Contents - Index

3D Machining Tips


File Size
3D machining code files can be very long: tens of thousands of lines of code. When any code file has been selected, the WELmill software calculates all the machining moves required to manufacture the product: this is what is happening when the screen says that the program is analysing a file. With very long files this process can take tens of minutes if the PC is slow, say an elderly 133MHz Pentium 1. The machining moves are subsequently "drip fed" to the WELmill controller circuit board.

An analysed file is automatically saved with the extension NCX. An NCX file for 3D machining can be very large: several megabytes. It is as well to check, now and again, that the computer's hard drive has plenty of free space.

Shortly before machining commences, further file processing takes place as the WELmill software adds data about the X, Y, Z origin location to the analysed file.

Machining Time
3D machining takes much longer than 2.5D machining: it can easily run into hours. For this reason it is a good idea to machine materials where high feed rates can be used: for example, high density closed cell rigid modelling foam. However, the highest feed rates do not give the shortest machining time. This is because acceleration/deceleration control is applied to machine movements. You can hear and see the acceleration/deceleration control applied by WELmill if you do a manual rapid traverse movement of one of the axes. The axis moves almost 1mm before it reaches the rapid traverse speed.

Feed Rates
Imagine you are to machine a surface which has many curves, lumps and bumps and no horizontal flat areas. The tip of the cutting tool will be constantly changing direction. It will travel very short distances in any given direction, perhaps only a few hundreths of a millimetre. Every time the tool changes direction it will accelerate from zero speed towards the set feed rate. However, before it reaches the set feed rate it has to decelerate and stop for another change of direction. The tool never reaches the set feed rate. The net result is that the actual feed rate of the cutter through the material is very low, perhaps only tens of millimetres per minute rather than the hundreds of millimeters per minute stated in the code.

As a general rule, the higher the feed rate, the great the impact acceleration/deceleration has on the machining time. By choosing a lower feed rate, say 350mm/min rather than 450mm/min, the machining time can be considerably reduced.

If the surface to be machined is predominantly flat and horizontal and with few lumps and bumps, then very high feed rates can be used to do the job quickly. If the surface lacks significant flat, horizontal areas then a lower feed rate will give a shorter machining time.

Feed rates should always be appropriate for the material being machined and the cutter being used.

Estimated Machining Time
CADCAM software will often give an estimate of machining time. It calculates this from the total length of the tool paths and the feed rates being used. It cannot take account of the acceleration/deceleration control applied by the machine's control software. Therefore any estimate of machining time must be taken with a pinch of salt. It is best to read it as saying that machining will take longer than the time stated - possibly a lot longer!

Cutters
The cutters usually used for 3D machining are ball-ended - they are like normal slot drills but, instead of a flat end, these cutters have an hemispherical end. Because 3D machining often involves cutting deep into a thick billet, long-reach cutters may be required. These have a longer flute length than standard cutters.

Whatever type of cutter is used, care must be taken to ensure that, when cutting deep into a thick billet, the shank of the cutter is not being forced through uncut material, even modelling foam. Take account of the flute length of a cutter when planning depths of cut.

Ball-ended long reach cutters are available from tooling suppliers and they are also available from TechSoft UK Ltd: see www.techsoftuk.co.uk

Take Care with Ball End Cutters
If 3D machining is being done with a ball-ended cutter, the cutter will cut into the material on which the billet is standing. This ensures that the profile of the part is completely cut. It is essential that the billet is standing on some suitable thickness (at least equal to the diameter of the cutter) sacrificial material which will not damage the cutter. It is likely that the cutter will need a safe, clear space all round the part. Plan work-holding with great care!

3D Machining Strategies Illustrated
                              
                              

Go to www.mecsoft.com and download and install the demonstration version of their program VisualMill 4. Run the Tutorial and find the section called VisualMill Features - Toolpath Strategies. Here you will find some excellent explanations of 3D machining strategies: example above. Please note that not all 3D machining software offers the range of advanced strategies found within VisualMill.


2.5D and 3D Machining
3D Machining Strategies Comparison
PC Specification for Running WELmill
Setting the Origin for Machining