Canadian Airgun Forum

The #1 Community for Airguns in Canada!
It is currently Tue Nov 12, 2019 1:02 pm

All times are UTC - 5 hours


The Canadian Airgun Forums are a place for people to discuss and learn about airguns and the airgunning sport in Canada. There are lots of discussions about airguns, airgun accessories, reviews, modification and repair information, airgun events, field target and free classifieds!

 

You need to register before you can post: click the register link to proceed. Before you register, please read the forum rules. By joining our free community you will have access to post topics, communicate privately with other members (PM), respond to polls, upload your own pictures, and access many other special features. Registration is fast, simple, and absolutely free! To start viewing messages, select the forum that you want to visit from the selection below.








Post new topic Reply to topic  [ 40 posts ]  Go to page Previous  1, 2, 3
Author Message
PostPosted: Wed Dec 31, 2014 2:33 pm 
Offline
User avatar

Joined: Sat Nov 03, 2007 4:19 pm
Posts: 8973
Location: Coalmont BC
Adjusting the head space on the FTP is pretty easy.... Remove the main pivot pin and keep lengthening the piston until you can't get the hole to line up when checked with a 3/16" rod (something that just slides nicely through the holes).... If you can just slide the pin in with the pump closed, you will be very close (but a whisker short).... The exact adjustment is one where after the gun is fired (no pressure in the valve) you can feel resistance on closing the pump about 3/4" from closed at the end of the handle.... Set to 1/2" is may fall open when fired, and set to 1" you will be over-stressing the pins and linkage....

BTW, at high pump numbers (pressure) you will notice a difference between the plastic piston with the flat face insert and the all metal adjustable piston, even if they are both the same length.... The plastic piston compresses, gets shorter, and increases the headspace, preventing the pressure from building as fast or as far....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


Top
 Profile  
 
PostPosted: Wed Dec 31, 2014 3:51 pm 
Offline
User avatar

Joined: Sun Feb 10, 2013 1:16 pm
Posts: 1787
Hi Bob,

I was wondering what the right amount of "pre-load" was to put in the piston. I discovered the falls-open-if-not-enough thing pretty quickly :)

I set the length much as you suggested, except I put 12 holes around the circumference of the piston head so that I can adjust the length in situ. With 12 I can always see the next hole when the previous one meets the edge of the slot. There's a set screw at the rear end of the piston with a delrin plug to lock against the threaded rod once the length is set. I took the o-ring off the piston so that I could more easily feel when contact was made.

The pressed ridge back-stop in the tube I'm using seems very shallow and only engages a small fraction of the back of the valve. That area is deforming and giving way, so I will likely have to machine some space and fit a steel washer to spread the load to prevent the valve moving back. I did try hammering on the backstop to try and set it in a little further, but I don't think I shifted it at all.

I had a first go at making a pin for the relief valve.
Attachment:
IMG_8134.JPG
IMG_8134.JPG [ 117.86 KiB | Viewed 259 times ]

Attachment:
IMG_8136.JPG
IMG_8136.JPG [ 82.61 KiB | Viewed 259 times ]

Attachment:
IMG_8138.JPG
IMG_8138.JPG [ 93.05 KiB | Viewed 259 times ]

I have a fairly stiff spring with .25" OD so I thought I'd start with about 3/4" of that, with a screw in adjuster at the rear. I'll build the first version in a 1/2" diameter body so that if I need to go to a bigger diameter spring I will have space to increase the bore. It all seems rather big and clunky, but once I get a feel for what works I can look at building a smaller one.

Jim


Top
 Profile  
 
PostPosted: Wed Dec 31, 2014 6:03 pm 
Offline

Joined: Thu Nov 02, 2006 4:57 pm
Posts: 1754
Location: mb
Whitewolf wrote:
Have you made any calculations as to your valve space interior?


EverHopeful wrote:
No, I haven't done that. It was intended to be smaller than standard, but obviously the gauge block doesn't help there. It will be interesting to see if I can get to the target fps with perhaps one less pump once the gauge is removed and replaced with the relief valve. I'm hoping I can make that with minimal extra volume....


Quite interesting project, and very nice work as well. . . . .

When measuring Valve Volume, a small (thin barrelled) syringe can greatly aid the process. . . . .
Do the volume calculations on the measurable internals of the valve(check and valve head)
and subtract them from the measured cc's you can inject into the valve(attached adaptors/additional
housing/chambers/etc.). . . . Leave whatever springs the system uses, in the valve when you
insert the water, and then it's minute volume is accounted for in the final calculation.

1ml=1cc=0.0610237441ci
EverHopeful wrote:
It all seems rather big and clunky, but once I get a feel for what works I can look at building a smaller one.

That; might even be marketable. . . . .

