# Advice needed, how to reduce diameter of aluminium bar?



## Helius (Dec 8, 2006)

I have an aluminium bar with a diameter of 32mm which should fit inside an aluminium tube. Problem is the bars diameter is 0.03mm wider than the tubes inside diameter.

I don't have a lathe or any milling machinery so I am looking for ideas. Reduce the diameter of the bar or increase the inside diameter of the tube?

I was thinking of cutting a scrap bit of tube in half, lengthways and putting the two halfs around the bar with a bit of emery paper inbetween. With a lot of vigorous twisting of the bar I may be able to reduce the OD of the bar with a reasonable and consistent accuracy.

I don't want to use heat and cold since I need to be able to disassemble the bar and tube when needed.

Any better ideas?

Thanks.


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## JimmyM (Dec 8, 2006)

Sorry. I'm on english units. 0.03/25.4=0.0011"/2=0.00055"
You'd only have to remove 0.0006" from the surface to reduce the diam by .0011. Figure printer paper is over 0.0010 thick. That shouldn't be a problem the way you suggested. Use several hose clamps around the length of it to keep pressure more even. Use twisting and sliding to even things out. Sure, it won't be perfect, but if it has to be, then take it to a machine shop.


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## LEDcandle (Dec 8, 2006)

I'm no machinist, but I guess I would either wrap the bar round with sandpaper and start 'stroking' it, or use a round file and work on the insides of the alu tube, depending if the file is long enough to file the entire tube (or file by halves).

Of course, the end result will most prob not be perfectly round, depending on your manual skill. I hand-bored my 4D mag by halves, to fit AA batts inside 4 across, only to discover that at the right angle, the batts would fit without boring  The boring took me a good 4-5 days, a few hours each day, to trim away about 1+ mm (est).


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## modamag (Dec 8, 2006)

500 grit sand paper would do nicely like LEDcandle said. It would only take 5-10 minute depending on how long your rod is & Al alloy.

Another way is to use thermal gradient. Aluminum will expand ~ 0.1 % / 25C. So if you have a temperature gradient of 50C that will do the trick. The problem is it'll be near impossible to remove it once you get it in.

Good luck.


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## Anglepoise (Dec 8, 2006)

Work on the bar, not the tube.
Your idea of the split tube will work fine. Take your time and you will have a perfect fit.


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## Helius (Dec 9, 2006)

Thanks for the advice so far. I will give my split tube idea a go first of all. I want to see if it works since I am developing an idea for diy LED Bike lights that reqires the minimum amount of mechanical working. This should make it accessible to a wide range of people who only have a small cold garage and a wibbly wobbly hack saw.

If all else fails, I have discovered that my local college runs a model engineering and basic engineering skills course on tuesday evenings. To late for me to join, but if I hang around outside with my aluminium rod and some bottles of beer (as persuasion) I might get it reduced down.


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