Very light but deep boring

How are you holding the work? Seems like a 3 or 4 jaw chuck would want to deform the tube and therefore result in an odd bore when released. If you've got a 6 jaw that might work.
 
Dave, this is a tall order. You're looking at taking a 0.0025" DOC over 12" and require a precision fit all the way down the bar. If you have even a tiny bit of deflection of the bar, and you will at that length, then you are unlikely to hit the required ID with the precision you require. Line boring "might" do it if the set up is perfect.

Thinking about it, I see three possible options. One, you could start with a solid bar and bore it with a solid carbide boring bar so that you can control depths of cut. A solid carbide 3/4" bar will get you half way and you can flip it and bore from the other side. If the set up is precisely controlled, you might meet in the middle. Two, you can try using your current bar and run a reamer through it from both ends. Not sure this would work but working with a reamer manufacturer might get you there. Third, what about lapping it to size? I'm not sure how you would do it but I imagine a line boring set up but use abrasive instead of a boring bit. This would allow you to cut and check, cut and check. If the lapping bar is solid enough and you run slow enough, it might work ... maybe.

I've done #1 and #2 but not over a 12" length so I cannot say for sure that it will work. Good luck with this one - as I said, tall order.
 
mikey said:
Third, what about lapping it to size? I'm not sure how you would do it but I imagine a line boring set up but use abrasive instead of a boring bit. This would allow you to cut and check, cut and check. If the lapping bar is solid enough and you run slow enough, it might work ... maybe.
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A brake cylinder hone would be the right size. The arms are spring loaded. can usually go down to 1/2"
 

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mikey said:
Dave, this is a tall order. You're looking at taking a 0.0025" DOC over 12" and require a precision fit all the way down the bar. If you have even a tiny bit of deflection of the bar, and you will at that length, then you are unlikely to hit the required ID with the precision you require. Line boring "might" do it if the set up is perfect.
Click to expand...

A tall order indeed. First questions first: Is the original piece already round both inside and outside to 0.001" if it is not, it is not likely to turn out well.
 
How about making a broach and pushing or pulling it through the tube. It will probably change the outside diameter but that can be fixed easily.
You could make a mandrel that the broach bolts onto the end. Either make a radius or just a shallow angle. You may have to start a little small and possibly make it larger to get your required ID.

Joe
 
If this is extruded Aluminum Tube, then odds are it is not round or concentric.

Is your tube already cut to final length or do you have an extra inch or 2 to work with?
As mentioned a 3 or 4 jaw chuck will distort the end and it will not be round once the chuck is opened.
If you have a bit of extra material you could make a light press fit plug for the very end of the tube to hold it round in the chuck. This plug could be as thin as 1/4 inch, but the longer the better. Just be sure that you press it in square.
The other end will need either follow or steady rest to hold it still.
Because of the light cut the reamer would probably work best, go very slow and lots of lube.
 
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