All opinions welcome. I am thinking of making a new piston for testing in my "industry sp2" .22 pistol my line of thinking is a lighter piston might accelerate faster and work smoother than the crude cast iron one that comes stock. I am also thinking that this would reduce the kick/bounce/??? of the springer action.
So a few questions: I have a small taig lathe and a taig CNC milling machine and a supply of T6061 aluminum stock & some 12L14 free machining steel. The 12L14 steel can not be hardened but should stand up to the trigger sear for a while and I can always make another replacement part later or make several on the first round.
Probably going to stick to the stock leather seals, I suppose I could find a 1" seal for a brake cylinder and use that but it's probably not worth it in an imperfect cylinder.
Will make the new cylinder walls thicker so there is room for counter bored socket head cap screws to secure the trigger sear part and just a bit of clearance above the spring diameter.
I could probably make an entire new breach out of aluminum but then would have to redesign the entire thing, trigger etc
So your thoughts on this?
Hey it's a hobby so cost is not the object here I know if I add up my time/machines/design I am taking a $20 pistol into several hundred dollar range but I am having fun (just wish it was fun plinking rather than thinking about this)
Since it holds the force of the spring. That is why it is a one-piece casting, but I am not sure if you were thinking to make them separate. Certainly there is a LOT of shear area in the factory piece that could not be easily replaced with fasteners. Let me know if I misunderstood what you were saying.
If you make the sear out of steel, then case harden it with something like this -http://www.brownells.com/.aspx/pid=7626/Product/KASENIT - and figure out a way to attach it sufficiently to an aluminum piston, that would be the lightweight setup! Let us know how it goes!
I would probably mill a slot/flat on the bottom of the piston to take the steel sear part, thus eliminating the shear factor. I certainly would not depend on 2 or 3 6-32 SHCS's to hold it. Possibly even use roll pins perpendicular to the sear to fasten it as I think there would be enough "meat" to inset and drill through if I keep the ID of the piston just enough to keep the spring from rubbing.
Kasinet is a good product, used to have some years back... heat the part dull red and plunge into the powder as I recall and it did case harden the first few thousanth's of steel (my older brother is a flintlock/percussion colector but not a machinest so I used to help him out)
So do you have any feel if a lighter piston/sear assembily would increase the avalable power? My gut feeling is less dead mass to move = more velocity avalable at the breach but I could be all wrong, math is not my strong point.
I asked in another thread about presure developed in a springer, like I said my math skills s***, but thinking a 1" piston area = 0.78535sq/in and if the stroke is say 1.5 it would be 4.7123889803850005 cubic inches so I would give or take at 1 atmosphere = 14.696 x 4.7123889803850005 = 69.247552 PSIG. Again no idea if this is a valid theory or close.
So now another question would be is it pressure or volume that affects the final velocity of a pellet?
Taking the previous example of the volume of the cylinder 4.7 cu in and applying it to the volume of the pellet moving 1.5" with a pellet volume of 0.147 cu in 4.7/.147= 31.9 as the ratio of the volume. Seems like the pellet would be long gone from the muzzle before all that volume of air was discharged.
A spring-powered airgun is basically a diesel engine. To increase power in spring-powered airguns, the best way isn't to make piston speeds faster, it's to increase bore size or stroke length or both. The simplest way to increase stroke is to replace the stock leather seal with a thinner one. If the chamber is the standard 25mm/1" Chinese chamber size, you can us the much thinner and higher quality Apex seal from Maccari for the TX-200. Last time I checked they were $9 and all you'd have to do is turn an dovetail adapter that would screw on the stock piston. Since you have lathe, you could do some math and see exactly how much you could shorten the piston with the new seal to maximize the piston stroke.
To soften the recoil on most Chinese spring guns, the best improvement would be to make the transfer port about .125" inches in diameter. This seems to be the optimum diameter for 25mm/1" bores. Most the Chinese transfer ports are over .130" and aren't very smoothly finished. If you don't have a copy Cardew's book, Trigger to Target, you should get a copy. It's a great read for the novice airgun tuner. After all that work on the piston and cylinder you may actually need a little more spring preload and more piston weight, Many of the UK export guns have spring piston weight or tophat to increase their power over the 12FPE energy level. I can see with good tuning, one of the .22 Chinese pistols pushing .22 RWS hobbies at 475-500 fps. That would make it a dandy 20-25 yard squirrel and bunny buster with a homemade wire stock.
see my above post about cylinder volume etc and see if I am on the right track, seems the port is larger than 5/32" or .160 or larger! Yikes it's way big.
