Pocket NC, Affordable and Accurate?
Since the beginning, the Pocket NC mill has been based around two core values, accuracy and affordability. While the Pocket NC uses many off the shelf components to remain affordable, it also employs several techniques that, when combined, have a greater impact on accuracy than that of expensive components.
A good example of this is the ball screw versus lead screw debate. In general, ball screws are associated with accuracy. A machine with ball screws must be more accurate than one with lead screws. In actuality, lead screws can have about the same accuracy as rolled ball screws, as both use similar technologies in the manufacturing process. A high end ball screw only has an increased accuracy of ~0.0003 inches per inch or ~0.0036 inches per foot over that of a good lead screw. While they can improve accuracy, ball screws are usually adopted for the increase in lifespan, linear forces and noise reduction. For this reason the Pocket NC mill uses preloaded lead screws with high life span nuts, reducing the cost by about $1000 per machine, without any major sacrifice in accuracy.
Some components used on the machine are extremely expensive, like the linear guides. So why use linear guides instead of lower cost guide rods? Guide rods in general are very good for linear applications with similar tolerances to linear guides. At Pocket NC we use linear guides because the mounting strategy of a linear guide allows each rail to conform to the squareness of its mounting surface.
Squareness is perhaps the most important aspect of a machine as a deviation in squareness for any given axis directly relates to the accuracy of the machine. An axis which deviates by 0.001 inches per inch will have 300% more effect on machine accuracy than that of a lead screw and thus is much more important to consider.
So how does Pocket NC create accurate machines? For starters each machine is made from billet aluminum and allowed to flex and bow throughout the machining process so that its final surface cuts have minimal effect on squareness. The use of a rotary axis during manufacturing assures minimum human contact where problems are likely to occur. Each part is then checked using precision granite and tooling before anodized plating and assembly. Parts which do not conform to tolerance bands are re-machined or recycled.
Throughout the assembly process, subassemblies of the machine are measured to ensure squareness.
Once assembled, each machine is calibrated and tested to ensure accuracy. The entire process takes about 15 hours. This includes machining, measuring, assembly, alignment, wiring, calibration and testing.
Is the Pocket NC mill accurate enough or right for your application? In general the Pocket NC mill is recommended for light manufacturing. This means high part counts for soft materials like Delrin or wax or low part counts for harder materials like Aluminum. The machine is also recommended for applications where +/- 0.005 inches feature locational tolerance is acceptable. While better tolerances are possible, greater attention will be required during the machining process.