor circuits and had to be very carefully adjusted to be sensitive to the desired perturbations within the bullet. Carefully set up and carefully used, one could pick out those bullets with thickness abnormalities and air bubbles between cladding and core.
I think there are a few folks using Juenge machines that have found a lot of success in grading out duff bullets!

Well the theory might be valid , but it doesn't work. you might be better served to actually try making bullets and testing with the juenke before giving results of tests that were not preformed. I have taken jackets checked on a mandrel for t.i.r then seated cores with and without oil and varied core seating pressures then checked on a juenke. then pointed up and rechecked NOTHING is repeatable. as I said before if you think you are finding voids you are mistaken. you can get great numbers with BAD jackets and bad numbers with great jackets. Or even better is a test shooting them had bullets with 15 + deviation shoot great and ones with 1-2 deviation shoot horrible. so tell me what its showing. if you want to check for core adhesion your better off cutting jacket off and looking at core seating process. If your testing factory bullets with a juenke well I'll leave that one alone....

 

[ThunderDownUnder]

I repaired a Juenke machine for a member of the Aus F-Open team a few years back. He asked me for my thoughts on it once I had it working again. I said it looked like a cheap basic electronics kit from back in the 1970s and really would not want to use it to sort bullets!

Testing the dynamic balance of bullets may not have the voodoo attraction of the Juenke machine, but it certainly has the science to support it.

Using an accelerometer and vibration analysis will provide the detail needed to know how the bullet is balanced internally and by measuring and weighing the external metrics of the bullet, the complete picture of how uniform the bullet is constructed will be known.

Most do the measure and weigh for bullets to be used in important competitions but I dont know of any F-Open shooter checking the dynamic balance.

Here is a short guide on dynamic and static balance without maths thats worth a read for those that are interested.

https://reliabilityweb.com/articles..._basics_of_balancing_and_measuring_techniques

 

[dgeesaman]

Most do the measure and weigh for bullets to be used in important competitions but I dont know of any F-Open shooter checking the dynamic balance.

Here is a short guide on dynamic and static balance without maths thats worth a read for those that are interested.

https://reliabilityweb.com/articles..._basics_of_balancing_and_measuring_techniques

I assume you're after two-plane imbalance measurement when you say dynamic?

 

[M995]

Here is an example of a machine used for testing dynamic balance for engineering students. Illustrates the use of soft mounts on the test bed, hard mounts also work.

It clearly shows that if the item under test is statically balanced that it does not necessarily mean it is also dynamically balanced.

Here is another interesting video which uses modified low velocity 22 bullets to illustrate imbalance. Its amazing to watch that one bullet with symmetrical cut outs actually flies pretty straight. I'm sure there is a lot of other forces going on with these modded bullets but its still an interesting vid.

The bottom video shown is very impressive & clears up a lot of thought. Now imagine the bullet spinning 220k RPM's

 

[ThunderDownUnder]

I assume you're after two-plane imbalance measurement when you say dynamic?

Not necessarily, I'm thinking single plane should do the trick. Bullets are short (under 100mm) and the length to width ratio would lead me to believe single plane should work ok.

I dont need to locate the imbalance to correct it on the bullet, (waste of time, probably impossible) I just need to find the magnitude of the imbalance of the bullets under test. If there is an appreciable difference between bullets leading to suspect an imbalance, then I start gathering data on boxes of bullets and see where it leads.

I'm still waiting on some parts to make a serious prototype but not too far off carrying out some testing. I am just trying to find out if anyone else has done this sort of thing before, while waiting for the parts to turn up.

I'm not expecting to find a large number of bullets out of balance because my shooting results on paper tell me its a rare thing. The 230s though have a higher incidence of flyers amongst shooters I know, to the extent they have stopped using them and now use the 215s instead.

Its just another thing to look into and I'm curious to see what I find. I can build the device without too much hassle and I'm enjoying the challenge. If dynamic balance testing uncovers a problem with bullets then we have another device up our sleeves to weed out inaccurate bullets. Its just measuring another bullet metric that most shooters dont and cant perform. There is no data to suggest its needed or necessary, so its all just conjecture at this stage.

 

[dgeesaman]

Not necessarily, I'm thinking single plane should do the trick. Bullets are short (under 100mm) and the length to width ratio would lead me to believe single plane should work ok.

Admittedly I've only begun to think about this topic, but I assumed that the wobble you referred to comes from two-plane imbalance, not simple single plane imbalance.

That said, the chances of a bullet having two flaws creating equal and opposite imbalance seem impossibly low. So screening based on single plane imbalance certainly should help.

 

[geo.ulrich]

After rereading the initial post I have some questions. you say bullets are dropping or dispersing a couple of m.o.a. and other shooters are experiencing the same issues are the shots mostly low. If it was a dynamic balance issue bullets would disperse in all directions not in one direction mostly. As I said before if your using factory bullets well your getting what you get. I do find it interesting all the discussions on trapped air in bullets. I have tried to get air trapped and if core seating punch is correct I have not been able to do it, even with cores that where so oversize they would barely start in jacket. after core seating I cut down two sides of jacket and peeled it like a banana and never found anything but good adhesion.