Think the "best way" is to machine out the port and make an insert to get to .120-.125 dia? I can probably make something like a carb jet thing and thread it in and it would snug up the breach/barrel connection.
I have been playing with one of these and have some SS tubing to reduce the diameter to 3mm, seems to add 50fps to a stock piston and seal...didn't try to replace the seal but will also give that atry when I get the time...the Apex does fit, got that far but ran out of play time...
By reducing the transfer port to .125" your going to decrease port volume by 28%! That will increase your peak pressure by quite a bit. It takes quite a bit pressure to properly obdurate the skirt into the grooves of the rifling. Better skirt sealing, better volumetric efficiency. Your idea of the carb style jet sounds practical. The chamber side should reamed/chamfered for smooth airflow. The barrel side should be center aligned with the bore centerline of the barrel. You'll want as perfect a seal on the breach as possible. Last but not least is the muzzle's crown should be very closely examined for imperfections. I mentioned the RWS Hobbys for a reason. Even with all these improvements, you still won't have a lot of volume to create power. The Hobby's are light will probably make better power than any other pellet in these springers.
Looking a the breach plug I see at least .05" that are extending into the chamber, possibly as much as .10". In a short stroke pistol like the Super, that's a substantial amount of potential volume. Nice job mounting the new piston seal. If there are no threads extending into the chamber past the where the threads on the plug end, I'd shave that plug extension to maybe .002 past the threads and put fine cross hatch finish on the breach plug and cylinder walls. This also shortens the transfer port volume. Smaller port volume equals higher compression which equals higher peak pressure. The stock piston looks like a heavy stout piece. I bet taking off a few grams of weight by turning it the middle would likely also be worth some smoothness especially with .177 pellets. I've heard of tuned version of the original gun it was cloned from the RWS 5, shooting light pellets in the 600fps range. I think that 525-550 with 7-8.0 grain pellets is very achievable.
Looks like shaving off .05" would add about 0.157cu in. volume with my example in the above of 4.7cu in would be 3% gain. Looking at Tunajussi's photos was interesting to see how the breach and cylinder are asssembled, what keeps them from unscrewing thread locking compound? Guess if that's the case the cylinder and breach should be heated before trying to unscrew it. Certainly makes it easier to work on the transfer port.
Tunajussi, just curious why you detached the trigger frame from the cylinder and how you plan to reattach it?
Carlos, still trying to get my brain around the transfer port thing. I can see where reducing the diameter would restrict the air flow and increase the velocity going through it but I think pressure would remain the same as that would be a function of the spring thrusting the piston forward. With a smaller dia port the piston at least in theory should move slower resulting in less impact at the end of it's stroke. Yesterday I put some layout dye on several pellets and test fired into a 3 foot 2" dia pvc pipe full of water, my digital camera doesn't do that magnification for close ups but I can clearly see the rifiling marks in the top and skirt. Don't know exactly how deep these should be but they look distinct under a magnifying glass.
My reason for thinking of going through the exercise of making a new piston was only because the original ones are such a crummy fit. They rotate loosly in the slot at the bottom of the cylinder which in turn makes the trigger sear engagement random and lack any form of skirt at the back end to keep the piston aligned in the cylinder. I am going to try building up the sides with a bit of brazing rod and then finish that down for a better fit. Figure anything to take out slop will improve accuracy or at least consistancy.
Suggestions on how to plug off the old transfer port? I was thinking of drilling it out and tapping for a 1/4-20 rod then braze the end and start over.
After removing the block from the compression tube, check how far the threads extend into the compression chamber. Once determine how much of the breach face you can remove if you can at all. After shortening the breach plug, I would then tap the transfer port with an appropriate sized tap. Using a brass or aluminum screw to plug the hole with some very strong adhesive glue. Them drill a .118" hole center aligned with barrel from the barrel side of the breach plug. I would not drill completely through the plug from the barrel side, Just enough to see the bit come through the chamber side. I'd then flip the breach over and ream the hole to no more than .125 from the chamber side. Putting a very slight chamfer on the transfer port on the chamber size with a sharp edge on the barrel side. The sharp step will provide some turbulence but if you've ever studied gas flow in engines, the exhaust port should always be slightly smaller than exhaust pipe to minimize the anti-reversion wave or backflow. Air at 4000 psi is about 1/4 way to become a liquid instead of gas.
To take up piston slop, you'll want to add grooves for PTFE slipper rings or holes for buttons, This is how the tighten up the fit on AirArms TX-200 pistons.
