31.6 ID steel tubing

So now that my Winter frame project is almost done I am starting to think about the next job, making a SPT. For those young ones a SPT is a stem like device that hangs off the back of a seat post and allows a handle bar bag to be mounted behind the post.

The post diameter I need to fit is 31.6. In looking at the commonly available 4130 tubing I don't find anything without a heavy wall. The closest i've seen is 1.375"x.049". This is both a bit thicker walled then i'd like as well as slightly larger an ID then best. In an ideal world I'd like to use something like a .035" wall if I could.

My question for you all is do you know of a steel tube with an ID of 31.6 (or a bit less) and a wall of .035 (or slightly thicker)? A second question is whether any one would turn down a aprox 6" length of tubing (like 1.375"x.065") to a .035"/.040" wall?

My lathe is in storage so I can't do the machining work. Thanks, Andy.

Quote:

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Originally Posted by Andrew R Stewart

So now that my Winter frame project is almost done I am starting to think about the next job, making a SPT. For those young ones a SPT is a stem like device that hangs off the back of a seat post and allows a handle bar bag to be mounted behind the post.

The post diameter I need to fit is 31.6. In looking at the commonly available 4130 tubing I don't find anything without a heavy wall. The closest i've seen is 1.375"x.049". This is both a bit thicker walled then i'd like as well as slightly larger an ID then best. In an ideal world I'd like to use something like a .035" wall if I could.

My question for you all is do you know of a steel tube with an ID of 31.6 (or a bit less) and a wall of .035 (or slightly thicker)? A second question is whether any one would turn down a aprox 6" length of tubing (like 1.375"x.065") to a .035"/.040" wall?

My lathe is in storage so I can't do the machining work. Thanks, Andy.

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I would use the 1.375x0.65 and cut/drill windows in the sides.

Online metals has 1.375 x 0.035 4130
Order Alloy Steel 4130 Tube in Small Quantities at OnlineMetals.com

but the ID is 1.305
Shiggy's 0.065 suggestion has ID of 1.245, should just fit 31.6mm/1.24409
Order Alloy Steel 4130 Tube in Small Quantities at OnlineMetals.com
flex hone it to fit?
cheers
andy walker

going to a smaller 1.25 tube, the ID of that with 0.065 wall thickness is 1.2"
might work that to get to 1.24409,
how long would that take?
Order Alloy Steel 4130 Tube in Small Quantities at OnlineMetals.com
cheers
andy walker

I think reducing the wall thickness is the wrong place to try to save weight. I have made similar weight-bearing items and had the metal tear.

My design would be something like this, with the full thickness on the clamp tube.

(Attachment coming)

Quote:

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Originally Posted by shiggy

I think reducing the wall thickness is the wrong place to try to save weight. I have made similar weight-bearing items and had the metal tear.

My design would be something like this, with the full thickness on the clamp tube.

(Attachment coming)

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Attachment 50778
Crude drawing but you get the idea

Attachment 50784Attachment 50785Shiggy- I have built this kind of device more then a few times. The wall thickness (.035") has been found to be quite sufficient. Your suggestion of adding (subtracting?) lightening holes is also something I've done before. I'll try to add a couple of photos of two different SPTs to show my approach (but not sure if photos will attach this time. Have been unable to do so for a while.) Andy.