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Parting off

Parting off
 

Parting off is mainly performed in bar feed machines, often used in mass production. The parting off operation is usually only a small part of the component's total machining time and therefore not usually seen as a target area for time savings. However, after the parting sequence, some components still have additional features to be machined while in the sub spindle. If chips wrap around the component they can prevent the clamping of the next component in the sub spindle. If this occurs, the machine will either produce scrap or stop. Chips wrapping around the finished component can cause problems in consecutive operations (heat treatments, deburring, assembly, etc.) and destroy the surface finish. Since parting off is usually one of the last operations to perform on the component, security is of great importance. If the tool breaks during parting, the component is usually scrapped. This may lead to long machine stoppages.

Parting off tips: ho​​w​ to choose parting tools

how-to-choose-parting-tools

1. Shallow parting2. Medium parting3. Deep parting

As process security is of high importance, it is preferable to choose a tool with precision coolant. This will evacuate chips, reduce temperature and insert wear and improve surface finish. Use a tool with the shortest possible overhang and a secure insert clamping for best stability. When parting off a component, bar or tube, it is important to save material and minimize cutting force. A narrow insert creates lower cutting forces and saves material. Choose an insert geometry designed for parting off. These specialized inserts produce chips that are narrower than the groove. This results in a parting off operation with good chip control and surface finish.

First choi​​ce f​o​​r parting off

One- and two-edge solutions should be considered as the first choice for different parting off operations. Use inserts which are designed to produce chips narrower than the groove.

Shallow parting
For shallow parting (diameters ≤ 12 mm (0.47 inches)), use 3-edge inserts for economic parting in mass production.
Medium parting

For medium parting (diameters ≤ 40 mm (1.57 inches)) use screw-clamp and spring-lock holders with 2-edge inserts.

Deep parting

For deep parting (diameters ≤ 112 mm (4.41 inches)), insert stability is crucial as this type of cut places high forces on the insert. Hence, a blade with a stable spring clamp and a single edge insert is the best choice.

Pip and burr free parting

For pip and burr free parting, use a sharp geometry with small corner radii. A front angled insert will also reduce pip and burr. Front angled inserts will create side forces and are recommended only for short overhangs (<13 mm (0.51 inch)).

Small part machining

For small part machining, use an insert with the smallest width and sharpest cutting edges. For best process security, use a tool with precision coolant. If superior tool edge sharpness is not necessary, choose a 3-edge or 2-edge insert for a more economical solution, or for larger diameters.

Parting off tips: how to apply parting off operations

Parting off bars and tubes

Overhang (OH)

Minimize overhang. When parting off bars, a shorter overhang and higher blade both decrease bending down (δ) by cubic function.

minimize-overhang-1 
minimize-overhang 
minimize-overhang-2
 
short-overhang

A short overhang increases stability dramatically. Part off close to the chuck. This is especially important when parting off with thin inserts which require stable conditions and gentle handling.

  • For an unavoidable long OH use a light cutting geometry
  • If the OH is less than 1.5xH, use the recommended feed for the geometry
  • If the OH exceeds 1.5xH, reduce feed rate to the lower end of the recommended feed for the geometry
 

Tool centre height

It is important to have the centre height correct ±0.1 mm (±0.004 inch), especially when parting off to centre. For long overhangs use a maximum deviation of +0.1 mm (+0.004 inch) above centre to compensate for bending down.

Positioning below centre causes: Positioning at centre causes:
positioning-below-centre positioning-at-centre
  • Increased pip
  • Breakage (unfavourable cutting forces)
  • Breakage (pushing through centre)
  • Rapid flank wear (small clearance)
 

Feed rate

Cutting through the centre of a bar creates unnecessary toughness demands that can lead to insert breakage. Here, the insert is exposed to rubbing as the workpiece starts to move in the opposite cutting direction. This action exposes the insert to tensile stresses resulting in breakages.

feed-rate
Calculating speed:
cutting-speed-mcutting-speed-i
 
 
 

Reduce feed by up to 75% approx. 2 mm (0.08 inch) before centre. Lower feed at the centre reduces cutting forces and drastically increases tool life, while higher feed in the periphery improves productivity and tool life. To avoid breakage, stop feed approx. 0.5 mm (0.02 inch) before the centre of the bar and the cut-off part will drop due to its weight and length.

lower-feed
Feeding through center causes breakage
 
 

Sub spindle

When parting off bars, a sub spindle can be used to pull the part off while significantly reducing toughness demands and increasing tool life. It can also allow for using a more wear resistant grade which in turn improves tool life even more. Perform the parting operation but stop feed approx. 1 mm (0.04 inch) before centre. Then use a sub spindle to pull the part off.

sub-spindle
 
 

Insert width

Use an insert that is as narrow as possible to save bar material and minimize cutting forces and environmental pollution.

increase-width 

Use the table to choose insert width, CW, depending on component diameter, D:

CW
​ D mm (inch) CW mm​
 -10 (-0.4)​  1.0​
 10-25 (0.4-1.0)​  1.5​
 25-40 (1.0-1.6)​  2.0​
 40-50 (1.6-2.0)​  2.5​
 50-65 (2.0-2.6)​  3.0​
 
 

Pip and burr free parting

Choose a left- or right-hand front-angled insert to control the pip or burr when parting off bars or tubes. A large front-angled insert decreases pips and burrs but may not produce a straight cut and can result in decreased chip control, poor surface finish and short tool life. Use as small a front-angled insert as possible. For longer overhangs use neutral inserts - the longer the tool, the bigger the problems with front-angled inserts.

