Hatsan Air Stripper Adjustment

[rsterne]

Quite a long time ago I made some alignment tools (pilots) out of wood to allow me to set the stripper cone parallel to the bore on installations where the alignment wasn't great, and I had drilled out the outer stripper housing and installed six setscrews to allow me to move the cone around....



While these did a great job of what they were intended for, adjustment of the gap was primarily an eyeball thing.... A few months ago, when I changed over my Hatsan AT-44 S-10 Long to shooting the 34 gr. JSB Heavies, I checked the alignment of the stock Hatsan Stripper with one of my pilots, and found it was virtually perfect.... At the time, I had been reading up on Barrel Harmonic Tuners, and several people told me that just a few thou change in the position of the adjusting weight can drastically affect group size.... Simultaneously, I read a few articles about Air Strippers, and the opinion is split about which is the operative factor, the gap between the muzzle and the stripper cone, or the effect of moving the weight of the cone back and forth.... I think it may be a combination of the two, but in reality DOES IT REALLY MATTER?.... I mean surely the goal is to reduce the group size as much as possible by moving the cone back and forth until you find the sweet spot where the group is the smallest.... So, I decided to design an adjustable version of the wooden pilot above, that would allow me to change the position the cone by just a few thou at a time, and would also be repeatable.... I came up with the following design, but it had to wait until the Motel wasn't so busy and I had some time to get into the shop to make it....

Instead of all wood, the core of the new pilot is turned from a 1/2" steel rod.... The mid-body is a precise slide fit in the stripper cone, which will vary with caliber.... There is a shoulder above that which is just under 12mm (the OD of the brass cone) so that it can push the cone into the hole in the stripper body when required.... Behind the mid-body the steel shaft is turned down to 3/16", and a piece of hardwood dowel, drilled out to 3/16", is pressed onto it.... The OD of the dowel is then turned down until it just slides into the muzzle.... The center hole in the end of the steel shaft insures it remains concentric with the midbody of the pilot that slides into the cone while turning the OD of the wooden bushing.... This insures the cone and bore are concentric and parallel....



The aluminum "nut" and the front of the steel shaft are threaded 1/4"-28, so that each turn is 0.036" of travel.... That works out so that 10 deg. of rotation is 0.001".... There is an 8-32 setscrew pushing lightly on a short piece of 1/8" Delrin rod against the threaded shaft to act as a brake to keep it from self-adjusting, and a screw slot in the end of the shaft for adjustment.... Turning the shaft changes the position of the shoulder on the shaft relative to the end of the aluminum collar, which in turn pushes the cone deeper into the body of the stripper when the aluminum collar is pushed against the end of the stripper body.... In the photo below, the cone is at the deepest position, with about a 1/2 caliber gap between the muzzle and the end of the cone....



In the photo below, the cone is slid out far as it can go, and the gap is about 2.5 calibers.... The cone has a slot which limits it's travel before it hits the setscrew in the stripper body.... There is a 1/2" diameter milled pocket that is 0.4" deep in the aluminum collar to allow the shoulder on the pilot a place to go when the collar is screwed in to retract the cone position.... The gap you see between the collar and the stripper body is the total adjustment range, which is 1/2"...



With the collar threaded down until the cone has the largest possible gap, it looks like the photo below.... You can see the shoulder on the pilot up inside the aluminum collar, and the center hole in the end of the steel pilot shaft with the wood bushing to protect the bore.... If you look closely, you can see a center punch mark at the end of the screwdriver slot, so I can tell full turns from half turns, and of course I can also measure how far the threaded shaft sticks out of the aluminum.... This is not a difficult tool to make on a lathe, but it is surprising how much time even a simple project can eat up.... about 4 hours in this case, plus the design time.... and then the time to write up this report of course....



