Hacking The Benjamin Titan NP Break Barrel

[George Schmermund]

Now that an effort to improve scope mounting methods seems reasonable (to me) I've decided to do some testing. The photo shows a single degree of freedom optical rail arrangement for doing a transfer function measurement on one of my stray scopes.

The plan is to attach an accelerometer to the scope's body and then use an instrumented hammer to excite the scope's response modes. The frequency and amplitude characteristics can then be determined.

This simple lash-up should provide some quick information to use as guidelines for vibration isolation and damping experiments.

[WhatUPSbox?]

I don't think creating compliance is an issue. It is interesting how the two commercial mounts chose to provide very different ranges of motion as seen in the two videos.


I have not found a parts diagram of the DM60 but post 14 in this thread describes it well https://www.airgunforum.co.uk/community/index.php?threads/sportsmatch-dampa-dm60-question.289657/.

I think the more difficult part is maintaining line of sight alignment. If you are mounting a scope with 1/4 MOA adjustments, you probably want the mount to be that or better, about a thou over 4".  I'm not sure how you measure drift of that scale on a springer.

[George Schmermund]

Stan - I'm not sure how compliance (1/stiffness) plays a roll in these transfer function measurements. Could you elaborate some on it's applicability in the proposed experiments?

[WhatUPSbox?]

The compliance observation and the reference to the two commercial designs was related to your mention of the scope mounts brought up by Marcos in the previous post, not your planned experiment.

[George Schmermund]

Thanks for the clarification. I agree that compliance itself isn't a major issue with the mounts shown in the videos. But both do show their foibles when the videos are slowed down to .25 speed. Repeated views of just the shot cycles demonstrate the amount of flexure in the scopes due to resonances. The double trigger pull in the Dampa one is interesting. Also, the piece of masking tape in the DM60 demo was less than quantitative.

A mount's return to mechanical "zero" after a shot cycle could be checked using fixed blocks on the barrel's scope rail and some feeler gauges. That would be pretty easy and there would be no moving parts to get out of adjustment between shots. The measurement could get much more advances than that, but not simpler.

Both mounts are attempts at addressing the recoil issue, but neither seem to do anything about the stored energy in resonances. A scope that won't hold zero is no better than one that's broken.

 

 

[WhatUPSbox?]

I did some rainy day fiddling with the instrumented 2240 that I showed in posts 176 and 178. I drilled a hole in the power adjustor for a wire probe to reach the back of the hammer (first image). This acts as a simple switch when the hammer is released and the shot cycle starts. The laser trap still marks the pellet exit. Image two shows a trace with all of that information captured. Channel 4 (blue) and Channel 1 (yellow) are the hammer start and pellet exit. Ch 2 (green) and 3 (purple) are the vertical and horizontal accelerometers. From hammer start to pellet exit is 17.4 ms and the hammer flight takes up most of that. The trace also show what appears to be multiple hammer bounces.

This test was with a long soft hammer spring (third image) and the power adjustor moved out (stock dimensions shown in the picture) to provide about 350 ft/s for target shooting. The longer spring is 50 mm Vs. the stock 33.8 and for me the stiffness measured out at .84 N/mm Vs the stock 3.0 N/mm (sorry about the units) The plan is to  get measurements using each spring with the power adjustor set to equivalent muzzle speed.

The hope is that the accel traces can be manipulated in Excel to get muzzle vertical speed and potentially motion. The fourth image shows the accel trace and the vertical velocity and displacement of the muzzle. The fifth image shows a comparison of two different shots. One shot's sample started earlier so it affects the integrals and generates the offset. These are just first examples, the process needs to get checked out.

The flight time for a 10m target shot is .09 to .1 sec so if the vertical velocity at the time the pellet leaves the muzzle is 6 cm/s, that is 6mm on target. Of course, these were loosely held shots into a table top trap, and what matters is the shot to shot variability (like any hold). But to me it is interesting enough to pursue to see if this is a useful tuning parameter for pistols. I'm already sure it is a primary suspect (scapegoat) for missed shots.   