Front angle Neutral
Stability and tool life​ Bad Good
Radial cutting forces​ Low High
Axial cutting forces​ High Low
Pip/burr​ Small Large
Risk of vibration​ High Low
Surface finish and flatness​ Bad Good
Chip flow​ Bad Good
pip-and-burr-free-parting

Parting small diameter bars

Make sure the lowest possible forces are generated. Use an insert with the smallest possible width and sharpest edges.

Never use a tool as a bar stop!

bar-stop

This is not good for any tool but for small insert widths it will damage the tool.

 
 

Parting into a drilled hole

drilled-hole

Avoid parting off into the conical area because this causes blade deflection and can lead to tool breakage.

 
 

Thin walled tubes

thin-walled-tubes

When parting off into thin-walled tubes, ensure the lowest possible cutting forces are generated. Use inserts with the smallest possible width and sharpest cutting edges.

 
 

Coolant

coolant

The use of coolant and lubrication has a large impact on process security in parting and grooving. In the Cutting fluid and coolant section you will find more information and recommendations regarding over and under coolant, precision coolant and pressure.

 

Y-axis parting

y-axis-parting-with-coolant

Y-axis parting is a completely new way of parting off. In conventional parting off most of the cutting force is generated by the cutting speed, the rest by the cutting feed. The resulting force is directed approximately 30 degrees into the tool. Thereby the parting off blades are loaded in their second weakest direction. The best way to overcome this is to reduce the blade overhang and/or increase the blade height. By turning the tip seat 90 degrees and utilizing the Y-axis on multi-task machines and turning centres to feed the tool, the resulting cutting force will be directed into the strongest direction of the tool. At a 60 mm (2.36 inch) blade overhang this increases the bending stiffness more than six times. This results in a much more stable, silent and vibration-free cutting process that gives better surface finish and allows higher feed rates and longer tool overhang. CoroCut® QD for Y-axis parting is the first choice for parting off in turning centres and multi-task machines with Y-axis. It can machine large diameters, up to 180 mm (7 inch), and is particularly suitable for long overhangs to reach between main- and sub-chuck. It is always beneficial to use Y-axis parting off blades if the overhang is larger than the blade height.

 

How to apply

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Y-axis parting in multi-task machines

multi-task-machine-axis

The tool assembly is often long to reach between main- and sub chuck. This means the total set-up is weak in the X direction compared to the Y-axis load, where the cutting force is directed into the tool assembly and into the machine spindle.

y-axis-parting-2-1-multi-task-parting.jpg y-axis-parting-2-1-multi-task-parting.jpg
Conventional set-up
 
multi-task-parting-y-axis y-axis-blade-capto
Set-up for Y-axis parting
 

The tool length is normally measured on an optical device outside the machine. The tool length will become the tool centre height when parting off and it is important to have the centre height correct, especially when parting off to centre.

optical-measuring
Optical measuring outside the machine

If it is difficult to see the cutting edge there is also a gauge plane on the tool. The plane is located:

  • 5±0.05 mm (0.197±0.002 inch) above cutting edge on G-tip seat (3 mm (0.118 inch))
  • 6±0.05 mm (0.236±0.002 inch) above cutting edge on H-tip seat (4 mm (0.157 inch))

The cutting edge with blade mounted in a standard adaptor is located 7 mm (0.276 inch) above Y=0. This must be compensated for as tool offset to get correct cutting speed. This information is marked on the tool.

 

Y-axis parting in turning centres

turning-centre-axis

The tool assembly is often long and slender to reach between main- and sub chuck and allow parting off close to chuck. The total set-up is weak in the X-direction compared to Y-axis load where the cutting force is directed into the tool assembly and into the turret.

bolt-on-macu-revolver-eps eps
Conventional set-up
 
turning-centre-parting y-axis-blade-vdi
Set-up for Y-axis parting
 

The tool length is normally measured with a probe inside the machine. The tool length will become the tool centre height when parting off and it is important to have the centre height correct, especially when parting off to centre.

If it is difficult to see the cutting edge there is also a gauge plane on the tool. The plane is located:

5±0.05 mm (0.197±0.002 inch) above cutting edge on G-tip seat (3 mm (0.118 inch))

6±0.05 mm (0.236±0.002 inch) above cutting edge on H-tip seat (4 mm (0.157 inch))

The cutting edge with blade mounted in a standard adaptor is located 7 mm (0.276 inch) above Y=0.

This must be compensated for as tool offset to get correct cutting speed. This information is marked on the tool.

 
probe-measuring y-axis-parting-probing-insert probing-reference-plane
Measuring with a probe inside the machine
 

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