Turning a long thin airgun barrel

[Dave_424]

Hello, I have to turn an airgun barrel's OD true to the bore. I was thinking of putting the barrel between centres and using a travelling steady and a really sharp tool bit. Is this the best way? Also, its either my headstock is out of alignment or my ways are worn but I think my lathe turns a taper, however this might just be the work flexing away from the tool. My lathe is a Colchester Chipmaster.

Thanks

Dave

 

[rake60]

I have never operated any lathe that would cut the same size over a long length.

CNC lathes are easy. You just lie to it and program different diameters where it cuts
bigger or smaller so it will cut straight.

On a manual lathe it turns a bit low tech.
Take a light test cut and mic the diameter every 1/2".
I use a black Sharpie Marker to mark those locations on the piece.
Where the size is .001 over I bump it is .0005 or bump it out if it's under at that point.

I'm sure someone has a lathe that will cut within .00001" over a 3 foot length.
Just ask to visit his shop for a demonstration.

Rick

 

[stevehuckss396]

I'm sure someone has a lathe that will cut within .00001" over a 3 foot length.

I can't get within 3ft in a .00001 length!!

 

[Entropy455]

Consider for a moment that steel has a thermal expansion coefficient of about 6 millionths of an inch, per inch, per degree F.

This means that a perfectly round 1-inch diameter steel bar at 68 degrees F, will grow in diameter by 0.000012 inches, if it is warmed up by only 2 degrees F.

The rise in temperature from machining operations is normally much more than 2-degrees F – which completely blows the tolerances you’re attempting to achieve.

The type of precision you’re seeking is possible, with temperature controlled flood-coolant, and grinding between centers, on extreme-high-precision machines.

The question is why in the world do you need this level of precision?

A 17 degree change in temperature will cause a 1” bar of steel to grow (or shrink) in diameter by ten times the tolarance you're seeking to achieve. . . .

 

[stevehuckss396]

The type of precision you're seeking is possible, with temperature controlled flood-coolant, and grinding between centers, on extreme-high-precision machines.

The original poster never mentioned tolerance, just wanted to know the best way to go about it. I believe the only tolerance mentioned was a hypothetical situation.

 

[Tin Falcon]

Hello, I have to turn an airgun barrel's OD true to the bore. I was thinking of putting the barrel between centres and using a travelling steady and a really sharp tool bit. Is this the best way?

Never heard of a traveling steady rest . It may be called that in some parts of the world. You need a follow rest. and you need to set your tail stock so it is true on both axis . IIRC there have been some build posts on follow rests for a quick change.
Tin

 

[rake60]

Never heard of a traveling steady rest . It may be called that in some parts of the world. You need a follow rest. and you need to set your tail stock so it is true on both axis . IIRC there have been some build posts on follow rests for a quick change.
Tin

I'm sure Dave was referring to a follow rest.
I have two of them. Tried to use both of them several times.
Some of those nicks in the paint on the walls of my basement hobby shop are indirect
results of those attempts.

They didn't work well, I was unhappy with the result, remove the follow rest
and throw it at the wall.......

Come on now, like I'm the ONLY ONE who does that? LOL

Rick

 

[Dave_424]

There is no tolerance mentioned, but closest is best I know that dimensions will change with temperature and a change in length will cause the barrel to bow, however this isn't an ultra high precision piece, but I would like to get it as accurate as I can (pun intended)

I couldn't remember what they were called but yes a follow rest, but its not a roller bearing type, but either brass or copper, I haven't had a proper look at it yet. If it does cut a taper it doesn't matter so much, also the barrel isn't cut along the whole length. About a 10" length of the 18" barrel. A taper isn't bad because there are 2 bushings that are pressed onto the cut down portion, one up at the shoulder of the cut and another half way along the cut. Its more important that the OD of the taper is concentric to the bore. I will also true up the bushes with the bore once they are on the barrel.

I'll just take it nice and slow, making sure that the centre in the headstock and the centre tailstock are lined up, use a nice sharp honed toolbit with lots of coolant and see how it goes.

Thanks for the input

Dave

 

[Mike N]

Hello, I have to turn an airgun barrel's OD true to the bore. I was thinking of putting the barrel between centres and using a travelling steady and a really sharp tool bit. Is this the best way? Also, its either my headstock is out of alignment or my ways are worn but I think my lathe turns a taper, however this might just be the work flexing away from the tool. My lathe is a Colchester Chipmaster.

Thanks

Dave

What is the ID, OD & length of the barrel. What tolerance are you trying to hold?