[George Schmermund]

The distraction of having too many projects going on at the same time has taken its toll on the break barrel measurements. The force transducer has finally come to life and has produces some results that I've been staring at for several days now.

There were a couple of questions early on about when the shot cycle actually starts. At the time I only had the DSO data from an accelerator and force transducer that were attached to a proxy scope arrangement. The DSO's trigger was set to capture the force and vibration signals as an attached scope might experience them. Some rearrangements with the sensors was tried and the data collected in various assortments. None represented the full shot cycle from start to finish.

This new batch of tests utilizes the custom force transducer (FT for brevity) that is now installed inside the power plant and directly behind the cylinder body of the air spring. The DSO images show the signal from the FT on CH1 and the accel attached to the scope proxy on CH3. The vertical green line is CH2 which is the muzzle trip wire signal for the pellet exit timing. The real numbers are for the horizontal timing. The magnitude values are arbitrary. The trigger timing is the carrot at the top left side of the screen and set to CH1.

The 2 DSO images are of the same trace data with and w/o a 10 kHz low pass filter. They're separated some for clarity. The polarity of each signal is correct.

[WhatUPSbox?]

Thanks for posting an update.

The force transducer provides an interesting trace. Just to understand it better, is the transducer a transient device where, even under the starting preload, it provides a zero output? In your trace for the FT, is compression positive? I was expecting the force transducer to follow the drop in the gas ram pressure with stroke, until the shock of the impact event.

[George Schmermund]

Stan - Thanks for the question. I've been trying to understand what's happening too. The FT settles to zero under static load. The preload when the rifle is cocked is ~ 200 lbs. The FT's output is positive when compressed and negative in tension. When the gas spring (GS) is fully extended it has a preload of ~ 140 lbs remaining. When the FT is installed into the receiver it is compressed slightly (?) more.

The accel's signal is positive when the force is applied up through its base. The DSO image shows the accel's response to the initial recoil as being positive as would be expected. Puzzling over several of these images (in my usual brain fog) I seem to have misrepresented the FT's polarity. It would make much more sense if the curve was inverted. The FT's signal should decrease, as you've suggested, when the GS is extending. The curve should then start increasing as the compression in the rifle's cylinder increases until the rifle's piston reaches the end of the cylinder. What happens after that is still unclear to me.

I'm going to do a few more tests today and reverse the FT polarity in the measurements and see what it looks like. I've got a dummy pressure transducer (PT) installed in the transfer port for the time being. I'm not ready to put a working PT into service yet because of the potential for dieseling to occur from the lube this soon after a rebuild. Actual pressure numbers during the shot cycle will be very useful.

[George Schmermund]

The polarity for both DSO channels should finally be right in this pair of images. I'm now using Fomblin PFPE oil for the rifle's piston/cylinder lubricant. It's an extravagance, but only takes a few drops and seems to be doing a good job so far. I didn't want to take a chance on any dieseling destroying the transfer port's pressure transducer.

The DSO's CH2 is now the accel and the scale is 100g's/V. CH3 is now the exit trip wire signal. Installing the FT into the rear end of the power plant seems to have tightened up a few thing's that might have been banging around in the earlier tests. The event timing seems to be about the same.

When the pressure measurements are added to the mix the full shot cycle dynamics record should add much more clarity to future interpretations. I'm counting on Stan to continue getting me back on course when necessary.   

[WhatUPSbox?]

Nice, the FT provides a clean trace. Is that the raw output of the piezo? It does not have some of the noise of the accel. I'm not sure I fully understand the initial drop but the roll offs prior to the two impacts make sense to me.

To be honest, I'm still trying to figure out the vertical recoil traces I posted earlier.

Great work George.

[George Schmermund]

Stan - your green trace appears to be a relatively hi Q ringing of a spring-mass system, obviously. You could expand the time base and measure the frequency in the largest envelope. Then put the gun on a foam pad and start exploring with a small tapping hammer. Something would probably give you a strong hint as to the source of the vibration. No need to actually fire the gun to get good signals, though dry firing it would be interesting.

