A fixture attachment system for CNC3020T

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I found it convenient to have a table with a grid of nuts for attaching the workpieces, that could double as victim material. In this note I describe how to make such a table out of HDPE.

Introduction

My first prototype used a plywood board, however, that doesn’t work well with water as coolant; also, plywood tends to deform in unexpected and non-uniform ways. HDPE is cheap, easily machined, works well as victim material, and resists deformation well.

This note describes a table with uniform 8x7 nut array, spaced 30mm between nuts, accepting M4 screws; there is at least 3mm of victim material at any point (5.2mm over nuts, 3mm over mounting screws).

All G-code is parametric (it thus requires LinuxCNC) and can be easily customized if other parameters are needed.

Tools

jig saw
DEXTER IC400JS
jig saw blade
Bosch T301CD
drill press
PRACTYL 500W with 16mm collet
d=6mm wood drill bit
generic
CNC mill
CNC3020T
d=3.175mm cylindrical endmill
generic
d=1.5mm l=7.5mm cylindrical endmill
generic

Materials

HDPE sheet, 300x300x12.7mm
generic
{M6x40 screw, M6 wing-nut, M6 washer} ×4
generic
{M6x12 bolt, M6 nut} ×6
generic
M4 nut ×42
generic

Process

Sheet was cut to fit CNC3020T’s table of 300x200mm using a jig saw.

Mounting holes

Sheet was marked for drilling on a drill press using d=1.5mm endmill. Sheet wasn’t drilled on the mill itself to avoid crashing the endmill into the table.

Marking was done using the following G-code script. Origin corresponds to the top of sheet, center of nearest leftmost mounting hole.

mark.ngc (download)
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S2500
M3
G0  Z6
G98
G81 X0   Y0   R1 Z-2 F300
    X0   Y130
    X0   Y260
    X150 Y0
    X150 Y130
    X150 Y260
M2

After drilling, pockets for screw heads were milled using d=3.175mm endmill. Initially, commands for milling first and last pocket were commented out and the sheet was attached using a M6x40 bolt in the T-slot, washer and a wingnut. After that, the sheet was reattached properly, i.e. using M6x12 screw and a nut in the T-slot, and the remaining pockets were milled.

screw-heads.ngc (download)
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; === HELICAL MILLING ===

; Helix-mill a hole.
; Assumes metric, XY plane, feedrate, and tool compensation are set.
O<helix_mill> sub
  #<xcenter> = #1 ; [mm] X center
  #<ycenter> = #2 ; [mm] Y center
  #<radius>  = #3 ; [mm] Hole radius
  #<zstart>  = #4 ; [mm] Z start position
  #<zend>    = #5 ; [mm] Z end position

  G0 X[ #<xcenter> - #<radius> ] Y#<ycenter>

  G0 Z#<zstart>
  #<znow> = #<zstart>

  ; Mill out body with several full helical turns
  O100 while [ #<znow> GT [ #<zend> + #<_zstep> ] ]

    #<znow> = [ #<znow> - #<_zstep> ]
    G2 I#<radius> J0 Z#<znow>

  O100 endwhile

  G2 I#<radius> J0 Z#<zend>

  ; Flat out the bottom
  G2 I#<radius> J0

O<helix_mill> endsub

; === SCREW/NUT POCKETING ===

; Mill a pocket for a screw head.
; Assumes metric, XY plane, feedrate, and tool diameter are set.
O<screw_head_pocket> sub

  #<xcenter>     = #1 ; [mm] X center
  #<ycenter>     = #2 ; [mm] Y center
  #<head_radius> = #3 ; [mm] Screw head thickness
  #<head_depth>  = #4 ; [mm] Screw head depth

  ; Initial position
  G0 X#<xcenter> Y#<ycenter>

  ; Cutter compensation to the right.
  G42

  ; Make a pocket for screw head.
  #<rnow> = #<head_radius>
  O100 while [ #<rnow> GT [ #5410 / 2 ] ]

    O<helix_mill> call [#<xcenter>] [#<ycenter>] [#<rnow>] [0.] [-#<head_depth>]

    #<rnow> = [ #<rnow> - #5410 * 0.9 ] ; Subtract 90% tool diameter

    G0 Z0.0 ; Get tool up for another plunge

  O100 endwhile

  ; Cleanup.
  G40
  G0 Z#<_zsafe>

O<screw_head_pocket> endsub

; --- BEGIN CONFIGURATION ---

; Don't forget to set up tool table.

