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Rigid tapping

Why synchronous thread production with rigid collet holders will not result in optimum tool lives.

When producing a thread on a CNC machine with taps or cold-forming taps (for simplicity´s sake, we will call them threading tools in the following) the speed of the rotation movement of the machine spindle with the speed of the feed axis must be registered, accounted and synchronised.
When accounting the threading tool pitch and the cutting speed – giving the feed speed, faults may occur caused by parameters not being considered during the control.

Two main influencing variables are:

  1. Influencing factors by the CNC machining centre
    computer speed, resolution of the axis detection (linear axis, turning axis, C-axis), mechanical condition of the machine.
  2. Influencing factors by the threading tool
    a) Tolerances of the thread pitch acc. to DIN EN 22857
    b) Change of thread pitch and length of the threading tool when tWork ≠ tMeasurement

 
1. Influencing factors by the CNC machining centre

Regarding the formfitting between tool and workpiece, the cutting and forming of threads with synchronous spindles requires permanent µ-exact control and adjusting of the feed axis movement in relation to the rotation movement of the tool spindle.
Thus the thread production differs from other known kinds of machining eg drilling, reaming or milling. These processings only require an exact linear movement of the control for positioning purposes, as these tools are not connected formfitting with the workpiece. Consequently, the main emphasis of machine manufacturers is on the control of the linear axis. In practice today simply rotary pick-ups with 256 impulses per spindle rotation (360°) are used to control the rotation axis. This corresponds to an angle and so a control gap of 1.4° per impulse.
→ Axial forces during thread machining arise caused by control faults or control inaccuracies.

Example:
Tap: M10
Thread pitch: 1.5 mm
Possible uncontrolled spindle rotation 1.4°
→ possible axial position fault of about 5.8 µm between threading tool specified position and machine spindle real position.

Graph machine spindle turning position fault /axial pitch fault (depends on thread pitch)
Effect of machine turning movement fault on the tool

Additionally, the computer speed of modern CNC machining centers is not sufficient to handle a higher number of impulses of the rotary pick-up in the range of n = 0 up to the max. spindle speed and to adjust the axis to be synchronised.
The example of a CNC machining center with 256 impulses per spindle rotation shows that the axial force working on the tool flanks, increases with growing cutting speed. The following graphs show that the axial force for forming an M10 thread with 500 rpm (about 15.7 m/min) is at about 1900 N; with an increase of the speed to 2000 rpm (about 62.8 m/min) at over 2500 N. This clearly shows that the arising axial force, caused by the synchronisation fault, depends on the speed.

Speed 500 rpm Cold-forming tap M10 in C45
 

Speed 2000 rpm Cold-forming tap M10 in C45

2. Influencing factors by the threading tool
a) Tolerances of the thread pitch

For threading tools the European standard DIN EN 22857 defines the dimensions and tolerances for ground threads. Extract from the standard DIN EN 22857

For the tool tolerance the standard allows a smallest deviation of ± 8 µm referred to a defined number of threads.

Example:
Tap: M10
Thread pitch: 1.5 mm
Check length: 7 threads
→ allowed pitch tolerance ± 8 µm

Force/Movement graph
Required force for the length change of threading tool with shank diameter 10 mm

 b) Change of thread pitch and length of the threading tool when tWork ≠ tMeasurement
Each tool temperature – differing from the measuring temperature 20°C – causes a change in length. For an M10 tap with 100 mm length the temperature change from 20 °C to eg 40 °C a causes length change of 32 µm.
Considering a check length of 7 threads acc. to standard DIN EN 22857 the following example results:

Tap: M10
Thread pitch: 1.5 mm
Tap length: 100 mm
Check length: 7 threads = 10.5 mm
→ axial growth of the tool and thread pitch of 3.4 µm

Temperature change development of a threading tool M10
Length 100 mm, temperature change 20 °C, length change 32 µm

Referred to a check length of 7 threads acc. to DIN EN 22857 and a pitch of 1.5 mm the axial length would change by 3.4 µm.
The proof of a change in temperature of the threading tool can be given by measuring the cutting face being heaviest used during the thread production. The following graph shows the temperature of the cutting face for a threading tool M10 with various cutting speeds. Material used is C45, coolant-lubricant is 5% emulsion.

Temperature progressing on the tool tooth face (M10), emulsion as coolant-lubricant

Summary:
To realize the total effect of the individual influencing factors mentioned before on the axial force component of the thread producing process, the shown possible position faults, length changes resp. the forces causing length changes must be combined.

The following graph shows:

  • with an addition of possible axial faults caused by machine pitch tolerance or temperature influencing factors a position fault between specified position of the tap and real position of the machine spindle of more then 17 µm may arise
  • this position fault results in an axial force of about 2800 N in the shown example with a threading tool M10.
  • this force is taken up by the flanks of the tool resulting in increasing flank friction and increased tool wear.

Force/Movement graph
Required force for the length change of threading tool with shank diameter 10 mm

  1. possible temperature-caused pitch fault
  2. possible machine-caused pitch fault
  3. possible standard resp. threading tool caused pitch fault
  4. possible axial force on the tool flanks

These perhaps theoretical reflections of the processes during production of a thread can be proven in practice.

As an example an M10 thread with three different tool holders is formed in material C45. The axial forces were recorded at two speeds which were 500 rpm = 15.7 m/min and 2000 rpm = 62.8 m/min. The following collet adaptations have been tested:

a) rigid synchronous collet adaptation
b) EMUGE collet adaptation Softsynchro® size 1 with minimum length compensation on compression and tension
c) Synchronous collet adaptation of a competitor with minimum length compensation with axial damping

With all tested collet adaptations a collet type ER20-GB with integrated square was used.

Speed 500 rpm Cold-forming tap M10 in C45


Speed 2000 rpm
Cold-forming tap M10 in C45

The following results were verified in these tests:

  • axial forces increase with the raise of speed
  • the forces which come into play in the coldforming of threads are considerably higher with a rigid collet holder than with an EMUGE collet holder type Softsynchro®
  • the competition collet holder can absorb the upcoming forces only lightly, in comparison with the rigid collet holder

What is the reason for the outstanding axial force performance of the EMUGE Softsynchro® tap holders with minimum length compensation?
Important feature is the patented designed separation of torque and axial force transmission.

Further design features of the EMUGE Softsynchro® tap holders are:

  • Clearance-free C-axes by formfitting torque transmission over steel balls.
  • Smooth response of the pre-stressed minimum length compensation after exceeding the constructive defined guiding force by nearly loss-free roll friction of the torque transmission balls in their ball tracks.
  • Minimum length compensation and axial force transmission over pre-stressed elastomer springs.
  • Elastomer springs preventing the tool cutting edge from bracing by their damping characteristics.


If the separation of torque and axial force transmission is disregarded, an axial fault is caused immediately when starting the thread cutting process, see example of the competition collet holder. Consequently, the axial force immediately increases heavily, see graphs on the preceding page. This is avoided by the practical-related design of the Softsynchro®.

For machine tools not providing the feature of synchronous thread machining it is necessary to use a larger length compensation than the minimum length compensation of the Softsynchro® holders.

EMUGE supplies length compensation holders KSN/HD with collet adaptation and internal coolant supply. The advantages of clamping the tool over collets are combined with those of a classic length compensation holder.

 

 
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