Turning a 'dome' or half sphere..

[hotponyshoes]

Hi all,
I have to make up some aluminium parts, these are basicaly feet for a table.
The legs are made from 2" tube and I have spun up the top ends from alloy so they are the same o/d as the tube with a step down so they press fit into the tube.
1/2" inside the tube 1 1/2" outside the tube.

I would like to do the same for the feet only have the parts rounded, so it looks like half a ball on th eend of the tube.

The best effort I have made so far consists of turning a load of 'steps' into the part and finishing it off with a grinder!

Is there a good way of turning this on a manual lathe?
The only other way I have found is to draw a shape onto a bit of card, tape it to the bed by the cross slide and try to follow it by eye to shape the part. Again, It needed much work with the grinder!

Thanks!

[Kenre]

Easy! a piece of small dia rod, say 1/4, with the ends sharpened, placed between the head stock and cross slide. U will need a small hole in each to hold the rod. hold the carrige against the rod while winding the cross slide out.

Shorter the rod the more pronounced the dome. Experiment for best results.

[ErnieD]

Hi,

There are radius cutters that you can buy and also many that you can make yourself. If you use the search function on this forum you will find many references to these tools.

ErnieD

[Mcgyver]

your original approach, a bunch of steps, is a very good way to get the result you want, you just need to bring Pythagorean into the picture. Think of the arc the tool wants to make to cut a 1/2 sphere as a series of right angle triangles with the hypotenuse = radius. for any given advance of the carriage, its easy to compute the feed in of the cross slide so that the steps perfectly follow the arc of a sphere. if you make all the X steps say 5 thou, you end up with a stepped turning with very small steps. A few secs with a file and emery, and you have a near perfect sphere section. this works so well i've never bothered to make a radius attachment

kind of MNC instead of CNC


I've attached the spreadsheet i use. plug in the criteria and it tells you what to move. you'll need a graduated handle on the lathe lead screw (a useful accessory if you haven't done so yet) or you can use the compound.

[Geof]

The nice thing about Mcgyver's method is that you can do it with any curve that can be calculated; circle, ellipse, parabola. I have done it to make a mold for casting a parabolic shape in plastic.

But it is TEDIOUS .

[pepo]

The only thing I would add is if you use the step turning method is to use a tool with a large radius. It will help blend the steps in and remove some of the material you are now having to hand finish off. I have used the "pointed rod the length of the radius" method and it is quick,easy and amazingly accurate. Another way is to loosen the compound so it can rotate and get the tool behind the pivot point. Use the compounds dial to adjust radius.This will only work if the forces are over the compounds footprint.

[ErnieD]

Go to www.littlemachineshop.com and do a search for a ball turning attachment. Once you see it you will be able to make your own.

ErnieD