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Machining PTFE, as we have touched upon before,
is never a straightforward process. Most machining handbooks will suggest that
PTFE should be treated much like wood when it comes to machining, as this is
the material it most closely behaves like when machined. And while this is a
good starting point for tool selection and CNC programme settings, as we delve
deeper into the aspects of machining PTFE, we see that it behaves much like
it’s own material. So learning by doing becomes the only option – since PTFE is
a niche product (when compared with other known polymers).
Recently, we faced an interesting issue when
creating a rather complex part. The part is approximately 200 Grams in weight
and machining it involved multiple operations including CNC turning, CNC
milling, drilling and finally tapping. All in all, the drawing highlighted over
28 dimensions that needed to be within a strict tolerance and it took us the
better part of a week to just get 10 prototypes ready.
We were pretty happy with the result:
everything measured, as it should. We almost didn’t check the tapping – which
called for an M3 tap in two places. The M3 taps used were brand new and the
first tap was done on the VMC as part of the programme – so there was no way it
could be an issue, we thought. But we were wrong.
The no-go gauge entered in the hole all too
easily and we were pretty shocked to realise that even an M3 bolt was sitting
loose in the hole. At first we though we had the wrong tap – which we didn’t.
We then argued that the gauge would always enter – as it was designed mainly
for harder materials and PTFE would yield all too easily, since it was much
softer. To check this we used the same taps on a mild steel plate and confirmed
that the no-go did not enter. But this still did not answer why the bolt itself
was loose.
We searched extensively for an answer online,
but there was very little information on tapping and even less on the issue we
in particular were facing. We then decided to start experimenting with
different combinations of taps and drill holes.
On the part, we had used a 2.2mm drill with all
3 taps. The first tap was done on our VMC, while the next 2 were done by hand.
We tried the following combinations:
Drill
Hole
|
Tap 1
|
Tap 2
|
Tap 3
|
Result
|
Remark
|
1.5
|
Y
|
Y
|
Y
|
Reject
|
Bolt
loose
|
1.5
|
Y
|
Reject
|
Bolt
loose
|
||
1.5
|
Y
|
Reject
|
Bolt
tight
|
||
2
|
Y
|
Y
|
Y
|
Reject
|
Bolt
tight
|
2
|
Y
|
Reject
|
Bolt
loose
|
||
2.2
|
Y
|
Y
|
Y
|
Reject
|
Bolt
loose
|
2.2
|
Y
|
Reject
|
Bolt
loose
|
In a couple of cases – where we used only the 3rd
(finest) tap, the bolt was tight. However, none of the holes were answering to
the no-go gauge, which passed equally easily in all the holes. We once again
argued that this was a PTFE related issue and that as long as the bolt was
tight, it should not be a problem. But many of the consultants and experts I
spoke with said that they had come across parts in PTFE that answered to the
no-go gauge, and hence there must be a way to machine such a part.
The problem was finally solved when an engineer
in our client’s side suggested we use a “Form Tap”. I had never heard of a form
tap and when I searched it, it seemed to apply mainly to tapping soft metals
(such a aluminium). There was no mention of applications to PTFE. Nonetheless,
it was our last shot, so we tried it and were pleasantly surprised.
We eventually went with a 2.0mm drill and an M3
form tap to get a result that was both functionally good and which answered to
the gauge.
The reason the form tap works, is because
unlike a regular tap, it does not bore into the PTFE, taking out material as it
does. Instead, it merely forms the tap profile within the drilled hole and as
PTFE is soft, it yields quite easily. The result is that the tapped hole is
much fuller than when a normal M3 tap
is used – making it tighter and ensuring the pitch profile does not yield to
the no-go gauge.
Surprisingly, this does again strengthen the
PTFE-Wood similarity in machining. Tapping is unheard of in wood; a screw can
be passed through a drilled hole and sit tight forever! In many ways, a form
tap is nothing more than passing a screw/bolt into the PTFE to imprint its
profile within the hole. Only that the form tap is possibly more exact and can
ensure that the resulting tap is accepted when inspected with the correct
gauges!