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Advice required

Forming a radius on the ends of fine bore stainless tube

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Kevin Muir22/09/2020 11:01:12
8 forum posts

Hi for a lurker who has been learning a lot from the excellent content on this forum.

I have a project which requires me to round over and polish the ends of small bore stainless tube to leave a perfectly smooth finish on each end of the stainless tube. The tubes are in two different diameters, 1.6mm OD x 0.8mm ID and 1.4mm OD x 0.6mm ID. The outside radius is very easily achieved with fine files, home made emery boards with 1200 grit wet and dry paper and polishing paste and a suitable polishing strop made from leather.

Does anyone have advice on how I can form the inner radius of the stainless tubes?

My thoughts on a possible solution for this problem could be a long way away from the best solution but I will share my thoughts with you all as I have no previous experience of this type of task.

I thought about making a form tool from silver steel with the correct radius (or as near as I can achieve) then harden and temper the tool. I will then use this tool with fine grinding paste to grind the inner radius.

The actual volume of stainless that has to be removed is very small hence the form tool and fine grinding paste approach. The inner radius geometry is not super critical but a smooth burr free finish is very important.

If anyone has any advice on a better approach to this task, please share your experience, all advice greatly received. Thanks Kevin

roy entwistle22/09/2020 11:07:22
1245 forum posts

What, if any, machinery do you have access to ?

Emgee22/09/2020 11:11:06
1695 forum posts
225 photos

As the internal radius geometry is not critical could a 60 deg angle be cut on the end of the bore, this could be achieved with a BS0 centre drill. because of the very small wall thickness the tool would only need touching lightly on the rotating tube, finishing to a smooth surface with your chosen method.

Emgee

Tony Pratt 122/09/2020 11:24:46
1222 forum posts
5 photos

Sounds like you have a lathe, so your method with grinding paste should work, or grind a needle file to a suitable shape as a 'graver' and use that to remove the metal then polish with wet/dry to finish.

Tony

Kevin Muir22/09/2020 11:27:42
8 forum posts

I have an accurate lathe boxford 4 1/2", I have a mill but it is not up and running yet and a pillar drill. I also thought about making a collet for a variable speed "dremel" to hold each tube to assist with the grinding of the inner profile against the form tool with the grinding paste and against the strop with polishing compound . The inner profile at the ends of each tube does needs to be a radius and not a countersink, apologies

I should have made that clearer. Thanks Kevin

Edited By Kevin Muir on 22/09/2020 11:28:23

Edited By Kevin Muir on 22/09/2020 11:28:48

Edited By Kevin Muir on 22/09/2020 11:29:18

Baz22/09/2020 12:25:38
468 forum posts

Grind the teeth off a three square needle file and use it as a graver, with a bit of a polish with scotchbrite it will be lovely and smooth

Andy Gray 322/09/2020 16:58:44
17 forum posts

A cocktail stick (or other tapered sliver of wood) pushed into the end of the rotating tube and some polishing paste? 

If you apply some pressure to the end of the cocktail stick, the fibres in the taper will naturally compress leaving a sort of radiused transition where it enters the tube.

Edited By Andy Gray 3 on 22/09/2020 17:01:32

Bob Worsley16/10/2020 18:00:35
54 forum posts

The end of the tube belled out?

Easiest is to just use a centre punch and tap it with a hammer. The secret of success lies in making a clamp for the tube by clamping two pieces of steel together and drilling through the join with the outside diameter of the tube. I have only done larger diameters but the hole the same size as the od will grip enough. If necessary put a tiny bell mouth on the hole so the tube splays out.

Holding steel doesn't need to be more than 1/2" wide, I used 1.1/2" by 1/2" but was forming tube up to 1/2" diameter.

For a closing in forming use the same tube clamp but hit it with a Vee shaped hole in the end of a bar, I used a centre drill but you will need a busted one reground.

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