Machining a curve

I'm trying to figure out how to make the part below. It's a "bell" that is used for crimping crown caps onto glass bottles. Most of it is simple turning and threading, but I'm not sure how to tackle the "bell mouth". My best guess so far so to successively approximate the curve using the compound, and then use some kind of abrasive to blend it together.

I don't think the exact nature of the curve is critical, just that it is smooth and gradual, and that the minimum internal diameter is spot on.

Is there a better way?

With care you can probably do it with the top slide & cut an angle then blend with a decent half round file.

Tony

I'd approximate it with a few tangents, 4 or 5 should do it, then blend them with a file.

Ok, done. Thanks for the prompt feedback.

You could also use a full size filed sheetmetal template clamped to the bed of the lathe as a guide, with a follower "tooth" or stylus on the compound. The cuts are made as you would with X Y movements to do the cuts, but by aligning the tooth to a guide rather than by using the dials to approximate the curve as you mentioned.

Easiest way to make such a template is to print out your section from CAD, check the print is to scale, then use spray glue or glue stick to attach the paper to the sheetmetal. Cut and file the sheetmetal to your printed line. This method can yield very accurate curves.

I'd do it by eye, but cutting outwards so that the critical inner diameter was correct.

Surely a bottle opener rather than a closer is going to be more useful this Christmas?

Neil

I'd draw a series of lines on your CAD model at regular intervals and then measure their lengths. You then have a set of co-ordinates to cut to moving the topslide out in equal amounts and the cross slide adjusted to the measured lengths. This will give the profile ias a series of steps that can then be blended, in this case internal are best done with a hand graver and external can be done with files.

This sort of thing

Bit hard to see the 10thou steps on this one

And blended

Edited By JasonB on 16/12/2019 16:28:08

Not seen as such a challenge but G02 and G03 work best for me.

Emgee

Don't see why it needs to be a curve. A plain taper and final parallel section would work just as well.

Andrew

Variation on the above answers. Not intended as a definitive answer I ask because am not sure.

Could you rough it out as suggested and then use a form tool ? If too much chatter then I have read that chatter could be reduced if run the lathe backwards and tool mounted on back toolpost ?

Variation on form tool - curved piece of wood with sandpaper wrapped around it ?

Bill

Bill

Thread started at 15:47:45.

“Ok, done. Thanks for the prompt feedback.” That post was made less than 10 1/2 minutes later at 15:58:15!

So, not a difficult solution? Doing it must have been a simple operation.