 

[Dave_424]

ID is 6.35mm, OD is about 13mm length is about 450mm, I'm at university at the moment and don't have the part in front of me but going home in 2 days and want something to do over christmas.

Dave

 

[Ken I]

Never heard of a traveling steady rest . It may be called that in some parts of the world.

I've only ever heard it called a "travelling steady" in 40 years in engineering - so must be local vernacular.

Ken

 

[Tin Falcon]

Well Ken we are from different parts of the world ;D

 

[Springbok]

Firstly a travel steady to put it simple terms is a device that the rod goese through the 3 pins are lightly set up to hold the work steady the travel is attatched to the bed of the lathe. Anyway why would you want to make a gun they only kill things and are illegal in my country.

Have a nice christmas and a good new year

Bob.

 

[Tin Falcon]

Anyway why would you want to make a gun they only kill things and are illegal in my country.

please keep the political comments to yourself such comments get thing stirred up way to fast.
But please do not break local laws.
Tin

 

[rustyknife]

If I have a piece that I want to cut a longish section and make it pretty darn close in tolerance, I generally try to turn small portions like 1/2 to 1 inch at a time. Measure often, move over and cut again, until I have cut my entire piece.

For instance if I want 6 inches of a rod turned down, I may do it in 6 separate cutting operations, then polish with emery. Its slow and tedious but obtains good results.

I've even done this with small pieces that I want very true.

 

[Springbok]

Tin
Please Please Please do not regard that as a political comment as I have had to many old friends killed.

So Back to the subject.
I suspect that this lathe is 2MT so have you 2 good fixed centres so you can put into both ends of the lathe this is a chinese model so will be extremely out of true bring up both points together and adjust you tailstock accordingly. There are other methods useing a known true bar. The fixed steady is used inbetween the long bar of metal to stop the bar flexing without changing any settings
this is a device that fixes to the bed usually on the front of the tailstock and has normally 3 prongs of copper this when loosened can be slid up and down the bed. Do not loosen the the steady prongs untill you have completed. This hopefully will give you an extremely accurate rod. Knowing armaments as I do has it any gunsight calibration equipment or threading on it, if so you have problems and anyway if you start turning a bit of gunmetal without tempering you will end up with a banana as a barrel

Merry Christmas

Bob

 

[steamboatmodel]

Hello, I have to turn an airgun barrel's OD true to the bore. I was thinking of putting the barrel between centres and using a travelling steady and a really sharp tool bit. Is this the best way? Also, its either my headstock is out of alignment or my ways are worn but I think my lathe turns a taper, however this might just be the work flexing away from the tool. My lathe is a Colchester Chipmaster.

Thanks

Dave

Hi Dave,
I am curious as to why you are doing this?
Regards,
Gerald.

 

[woodchip85]

Just remember, if your changing a barrel over in a air rifle and increase the length (I know you've only asked about how to turn the OD) you will also increase the ft-lbs of said !!!FIREARM!!! I say fire arm because most newer air rifles especially PCPs are extremely near the legal 12ft-lbs in the UK and going such a mod may put you over, in which case you will need a firearms certificate. I would recommend speaking to a gunsmith and having you rifle crono-graphed before doing anything.

 

[kwoodhands]

I'm sure Dave was referring to a follow rest.
I have two of them. Tried to use both of them several times.
Some of those nicks in the paint on the walls of my basement hobby shop are indirect
results of those attempts.

They didn't work well, I was unhappy with the result, remove the follow rest
and throw it at the wall.......

Come on now, like I'm the ONLY ONE who does that? LOL

Rick

I have never thrown my follow rest against the wall,thought about it though.
Mine sits on a shelf ,hopefully never to be used.
mike

 

[Dave_424]

Guns only kill things if you shoot at a living thing. What about target shooting. Also what if part of your job is pest control and you have to dispatch pests?

I know that lengthening a barrel will increase its power, usually about 25fps per inch of barrel, in .22 anyways. The barrel and valve parts are being made by me so it will all be tuned below the legal limit of 12 ft/lbs when complete.

The reason in turning down the OD is that the barrel is actually held in place by the OD of the barrel using some bushings about half way down the barrel. Ive decided to attach the bushings to the barrel, then true up the OD of the bushings relative to the bore rather than doing the whole barrel.

This isn't my first airgun I've built, probably about my fifth.

The barrels are made of a high lead content steel so are easy to work with and no tempering is needed. Why would threading be a problem.

Im not using a chinese mini lathe, its a medium sized piece of good old heavy english steel.

I'll get a barrel ordered after christmas and see how it goes

Dave