Threading Technologies Part 3 docx

Albeit for an internal thread production, there must be sufficient working space for the cutter to be able to perform the thread milling task.. Tooth Profile and Dimensions The thread mi

removal rate may increase by up to 300% at each suc-cessive  infeed.  Therefore,  in  order  to  minimise  these 

induced stresses on the cutting edge and keep them as 

uniform as possible, the DOC must be reduced with each 

pass along the workpiece. 

Thread Finishing and Close Tolerancing

In  order  to  obtain  a  good  surface  finish/texture,  or 

a  close  tolerance  on  the  finished  thread  flanks,  the 

CNC  machine  tool  can  be  programmed  to  make 

ei-

ther one, or two additional finishing passes. These ad-ditional passes are termed ‘spring-cuts’ and improve 

both accuracy and precision in the final thread form. 

‘Spring-cuts’ cannot be utilised on workpiece materi-als that have a tendency to work-harden, for example 

when thread turning stainless steels and so on, as they 

cause high tool wear. With work-hardening materials, 

cuts of <0.03 mm should be avoided, as these materials 

elastically-deform instead of being cut. The severity of 

the problem of work-hardening is even greater when 

thread turning austenitic steels and their equivalents 

and here, it is recommended that an infeed pass should 

always be >0.08 mm. These comments are confined to 

steels  and  their  alloys  and  even  here,  an  appropriate 

number  of  infeeds  by  trial-and-error  may  be 

neces-sary. When a threading insert breaks, it is normally the 

result of induced high stresses, so the remedy is usually 

to increase the number of infeed threading passes. This 

solution is also true for machining threads in most cast 

iron grades, the exception here being for austempered 

ductile irons (ADF). It seems somewhat obvious that 

the greater the number of passes the threading opera-tion is divided into, the smaller the DOC and the stresses 

on the threading insert’s tip. Moreover, if this philoso-phy is pursued too far and very many infeed passes are 

programmed,  the  tool  will  simply  not  be  able  to  cut 

at all – owing to insufficient DOC and this in turn, will 

result in simply elastic deformation of the workpiece 

material. Such a ‘timid approach’ to thread cutting will 

lead to a higher wear-rate, so it becomes necessary to 

further reduce the number of passes – thereby becom-ing a self-defeating machining objective!

  ‘Spring-cuts’ , are cuts that create a very light tool pressure on 

the threaded workpiece, to minimise the elastic deflections in 

the  ‘loop’  produced  by  the  tool-machine-workpiece  system, 

thereby improving machined quality.

Cutting Forces

If  a  comparison  is  made  between  the  cutting  forces  for a threading operation with that of external turn-ing, then the power requirements are higher for thread  formation,  especially  when  the  chip  thicknesses  are  small. If however, the chip thickness is increased, then  the  values  for  plain  turning  are  approached.  Hence, 

one should always attempt to utilise higher chip thick-nesses, as the benefits are two-fold: a decrease in the  power consumption, combined with an increase in the  subsequent production rate. 

General Comments on Cutting Inserts

Thread cutting demands an insert with a: sharp cut- ting edge, good wear resistance and the ability to with-stand temperature fluctuations. A sharp insert cutting  edge combined with a favourable geometry are neces-sary, so that a good final thread surface finish occurs,  while simultaneously reducing vibrational tendencies.  High wear resistance is crucial, as otherwise the sur- face finish would be impaired and thread tolerance de-viations would occur. Temperature fluctuations must 

be  withstood,  as  the  very  operation  of  threading  in-troduces fluctuations in the insert’s edge temperature:  the fast machining pass along the thread causes heat-ing, then the tool is rapidly withdrawn and returned 

to the start-point – during which time the edge cools.  This  cyclical  process  is  continuously  repeated  up  to 

20 times in quick succession, which could potentially  promote  fatigue  cracks  in  the  insert  –  termed  ‘comb  cracks’ (i.e often previously present after milling oper-ations – as they had finite fine lines at regular intervals 

on the tool’s edge, giving them the physical appearance 

of a ‘comb effect’). This was a real problem some years  ago  –  which  has  now  been  overcome  with  suitable  coating technology – and could in the past may poten-tially shorten the insert’s useful life. 

NB  See Appendix 8 for a Trouble-shooting Guide to 

conventional thread turning problems and remedies

Figure 107 Threadmilling cutters and typical thread generation operations [Courtesy of Sandvik Coromant]

.

5.6 Thread Milling

Introduction

Thread milling geometry in contrast to that of a basic 

tap (i.e. having a single spiral shaped tooth Fig. 107ai), 

has a series of teeth which do not form a spiral, but 

are  configured  without  pitch  (Fig.  107aii).  This 

fun-damental difference in tool design is attributed to the 

different thread production processes, explained ear-lier. Not only can a thread milling cutter have a similar 

visual geometry to that of a machine tap (Fig. 107aii), 

but it can occur with a single radially-mounted blade 

for  milling  both  external  and  internal  threads  (Fig. 

