Siemens CNC Milling | RND-CHF-CHR | Rounding and Chamfer

RND-CHF-CHR Introduction

You can insert the chamfer (CHF or CHR) or rounding (RND) elements into a contour corner. If you wish to round several contour corners sequentially by the same method, use “Modal rounding” (RNDM).

You can program the feedrate for the chamfer/rounding with FRC (non-modal) or FRCM (modal). If FRC/FRCM is not programmed, the normal feedrate F is applied.


CHF=… ; Insert chamfer, value: Length of chamfer
CHR=… ; Insert chamfer, value: Side length of the chamfer
RND=… ; Insert rounding, value: Radius of chamfer
RNDM=… ; Modal rounding: Value >0: Radius of chamfer, modal rounding ON; This rounding is inserted in all contour corners. ; Value = 0: Modal rounding OFF…
FRC=… ; Non-modal feedrate for chamfer/rounding; Value >0, feedrate in mm/min (G94) or mm/rev. (G95)
FRCM=… ; Modal feedrate for chamfer/rounding: Value >0: Feedrate in mm/min (G94) or mm/rev. (G95),; Modal feedrate for chamfer/rounding ON; Value = 0: Modal feedrate for chamfer/rounding OFF; Feedrate F applies to the chamfer/rounding.

The chamfer/rounding functions are executed in the current planes G17 to G19.

The appropriate instruction CHF= … or CHR=… or RND=… or RNDM=… is written in the block with axis movements leading to the corner.

The programmed value for chamfer and rounding is automatically reduced if the contour length of an involved block is insufficient.

No chamfer/rounding is inserted, if

  • more than three blocks in the connection are programmed that do not contain any information for traversing in the plane,
  • or a plane change is carried out.

F, FRC,FRCM are not active when a chamfer is traversed with G0.

If the feedrate F is active for chamfer/rounding, it is by default the value from the block which leads away from the corner. Other settings can be configured via machine data.

CHF or CHR (Chamfer)

A linear contour element is inserted between linear and circle contours in any combination. The edge is broken.

See the following illustration for inserting a chamfer with CHF using the example: Between two straight lines.

See the following illustration for inserting a chamfer with CHR using the example: Between two straight lines.

CHF and CHR Example

N5 G17 G94 F300 G0 X100 Y100
N10 G1 X85 CHF=5 ; Insert chamfer with chamfer length of 5 mm
N20 X70 Y70
N30 G0 X60 Y60
N100 G1 X50 CHR=7 ; Insert chamfer with leg length of 7 mm
N110 X40 Y40
N200 G1 FRC=200 X30 CHR=4 ; Insert chamfer with feedrate FRC
N210 X20 Y20

RND or RNDM (Rounding)

A circle contour element can be inserted with tangential connection between the linear and circle contours in any combination.

See the following examples for inserting roundings:

RND and RNDM Example

N10 G17 G94 F300 G0 X100 Y100
N20 G1 X85 RND=8 ; Insert 1 rounding with radius 8 mm, feedrate F
N30 X70 Y70
N40 G0 X60 Y60
N50 G1 X50 FRCM= 200 RNDM=7.3 ; Modal rounding, radius 7.3 mm with special feedrate FRCM (modal)
N60 G3 X40 Y40 CR=20 ; continue inserting this rounding – to N70
N70 G1 X30 Y30 RNDM=0 ; Modal rounding OFF
N80 X20 Y20
N90 M30