Personal CNC Cutting Ability
The machine is capable of cutting most materials at or near their recommended feeds and speeds. For example, for fast metal removal on 6061 aluminum we will run a 1/2” diameter 2 flute cutter at around 18 IPM (inches per minute) and 3000 RPM, using a full 1/2” depth of cut. That’s a pretty good volumetric rate of metal removal so it’s essential to clear chips with a flood coolant. We will run smaller cutters when we’re not trying to remove large amounts in a hurry. For most aluminum work we use 3/8”. The example above, using a 1/2” cutter, results in a surface speed of 390 SFM (surface feed per minute). A 3/8” cutter needs 4000 RPM to get the same surface speed, well within the performance envelope of the machine.
Cutting steel and iron needs a lower volumetric rate, thus slower feed and speed. The PCNC 1100 will run best using smaller cutters when working with tougher materials. For example, the general machining recommendation for some oil hardening steels is 30 SFM. Doing this with a 1/2” end mill, the surface speeds calculation indicates 230 RPM, but that’s near the minimum spindle speed of the PCNC 1100. By switching to a 1/4” end mill the recommended spindle speed becomes 460 RPM, well within the capability of the PCNC 1100. By keeping close to general machining recommendations your tools will last longer and you’ll have a better cut. For examples of cutting performance, please refer to Tormach’s Machining Videos.
Understanding PCNC Accuracy
While a machine tool may seem absolutely rigid, the truth of the matter is that everything has some elasticity. Related to elasticity is the compressibility of components such as ball nuts and bearings. Preloading of bearings and ballscrews can remove the physical open space between moving parts, but the technique cannot eliminate compressibility. The key to achieving maximum accuracy is understanding and controlling the magnitude and direction of forces. Maximum accuracy is achieved when the forces are minimized, as occurs in a finishing cut. Maximum repeatability is achieved when the forces are repeatable, both in magnitude and direction.
Resolution, Accuracy, and Repeatability of the PCNC
The minimum discrete position move is 0.0001” – this is the resolution of motion. Machine accuracy is closely related to ballscrew accuracy. Our ballscrews are accurate to 0.0006” per foot, but considering all the other factors that come into play, we prefer to keep accuracy expectations to 0.0013” per foot. Repeatability will be better than 0.001” per foot. Machining is a mix of science, skill, and art. The caveat in stating accuracy and repeatability is that these factors depend on the techniques used by the machinist. A skilled machinist can often deliver accuracy that exceeds the accuracy specified by the machine builder, while an inexperienced machinist may have difficulty delivering the expected accuracy. With this understanding, we cannot tell you what accuracy you will be able to achieve in your own work. Never the less, the accuracy specified by a machine builder remains an important reference point. Our minimum specification for accuracy is stated in the following quality control document below:
– Download TD10105: PCNC 1100 Certificate of Inspection
– Download TD10104: PCNC 770 Certificate of Inspection