108b). Albeit for an internal thread production, there 

must be sufficient working space for the cutter to be 

able to perform the thread milling task. 

Tooth Profile and Dimensions

The thread milling cutter profile usually conforms to 

that of the thread to be milled. In certain cases, it may 

be essential to correct the milled thread’s profile. This 

being the case, when the diameter of the thread to be 

milled does not have a definite ratio to the diameter 

of the thread milling cutter. A major advantage of em-ploying  thread  milling  in  the  production  of  threads, 

is that it can mill a range of threads of differing diam-eters. The one limitation here being that modifications 

of the thread’s pitch is not practicable

If one discounts the tool’s thread pitch, then the de-sign  of  a  thread  milling  cutter  is  remarkably  similar 

to that of a machine tap (Fig. 107a). A typical thread 

milling  cutter  (Fig.  107aii),  is  characterised  by  its 

cutting  section’s  size  and  dimensions.  The  total  tool 

length and its associated thread length are also part of 

the cutter’s dimensions. Thread milling cutter designs 

can  also  incorporate  either  a  collar,  or  not  –  as  the 

milling situation dictates, together with either a coun-

tersinking chamfer, or not. Therefore, the thread mill-ing cutter’s cutting section (Fig. 107aii), consists of its: 

flute length, flute profile, tooth form together with its 

associated form relief. In a similar fashion to that of a 

machine tap, the flute length usually incorporates run-out of the flutes, although this flute run-out does not 

have to be as great as that found on machine taps, due 

to the smaller chips that are produced. Thread milled 

chips  do  not  remain  in  the  cutter’s  flutes  during  the 

thread  milling  process,  and  as  such,  will  not  restrict 

further  chip  development.  The  tooth  width  is  larger  than that found on machine taps, with relief grinding  creating  the  necessary  clearance  angles,  required  for  milling threads

Interference Ratio

If  the  thread  milling  cutter  diameter  to  that  of  the  nominal thread diameter ratio of 70% is adhered to,  then  no  milled  thread  profile  distortion  should  take  place  (i.e.  see  Fig.  109a),  irrespective  of  the  thread’s  depth – this fact has been consistently well proven by  industrial applications. 

In Fig. 109a, the illustration depicts the fact that the  diameter  of  the  thread  milling  cutter  and  its  associ-ated profile depth, determine the pressure angle of the  thread’s diameter

Helical Interpolation

Helical  interpolation  (Fig.  108a),  is  the  amalgama-tion of two kinematic motions, these being: linear and  circular  interpolations.  Therefore,  in  thread  milling,  different threads can be manufactured by the form of  overlaying the pitch direction with that of the direc-tion of rotation of the circular movement. 

Thread  milling  cutters  are  normally  designed  for  right-hand cutting, with the direction of rotation be-ing generally clockwise. However, by altering a range 

of  kinematic  motions,  such  as:  the  axial  direction  of  the  feed,  reverse  cutter  rotation,  or  by  synchronous  milling, all thread combinations can be manufactured  – some of which are depicted in Fig. 107c. Depending  upon the component features to be thread milled, such 

as into blind, or through holes and whether horizon- tal, or vertical machining techniques are to be incor-porated,  together  with  the  lubrication  type  and  chip  removal  strategies,  these  will  determine  the  correct  choice of milling procedure to be adopted. Generally, 

for  thread  milling  production,  synchronous milling methods (i.e. Fig. 109b) should be applied whenever  possible, as they achieve the following intrinsic ben-efits:  lower  cutting  forces,  improved  chip  formation,  longer tool life and improved surface quality

  Synchronous milling methods,  can  be  identified  when  the 

thread milling cutting edge emerges with a chip thickness of  zero (i.e. h = 0).

Speed Ratio

When  thread  milling,  the  cutter  edge’s  speed  is 

cal-culated by the cutting speed (i.e. revolutions) and the 

feedrate per tooth. With linear movement, the cutting 

edge’s feedrate is identical to that at the tool’s centre. 

However, with helical interpolation, it follows a path 

of a circle in the plane (Fig. 108a). All machine tool 

CNC controllers will calculate speeds from the tool’s 

centre, it is necessary to program a command for con-

verting the cutting speed (i.e. a contour-related pro-gram). When such a program does not exist, or the  central point is programmed, it is necessary to con-vert the feedrate accordingly. It should be mentioned,  that the interactive control at the CNC control panel  will always indicate the speed at the centre point of  the  tool  and,  when  running  with  no  load  (i.e.  usu-ally termed a ‘dry-run’), this speed is simple to check.  Furthermore, if this speed is disregarded, the thread  milling cutter will run at a speed many times greater  than that of the feed, which shortly leads to the cut-ter’s breakage

Figure 108 Thread milling using a single-edged insert for either internal/external threading operations, can be achieved via a

complex simultaneous circular interpolation of the ‘x’ and ‘y’ axes and a ‘z’ axis linear motion [Courtesy of Stellram]

.