Here's to the continuation of the successes of your project in the New Year! :drinkers:

_________________
Don't Be Sexist - Broads Hate That.


Top
 Profile  
 
PostPosted: Thu Jan 01, 2015 1:48 pm 
Offline
User avatar

Joined: Sun Feb 10, 2013 1:16 pm
Posts: 1787
Oh well, prototype 1 is a fail. I couldn't get a seal. The first attempt seemed to almost be there, just a slow leak, so I took it apart and gave the inner surface a polish. When I put it back together again I must have got it off centre as there was no seal at all and now it's all jammed up in there so I can't get the needle part or the spring back out again. Oops. I'll have another look at that after lunch :)


Top
 Profile  
 
PostPosted: Thu Jan 01, 2015 5:30 pm 
Offline
User avatar

Joined: Sun Feb 10, 2013 1:16 pm
Posts: 1787
Success! I managed to get the stuck valve out, it had indeed gone in at an angle - too much play in the cylindrical section to guarantee lining up with the 3/64 inlet hole. I re-assembled more carefully and after a bit of fiddling achieved a partial success. It sealed at low pressures and opened at high, but the transition between open and closed again was very very slow. I was hoping for a much crisper transition. I thought perhaps the choice of a needle shape was the wrong one, so I decided to try a flat delrin poppet like the many others we use. I recessed the valve face with the concave ground drill bit that I described earlier in this thread, and made up a simple poppet to fit. It still has a much slower transition than I was hoping for, but it seems to be significantly better than the steel pin. I did some velocity testing and everything looks very promising. The range of adjustment covers the 500 fps point very well, and there doesn't seem to be any serious leak at pressures below the set point. There is significantly less extra volume than in the pressure gauge, so I'm getting 423 fps on three pumps and 490 on four. In my quick testing I managed to get the valve to open at five pumps or more, with a set pressure of just over 500 fps. I'll need to improve on that slightly in the final version. The biggest problem is at one pump over normal, as the valve opens only a very little and it takes quite a while for the pressure to settle back to the set point. I'm wondering if a larger inlet hole will make the transition back to the set point quicker. The larger area will require more spring force, so I'd better check that I have enough adjustment left in the spring, or find a stronger one.

The prototype is indeed huge and unwieldy at 2" long, but the adjuster is about half way down the valve at the set point, so I could chop 1/2" to 3/4" of inch off straight away, and the OD can be reduced a fair bit if I stick to this spring. There's an 1/8" that can come off the nose of the valve, as with that thickness delrin washer the body clears the gas tube. I think with the delrin poppet, which doesn't have the same alignment issues that the steel one had, I could shorten both the larger diameter cylindrical section and the spring guide part. I'm not sure if I can shorten the spring or not - I will have to measure how much it is being compressed and then see how much spring I need to achieve the same compression without coil bind. It sure was great to see it basically working as planned though, and the tune that this allows is *much* nicer than the one I'm currently using.

Here's a pick of the valve body, delrin poppet, spring and adjustment screw, with the first steel pin valve below
Attachment:
IMG_8141.JPG
IMG_8141.JPG [ 163.18 KiB | Viewed 224 times ]

Hope I'm not boring you all with my ramblings, but I've been having great fun and I hope it's of interest to a few of you.

All the best,
Jim


Top
 Profile  
 
PostPosted: Fri Jan 02, 2015 7:11 am 
Offline
User avatar

Joined: Sun Feb 10, 2013 1:16 pm
Posts: 1787
Turns out the existing spring was very close to coil bind at the set point, so I'm not going to be able to shorten it appreciably. It occurred to me that this is probably a good application for bellevilles, so I've put in an order for a few to try.


Top
 Profile  
 
PostPosted: Fri Jan 02, 2015 7:27 am 
Offline
User avatar

Joined: Tue Jun 03, 2008 5:46 pm
Posts: 2814
Location: Canada
May have missed it, but if you're going to re-make the body to fit bellevilles.....can you reduce the seat diameter? Would reduce the force required of the spring.

Al


Top
 Profile  
 
PostPosted: Fri Jan 02, 2015 7:53 am 
Offline
User avatar

Joined: Sun Feb 10, 2013 1:16 pm
Posts: 1787
I think the seat is already fairly minimal, the inlet is a 3/64 and I didn't grind a flat this time, so the cone around the bore is fairly sharp still. I was actually thinking of testing with a larger bore and more of a flat to see if I could get a sharper transition. It only needs to hold ~ 600 psi which comes out at only a pound or so of force, so I'm hoping it will be fairly easy for belleville discs.

Jim

[edit - on further thought, 3/64 is only small compared to what I'm used to, and I have number drills that would go a fair bit smaller. I shall keep it in mind when I make the next body, after all, there's no harm in starting small and drilling out if necessary.]


Top
 Profile  
 
PostPosted: Fri Jan 02, 2015 11:45 am 
Offline
User avatar

Joined: Sat Nov 03, 2007 4:19 pm
Posts: 8973
Location: Coalmont BC
My guess is that the seal isn't sealing at the 3/64" hole diameter, but much further out on the seal surface.... so much more area, and therefore spring force required....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


Top
 Profile  
 
PostPosted: Fri Jan 02, 2015 5:23 pm 
Offline

Joined: Thu Dec 05, 2013 11:49 pm
Posts: 132
Location: Winnipeg
I used to pull the bleed valve in 80's era 760's all the time. Also used to pin them. I can't remember what was inside, sorry. See if you can snag one somehow, or get a shot of the guts for some insight.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 40 posts ]  Go to page Previous  1, 2, 3

All times are UTC - 5 hours


Who is online

Users browsing this forum: Google Adsense [Bot] and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
News News Site map Site map SitemapIndex SitemapIndex RSS Feed RSS Feed Channel list Channel list

Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group

phpBB SEO