Tormach LLC - Personal CNC Machines, CNC Tools and CNC Accessories

Quality

There are thousands of small decisions to be made as the design and manufacturing process for a product evolves. These decisions generally revolve around picking the appropriate compromise among a variety of competing factors; factors such as cost, speed, appearance, convenience, et cetera. These decisions are made easier when you pick one factor and "put a stake in the ground." For us, that factor was quality.

The stories and images below are presented as our way of providing you some insight into the detail and depth of our attention to quality. When you get to the heart of the matter, quality is not about forms, meetings, tests, or ISO certifications; instead quality is an attitude, a discipline, and a perspective. Techniques and tools related to quality, such as ISO 9001 procedures, are like any other tools; they can be used correctly or used poorly.

Many of our sources are from China and Taiwan, places that are occasionally known for a lack of quality. We found many factories that fail to understand quality and a few that "get it". Direct experience with a supplier and their products is the best way to tell the difference. Finding the right suppliers takes a great deal of time and money, but it's an investment that provides a competitive advantage. Please don't bother to ask us who our suppliers are, in most cases we are not willing to discuss the subject.

Machine Production

To start with, let's recognize that ultimate machine precision is not a suitable goal, unless of course, cost is not a concern. The image to the right is a link to a QC document that defines the level of precision appropriate for the PCNC 1100. In this context, quality is not the degree of precision the machine is designed for; rather it is the ability to always meet the precision specifications.

The tests described in the QC document are performed on every machine, with the exception of the Optodyne test described near the end of the document. That test is done on every 20th machine. The Optotyne laser Doppler displacement measuring system (www.optodyne.com) is an exceptional tool, normally only used to calibrate high-end machine tools. The link below is a 6 meg video, a test on basic linearity for the X axis. The machine moves 10 mm at a time while the Optodyne is recording position. Two readouts on the laptop are command position and actual position, in mm.

One simple principle we used when looking for suppliers was to try and make sure that the product we were interested in was not the best thing they have ever made. The photo below is from a typical Chinese drill/mill factory, where the best product they know how to make is a simple drill/mill, similar to those found in discount catalogs. We worked with several such companies, all the way to delivering prototype CNC mills. None of them were good enough.

The manufacturer we partnered with for production of the PCNC 1100 has extensive experience producing larger CNC machine tools for the domestic market in China. One of their VMCs is shown here, behind our friend and interpreter, Ms Chi Jing. One of the Chinese engineers put it pretty well when we were discussing some difficult details of producing the PCNC 1100, he used the American idiom “Piece of cake”. For them it was true, for others we had presented challenges they simply could not meet.

Ballscrews

Ballscrews are a core element of any CNC machine tool. We visited several Chinese ballscrew factories. Here again, we looked for the ability to deliver higher precision than the PCNC 1100 required. Ballscrews on routers or low end machine tools are commonly C7 grade, intended for utility motion only. We specify P4, a basic precision level common to many machine tools. The photo below shows Swiss manufactured (Reishauer) ballscrew grinder designed to produce P1 ballscrews up to 2 meters in length. This was just one of the many screw grinders they had. P1 ballscrews are exceptionally expensive, with an allowable mean deviation about 1/3 of that on a P4 and 1/8 of the allowable deviation of a C7 grade.

The P4 screws needed for the PCNC 1100 would probably not be made on the Reishauer grinder. A more typical grinder is shown below, along with a laser QC system used to check production quality.

Motors

The PCNC 1100 spindle is driven by a standard 3-phase induction motor. Most standard motors look alike on the outside, but there are important differences on the inside. The company that manufactures our spindle drive motor does not make any single phase motors, their main market is larger industrial motors. A large portion of their motors are private labeled to a couple of well known international corporations, one is Japanese, one is German. The photo below shows the initial stages of building a stator winding. The blue paper is called “phase paper”. It’s an advanced construction technique that helps prevent insulation breakdown on higher voltage motors (460V and above). All of our spindle motors have phase paper.

The windings in inexpensive motors are typically just dipped in resin to lock the wires in place. The factory that builds our motors has a VPI (vacuum pressure impregnation) system. This is a system that submerses the stators in resin, then pulls a vacuum, drawing all the air out of the windings. When the vacuum is released the resin moves deep into the windings, forming a much more solid stator winding (Google "vpi resin windings" for details). VPI is far superior to dipping, but they need this system at the factory because they make 400 Hz high-speed spindle motors. They use the system when building our standard motor simply because they have it.

The final motor photo shows some rotors, just before balancing. All of our rotors are balanced to 3600 RPM. The blue coating on the rotors prevents rust. One of the more common motor failure modes is bearing failure, normally from contamination. Very often the contamination is generated inside the motor, from rust on the surface of the rotor.

Summary

These are just a few examples of the depth and detail of our concern for the quality of the PCNC 1100 and the components that go into it. We have similar stories on stepper motors, ball bearings, tooling, iron foundries, electronics suppliers, and much more. Our concerns were not limited to East Asian sources. There were several USA based power electronics firms that failed to meet our need for quality and reliability. The process of selecting suppliers and components has been expensive and time consuming, but we feel that the performance and reliability of the product depends on quality components. Our company name goes on the outside of the machine, we’re concerned about what goes inside the machine.