Q: How to slice off very short lengths of metal tubing?

[mmagliaro]

I want to make some very very short spacer bushings for my steam loco driver's crankpins.  I have lengths of very small diameter hypodermic stainless steel tubing (about .039" OD).  I want to slice off little "rings" of it, perhaps .025" long or so.
Needless to say, being stainless steel, it does not cut easily.  I have had success in the past cutting this stuff with a Dremel cutoff disk or even a jeweler's saw (like a Zona).  The problem here is that the little pieces I want to cut are so short, and additionally, I really need to get square, cleanly-cut ends.  These little rings are going to go around my crankpins as bushings for the siderod holes.

I am working on a jig in a block of metal.  (Block of metal with a hole drilled in it for the tubing, with a thin slot cut in it just "so", so that I can insert the tubing and then guide a saw blade in the slot to cut off neat little bits).  But it's still going to be a bit of a crap shoot to get perfect spacers.

Any better ideas?  Is there some simple way to cut off neat little bits of this stuff that I don't see?
Thanks.

[Chris333]

That micro lux (or Harbor Freight) mini miter saw is the only thing I can think of.
http://www.harborfreight.com/2-in-mini-bench-top-cut-off-saw-62136.html

I've never owned one though.

[mark.hinds]

When I have done this sort of thing in the past, I have done it in 2 steps.  However, I have been doing this with either brass or plastic, and not stainless steel.  Whether you can achieve the precision you need using my crude technique is the question. 

 1. Chuck the tubing into a rotating tool (electric drill, Dremel, or whatever), and square up the end with a file.  Obviously, the fact that the tubing is rotating allows any reasonably skilled person to get a good result on this first step.
 
 2. However, the problem is the second step.  With the tubing still in the rotating tool, I then cut off a section by eye (…) slightly longer than I need.  Then I take that piece, and file it down, painstakingly on a sharp, fine-grained metal file, clamped to my workbench.  I push down on the piece with my index finger, with the rough side down, and go very slowly, rotating the piece frequently to cancel out the inevitable uneven finger pressure, checking periodically with a micrometer.  This is also hard on the finger if one is doing many such pieces. 

 3. Instead of my step 2 above, perhaps you could temporarily adhere the unfinished piece to a metal rod (of a diameter which fits snugly inside the section of tubing), chuck the rod into the rotating tool, and gently clean up the rough end with a file, periodically checking the length.  Then un-adhere the piece from the rod. 

[Burlington Bob]

I agree with Mark.  If you had a way to use a steel block to hold the cutoff, you might be able to file the other end like you would if you were using a Fast Tracks jig.

[mark.hinds]

I agree with Mark.  If you had a way to use a steel block to hold the cutoff, you might be able to file the other end like you would if you were using a Fast Tracks jig.

The key to getting the end square is to have the material being worked on rotate, of course.  Probably obvious, but I just wanted to emphasize that. 

[Burlington Bob]

Use a steel block AFTER the first step of using a drill and file or a lathe and file or tooling.  Then place the excess length tube in a steel block with a slightly bigger hole that has a plug inserted to hold the tube at the correct length.  Putting the square end down will allow the non-square end to protrude above the surface of the steel block for filing to the correct length and also be square to the other end.  This is dependent upon the hole being perpendicular to the steel block, of course.

[Burlington Bob]

Exactly how robust do these have to be?  If you have a wall thickness of ,005"  that will leave an opening of .029", which means a pin diameter of less than .027".  That's very tight tolerances for something like that.  And VERY delicate!

[mark.hinds]

Sorry Bob; I didn't realize that you were suggesting an alternative for my "step 2".  That could work. 

Another alternative to my step 2 might be the following.  Clamp the rotating tool (drill / Dremel) sideways to the workbench, with the axis of rotation horizontal.  Chuck the tubing into the tool, after the free end has been "trued up" as in my step 1.  At the free end of the tubing, you have a metal or plastic block, with a shallow hole drilled in it (less than .025"), into which the tubing is inserted; this provides support at the free end.  Clamp a shim to the block, sufficiently thick so that with the tubing inserted into the hole, and with a saw blade held against the shim, you can gently press the saw blade down into the rotating tubing to cut a piece of it off.  With some trial and error, you could adjust the shim thickness to allow the saw blade to reliably cut off a section of tubing to the desired .025" length.  When I do something like this, I use my resistance soldering foot switch to power the rotating tool (drill / Dremel), so I have 2 hands free.   if needed, clean up the part on a file, as in my original step 2. 

MH

[Burlington Bob]

No problem, Mark.  I reread the post and realized these are just spacers for the crank pins.  Hopefully, one way or another, these will get made.

[Sokramiketes]

Here are the suggested methods for Ngineering stainless steel tubing:

http://www.ngineering.com/tubing_cutting_tips.htm

[mmagliaro]

Use a steel block AFTER the first step of using a drill and file or a lathe and file or tooling.  Then place the excess length tube in a steel block with a slightly bigger hole that has a plug inserted to hold the tube at the correct length.  Putting the square end down will allow the non-square end to protrude above the surface of the steel block for filing to the correct length and also be square to the other end.  This is dependent upon the hole being perpendicular to the steel block, of course.

Love this idea!  This could really work.
And as to the tolerances.. the crankpins are M0.7 (yes, they do make screws and bolts smaller than M1.0 !! )
so they are 0.0275" and they just slip inside this stainless tubing, which is just exactly what I want.  These are crankpins that have to actually turn the drivers, on a little 0-6-0 switcher, so they need to be in scale, strong, and extremely accurate.

[mark.hinds]

Love this idea!  This could really work.
And as to the tolerances.. the crankpins are M0.7 (yes, they do make screws and bolts smaller than M1.0 !! )
so they are 0.0275" and they just slip inside this stainless tubing, which is just exactly what I want.  These are crankpins that have to actually turn the drivers, on a little 0-6-0 switcher, so they need to be in scale, strong, and extremely accurate.

One thing which you may encounter if trying this is, because of tolerances between the OD of the tubing and the ID of the hole, and because the .025" length of the tubing is less than its .039" diameter, the tubing may twist slightly in the hole when being filed.  This might result in the "near" end being filed more than the "far" end, so the final surface may not be exactly square.  To minimize this, it might be necessary to file from several directions. 

[Burlington Bob]

Thanks and hopefully it works.  Let us know.

[mmagliaro]

Actually, I think it does work.  I made one tonight.  Yes, I already planned to go round and round filing from multiple directions.    I had to drill a few holes before I got one just right so that the little piece of tubing sat down in it and, after filing it as flush as I could with the face of the metal block, it would come out about .025" long within a few thou.

Let me make a few more.  I'll post photos in a day or two.

[Burlington Bob]

That sounds very encouraging!  Waiting for pictures.