polyurethane for seals

[greenwayjames]

Hi James

From JB's May 1991 article it reads as he cuts the Polyurethane with HSS lathe tools but says you have to experiment with the cutting angle by grinding the tool ?



All the best Mick

hi Mick
The device I saw him use to cut a groove for the O ring was definitely a high speed grinder fixed on the slide in the position that a toolpost would normally occupy. I think that is why they are called a tool post grinder

[greenwayjames]

[QUOTE=NickG;6420235]What on earth is a tool post grinder ! sounds like a complete bodge to me . If you mean he has tried to turn his lathe into an external grinder, then accuracy will go out the window.



See my reply to Mick.

[NickG]

[QUOTE=greenwayjames;6420277]

What on earth is a tool post grinder ! sounds like a complete bodge to me . If you mean he has tried to turn his lathe into an external grinder, then accuracy will go out the window.



See my reply to Mick.

sounds like he was just trying to put a rad on the front of a seal, OVERKILL! just use a radiused cutting tool. However this has nothing to do with my original post, all I am trying to say is , if you have a lathe a little skill , and some polyurethane, you can make your own seals, The difficult part is doing the rear where it mounts to the piston, and that I have done,

[bigtoe01]

poly has to be kept very cool, a special coolant /cutting fluid is used and extremely sharp tooling....unless you do it this way tolerances are all over the place. So...you then mould it, which is again hit and miss to start until you get the mould the right size

if you go on youtube you will see seals can be cut extremely accurately on a lathe with the correct sharp tooling etc.

[greenwayjames]

[QUOTE=NickG;6420235]What on earth is a tool post grinder ! sounds like a complete bodge to me .



I hope this lifts the veil from your eyes and saves a bit of googling to find out what a toolpost grinder consists of and how it works. It looks to be quite a handy accessory

http://www.smithy.com/machining-hand...pter-3/page/26

[brucegill]

[QUOTE=greenwayjames;6420756]

What on earth is a tool post grinder ! sounds like a complete bodge to me .



I hope this lifts the veil from your eyes and saves a bit of googling to find out what a toolpost grinder consists of and how it works. It looks to be quite a handy accessory

http://www.smithy.com/machining-hand...pter-3/page/26

A cracking and very useful tool. Would love one but not cheap. Think you need one to reduce the OD on the 77/97 hardened comp tubes to ad bearings like the lazer glide. Defo not a "Bodge" tool

[NickG]

[QUOTE=greenwayjames;6420756]

What on earth is a tool post grinder ! sounds like a complete bodge to me .



I hope this lifts the veil from your eyes and saves a bit of googling to find out what a toolpost grinder consists of and how it works. It looks to be quite a handy accessory

http://www.smithy.com/machining-hand...pter-3/page/26

That's a proper grinding spindle.

[vobster]

Thanks for making me one, I'll let you know how it goes

[Artfull-Bodger]

How are you finding polyurethane ?, it's not a particularly low friction material and does expand when warmed up?

[NickG]

How are you finding polyurethane ?, it's not a particularly low friction material and does expand when warmed up?

Its un tested as yet the first one is going into vobsters gun. We will see.

[nced]

After reading about the limited availability of 25mm seals for the TX , I bought some polyurethane and had a go at making some, not for the TX I grant you , I have made a piston head with an o ring and HW fitment for an idea I have for converting the 97 to 25 mm. Pics below hopefully. It does open up the potential for making walther style seals for the TX http://i1346.photobucket.com/albums/...ps8b590257.jpg http://i1346.photobucket.com/albums/...psd5a32f0f.jpg

I'm a "Yank" so my R9 (rebadged HW95) was tuned to about 14.5 fpe and I found that the polyurethane seals eroded rather quickly at the transfer port area. While the edges of the seal were still pliable, the center of the seal was eroded and hardened like a cinder. It was at that time a couple decades ago that I started messing with aluminum oring sealed piston caps to solve the "erosion issue" and have been using the alloy caps since. Here are a couple pics of eroded HW piston seals from that time..
The samples above weren't polyurethane and MYpoly piston seals eroded MUCH worse. In the beginning I would cut my piston caps with enough clearance between the piston face and seal to fit a thin polyurethane washer as a sort of "shock absorber". After a while I found that when properly balanced there was no need for this bumper since the piston decelerates on the high pressure column of air just before the pellet starts moving through the barrel. When I first started cutting piston caps I pulled apart my R9 after several thousand shots to inspect the oring and this is what I found.......

No impact damage at all on the face of the aluminum piston cap with power levels of 14.5fpe.....just some "combustion coloring" from the molly bearing lube that was used.

