Couldn't find my old notes, sorry, but I do recall that I wrote a custom program to make fresnels- the problem I had was tool radius compensation: as you know the individual facets of the fresnel lens are pointy triangles, with one face being parallel to the Z axis and the other face being angled tothe Z axis and decreasing in angle as you travel away from the lens centre (X0) these individual "prisms" add up to act like a lens. The machine I was using would try to apply tool radius compensation at the interface between these two surfaces causing the tool to move in X and plunging the flat facet.
I think the way I got round it was to get a half radius tool (from Contour Fine Tooling). I then "lied" to the machine that the tool radius was zero and factored in the compensation into my X and Z co-ordinates and used no tool radius compensation in the M/G-Code.
Note that because the half radius has to be very small you
will get massive tool wear resulting in a poor finish. I made this a
bit better by post polishing with "Magic" polishing compound and a cotton bud (serious) and hard coating it.
By the end of the project though, I was seriously wishing I could use a form tool and rotate it for each facet and just "poke" cut (like an "r theta" machine). I even thought of sticking an adjustable wedge underneath the component and tweaking it for each facet, butbalancing that would have been a nightmare.
Contour might be able to give you more info- the guy who heads up Engineering there worked on fresnels with me at a different company- he'd probably be interested in helping you since he supplies the tools- they aint cheap, about 400 quid each, but they can be re-lapped at about 25 quid a pop. PM me and I'll give you their details.
I have no commercial interest in Contour, just a very satisfied customer, there are two other suppliers I know of, but I've stuck with Contour for 20 years
Oh, if you are buying one of these tools buy two or three- you will crash at least once (don't ask how I know this lol), you need monocrystaline natural diamond- polycrystaline gives a poorer finish, you want a conical controlled radius with a top rake of zero for plastics and aluminium/ copper/ brass. [Re-Edit] Oh, watch your front clearance if it's a small diameter frensel you're making for although the typical fresnel had a spherical surface at X0, Z0, as you go out to X+ve on the primary facets the bottom of the tool (the shank as I call it) can hit the surface you just cut causing the part to fly off the spindle at a rapid rae of knots, generaly in the direction of your face as you peer intently at it to figure out what the noise is (again, don't ask lol).
You cannot diamond turn any ferous metals by the way, the diamond interacts with the carbon and changes bonding at the atomic level.
[Edit]Forgot: there's only one other guy on these forums that I know of who does diamond turning- PM me and I'll send you his name.[/Edit]
[re-re-Edit]Oh, and another thing: keep the humidity in your shop high or use spray mist with water to stop static build up on your diamond- plastics will wear your diamond tool about 5 times faster than, say, copper or brass due tostatic charge.[/re-re-Edit]
09-21-2010, 01:39 PM
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