Cnc machining nm09/2
Points on CNC lathe programming
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- 4.23.3 Roughing out tapers
- 4.23.4 Roughing out radii and fillets
- 4.23.5 Roughing out chamfers
- 4.23.6 Programming the finishing cut
4.23 Points on CNC lathe programming4.23.1 FacingAs a general rule the overall length of the workpiece is faced to length as the first machining operation. In this case the direction of feed is normally towards the centre of the work with a width of cut of no more than 1 mm. Refer below. When facing cuts are taken, the first cut is a light reference cut as the end of the bar stock rarely has a perfectly square face. The facing out is driven past the centre line of the work to a point equal to twice the nose radius of the insert. For example, if the insert has a 0.8 mm nose radius then the facing cut will be G01XC-1.6 
Figure 4.46 Note: The machine has no way of knowing if the workpiece blank is too long. Consequently, if the job was 5 mm longer than what you have programmed as a Z axis value, the lathe will attempt a 5 mm facing cut and so creating a very dangerous situation. When facing to a shoulder as shown below, the direction of the cut is normally away from the centre of the work with a depth of cut of no more than 0.1 mm. The reason for this is that most finishing tools have a 3 to 5 degree negative approach angle which is used to finish up a shoulder.
Roughing out a profileTool number 1 is the roughing tool used for facing and turning. This tool can take a 3 mm depth of cut using a feed rate of approximately 0.2 to 0.35 mm/rev. When roughing remember to leave the work 1 to 2 mm oversize for the finishing cut as carbide tools do not perform satisfactory when taking light cuts below 0.5 mm depth. When roughing machining large diameter stock remember to include a G50 command line to limit the RPM to a safe value which will not throw the work out of the chuck jaws. Remember that this command requires a speed limit value. For example G96 S200 = constant cutting speed of200 m/min, G50 S2000 = a maximum RPM of 2000. Also remember when machining black bar it is never exactly the size stated, it is always larger. For example, Ø 100 black bar is probably Ø 101 or larger. It is best to go and measure your material to check this out, then allow for this in your depths ofuts. The material may also be out of round. Note: It is strongly recommended that the tool path co-ordinates for roughing are establishes by drawing the component on graph paper using a 2: 1 scale and then plotting each roughing cut on the graph paper. 4.23.3 Roughing out tapersWhen roughing tapers, it is important that the finishing tool has a reasonably even depth of cut. ‘Steps’ in tapers should be avoided. 
Figure 4.48 Method oneUsing ‘method one’ the steps are removed by programming the tool movement in such a way that, at the end of each successive cut, the tool is fed from the end point of one cut to the end point of the previous cut along a line approximately the same as the required taper. 
Figure 4.49 Method twoWhen the last depth of cut is programmed, feed up the taper leaving a suitable finishing allowance. 
Figure 4.50 4.23.4 Roughing out radii and filletsA method similar to that of roughing out tapers is also used to rough out radii and fillets. Again the important factor to remember is that steps should not be left for the finishing tool to clean up. These steps must be eliminated during the roughing process. 
Figure 4.51 Method 1Perhaps the simplest method of roughing out a radius as shown is to first draw the profile on a sheet of graph paper to a scale of 2: 1. Having done this, the procedure now only requires a series of point to point coordinates from the end of point of one cut to the end point of the previous cut. This generates an approximate radius without the need for the calculations associated with true form circular interpolation. 
Figure 4.52 Method twoThis uses a preliminary roughing out cycle in which the bulk of the material is removed in a manner which will leave undesirable steps. The final roughing cut, is programmed to remove these steps by cutting a true form radius smaller than the design size using the circular interpolation function. 
Figure 4.53 4.23.5 Roughing out chamfersMethod oneBecause chamfers generally require only a small amount of material to be removed, the simplest way of cutting chamfers is to program each chamfer, small fillet or radius as part of the finishing cut as shown. 
Figure 4.54 An alternative method is shown. This method which is slower, provides for a greater dimensional accuracy in that any backlash which might be present is eliminated even though in theory there should be no backlash present. The method illustrated here should only be considered if the dimensional accuracy of the diameters of the shaft were critical. 
Figure 4.55 4.23.6 Programming the finishing cutWhen programming the finishing cut for a component the programmer has to consider four basic things:
Finishing sizeWhen programming the diameter of a finished size, the size programmed should always be the mid point between the high limit and the low limit of size. 
Figure 4.56 Finishing toolThe practice of only using one tool for all roughing and finishing operations is one which is not generally recommended because the one tool would wear quickly and as a result the dimensional accuracy of the tool will be effected. In addition surface finish would also be effected. Usually one tool is used for roughing, leaving a small amount of material to be removed by the finishing tool. With this method the finishing tool maintains a working tolerance of size and finish over a greater number of components. Finishing speedsWhen using finishing tools, speeds should be increased. For example, when finished machining mild steel and coated carbide tools, use a cutting speed of200-300 m/min as a starting point. Feed rates for finishing vary, depending on the tip radius and surface finish required. As a rule of thumb finishing feeds should not exceed one quarter of the Tool Nose Radius -the feed rate for a tool with a 0.4 nose radius would therefore not exceed 0.1 mm/rev. Directory: file -> view view -> Western Europe after wwii view -> Author study: paul fleischman view -> Qpa1 7ela study Guide view -> Arctic Oil/Gas Neg view -> November 1970 Approximately 150,000-500,000 deaths Bhola Cyclone Bangladesh view -> Grade Level: 6 Unit of Study: Caribbean glce view -> Citation: Duffield, Katy view -> Table of Contents Lesson # view -> Chinese Wind Energy Disad Download 0.53 Mb. Share with your friends:
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