Spring Gun tuning - Top Hat advice

[NickG]

This is interesting , it doesn't really matter if your long bit is at the front or the rear, if its a thin walled guide the piston rod actually helps strengthen it if at the front , however you do it make sure both ends are located centrally and make them so they just contact when cocked , full support and no odd coil sticking proud when cocked .

[Mr.Fixit-Norm]

I’m just a bit pissed off now , I received the delrin rod today and despite the listing being for 10mm & 12mm diameter, the 10mm as suggested is a bit too loose and the 12mm is actually 12.22mm, 0.12mm BIGGER than the ID of the spring....grrrr

There’s no mention on the site of the diameters being ‘nominal’ so it looks like a phone call on Monday

The company in question is HPC Gears of Chesterfield.

I’m not sure I’d be comfortable machining 0.22mm of the length of it ( 200mm ) on the lathe although I do have a live centre for the tail stock so it might be within my means.
I only need a piece 125mm long so unless they can provide me with another bit of exactly 12mm diameter it’ll mean a bit of careful work.

The 10mm piece is also over at 10.15mm but too loose

Norm

[evert]

That extra material makes it possible to machine it to spec.
You might need to do some fine adjustments on the lathe to eliminate taper when you machine it to size.

[Mr.Fixit-Norm]

That extra material makes it possible to machine it to spec.
You might need to do some fine adjustments on the lathe to eliminate taper when you machine it to size.

Yes you’re right, and it looks like I owe HPC a mental apology, much further down the listing past all the other adverts is a bit about the tolerances and 12mm is +0.2-0.7mm. Looks like I’ve some machining to do

Norm

[evert]

Looks like I’ve some machining to do

Thats the fun part

[Mr.Fixit-Norm]

Thats the fun part

Haha , yes, but only when you get it right

[Mr.Fixit-Norm]

I managed to spend some time in the workshop this afternoon and successfully machined the 12.2mm delrin rod down to 11.95mm, this gave an approximate 0.05mm gap between that and the spring. It wasn’t that easy to do, even using a live tail stock the rod had a tendency to spring away from the cutter when I was machining the middle part. ( I’m guessing that’s what evert was referring to ? )

I overcame this by taking very fine cuts, ending up with a rod that was a nice smooth fit in the spring. I then machined the old guide off the plug and while it was still centred in the lathe, drilled a 10mm hole right through.

I then machined 15mm of the end of the Rod down to 10.1mm which allowed for a tight fit in the plug ( careful application of a mallet )

I trimmed the guide down to 115mm which I guessed to be the maximum I could get away with but that was a bit too close for comfort when cocked so I took another 5mm off.

I found a nice new shiny 2mm thick plated washer which was a perfect fit in the end of the piston, not to tight so I couldn’t get it out but tight enough so it won’t move sideways , with the top hat thickness this gives 5mm of pre-load.

Once assembled with a decent amount of moly grease on the spring, guide and top hat, and back in the stock I found it was marginally harder to cock but not enough to be an issue. When I fired it there didn’t seem to be any twang whatsoever and despite the heavy trigger it seemed smoother and with virtually no recoil !
Not sure if the lack of recoil is a real effect or due to my rose tinted safety specs - lol

I didn’t have a chance to chrono it but I’ll try that tomorrow. I don’t know if what I’ve done will affect the power, I’m not worried if it doesn’t as the exercise was to quieten and smooth it, but it’ll be interesting to see what the result is.

Unfortunately I didn’t take any pics of the spring guide but it looks just like the original one but 110mm long rather than 49mm

Norm