T2 M6 S2500 F300 ; 3.175mm cylindrical

#<_zsafe>       = 10.  ; [mm] Safe Z
#<_zstep>       = 3.   ; [mm] Z step

#<_thickness>   = 12.7 ; [mm] Sheet thickness
#<_head_radius> = 6.   ; [mm] Screw head radius
#<_head_depth>  = 8.   ; [mm] Screw head depth

; --- END CONFIGURATION ---

G21 G90 G17 ; Metric, Absolute, XY plane
M3

/O<screw_head_pocket> call [0]   [0]   [#<_head_radius>] [#<_head_depth>]
O<screw_head_pocket> call [0]   [130] [#<_head_radius>] [#<_head_depth>]
O<screw_head_pocket> call [0]   [260] [#<_head_radius>] [#<_head_depth>]
O<screw_head_pocket> call [150] [0]   [#<_head_radius>] [#<_head_depth>]
O<screw_head_pocket> call [150] [130] [#<_head_radius>] [#<_head_depth>]
/O<screw_head_pocket> call [150] [260] [#<_head_radius>] [#<_head_depth>]

G0 Z#<_zsafe>
M2

Nut slots

Sheet was fixed upside-down on the table. To avoid crashing the endmill into the table, a washer was placed between the table and the sheet, offsetting it by 2mm. In other words, a vertical slice through the mounting hole looks like: bolt head : T-slot : washer : sheet : washer : wing-nut.

The middle bolts should be removed despite being present on the picture. I discovered they would interfere with the spindle too late. I have replaced them with just a washer between table and sheet.

Pocketing was done using the following G-code script. The origin is at (13mm,45mm) from near left corner, owing to the asymmetry of the work area.

nut-pockets.ngc (download)
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; === HELICAL MILLING ===

; Helix-mill a hole with pecking.
; Assumes metric, XY plane, feedrate, and tool compensation are set.
O<helix_mill> sub
  #<xcenter> = #1 ; [mm] X center
  #<ycenter> = #2 ; [mm] Y center
  #<radius>  = #3 ; [mm] Hole radius
  #<zstart>  = #4 ; [mm] Z start position
  #<zend>    = #5 ; [mm] Z end position

  G0 X[ #<xcenter> - #<radius> ] Y#<ycenter>

  G0 Z#<zstart>
  #<znow> = #<zstart>

  ; Mill out body with several full helical turns
  O100 while [ #<znow> GT [ #<zend> + #<_zstep> ] ]

    #<znow> = [ #<znow> - #<_zstep> ]
    G2 I#<radius> J0 Z#<znow>

    ; Peck
    G0 Z#<zstart>
    G0 Z#<znow>

  O100 endwhile

  G2 I#<radius> J0 Z#<zend>

  ; Peck
  G0 Z#<zstart>
  G0 Z#<zend>

  ; Flat out the bottom
  G2 I#<radius> J0

O<helix_mill> endsub

; Make one polygonal milling turn.
; Assumes metric, XY plane, feedrate, and tool compensation are set.
O<polygon_turn> sub
  #<xcenter> = #1 ; [mm] X center
  #<ycenter> = #2 ; [mm] Y center
  #<radius>  = #3 ; [mm] Hole radius
  #<zstart>  = #4 ; [mm] Z start position
  #<zend>    = #5 ; [mm] Z end position
  #<edges>   = #6 ;      Edge count

  #<zedgestep> = [ [ #<zend> - #<zstart> ] / #<edges> ]
  #<anglestep> = [ 360.0 / #<edges> ]

  #<znow>      = #<zstart>
  #<angle>     = 120.

  O100 repeat [ #<edges> ]

    #<znow>  = [ #<znow>  + #<zedgestep> ]
    G1 X[ #<xcenter> + cos[#<angle>] * #<radius> ] Y[ #<ycenter> + sin[#<angle>] * #<radius> ] Z#<znow>
    #<angle> = [ #<angle> - #<anglestep> ]

  O100 endrepeat

O<polygon_turn> endsub

; Helix-mill a polygonal hole.
; Assumes metric, XY plane, feedrate, and tool compensation are set.
O<polygon_mill> sub
  #<xcenter> = #1 ; [mm] X center
  #<ycenter> = #2 ; [mm] Y center
  #<radius>  = #3 ; [mm] Hole radius
  #<zstart>  = #4 ; [mm] Z start position
  #<zend>    = #5 ; [mm] Z end position
  #<edges>   = #6 ;      Edge count

  G0 X[ #<xcenter> - #<radius> ] Y#<ycenter>

  G0 Z#<zstart>
  #<znow>  = #<zstart>
  #<zprev> = #<zstart>

  ; Mill out body with several full helical turns
  O100 while [ #<znow> GT [ #<zend> + #<_zstep> ] ]

    #<znow>  = [ #<znow> - #<_zstep> ]
    O<polygon_turn> call [#<xcenter>] [#<ycenter>] [#<radius>] [#<zprev>] [#<znow>] [#<edges>]
    #<zprev> = #<znow>

  O100 endwhile

  O<polygon_turn> call [#<xcenter>] [#<ycenter>] [#<radius>] [#<zprev>] [#<zend>] [#<edges>]

