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Overview - need for inspection
Shrinkage - details
Profile Scanning - for precision
Samples - output examples
Roll Testing - functional inspection
Tool Scope - visual inspection

Molded Gears NEED Inspection

     The physical dimensions of any molded gear are not cut or formed directly. A mold cavity is constructed, shrinkage of the part from the cavity walls is estimated, and the part is formed as shown in the figure below.

     Before anything else, the molder must estimate shrinkage, cut the cavity, mold the gear, and determine how close s/he came to the targeted dimensions, but these types of measurements are not trivial. The entire form of the gear, including the involute base circle and tooth thickness must be accurately measured. Currently, no standardized system exists for directly measuring gear shrinkage.

     Kleiss Gears, Inc. has developed its own system for this this measurement. We directly measure and control all aspects of plastic gear shrinkage. Simple outside diameter measurements or even roll testing with a calibrated master gear cannot characterize plastic gear shrinkage - the involute form must be measured independently. We employ a scanning Coordinate Measurement Machine (CMM, left) for this purpose.

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Shrinkage and Plastic Gears

     Molded gears do not shrink in any simple fashion such as a photographic reduction. There are a minimum of 4 distinct shrinkage rates for any gear. Please refer to our downloadable article for more information on molded gear shrinkage.

     Even simple features such as outside and root diameters must be carefully inspected. A simple caliper check will often miss important features. These diameters must be inspected for total form error as well as concentricity to the principle bore or datum. At Kleiss we probe the tip and root of each tooth and construct a best-fit diameter with respect to the gear datum (as shown below).

     Inspecting the gear involute profiles requires just as much attention to detail. Each tooth should be inspected since the molding process can result in errors anywhere on the gear. The actual form errors of the teeth should be measured directly so that these errors can be eliminated in the molding process or compensated for in the mold cavity.

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Gear Profile Inspection

     At Kleiss, we scan each tooth of our molded gear to determine the size and form error of the involute gear profile. The measurements are taken with a computer-controlled probe scanning at a constant rate with a constant force from the root of the tooth to its tip. Accuracy of our equipment is calibrated to 40µ inches with NIST standards on a yearly basis. The figure below graphically shows the method of scanning.

     After the data is collected for all the teeth, it is mathematically evaluated to determine profile form error and concentricity as well as base circle shrinkage error and actual tooth thickness.
     Below are two gear scan traces. The first report is for an actual measured gear that has significant shrinkage error. The second report is for a gear designed and molded as a replacement for the same application. The original, poorly molded gear had been purchased off-the-shelf from a commercial supply house.

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Gear Roll Testing

     Gear profile inspection is quite laborious and time consuming. It is absolutely required for properly sizing a gear but is not justifiable for production control of molded gearing. For this, gear roll testing must be used. Gear Roll testing consists of the following:

1. Fabricating a nearly perfect 'master gear'.
2. Placing the production gear against it with a slight preload.
3. Rotating the gear set while under this light load.
4. Recording both the actual center distance and the variation of center distance for the production gear.

     If it were a perfect gear, the center distance would be exactly where it is specified, and it should have no variation from that position. Eccentricity will cause a sinusoidal variation once per revolution. Molded anomolies will affect it as well as tooth thickness errors and variations. A picture of an actual roll tester and a schematic of its construction are shown below:

     At Kleiss the center distance variations are recorded, graphed, and kept as a history of the molding experience. A sample of such graphs are shown below:

     As can be seen in these traces, roll testing can give an indication of shrinkage errors but it cannot aid in determining the amount of error or correction needed. This type of measurement is best used to determine and validate process control.

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Werth Coordinate Measurement Machine
 

     Kleiss Gears has recently added a Werth Coordinate Measurment Machine with combined and integrated probe and vision capability in its metrology department. This machine will provide improved performance in traditional gear scanning as well as allow vision scanning of micro-miniature gears. Werth has outfitted this machine with a 4th axis rotary table to facilitate Kleiss requirements for precision and versatility in gear measurement. This machine is expected to provide new insights into the study and manufacture of precision molded gears.

Olympus Tool Scope
 

 

  And finally, we have in reserve an Olympus tool-makers microscope with magnification from 50x to 500x with Heidenhain scales on all axes. Sometimes nothing less than a tool scope focused and sighted by eye will do the required job.