Figure 109 Threadmilling interference ratio, plus cutter

positioning and feeding [Courtesy of Guhring]

.

Internal Thread Milling: Radial Positioning

to Nominal Diameter, Via Entry Cycles

The  thread  milling  cutter’s  radial  positioning  to  the 

nominal diameter at the start of the thread’s produc-tion, is achieved by so-called ‘entry cycle’0 (Fig. 109c), 

while the movement following the thread’s milling op-

eration is achieved by cutter motion from the nomi-nal diameter to the hole’s centre, via a corresponding 

‘exit cycle’. Thread milling cutter approaches to that of 

the start of the thread, via suitable ‘entry cycles’ can be 

achieved by several different ways, these are:

• Linear plunging (Fig. 109ci) – of the thread milling 

cutter  into  the  workpiece  material,  creates  a  very 

large  contact  angle  at  the  cutter’s  periphery, 

lead-ing to the undesirable situation of high tool loading 

and long chips. This problem is particularly acute 

when  the  differences  between  the  thread  milling 

cutter’s diameter to that of the hole’s size is small. 

Moreover,  this  radial  entry  linear  plunging 

tech-nique can leave a small ‘delay mark’ on a portion 

of the milled thread. 

NB Linear plunging

is not an advisable thread mill-ing  technique  for  the  production  of  accurate  and 

precise small threads.

• 90° quarter circle entry cycle (Fig. 109cii) – allows 

just a small difference in the diameter between the 

tool  and  the  thread  to  remove  a  large  part  of  the 

chip volume, during the linear section of the entry 

cycle.  This  particular  entry  cycle  strategy,  is 

nor-mally  only  utilised  for  relatively  large  differences 

in diameter between the hole size and the cutter’s 

diameter

NB  The 90° quarter circle entry cycle

has the advan-tage of a relatively short entry path, together with a 

simple CNC program.

• 180° semi-circle entry cycle

(Fig. 109ciii) – the cut-ting force loading of the tool is at its lowest when 

0  Entry-cycles, allow the thread milling cutter to be moved in a 

circular arc to the nominal thread’s diameter.

  Delay marks, are the result of a slight dwell, prior to the next 

command line activation in the thread milling program, caus-ing cutting forces to ‘slightly relax’ and then impinge into the 

machined thread’s surface.

the cutter is plunging, due to the contact angle be- ing relatively small during the complete cycle en-try. 

NB  The  180° semi-circle entry cycle necessitates 

a  slightly  more  sophisticated  CNC  program,  al- though it has been found to be the most cost-effi-cient thread milling technique overall. In Fig. 110, 

is  depicted  a  step-by-step  visual  interpretation  of  this actual thread milling process, along with a typ-ical programming example

One  distinct  advantage  that  utilising  thread milling tooling gives  to  the  quality  and  fitment  of  matching  threads, is that minute variations in the pitch and to a 

lesser degree its associated depth, can be programmed-in by  the  operator  to  modify  ‘worklesser degree its associated depth, can be programmed-ing clearances’. 

This has the distinct benefit of providing control over  the ‘backlash’ between the two mating thread milled  parts. 

5.7 Thread Rolling –

Introduction

It is normal to specify thread rolling when substantial 

quantities of threads need to be manufactured. In es-sence, the production process is one of ‘cold-forming’ , 

in which the threaded features on the workpiece are  formed  by  rolling  a  thread  blank  between  hardened  dies  (Fig.  111).  This  rolling  action,  causes  the  metal 

to flow radially into the required V-form profile (i.e.  see  Fig.  111a  –  inset  schematic  diagrammatic  com-parison between a cut and rolled thread – indicating 

the ‘directionality of the grain-flow’). Due to the fact  that no workpiece material is removed in the form of 

chips, there is no waste material – resulting in substan-  ‘Directionality of the grain-flow’ , this anisotropic behaviour 

of the manipulated grain structure after rolling is one of plas-tic deformation of the local material

(i.e. Fig.111a – inset dia-gram, indicating the V-from rolled thread). This local plastic  deformation,  raises  the  material’s:  hardness,  tensile  and  fa-tigue strengths, together with its proof stress. However, there 

is some ‘drop-off’ in both the thread’s creep strength and its  ductility as a result of rolling, but this is tolerated – due to the  major benefits described.

