New QB 78 on HPA - (Now .25 cal)

I've started working on another QB 78 setup to run on HPA.... The twist on this one is that the tank is reversed and will act as the forestock.... I did this on my Hayabusa project, and the QB 78 main tube was just the right length.... I'm using a 13 CI Ninja 3000 psi tank regulated to about 1600 psi and equipped with a 1.8K burst disc.... The tank block is machined from a block of 1" thick 6061-T6 aluminum that measures 1.37" x 2.63".... It is wide enough for a 3-groove Picatinny rail on the bottom for mounting a bipod and/or sling.... The tank is tucked up against the bottom of the main tube with minimal clearance.... The assembly attaches to the front of the main tube using the stock knurled nut with the hole enlarged to 1/2".... That is the only modification to the gun necessary to attach the tank, other than shortening the stock....

http://i378.photobucket.com/albums/o...BTankUnder.jpg

There is a brass fitting with O-ring that fits into the front of the QB main tube and is retained by the knurled nut.... The O-ring is moved to behind the threaded part of the tube for additional strength.... That fitting is attached to the tank block using a steel 1/8" NPT hex pipe nipple, with the knurled nut captured in between and free to rotate.... Below where the nipple screws into the block I machined a standard paintball fitting that the regulator output of the tank screws into.... There is a pin that presses on the pin valve in the regulator to open it.... The air passes around that pin and travels vertically through the block via a 1/8" hole to where the pipe nipple is.... and then from there into the main tube.... The bottom of the 1/8" hole is plugged by a 1/4" long 10-32 low profile SHCS recessed flush with the grooves in the Picatinny rail, and sealed by an O-ring....

http://i378.photobucket.com/albums/o...tsandValve.jpg

Also in the photo above is the highly moddified QB valve.... Basically, once you open up the front of the valve, all it does is provide a place for the spring to sit.... With that in mind, I machined away all the extra material, so the valve is now the entire volume of the main tube, about 50cc including the valve area.... This is a large enough volume of regulated air to support up to 50 FPE shots without losing efficiency.... Since the eventual plan is for this gun to be converted to .25 cal, that is ideal for my purposes....

Bob

Sick!!! I cant wait to see what she looks like when it's done .

Holy cow Bob....would you mind printing up a program so that we can keep up. :icon_eek: Two days ago we were talking about the leade on a Big Bore and now you already have 50% of a QB conversion done.:hypnotysed:

How are you machining the threads in the tank block for the paintball tank?

Painstakingly.... I lay out the position in my milling attachment while drilling the hole for the pipe threads and centerdrill it.... Then I chuck it in a 4-jaw, using the centerdrilled hole and a 60* center in the tailstock for alignment.... I drill it 1/8" to a depth of an inch, and then 1/2" to a depth of 0.7" to the point of the drill.... Then I use a 5/8" end mill to square off the hole to 0.700" deep.... I then use an 11/16" mill and bore in about 1/4" and check the size.... I'm looking for about 0.695" ID, and today I was lucky, so I just bored it in 0.700" and that gave me the shouldered portion for the O-ring to seal on....

I then used a boring bar and enlarged the hole to 0.740" ID to a depth of 0.400".... I then took an internal threading tool, put the point in at 0.400" and opened up a groove to 0.840".... That gave me a spot to stop the threads and also formed a tapered shoulder for the O-ring to guide on when entering the inner portion.... I then set the lathe up for 14 TPI and using five 0.010" cuts I threaded the outer portion until the point of the threading tool was at 0.840" ID.... I stop the feed about 0.010" before the tool touched the shoulder I made in the previous step (ie 0.390" below flush) so as to not damage it....

At this point I then use a 1/2" -14 NPS tap and run it in to clean out the threads, making sure I stop before the tip of the tap touches that all-important shoulder.... I then change to a bottoming tap (I ground one perfectly square on the end) and run it in exactly 5.5 turns, at which point (if I did it all correctly) you can feel it free up as it cleans out the bottom thread.... Again, DON'T touch the shoulder.... With a bit of luck, you don't damage anything (today was a good day)....

