I am trying to calculate the power/energy used when drilling in
various types of materials.  There are some good empirical
calculations in the "Machinery's Handbook" (I used the 24th edition)
for drilling in various metals.  The equations in this book determine
the power required to drill in a material using an experimentally
determined constant called the "Work Material Factor" , Kd.  In other
references the value is referred to as "Work Material Constant", K. 
The tables I have seen for this constant list only metals, and the
values range from 7000 for leaded brass to 34000 for hard steel.
I am interested, however, in calculating the power required to drill
in certain thermoplastics, especially polyvinyl chloride (PVC).  So I
am specifically looking for a Work Material Factor value for PVC or a
similar hard plastic.  Since I am trying to do a careful analysis it
will be necessary to provide a reference for the value.  Thanks for
your help.

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Request for Question Clarification by guillermo-ga on 23 Dec 2003 13:54 PST

Dear griffy-ga,

Thanks for such an interesting question. So far, I?ve found several
charts that show values for PVC ?tensile strength? ?what, I know, is
not what you?re looking for. However, I?ve also found an academic
paper introducing a theory of metal cutting that in one of its steps
relates ?tensile strength? to ?work material constant?, so a formula
to calculate the latter from the former is obtained. Would this
information altogether make an answer for you? Thanks for your help.

Regards,

Guillermo

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Clarification of Question by griffy-ga on 23 Dec 2003 14:40 PST

Guillermo-

Thank you for looking into this for me. 
The analysis in general has to be very specifically for drilling, not
cutting.  It turns out that the chisel edge of a drill removes the
metal by a very complex combination of extrusion and cutting, so the
equations as described in the Machinery's Handbook are different for
other cutting operations such as turning.

I had previously found an article by Toropov and Ko that is similar to
what you describe.  Unfortunately the numbers do not seem to apply to
the drilling case.  They state that essentially the constant, whch
they call Kf, is proportional to the tensile strength.  When I compare
PVC (tensile strength of around 40 MPa for rigid PVC) with Cast Iron
(tensile strength of 150 MPa) the K constant seems too very low to me.
 For drilling I have seen other analyses (by other methods) that show
PVC to be harder to drill than Cast Iron.  On the advice of an expert
I tried plotting Yield Strength for a number of materials and it
doesn't look right either.  So I believe there is more to the problem
than one single value.

I'm not sure there is an easy answer out there; I even called the
company that provided the table for the handbook and they couldn't
help me.  So perhaps we should call off the search.  I appreciate your
effort but I'm not sure this is worth the effort.

Hello, griffy-ga

Thank you for your prompt response. I post this as a comment so you
don?t need to post a reply. The article we are talking about seems to
be the same indeed ?same subject, same authors, same name for the
value of your interest (Kf, constant of plasticity). I had considered
probable that the values for cutting process would not be of use for
drilling, and also the fact that Tuporov-Ko?s theory is for metals,
not plastics. Anyway, it was the closest I had arrived to what you?d
asked for, and I wasn?t certain it wouldn?t work.
 
After your clarification I kept on searching other sources with no
better results. I?ve reached my blind alley in this search, maybe
another fellow researcher?s abilities will do better. Yet, I?ve
enjoyed this search: learned a lot. Thank you.

Guillermo