  ; Flat out the bottom
  O<polygon_turn> call [#<xcenter>] [#<ycenter>] [#<radius>] [#<zend>] [#<zend>] [#<edges>]

O<polygon_mill> endsub

; === SCREW/NUT POCKETING ===

; Mill a pocket for a nut, part #1 (nut pocket).
; Assumes metric, XY plane, feedrate, and tool diameter are set.
O<nut_pocket_1> sub
  #<xcenter>      = #1 ; [mm] X center
  #<ycenter>      = #2 ; [mm] Y center
  #<nut_depth>    = #3 ; [mm] Nut depth
  #<nut_radius>   = #4 ; [mm] Nut radius
  #<screw_radius> = #5 ; [mm] Screw radius

  ; Initial positioning
  G0 X#<xcenter> Y#<ycenter>

  ; Cutter compensation to the right.
  G42

  ; Make a pocket for nut.
  #<rnow> = #<nut_radius>
  O100 while [ #<rnow> GT #<screw_radius> ]

    O<polygon_mill> call [#<xcenter>] [#<ycenter>] [#<rnow>] [0.] [-#<nut_depth>] [6]

    #<rnow> = [ #<rnow> - #5410 * 0.9 ] ; Subtract 90% tool diameter

    G0 Z0.0 ; Get tool up for another plunge

  O100 endwhile

  ; Cleanup.
  G40
  G0 Z#<_zsafe>

O<nut_pocket_1> endsub

; Mill a nut pocket, part #2 (screw body pocket).
; Assumes metric, XY plane, feedrate, and tool diameter are set.
O<nut_pocket_2> sub
  #<xcenter>      = #1 ; [mm] X center
  #<ycenter>      = #2 ; [mm] Y center
  #<screw_radius> = #3 ; [mm] Screw radius
  #<thickness>    = #4 ; [mm] Sheet thickness

  #<hole_radius> = [ #<screw_radius> - #5410 / 2 ]
  O<helix_mill> call [#<xcenter>] [#<ycenter>] [#<hole_radius>] [0] [-#<thickness>]

  ; Cleanup.
  G0 Z#<_zsafe>

O<nut_pocket_2> endsub

; --- BEGIN CONFIGURATION ---

; Don't forget to set up tool table.

S2000 F250

#<_zsafe>        = 30.  ; [mm] Safe Z
#<_zstep>        = 3.5  ; [mm] Z step

#<_thickness>    = 13.5 ; [mm] Sheet thickness
#<_nut_radius>   = 3.95 ; [mm] Nut radius
#<_nut_depth>    = 7.5  ; [mm] Nut depth
#<_screw_radius> = 2.   ; [mm] Screw radius

#<_grid_step>    = 30.  ; [mm] Nut grid step
#<_grid_width>   = 180. ; [mm] Grid width
#<_grid_length>  = 210. ; [mm] Grid length

; --- END CONFIGURATION ---

G21 G90 G17 ; Metric, Absolute, XY plane

O<grid> sub
  #<gridx> = 0.
  O100 while [ #<gridx> LE #<_grid_width> ]
    #<gridy> = 0.
    O110 while [ #<gridy> LE #<_grid_length> ]
      O120 if [ #1 EQ 1 ]
        O<nut_pocket_1> call [#<gridx>] [#<gridy>] [#<_nut_depth>] [#<_nut_radius>] [#<_screw_radius>]
      O120 elseif [ #1 EQ 2 ]
        O<nut_pocket_2> call [#<gridx>] [#<gridy>] [#<_screw_radius>] [#<_thickness>]
      O120 endif
      #<gridy> = [ #<gridy> + #<_grid_step> ]
    O110 endwhile
    #<gridx> = [ #<gridx> + #<_grid_step> ]
  O100 endwhile
O<grid> endsub

G0 Z#<_zsafe>
T4 M6 G43 ; 1.5mm cylindrical
M3
O<grid> call [1]

G0 Z#<_zsafe>
T2 M6 G43 ; 3.175mm cylindrical
M3
O<grid> call [2]

G0 Z#<_zsafe>
M2

After pocketing, an M4 nut was driven inside each pocket.

Fixing workpieces

I have tried several methods of fixing workpieces in the past. The problem is that the forces inflicted by cutting can be quite great; if the only counteracting force is friction, it will get slightly displaced.

So the simple and reliable solution is: just drill some holes in it and screw it down. I drill holes with a d=5mm drill to allow for some inaccuracy in marking the holes and flexibility of material.

Conclusions

  • HDPE sheet is great for this kind of fixture; it is rigid and does not interact with coolant.
  • Nuts fit tightly inside the pockets and do not get displaced.
  • Offsetting nuts by 7.5mm allows a large margin for attaching materials of different thickness.
  • Placing nuts over the T-slots allows the coolant to escape rather than get stale inside the pockets; nevertheless, nuts can slowly corrode over time.

Want to discuss this note? Drop me a letter.