I then carefully deburr the outside edge of the hole and the edge of the shoulder, and thread in a tank without O-ring.... Assuming it bottomed (and it did) I double check the transition at the shoulder below the threads where the smooth portion starts for the O-ring and try the tank with O-ring.... If it feels good, and if, on removal, the O-ring looks good and not shredded then it will probably hold pressure.... This one was particulaly nice, so I'm confident it will be fine.... I've done 4-5 of these now, and while they are still nerve-wracking, they are getting better each time.... and even the first one I managed to salvage....

Bob

Kind of curious as to the point of diminishing returns on the valve cut away.

I'll ask it here (I may have missed reading it else where)... what increments did you use to work up flow to arrive at those openings? I'm interested in what the process was...

I have an interest in going to .25 cal at a later date, once I've got my 78 accurized...

Regards,

Doc Sharptail

Here is a previous valve I did.... compared to stock....

http://i378.photobucket.com/albums/o...HPA/QBMods.jpg

My reasoning is that once you remove the piercing pin, internal sleeve, and open up the valve.... you basically no longer have a fixed volume of valve anyway.... The pressure drop in the valve during a shot is then proportional to the volume of the entire tube.... I took this to the extreme in my Hayabusa.... Here is the valve arrangement for that gun....

http://i378.photobucket.com/albums/o...ountDetail.jpg

I have done a lot of work that all leads me to belive that if you are searching for high power, you need to get the air into the valve (ie to the valve seat) quicker than it can get out.... and the higher the pressure you can maintain during the shot cycle, the more performance possible.... That is why I have a design point of 1 cc of air "on deck" for every FPE of energy I want the gun to produce.... My Hayabusa has 110 cc of air between the regulator and the valve seat.... This new QB has 50-55 cc.... A QB 79, even if you installed my completely opened up valve in it only has about 10 cc.... basically just the volume of the opened up valve, there is no tube length.... That doesn't mean you can't get good power out of a QB79.... it just means you can get more from a QB78.... When I used the valve in the photo above in a QB78 I got over 31 FPE.... while the identical setup in a QB79 only produced 26 FPE....

Bob

I got the stock shortened today and everything bolted together for the first time.... and there were no leaks.... Here's what it looks like sitting on a bipod....

http://i378.photobucket.com/albums/o...stAssembly.jpg

I made the barrel band later in the day but it's not in this photo.... It fits in the recess in the top front of the tank block.... I machined the extra notch in the breech to hold the bolt back as this gun will be "cock on open" and it makes sure there is lots of room to load the pellet.... I also machined recesses in the breech and valve for a poly transfer port and finished modifying the valve, installing a Delrin seat on the poppet and boring the valve out around it for increased flow.... I also eased the corner at the bottom of the exhaust port and drilled the valve throat out to 0.223".... This gun will use the new hammer I developed for the last QB 78 I built....

http://i378.photobucket.com/albums/o...ammerParts.jpg

You can see the extra cocking notch for the bolt in that photo.... The upper hammer and cocking block are stock.... the lower ones are the new ones.... The sear catch point is moved forward 0.20" to increase the hammer travel, and the back of the hammer is shortened the same amount.... The spring guide is shortened about 1/8" and there is a hole in the bottom of the spring recess in the hammer to miss it and allow the hammer to go all the way back until it touches the cocking block.... That gives maximum possible preload room for the hammer spring....

I will be making a preload adjuster for this gun, but I haven't yet decided if I will just make it off center, or make a new rear block with screws top and bottom to retain it and the adjuster on the centerline.... I'll have a think on that tomorrow....

Bob

A work of art! You sir have Mad Skills !!!

could we see how that open valve would be mounted? I am trying to imagine how it sits in position.

Are you talking about the QB valve or the one in my Hayabusa?....

The one in the QB bolts in in the original location.... The O-ring seal is behind the cut away portion, so the aluminum part just sits inside the air tube as all it's doing is holding the valve spring in place....

In my Hayabusa, the brass portion (the valve) just bolts into the main tube, sealed with an O-ring.... The spring holder is attached to the front tank block with a long threaded rod and again, just sits in the air tube....

In both cases, the air tube itself serves as the valve chamber, basically just blocked at the back by the portion of the valve containing the seat and poppet....

Bob