A smooth firing stroke will produce more power with more consistent accuracy. This is easier to do with a lighter piston. A 420fps .22 pellet pistol that has smooth shooting cycle that's accurate will be a lot more fun than a harsh firing 500 fps .22 pistol that's not nearly as accurate.
After further pondering the photos of the piston, if after the chamber and piston seal changes, you still see just the piston when the gun is in the cocked position, I'd carefully shave some of the front of the piston until I see the back end of the seal at the edge of the chamber at the cocking slot. This may require you to remove some of the metal the inside curvature of cocking arm that's attached to the piston to make sure the piston is reaching the bottom of the chamber. A spring tophat can put weight back in your need it.
This message has been edited by crgintx on Jul 7, 2009 2:52 AM
seconds page first photo, original over and reworked under.
I drilled few holes on trigger frame and welded them with mig. I put new frame because I made new trigger consept, you can see sketch under new frame. Two stage, adjustable, with very light pull,,at least I hope so. Without main spring pressure it works fine so far.
Breech plug was threaded overtight, with "one more round"-method after enough, no factory glue. It's no fixed with plug weld, I'm not going to remowe it anymore.
Original transferport plugged with M8 bolt and soldered with tin.
This message has been edited by Tunajussi on Jul 7, 2009 4:07 AM
I think that piston is already too light, at least with stock spring. My piston weights 112 grams. Pushing pellet 1 mm inside to barrel increases velocity about 25 fps.
Just weighed mine that I had already added some bronze filler to to fix the side to side slop in the guide slot and it's 100 grams after all that!
Now on to the 1mm pre load, lets examine that, is it allowing the piston to get a head start to terminal velocity before building pressure? Seems you have some way to measure this stuff, so if you have time and inclination how about a 2mm and 3mm pre-load and results? Who knows perhaps all we need is a push rod device to preload the pellet a bit deeper? Kind of goes along with my pressure reliief theory to let the spring get a head start before the piston hits backpressure and much easier to do.
I believe the engine is so weak that pushing pellet to rifles is "too" hard to it. Normally that manual "pellet pushing" reduces velocity. So I think that 2 or 3 mm is bad idea.
By the way, my pistol is XS-X2. Workmanship is very good compared to pictures of Tunajussi.
In most cases, making a springer piston lighter will DECREASE power - not increase it.
July 6 2009, 4:56 PM
To understand why this is so, the first fact you need to understand is that, in a springer, it's not the mainspring that provides the force to compress the air charge. The mainspring can't - because it's not nearly powerful enough.
A perfect analogy is when you use a hammer to drive a nail. Your arm isn't strong enough to push a nail into wood directly - at least my arm isn't. Instead your arm accelerates the hammer and it's then the momentum and kinetic energy of the hammer that drives the nail. And the hammer must be heavy enough or it won't do the job efficiently.
Like driving a nail, compressing the air charge of a springer requires much more force than the mainspring can supply. All the mainspring does is accelerate the piston - which then slams into the air charge and generates the 1000s of psi required to drive the pellet down the barrel.
And, like the hammer, the piston must be heavy enough to do the job efficiently.
Not trying to be argumentative but somehow I can't see a springer reaching anywhere near that unless I am missing something in the physics. For example a .22 benjamin using CO2 claims "up to 430 fps" that is with CO2 which has a vapor pressure of 970 PSI @ 80 deg F.
Just for amusment I ran the spring wire diameter, free length, OD through a spring rate calculator and at full compression it has 128 lbf(pound-force)of force or a rate of 34.1 lbf/in., since the spring is not 100% compressed the spring force is probably in the 100 pound range.
Now there is also adiabatic heating of the air as it's compressed which will make the air more dense, for instance at atmoshpheric pressure 0PSIG it's like .075lb/cu ft heat it up to 300 deg at 100 psig and it .408lb/cu ft. No idea how this interacts but it does explain dieseling with the heated air.
I like the analogy of the hammer and it does make sense for piston weight to have enough inertia to follow through as the spring force weakens the further through the firing cycle things move.
Following along the same lines I wonder if a large releif port in the first 1/4" of piston travel would not assist in accelerating the piston before it hits the wall of air to compress. Similar to a brake master cylinder with the releif port. Something like that could probably even be incorporated in the transfer port with crossdrilling and a hardened ball & brass valve seat but the simplest would just be a hole in the cylinder that the seal passes.
I am not trying to re-invent the wheel here just tossing out some ideas and thoughts.
Kristin
PS- After the sun was setting in the west I thought of a couple more tests (gee I love messing around with experiments) I fired up the compressor that tops out at 120 PSIG, loaded a pellet into the spare barrel and using a blow gun with a brass tip popped a few into my foam block, average penetration 1.5-2" almost the same as my pre-cleaning experiment.