Figure 110 A typical threadmilling cutter operational sequence, with an illustrated series of cutter motions and a ‘practical’

word-address CNC program [Courtesy of Guhring]

.

Figure 111 Thread rolling techniques – produce a strong thread form

.

of  thread  rolling  on  CNC  machine  tools,  is  that  due 

to this cold-working process, rolled threads have high 

strength, are smoother and more wear resistant then 

there machined counterparts. The thread rolling pro-duction rates are fast, typically a complete thread can 

be formed in a second, with the thread quality being 

consistently high. 

A  principal  characteristic  of  a  thread  rolling 

op-eration  is  that  the  rolled  thread’s  diameter  is  always 

greater  than  the  original  blank  diameter.  If  the 

pro-spective thread must have an accurate ‘class of fit’ , then 

its blank diameter is marginally increased by 0.05 mm 

with respect to the thread pitch diameter. When it is 

desired to have say, the body of a bolt larger than the 

outside diameter of the rolled thread, then the thread’s 

blank diameter is produced smaller than the body. 

5.7.1 Thread Rolling Techniques

In  Fig.  111,  can  be  seen  the  three  basic  techniques 

used to thread roll employed on CNC machine tools, 

these are: 

• Two-roll tangential rolling (Fig. 111a), is a similar 

process to that of ‘knurling’. As the spindle turns, 

the workpiece’s pre-rolled diameter is progressively 

raised to its final shape, normally over the course 

of  between  20  to  30  revolutions.  The  tangential 

thread  rolls  are  fed  from  the  X-axis,  at  a  tangent 

to the workpiece. When the centreline of the rolls 

line-up with that of the centreline of the workpiece, 

the process is complete. Usually rolling a φ20 mm 

thread  at  1200 rpm,  takes  about  1  second, 

con-versely, a single-point turned thread would require 

  Thread rolling, is known as a ‘chipless operation’

and as a re-sult of the ‘cold rolling’ production process , the operation is 

cleaner and material savings in blank stock weight are of the 

order of between 15% to 20% – depending upon the size and 

length of the threaded feature manufactured. 

  Knurling (i.e  not  illustrated),  utilises  either  two,  or  three 

hardened  rotating  knurls  which  are  pressed  into  the 

previ-

ously turned outside diameter, thereby giving a ‘gripping’ sur-face pattern – and hence aids in purchase for one’s grip, with 

normally either a straight-, or diamond-shaped knurl.

NB  It is possible to utilise tangential sliding knurls to impart 

the desired ‘imprinted patterned surface’ onto the workpiece’s 

periphery.

10 times longer to manufacture the same threaded  feature,

• Three-roll radial rolling (Fig.  111b),  is  similar  in 

operation to tangential heads, in so far as the work-piece  is  normally  approached  from  the  side,  per-pendicular  to  the  major  thread’s  axis.  The  radial  rolls are sprung-loaded and when they are brought  over the workpiece, the tension is released, causing  the  rolls  to  rotate  and  produce  a  thread.  Flats  on  the rolls allow for work to be inserted and removed. 

In both the tangential and radial rolling techniques,  they are limited to thread lengths that are no greater  than the thread roll widths. The principal difference 

between these two heads, is that with radial heads the form is completed in just one

revolution, as op-posed  to  the  20  to  30  revolutions  necessary  with 

tangential rolling methods. This fact, makes the ra-dial rolling the fastest of all rolling techniques. For 

example, if the workpiece spindle is rotating at 1200  rpm, and a φ10 mm thread is to be rolled, it would  take just 0.5 seconds to complete,

• Two-roll axial rolling (Fig. 111c), these rolls engage 

the workpiece from its front, along the workpiece’s  centreline (i.e. Z-axis). This rolling action is analo-gous to a threading die, or thread-chaser, traversing  from one end of the workpiece to the other. Hence,  this  rolling  arrangement  is  capable  of  producing  very  long  threads,  or  threaded  portions  on  the  workpiece,  moreover,  the  axial  heads  support  the  part  during  the  thread’s  manufacture,  eliminating  the need of a supporting tailstock

In all these thread rolling processes, the operation of  thread rolling remains primarily identical in its final  rolled threaded feature on the workpiece and the pro-cess  of  imparting  threads  on  ductile  and  to  a  lesser  extent some work-hardening materials, should be en- couraged. There are other techniques for the produc-tion of rolled threads that have not been shown here,  including: reciprocating and flat die designs, planetary  rolling, etc., they have not been incorporated into this  review,  because  of  the  difficulty  of  utilising  them  on  CNC machine tools

References

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Hanson, K. Roll your Own [Thread Rolling]. Cutting Tool 

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Hazelton, J.L. Tapping the Hard Stuff. Cutting Tool Eng’g., 

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Lewis,  B.  Challenge on Tap  [Tapping  Problems].  Cutting 

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