I hope the photos and the description make it clear how it was made, and how it works....  My plan is to start like this, with the cone fully out, with the screw slot and center punch mark pointing at the set screw hole in the collar.... I will slide it in until the collar touches the end of the stripper body, snug the setscrew holding the cone in place, slide out the tool and shoot a group at 50 yards.... I will measure and record the group size, screw the threaded rod in one turn, slide the tool back into place, loosen the setscrew until the cone moves in 0.036", snug it up and shoot another group.... I will keep doing this until the cone is as far in as possible.... That will take 14 turns, so the initial test will be 15 targets.... Graphing the results should quickly narrow down the best position of the cone, and at that point I plan to repeat the process with 1/4 turn adjustments, or less if required.... If there are two (or more) sweet spots, I can explore them both, possibly at different ranges.... I think you could spend a whole day (or a few days) just tuning with this device.... I hope it's worth it.... 

Bob

[customcutter]

All I can say is WOW!  That is some piece of work, and I know a lot of thought went in to figuring out a process to do it.  I was wondering if air rifles had issues with barrel harmonics the other day when another member and I were talking about indexing barrels.  Apparently you must think so.  I hope all the hard work and effort is worth it as well.  Maybe you should patent it...

[AmBraCol]

Bob, you are my favorite mad scientist.  I love seeing what you come up with next.  Thanks for sharing your thought processes with us.    I'm looking forward to seeing how this project develops.

[Mod90]

Nice idea Bob. I used to have a contraption that I'd use to make adjustments on the air stripper that consisted of a flat metal seat at the base of the cone thay was drilled and threaded to accept a bolt that passes through the cone, with a riser that would fit around the brass on the exit end and another flat plate on top of that with just a hole drilled. With that setup, just turn the bolt and it pulls the cone forward. There were two downsides to this setup though, one being that it would only move the cone in one direction, outward,  and if i overshot the ideal position I'd need to restart the whole process. The other issue was there wasn't really any way to ensure that the cone of the air stripper was lined up perfectly with the bore as youve achieved using a wooden dowel, unless using a rather long bolt that was just a little smaller than the bore. That wouldn't be without its risks of possibly damaging the crown. I like that dowel idea, wish I'd thought of it.

Is your adjuster bidirectional or just one direction?

[rsterne]

It can only push the cone inwards (or rather limit how far you can push it outwards).... so you need to hold the cone against it while tightening the setscrew in the stripper body....

Bob

[HectorMedina]

Bob;

The weight of the brass cone is small enough that you can adjust in millimeter steps (for reasonably stiff barrels.- 0.625" OD and 14" length).

If your barrel is longer or thinner, you may need finer adjustments.

See how that part worked in a Walther LGV:

http://www.ctcustomairguns.com/hectors-airgun-blog/the-walther-lgv-a-second-look

Good instrument you created there!

Thanks!






Héctor

[rsterne]

I have no experience with how fine the adjustments need to be, but the stripper cone is indeed not very heavy, and the Hatsan barrel is fairly stiff.... so I can well believe that 1 mm adjustments is about right.... Each turn on my adjuster is 0.036" (0.9 mm) so it is likely I won't need any finer adjustments than that.... Good to know....

Bob

[Mod90]

True the Hatsan barrels are fairly stiff Bob, and thats one reason for using small adjustments.
But what I've found in my efforts to tune the air stripper is that best results are gained by making small adjustments simply because too much movement of the cone can have either a negative or positive effect, and it can take as little as one mm of movement to cause this. I do believe its possibly caused by either the change of weight position change possibly affecting the barrel harmonics, or the brass cone possibly leans in its aluminum housing depending on its position and the pressure applied to it by its single set screw. Or it's possible it could be a combination of all those factors. I really don't know.
But in general, as mentioned the finer adjustments  do seem to bring the best results.

[rsterne]

All I know is I wasn't able to get fine enough adjustments, and had no repeatability, so that is the reason for the new adjusting tool.... Hector knows his stuff, so if he feels 1 mm adustments are fine enough, using 1 turn on my adjuster should be fine....

Bob

[Machinist]

If a very lightweight plastic stripper were used, the effect of weight relocation would be reduced, and the cone to barrel relationship could be scrutinized independently no?

What is the ID of the brass cone Bob? Could the clearance be reduced?

Steve