I have moderated my R9 power level to about 12.5fpe for the last few years since I believe that the R9/HW95 simply "shoots better" at this power level so I'm wondering if "piston cap burning" would be an issue if the caps were made from molly filled 6/6 nylon (Nylatron) instead of aluminum. When first cutting polyurethane seals and noting the erosion of the seal face I did a little "heat resistance test". I played the flame of a propane torch over a piece of polyurethane and it actually melted rather quickly like a sticky wax. Matter of fact, I would melt the face of a short section of rod and stick it to a steel mandrel for spinning in my lathe for cutting piston seals. Later I did the same "flame test" on a piece of 6/6 nylon and found that it didn't melt, but after more heat than used to melt polyurethane.....the nylon simply charred into a cinder. I did the same test with acetal (Delrin) and the temperature resistance was between that of polyurethane and 6/6 nylon. Since the acetal machines very nicely whereas 6/6 nylon is tougher and harder to machine, perhaps it would be a good "bearing type replacement" for aluminum for an oring sealed piston cap.

[NickG]

I'm a "Yank" so my R9 (rebadged HW95) was tuned to about 14.5 fpe and I found that the polyurethane seals eroded rather quickly at the transfer port area. While the edges of the seal were still pliable, the center of the seal was eroded and hardened like a cinder. It was at that time a couple decades ago that I started messing with aluminum oring sealed piston caps to solve the "erosion issue" and have been using the alloy caps since. Here are a couple pics of eroded HW piston seals from that time..
The samples above weren't polyurethane and MYpoly piston seals eroded MUCH worse. In the beginning I would cut my piston caps with enough clearance between the piston face and seal to fit a thin polyurethane washer as a sort of "shock absorber". After a while I found that when properly balanced there was no need for this bumper since the piston decelerates on the high pressure column of air just before the pellet starts moving through the barrel. When I first started cutting piston caps I pulled apart my R9 after several thousand shots to inspect the oring and this is what I found.......

No impact damage at all on the face of the aluminum piston cap with power levels of 14.5fpe.....just some "combustion coloring" from the molly bearing lube that was used.

I have moderated my R9 power level to about 12.5fpe for the last few years since I believe that the R9/HW95 simply "shoots better" at this power level so I'm wondering if "piston cap burning" would be an issue if the caps were made from molly filled 6/6 nylon (Nylatron) instead of aluminum. When first cutting polyurethane seals and noting the erosion of the seal face I did a little "heat resistance test". I played the flame of a propane torch over a piece of polyurethane and it actually melted rather quickly like a sticky wax. Matter of fact, I would melt the face of a short section of rod and stick it to a steel mandrel for spinning in my lathe for cutting piston seals. Later I did the same "flame test" on a piece of 6/6 nylon and found that it didn't melt, but after more heat than used to melt polyurethane.....the nylon simply charred into a cinder. I did the same test with acetal (Delrin) and the temperature resistance was between that of polyurethane and 6/6 nylon. Since the acetal machines very nicely whereas 6/6 nylon is tougher and harder to machine, perhaps it would be a good "bearing type replacement" for aluminum for an oring sealed piston cap.

Very interesting post, I have been using delrin o ring heads on my pistons, to good effect, there is no discernible wear after thousands of shots, The poly is an attempt to improve the already tame firing cycle, maybe I am already there.

[T 20]

A cracking and very useful tool. Would love one but not cheap. Think you need one to reduce the OD on the 77/97 hardened comp tubes to ad bearings like the lazer glide. Defo not a "Bodge" tool

There is a cheap alternative Bruce.

This may sound like the biggest bodge anyone has ever admitted to on the BBS --- but it works.

Get a 4" angle grinder and mount it to a piece of steel to fit in the tool post of a lathe, I use the threaded holes for the grinder handle to mount off and put a jubilee clip round the body to give a third mounting point.

Set the grinder up in the tool post and run your lathe backwards and hey presto you have a tool post grinder.

I have used this setup to machine down the OD of bearing races and hardened steel bushes --- it's a bodge but it's also very accurate if you go easy with it.





All the best Mick

[nced]

Very interesting post, I have been using delrin o ring heads on my pistons, to good effect, there is no discernible wear after thousands of shots, The poly is an attempt to improve the already tame firing cycle, maybe I am already there.

Thanks for the feedback!

Think I'll give a Delrin piston cap a try so I'm wondering what side clearance you use.

Years ago I made a molly filled 6/6 nylon piston cap for my HW77k using my normal "piston cap to receiver tube" clearance which was .002 inches. The setup worked just fine till I accidently did a dry fire and after that the gun didn't function properly. Disassembling the "77" I found that the nylon piston cap had expanded with the impact of the dry fire and instead of havind a couple thou clearance it was a "tight fit". I later experimented with .005 inch side clearance with the nylon and deliberately dry fired the gun three times with no ill effects. The larger side clearance did work well, however I switched the cap back to 6061 T6 aluminum with my usual .002 clearance anyway.

[bmp01]

Nick,
I was interested in making some 'std' seals , I don't expect to be using O rings, do you mind sharing where you got the polyurethane from?

Thanks,
Byron.