Next I got a piece of leather, soaked it in water and sat it on a bit of wood, pressing the muzzle down fired the gun a few times w/o a pellet. Well it didn't blow a hole in the leather, barely marked it more than the pressure of me holding it to the floor and after a second or so when I released I could hear a hiss of escaping pressure.
So my conclusions about this point is the cylinder pressure is somewhere in the 100-200 PSI range, I may try tomorrow to get a better seal to the breach end and try again as I suspect there may have been some leakage as I was juggling the barrel & blow gun in the fading light. Begining to think it's more about the volume of air moved than the actual pressure and delivering that cleanly to the skirt of the pellet.
This message has been edited by kd006 on Jul 7, 2009 1:17 AM This message has been edited by kd006 on Jul 7, 2009 12:36 AM
Re: In most cases, making a springer piston lighter will DECREASE power - not increase it.
July 7 2009, 4:07 AM
The energy required to launch the pellet to its target comes from the main spring. It must, as the gun has no other source of energy. The rate of accelleration of the piston is a function of its weight and the force the spring can apply. A lighter piston will accellerate faster.
How this affects performance I'm not sure.
Can not inflate a tire to 40 PSIG without mechanical advantage! When I was young we had two mechanical pumps, one a straight cylinder leather piston one and another "foot" pump with a step on it. I do recall the foot operated one was much easier to use as it worked off of a lever system.
This is a very interesting topic as it seems we are getting at the heart of how springers work (or don't).
Basic hydraulic theory says if you use a small piston to pump up a larger one you can increase the force, and I can not see how a large piston driving a small one could result in higher force. What seems to be going on is we are taking a large volume of air and forcing it into a smaller volume and forcing out a lower mass piston (the pellet) and doing so increasing the velocity in the barrel. As mentioned by Carlos in a previous reply we are probably not looking at laminar flow of gasses but it may be advantageous to try for that ideal.
So what is going on here? We have a 4.7 cubic inch cylinder emptying into a 1.6 cubic inch area(barrel .22x 7" long) About a 3:1 ratio, dang it's been too many years since I studied laminar and turbulant flow and I almost can't see why the breach end of the port should not be just a hair less than .177 or .22, instinctivly I would think that would provide laminar flow. I am thinking a large port on the cylinder side tapering into a venturi and then out to the intended target?
Paul, I have to say you have a feel for this, "The energy required to launch the pellet to its target comes from the main spring. It must" as where else can any energy come from unlesss there is detonation of oil in the cylinder/chamber? With any detonation would come some variables that would preclude any repeatable result?
Kristin
This message has been edited by kd006 on Jul 7, 2009 5:50 AM
The question is one of efficiency. A typical springer transfers about 1/3rd of the energy
July 7 2009, 12:53 PM
...invested in cocking the mainspring to the kinetic energy of the pellet. The wasted 2/3rds of the mainspring's energy disappear down various avenues, but the largest is the kinetic energy retained by the piston after it bounces.
The efficiency of energy transfer to the pellet depends on a number of factors, one of which is the peak momentum of the piston. For any given kinetic energy imparted by the spring, the heavier the piston the greater its momentum.
This is true because...
1. energy = 1/2MV^2
2. momentum = MV.
Therefore...
3. momentum = square_root(2M x energy).
Therefore (within limits) for a given spring energy, heavier piston = more momentum = greater efficiency = more muzzle energy.
Will do some tests with 30 to 50 gram tophats and report back
This message has been edited by kd006 on Jul 7, 2009 7:39 PM This message has been edited by kd006 on Jul 7, 2009 5:52 PM This message has been edited by kd006 on Jul 7, 2009 5:49 PM This message has been edited by kd006 on Jul 7, 2009 5:20 PM
Should be interesting. Just be aware that there is a limit...
July 8 2009, 9:33 AM
...to how massive the piston can be made before it won't be able to bounce properly from the forces of compression and will instead impact the end of the compression tube.
Piston slam, besides being bad for both gun and firing cycle, will (for obvious reasons) reverse the relationship between piston mass and power.
Don't have any way to quanitify the results, but added something like 40 grams of tophat, very little free space there too thick a brim and the sear will not lock up before the spring is down solid on its coils. Ended up with perhaps .09" thick "brim" which I would expect to break off afrer a few shots and it's also stressing the spring even more overcompressing it. Probably the better way to go would be a spring of smaller dia wire/more coils to de-stress everything.
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