A STEP AHEAD With Patrick De Vos

”If the milling machinist works  with the same cutting conditions as the designer of the cutting edge, he  is able  to optimize and  predict the cutting behaviour.”

Patrick de Vos is Seco’s Global STEP (Seco Technical  Edu- cation Programme)  Master. In every  is- sue, Patrick will  be discussing a specific aspect of machining and sharing his know- how with  New Edge readers. In this issue Patrick discusses the role  of average chip thickness in milling operations.

The term “chip thickness” comes  from  the  theory  of gen- eral  machining.  It is the  thickness (measured  perpendicularly to the cutting   edge)  of  the  material  to  be machined.  The following relationship exists  between  the  feed  and  the  chip thickness:  h  = f  x  sin  K.  whereby  his  the  chip  thickness, f the feed  and K the  entering angle.  With  turning tools,   the   entering   angle   is   usually 93°  or  95°.  Feed  and  chip  thickness are then virtually  the same.

With milling, the chip thickness is determined in the same way. Because milling is a ‘two-dimensional’ operation the basic concept must be applied correctly. In a typical milling operation, the thickness of the material located just in front of the cutting edge changes constantly. The result of this is that with a certain feed the chip thickness changes accordingly.

That is why the term ‘average chip thickness’ was introduced: the thickness of a “theoretical” rectangular chip with the same surface area and length as the actual chip.

Second dimension

With milling, a relationship between the average chip thickness, the milling method, the feed per tooth and the entering angle can be defined. In this case, the ‘second dimension’ needs to be taken into account during the operation. This is achieved by applying the ae/Dc ratio, whereby Dc is the diameter of the cutter and ae is the radial cutting depth during the operation. 

Why is the average chip thickness so important? All research into the behaviour and possibilities of the different cutting    edge   geometries   are largely  based  on the  used  (or desired) average   chip   thickness.   All  sorts   of factors,   such  as  cutting  temperature, cutting  forces,  chip  formation and  re- moval, tool life, cutting edge wear and vibrations are strongly  affected  by the relationship between  the  cutting  edge geometry  and  the  average  chip  thick- ness.   If  the   milling   operator  works with   the  same  cutting   conditions   as the  designer  of the  cutting  edge, he is able  to  optimize  and  predict  the  cut- ting  behaviour.  And because  the  same average  chip  thickness  with  different operations   results   in   different   feeds per  tooth,  he is  able  to  maximize  the operation’s productivity.

The tools

Which  practical   tools  are  available? First  of  all,  the  ISO-name  of  the  in- sert.   In code  position  10 the  options with  regard to  the  geometric  cutting conditions (cutting   depth   and   feed) are given. Seco uses a code for this in which the  letter  represents  the  degree of difficulty  of the  operation  and  the number  represents the average   chip thickness under normal    operating conditions   in   traditional   workpiece materials.  For  example,  M14  means: operation  under  normal circumstances with  an average  chip  thickness  of 0.14 mm in standard steel or stainless steel. Another  tool is the cutting  edge graph,  in  which  the  degree  of  difficulty  of  the  operation is marked  off on  the  vertical  axis  and  the  correct chip thickness  on the horizontal  axis. With  this  graph  it  is possible  both  to select  a  specific  cutting   edge  geometry and to solve problems  during  the operation.  After  assessing  the  degree of  difficulty,   the   right   cutting   edge geometry  and  corresponding  average chip thickness can be determined.

Feed per tooth

The  cutting   method   is  taken  into account   (off  center   or   central  mill- ing), the  ae/Dc  ratio  and  the  entering angle  (and  indirectly  the  strength   of the  cutting edge used) of the cutter. Based  on  two  correction  factors  it is determined  which   feed  per  tooth   is suitable   for   a  specific   average   chip thickness.  Correction  factor C1  takes into  account  the  ae/Dc  ratio  during the operation  and the cutting method used.  Correction   factor   C2   consid- ers  the  entering  angle  of  the  cutter. A number  of safety factors  have been included  in  the  graph  to  ensure  that the  maximum chip  thickness,  that  is to be cut by the cutting  edge at a cer- tain moment, is not exceeded.

Example

A  100  mm  cutter   with   an  entering angle   of  90°  is  used   for   face  mill- ing of a surface  of  20 mm wide.  The milling  method  used  is  central   mill- ing.   The   average   chip   thickness   is known  (see  code  position  10  in  the name  of the  insert):  0.14 mm.  In  the chart  we see that  for an a /D ratio  of 20/100  (=20%)  C1  has  a  value  of  1; for  an  entering  angle  of 90°,  C2 also equals  1. This  means  that  the  feed to be  used  equals  0.14  x  1  x  1  = 0.14 mm/tooth.  If the  user decides  to  car- ry out the operation  with a 45° cutter the feed will be 0.14 x 1 x 1.4 = 0.20 mm/tooth.   Using  the  off  center  mill- ing method  the  feed would  be  0.14 x 2 x 1.4 = 0.40 mm/tooth.  This means an  increase  of the  initial  feed rate  by 186%   with   the   same   average   chip thickness,  or in other  words, with the same  cutting  edge  load,  cutting  tem- perature and tool life.

Conclusion

In  order  to  optimize  the  use  of  mill- ing tools one needs to know the right average  chip  thickness  and be  able  to convert  it  in  a  practical  manner  into the  right  cutting   conditions,  in  this case the  feed per tooth.  The tools  de- scribed above are essential for this.

The   term   average   chip   thickness and  the  different  ways  it  can  be used to  increase  the  productivity   of  mill- ing  operations are  discussed  in  great detail  during  the  STEP training  pro- gramme organized  by Seco Tools.

The   average   chip   thickness   is   the most  important  cutting  condition  for milling  tools  that  are  used  in,  for  ex- ample, difficult materials  (hard  milling) or in specific technological approaches, such as high-speed  milling.

E-mail: patrick.devos@secotools.com

• Follow @CNCTimes