Hemispherical 'dimples' in brass - tool geometry

Hi. I have been asked by a friend (a potter) to make a reproduction of an 18th century 'rollet', which is a tool for embossing clay. The tool is essentially a brass disc maybe 25-30mm in diameter with somewhere near (not great on tolerances these potter chaps!) 3mm diameter hemispherical 'dimples' cut into the periphery. It is to be used in much the same manner as a knurling tool is on metal, but to emboss a pattern in leather-hard clay revolving on a lathe. I don't have any ball-end slot drills of the right sort of size, and anyway would like to be able to customise the diameter of the 'dimples' to suit his aesthetic sensibilities. I'm thinking I should be able to make cutters from silver steel, but I'm pretty clueless as to the appropriate profile (rake angles etc) for the business end. Any advice would be much appreciated.

Regards, Bob.

Edited By Robin Graham on 09/03/2014 02:36:49

Robin,

Most Brass [*] cuts quite happily with zero rake, or negative rake tools.

For this job, I think you should only need to turn the apropriate radius on your Silver Steel and then grind away half the diameter. ... You may even get away without hardening it.

MichaelG.

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[*] "Hard Leaded Engraving Brass" is the nicest to work with, but you would probably be O.K. with a slice from any available bar stock.

Believe it or not but back in the day I used to be a potter and if we needed anything like this we made our own out of clay and fired it in the kiln giving it infinite durability. Quick and easy to make with designs only limited by the individuals imagination. A brass one! Absolute luxury. How times have changed. . mumble . . mumble. . .

Rick

Hello Robin,

I imagine you could do a perfectly satisfactory job using the side flutes of an ordinary end mill cutter, it won't be a very demanding application in any case.

Regards

Brian

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Brian,

That would make excellent semi-cylindrical grooves; but I don't see how it could cut hemispherical "dimples".

MichaelG.

Michael,

Maybe I misread the brief, it was the reference to making the impressions on the periphery that I visualised. Otherwise I agree of course, hemisherical dimples need a ball ended tool. The other suggestion of firing one was perhaps better for durability.

Brian

Another approach altogether would be to use a softer disc - say of copper, and to create the dimples using a ball-ended punch. The copper could even be a thin sheet (1mm or 1.5) which could be punched while straight and then curved round and joined to form the rollet surface. And I suggest that aluminium would be OK too.

Ready made punches are available in sets from jewellery tool suppliers.

Just a thought - Tim

Do you want upstanding dimples on the brass or on the clay ? dimples cut ?
Nobby

Michael's suggestion should work fine. A filed dome on the end of a silver steel bar, then file half away should create a tool that works well.

Neil

In the periphery surface of the disc, drill suitable sized holes at equal distances. Insert balls in the holes, Solder up or glue the balls in the holes.

I'm also thinking of those feed sprockets for tractor fed paper on the old dot matrix printers.

Steve

Thanks for all your suggestions / comments. I made a 3mm cutter this afternoon as per Michael G's suggestion (I did actually harden it) and it worked beautifully, leaving an almost polished finish. I really should spend some time learning how to grind cutting tools - the very few I have made all leave cleaner finishes than I can achieve with bought tools.

The guy I'm doing this for is showed me a version he'd made himself from plastic, but the problem was that he couldn't align the 'dimples' on the centre line between the edges of the disc accurately enough to produce the effect he wants. I imagine he'll have thought about making them from clay, but presumably he'd have the same problem. I'll mention it to him nonetheless.

The idea of using the side flutes of an end mill, while not appropriate to this particular geometry, may well prove useful in future - he's asked me to think about other patterns I could produce. Though I'm obviously going to have to go CNC now...

Regards, Rob.

Edit - some more replies have appeared between my starting this and pressing the 'POST' button. Sorry if I wasn't clear in my original description - the dimples in the rollet are hollows, so the pattern on the clay would be raised 'bumps'. Though maybe the other way round may be of future interest, so thanks for ideas for doing it that way too. Tim, your idea is interesting and hadn't occurred to me. This is at an an early stage, (I know even less about pottery than metalwork!) so I don't know if it would work, but shall suggest it to my potter friend.

R.

Edited By Robin Graham on 09/03/2014 20:57:54