AirGunTech: Re Engineering Spring Powered Air Rifles for smoothness, consistency, and to better suit UK power levels.

Small update on the HW80 build…..adding the compression cylinder sleeve

Work has started on the 80 build now, decided to start with the compression cylinder while waiting for parts.

The compression cylinder length is dictated be the distance from the breech to the cocking shoe aperture machined into the main cylinder…this happens to be 95.2mm on the gun I have here, the chosen stroke will be 80mm so I have around 15mm behind the nose of the piston when the gun is cocked for me to add fixings so the new sleeve is held tight against the face of the breech.

As mentioned earlier I chose to sleeve down to 25mm, the reason for this was so either the TX200 or LGV piston could be used, this would save a ton of machining (as long as the pistons can be adapted easily)

So…easy first cut is machine a length of tube down to 95.2mm, I actually went for 95.1mm to gain a midges of clearance.

Next step is to machining an O ring groove. I chose to use 30mm OD 2mm cross section O ring, groove was machined to give 0.25mm crush on the O ring, from the front face and from the outer cylinder …this was to ensure a good seal (I hope ;))

 

The actual machine marks have been polished out now, this is important for the O ring to give a consistent seal, I polished to 320grit but forgot to take a pic…you get the idea and reasons why though ;). Following pic shows the O ring installed.

 

 

 

 

 

 

 

 

 

 

 

Moving on, next was machine in the fixings for the sleeve…with having 14mm available I went for the roughly accurate approach, there is plenty of space to get these in so just marked the positions with a sharpy marker and drilled to 3.2mm…

 

The 2 holes are 13mm away from centre.

 

next step is de burr, then with the O ring removed from the sleeve insert the sleeve into the action. NOW…after struggling  with the method I used I realised it would be easier to add a 3rd hole to the sleeve around 2mm from the back edge so a hook can be inserted to withdraw the sleeve..2mm dia or so would be all thats needed and a bent TIG rod could be used to withdraw the sleeve. This 3rd hole obviously needs to be added to the sleeve before you insert it for the next stage which is marking the position for the fixing holes from the 2 holes already drilled into the action. Also note I added the old piston back into the action backwards and then screwed the trigger block so some pressure was applied to the sleeve to hold it in position while the additional fixing holes were drilled.

 

Now…once drilled, remove the sleeve tap the2 holes in the action M4, then again deburr the inside of the action, tap the sleeve for the 2 fixings M4 and again deburr the outer and the inner. Its a long winded process, you have to ensure the sleeve slides into the action smoothly with no burrs, this is also important for the O ring as any damage will lead to a leak at the breech face.

With the sleeve drilled and tapped and the action drilled and tapped and all de burred add the O ring.

 

Now we have 1 last job on the action before inserting the sleeve and loctiting its fixings…the transfer port.

 

I have no pics of this, but will explain why the transfer port diameter has been altered. Final stroke is hoped to land bang on 80mm, this gives a piston swept volume of 39.28CC

The HW80 transfer port is 11mm long, I want a final static compression ratio of 350:1 or as close as possible, so reverse calculation gives 3.6mm for the transfer port diameter needed.

Static compression ratio = Piston swept volume / transfer port volume.

so transfer port volume =  Piston swept volume / static compression ratio

 

Once you know the transfer port volume you can reverse the calculation for volume of a cylinder to give the radius of the transfer port which you double for the diameter.

Pi * (r squared) * h = V  so

V/h*Pi= r squared, so we need to hit the square root to get r then double it for the diameter.

Use as many decimal places as you can for accuracy…reverse checking though (otherwise known as the trial and error calculation method)is always good so…

Piston swept volume = 39.28571428571425CC

transfer swept volume with 3.6mm diameter and 11mm height = 0.11201142857142848CC

39.28571428571425/ 0.11201142857142848 = 350.729517

So pretty close to 350:1 😉

 

So…the transfer port is drilled out carefully to 3.6mm, then lightly polished with a bootlace soaked in Autosol polish before the action is washed out with degreaser and dried. Then last job for this update is grease the sleeve and O ring and press the sleeve into the action, compressing it while 2 x 4mm long M4 grub screws are blue loctited into place to hold the sleeve hard and tight against the inner breech face. The grub screws need to finish just below the inner surface of the inner sleeve so they do NOT foul on the piston seal when inserted 😉

 

This is pretty much where I am at right now, waiting for the piston to arrive and the new style cocking shoe that was shown in stock but was obviously not due to the week+ lead time for delivery gggrrrrrrrrr!!!

I have pressure tested the action for 30mins by blocking the transfer port from the inside of the action and pressing a hw77 piston into the sleeve…it held pressure well so im looking forward to the next stage of the build

 

 

 

 

 

 

 

 

 

 

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