Starrett - Manual and DCC

Manual CMMs

Starrett utilizes the exact rigid machine frame design and materials for its manual line of CMM’s as its Direct Computer Controlled versions. No area is compromised with our manual machines. The same quality scales and air bearings are used throughout the entire machine. Starrett manual machines also incorporate continuous, full-travel fine feed mechanisms with axis locks built in.

 

 

Direct Computer Control CMMs

In addition to the above prerequisites for both Manually operated CMM’s and Direct Computer Controlled (DCC) CMM’s, additional features are important for DCC style machines.

 

 

Drive System

Starrett uses friction drives exclusively on all three axes of every DCC machine that is produced. These drives incorporate a direct shaft drive to a precision drive band that totally eliminates hysteresis (backlash). This is a true "Zero Backlash System." (See drawing shown below.) When dealing with highly tuned DC servo-controlled motors, hysteresis in the drives and the machine frame cannot be tolerated for maximum positioning accuracy and repeatability. These drives are very low friction and induce minimum vibration into the machine. This eliminates the reciprocating shaft that induces vibration into the machine. This is commonly found with ball screw or lead screw drives. We also eliminate the hysteresis that is almost impossible to remove from the screw drives.

Sectional view of the Friction Band Drive mechanism.

In addition, Starrett incorporates a Z-axis drive isolation system on all of its DCC machines. The drive receives all of the forces of the cables and motor system and runs on an independent linear guideway which is aligned with the Z-axis. When the drive moves up and down, the linear forces are transmitted through a stiff air bearing system to the Z-axis ceramic ram. This totally eliminates the Z ram from the machine frame, which isolates all lateral forces resulting from cables and drive system misalignments. Ultimately the ram is prevented from being deflected, which would adversely affect linear straightness.

 

 

DCC Controller and Hardware

The positioning of a DCC CMM is accomplished by driving the motors of the CMM axes to the right location. This requires an efficient communications protocol between the controller and the CMM scales so that the proper information is current and precise. This information must be updated on-the-fly." To efficiently meet this requirement, Starrett developed a proprietary CMM interface board which is mounted directly onto the computer system motherboard. This direct communications link insures that not only an efficient and reliable link is established but the speed of information and message passing is limited only by the CPU processing speed.

 

 

Positioning Accuracy

It is not only important to produce accurate measurements, it is also important to deliver the probing system to an accurate position prior to contact with the object being measured. This will ensure that the MEASUREMENT point is always as accurately collected as possible. It will also ensure the ability to position the machine to an accurate location for difficult-to-access positions or into tight locations.

 

 

Tunneling Accuracy

This ability to drive in as straight a line as possible between the last position and the next position will not only insure collision-free motion but it will also improve speed and throughput performance.