How are you doing your 10M target timing?

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Both of my previous DSO images are with a 10 kHz filter in the line. Above that frequency it just gets unintelligibly noisy. The FT signal is cleaner because of it's rigid construction assembly using bolts instead of a pin. That end of the receiver is also bolted tightly to the stock which adds a lot of damping.

[WhatUPSbox?]

Yes, I do need to do some tap tests to explore that ringing.

The 10M timing is just a rough estimate based on muzzle velocity. It is meant to see what the ballpark effect of vertical muzzle recoil might be.

I've been trying to figure out how to get an independent measurement of the muzzle recoil vertical velocity for the pistol to compare with the integral of the accel data. I tried the geophone but I'm not sure I like that data. I looked at the specs of the audio exciter that Michael posted, but it looks like it has a resonance at 1.2 kHz.

[George Schmermund]

It might be worthwhile to reconsider the humble phono pickup cartridge. a simple preamp w/o the RIAA compensation is probably a 1 chip solution. The stylus tip has very low mass and you could 3D print a holder for making measurements w/o having to mount a fixed transducer to the DUT. Another advantage is its extended frequency response. You could even get a complete tonearm assembly for cheap on eBay and modify it to due your bidding.

[WhatUPSbox?]

If I understand the phono stylus/cartridge, I still need a suspended seismic mass to measure. I would like to get to where I can hold the pistol as normally fired on target and be able to record the pellet exit and muzzle vertical motion (a or V). I have a small portable 2 ch DSO that will let me do this away from the benchtop.

[George Schmermund]

I forgot that anyone shoots pellet guns anywhere besides clamped to a test bench.

[George Schmermund]

Here's a set of 4 DSO images of the same shot. I've misplaced a part for the pressure calibrator, so the actual value for Ch4 is guess work at this point. I'm sure that it's over 1,000 psi. Its double peak is a mystery at the moment. The pellet's exit time interval (~2ms) after the first force peak (Ch1) is what I use as a check value in the approximate overall shot-cycle timing. The DSO's timebase for the images is from 5ms/div down to 500µs/div.

This will all take some sorting out, but a guy needs something to do during a pandemic.

[mikeyb]

I expect your pressure peak may be over 2000 psi once you have confirmed a calibration reference.

I've captured pellets from PCP rifles charged to 2000 psi and found solid rifling marks but minimal to no skirt expansion. Captured pellets from a similar FPE-at-muzzle break-barrel rifle had more defined rifling and severely "ballooned" pellet skirts. That suggests to me that the peak psi experienced by those break-barrel pellets is well over 2000 psi.

While it could be something else, I think the small double peak in the pressure trace can be explained by the pellet finally "popping" the lead and just starting its acceleration down the bore.

Thanks for keeping the information flowing. Wish I had the time to participate.

[WhatUPSbox?]

George, great job getting the pressure sensor online.

The double peak is interesting. Going back to post 150, the pellet started moving at about 500 micro-sec before the accel peak. Also if I remember correctly from your CP measurements it took about 200-250 psi to get the pellet moving in that case. The double peak looks like it is in between events. I wonder if a piston seal leak would look like that. The other thing that is happening at that point is the temperature spike.

The pressure curve also shows very little pressure in the breach for the last milli-sec that the pellet is in the barrel. Some of this is because of the piston retracting but I wonder how this drop-off propagates down the barrel in this timeframe.

Thanks again.

[George Schmermund]

I've ordered another part for the pressure calibrator. It will probably be here before I manage to find the one that's missing. As it stands now the pressure curve is probably close to good up to ~ 1,000 psi. Each vertical division on Ch4 should be close to 200 psi. Stan's estimate of the pellet's release time and pressure is probably reasonable. Above 1,000 psi the transducer's electronics are most likely going to be nonlinear and may be the cause of the ragged double beaks. Stan's pointing out the possibility of thermal spiking also has merit.