This technical information has been contributed by
Gear Specialist and Merit Gear Company

Making The Difference In Heat Treat

You tell me. Do you think there's a difference? With more and more companies jobbing out or subcontracting vital parts and components to America's production specialists... our job shops, you can bet a job shop with an in-house heat treat can make a difference.

Today manufacturers are depending more and more on specialty shops for higher quality parts and just-in-time deliveries. They are moving away from in-house manufacturing of small quantities, and instead, relying on job shops for these production runs. The role of the heat treater in the manufacturing of these parts is vital and can make the difference for the job shop.

With most gears manufactured requiring heat treatment, you can see why the heat treater plays such an important role. The gear manufacturer has to put much trust in the operations performed by the heat treater. This is where an in-house heat treat can make the difference.

The Difference

A gear is used for a combination of turning, gear cutting, heat treating and grinding. Although the manufacturing processes involved in making this gear are few, if they are not coordinated to complement each other, the end result could be costly in scrap, rework, and/or customer returns.

Incorporating the use of CNC lathes, the parts are turned to closer tolerances than are required on the print without incurring additional costs. This results in an accurate area for the gear cutting fixtures to locate on. Once the parts are completely turned, the next operation is gear cutting. First, the internal spline was shaped. A change in size during heat treat was anticipated, so an adjustment was made to the between pin measurement on the internal spline using data collected from similar jobs.

The next operation was cutting of the two helical gear teeth. Since the gears are helical in design, and have a tendency to change angles during the heat treating process, it was decided at this time to cut a pilot run. Both gears were left hand and had a helix angle of 10 13' 20". We used the larger of the two gears for the example, but both gears were cut with adjustments to the helix angle. The part was cut to an increased angle of one thousandth of an inch. (Approximately one quarter the thickness of the average hair on a person's head.)

After both gears were cut on the pilot parts they were documented and sent to heat treat. All the parts were made from 8620 H material and heat treated to Rockwell C 58/63 with an effective case depth of 0.030 to 0.045.

During heat treat, we tried to simulate a full load by matching the pilot pieces as closely as possible to the dimensional sizes of the other parts in the load. This match also requires the exact same heat treatment to be used in regard to the case depth and Rockwell hardness. This way, we are able to control the quenching speed which is very important in determining the amount of size change.

Once the heat treating was completed the parts were reinspected. The desired result was an accurate lead angle of 10 13' 20", shown as a straight line. The reduction in pin size on the internal spline resulted in the proper over pin measurement. Based on these results the balance of the order was cut, sent to heat treat and returned. A sample of parts were inspected by statistical quality control methods and found to be within print specifications.

The Key

In the example shown, in-house heat treat was the key to the consistency in results from the pilot run to the production run. What makes all this possible is the use of various controls during the heat treat process. Controlling the fixturing of the loads, the use of programmable controllers, and requiring certified calibrations of our instruments are all examples of these controls. Having control of the process enables us to duplicate the identical heat treat pattern every time we run the same parts. This is the "key" difference Merit Gear has with an in-house heat benefiting our customers and us.

Five important controlling factors that make a difference in Merit Gear's heat treat are:

  1. Complete and thorough documentation of all test and production runs
  2. "Fixturing" of special loads
  3. The use of modern and sophisticated instrumentation
  4. A regularly scheduled preventive maintenance program
  5. Certified outside sources who calibrate and certify our instruments and hardness testers.

Complete and thorough documentation of the parts to be processed is very important. We begin each job by issuing a work order. On this work order is all the necessary information needed to process the parts through the heat treat, including any special instructions. A master work order is then kept on file by part number. Once the order is in process, documentation of the heat treat cycle is recorded. This information lists important factors such as the time required for the furnace to get up to heat, the length of the diffusion cycle, what type of quench was used, and the tempering temperature.

We also keep a log of all our furnace charts. Once a load enters the furnace, the operator is required to record the date, part number, his name, customer's name and number of pieces in the load on the chart. The chart gives us a visual record of the cycle time, and the furnace temperatures. These charts are retained for seven years.

One critical control at the hands of the heat treater is the "fixturing" of the load. How you make up your load makes the difference! You can speed up the quench or slow it down simply by adding or deleting parts. It is vital that you make up your load the same way every time to obtain the same results. All special fixturing instructions are noted on the work orders.

The use of modern and sophisticated instrumentation is another way Merit Gear controls its heat treat process. Carbon probes are used in our furnaces to maintain a consistent dew point. The probe regulates the dew point automatically. