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Break-throughs are through-milled openings in workpieces which may be of different geometrical shape and size. They may have a square or rectangular basic shape with smaller or larger radii at the "corners". Break-throughs are generally milled with relatively rough accuracy specifications, i.e. according to the corresponding application.
Figure 30 - Break-through
1 - workpiece
2 - break-through
For milling break-throughs it is necessary to predrill at a given point (end point) so that the cutting infeed movement can easily be carried out with a shank cutter.
If possible, for milling break-throughs the frame milling fixtures, usually used with column-and-knee type milling machines, are applied.
For a uniform machining surface the feed direction must be changed without stopping the feed.
For milling break-throughs the following milling machines are suited:
- vertical milling machines with frame milling fixture frame milling: change of the feed direction from
· X into Y-direction takes place without stopping the feed.
· X-direction corresponds to longitudinal feed.
· Y-direction corresponds to cross feed.
Figure 31 - Frame milling
1 - workpiece,
2 - break-through,
3 - directions of motion with frame milling
- horizontal milling machines with frame milling fixture:
· change of the feed direction from X into Z-direction
· X-direction corresponds to longitudinal feed.
· Z-direction corresponds to vertical feed.
The frame milling programme is set on the selector switch panel or the milling machine.
- Double-column piano-milling machines are used for workpieces which due to their size (dimensions) are not suited for being clamped on a column-and-knee type milling machine.
Setting up for and milling of break-throughs is carried out in the following sequence of operations:
- Check the functionality and operational safety of the milling machine to be used.- Lay out the necessary workpiece clamping and tool clamping means as well as measuring and testing means.
- Fix, align and set the workpiece clamping means (mainly machine vice or direct clamping on the table), clamp the workpiece, fix or chuck the milling tool.
- Mount the coolant system and the cutter safety guard.
- Position the milling machine table and the frame milling fixture, set the cutting values.
- Scratch and set the length and width of the break-through (stops at machine table and saddle or at machine table and knee), rough-mill the break-through with small cutting depth (0.5 - 1.0 mm).
- Carry out visual examination and dimensional check and, if necessary, corrections. Finish milling the break-through. The cutting infeed is made at the predrilled point when shank cutters are used.
- Unclamp or remove the workpiece, deburr the cut edges and clean the workpiece. Make a visual inspection, dimensional check and surface examination according to the specifications. Profile gauges or the counterpart are recommended for dimensional check.
- Change the workpiece in accordance with the requirements stipulated for proper clamping and labour safety regulations.
- After having finished milling of break-throughs, clean the milling machine, ail tools and auxiliary equipment used and the whole workplace.
What advantage does frame milling
offer?
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