The key principles and recommendations are covered below.
Machining quality is dependent on many criteria. The five criteria for success are:
1 ) Production equipment: condition and choice of equipment (machine, spindle, suction, workpiece clamping, choice of cutting tool, etc.) 2) Machining method and strategy: machining direction (conventional (up) or down), number of cuts, type of entrance into the cut (angular, tangential), use or not of sprayed lubricant, etc. 3) Human resources: training, level of experience of the technicians in using the production resources. 4) Material: type and quality of the material. 5) Environment: dust, vibration, temperature (workshop and material), etc. And also, required surface finish and target machining time.
In general, low-power spindles (0.5 to 1.5 kW) can reach high rotational speeds, but deliver very low torque at low speeds.
They should not, therefore, be fitted with tools whose diameter is more than 6 mm.
When machining thick materials, the number of cuts must be increased.
For cutters with a diameter of less than 4 mm, the axial depth of cut (Ap) should be equal to the Ø and be about 3 mm for cutters with a diameter of 5 to 6 mm.
The calculations (given in page 46 of this catalogue) used to determine the rotational speed of the spindle clearly show that when the Ø of the tool is larger, the rotational speed of the spindle needs to be reduced, irrespective of the material. The rotational speed should also be adjusted to suit the properties of the material.
For example: when machining soft materials, the rotational speed should be lower so as not to heat the material.
The rotational speed should also be reduced if the tool is long (since the potential out-of-balance is greater as is the risk of breakage and vibration).
A small-diameter tool is more susceptible to bending. The feed speed should therefore be set lower than that possible with a larger diameter.
The same principle applies for tools that have a long cutting length - the feed should be reduced since this type of tool generates a lot of bending.
When machining soft materials, the feed speed can be increased so as not to heat up the material.
Take care when calculating the feed speed: when you increase the number of teeth, you need to reduce the Fz value due to the impact of less effective chip evacuation (you cannot go three times quicker with three teeth than you can with one tooth).
The in-feed (or plunge) speed is normally half, or even a third, of the feed speed.
The impact on the machining time is not too significant, but this lower speed increases the service life of the tool (by protecting the tip) and the spindle.
(It can even be lower. For example: Ø 20 face cutter fed directly onto the material: in-feed (plunge) speed of about 50 mm/min)
There is no benefit in setting a very high feed for very small workpieces. The reason is that the machine only vary rarely reaches this speed; the gain in time and in surface finish is very small. However, the geometry of the workpieces and the service life of the cutters is degraded.
New tools being used for the first time will not produce their best surface finish until the tool has machined a few metres of material, due to the extremely sharp edges on new tools.
This is particularly true for one-flute tools used to machine plastics.
The 4053 series cuts less aggressively and does not need to be “run in”.
The useful length must be greater than the thickness to be cut, without being too long, since this leads to:
- A longer unsupported length,
- A less rigid and more breakable tool,
- An impaired surface finish and shorter tool life.
Upcut cutters with a right-handed cut tend to pull the machined workpiece towards the tool: the chips are very well evacuated, but the workpiece must be clamped securely to avoid any vibration problems.
Downcut cutters with a right-handed cut tend to push the machined workpiece against the table of the machine, which reduces clamping-related issues. There will be no delamination of the material near the surface of the workpiece, but the chips will be poorly evacuated (with a risk of chip jamming).
Excellent chip suction or providing clear space under the workpiece are recommended.
A number of criteria need to be satisfied to obtain a good surface finish, with feed speed far from being the only one.
- Securely holding the workpiece (extremely important).
- The right tool for the type and thickness of the material.
- Good condition of the machine (shafts, spindles, tapers, collets, etc.) and tool.
- Good chip suction.
- Good cutting conditions.
- Good machining strategies.
Removing 0.3 to 0.5 mm of material with a finishing cut is a good way to obtain a better surface finish for many materials. This eliminates any built-up edge-related issues and smooths out the effects of vibration. However, this is not true for all materials.
One-flute cutters, due to their geometry, do not produce the best surface finish in the bottom of pockets. Two-flute cutters have flatter tips and produce a better surface finish.
Smaller overlaps and lower speeds also greatly improve the surface finish.