On 2/4/2022 9:50 PM, James Waldby wrote:
Bob La Londe <none@none.com99> wrote:
Ordinarily I can order PTFE rod the exact size I need to make PTFE pins.
All I have to do is drill and tap it for an internal reinforcing
screw. I was not able to order the exact size I needed this time
around. I had to order oversize rod to turn down to size. I was
wondering if you guys had any tips for getting a decent finish.
My plan is just to hand grind a medium radius tip turning tool, and hand
finish the final sharpening on the fine and extra fine diamond bench
stones. Diameter can be 0 to -.005 (maybe worse) with little ill
affect, but smoother would be better.
I haven't done much with PTFE but have seen videos using carbide
tooling, one where the guy seemed to know what he was doing and got
good results, and another that is really hard to watch (looks like
things are going to explode any moment, sounds like scraping instead
of cutting, big dangerously arranged chuck).
Here's the 2:31 video with clean good-sounding cuts, good chip, good
finish: <https://www.youtube.com/watch?v=wIqib3j1otk> (On the negative
side, one of the commenters asserts "this is not teflon . it is pp/nylon...teflon chips are not like this", and poster basically
replies is-so.
And the hard-to-watch 2:59 one, <https://www.youtube.com/watch?v=YysC4Zj_4Os>
There are several inconsistent recommendations in the thread <https://www.practicalmachinist.com/vb/general/so-what-secret-getting-good-finish-teflon-141213/>
with several people giving detailed advice about grinding HSS, with
small radius and lots of relief being a recurrent theme; and others
saying modern carbides are ok; and one or two claims that PCD, polycrystalline diamond, is what to use, and if you can use Delrin
instead maybe the reinforcing screw wouldn't be needed, or if you
could use HDPE the material would be a lot less expensive. (Perhaps
the latter have already been ruled out for whatever reason.)
I posted this same query 4 different places and received from one to
several responses in all of them. In the amateur group I got a lot of
the usual "You didn't provide enough information," and "You should do
something else" responses I am accustomed to seeing from groups with a
lot of people who are anxious to show how smart they are, but don't
necessarily all have the experience. A couple did suggest cryo-grinding
(my word). That was a bit exotic for me, but I can see how it might
work. Many skipped right over my comment about an insert and jumped
right to it being to flexible to turn. Generally not a lot of real
world basic practical responses, but very entertaining.
In the mostly professional group they didn't even comment on stiffness
given I had already mentioned an insert. They were pretty much split
right down the middle between hand ground positive rake very sharp HSS
with a radius, and high polish positive rake aluminum inserts. One guy
went off on a tangent about Teflon (tm) being only a .003 coating, and
when I pointed out I stock solid PTFE rod stock, he tried to say yeah,
not really, but close enough, and then went off on a rant about safety, employers, and PPE. Except for him the direct replies basically all
said very sharp tool, smooth polish, positive rake, and machine to
dimension in one pass unless it was beyond the scope of the tool. Now
some of the replies to the replies got a little bogged down in petty
self measuring and one-upsmanship, but the direct replies were succinct
and to the point.
Your response is a little bit of both, but much more balanced with
recognition that I probably already discounted some of the "do this
instead" alternatives with reason. Maybe good reason. Thank you. It
was a balanced reasonable response.
Before posting I was pretty sure it would machine just fine with the
approach I intended. I may switch to a carbide insert because I do have
some high polish aluminum cutting inserts and that would save me the
time to grind a tool. I was just checking to see if there was anything
special to it that I didn't know. I have enough stock I could waste a
little.
And just for fun.
Teflon(tm) is a trade name. All Teflon is PTFE, but not all PTFE is
Teflon.
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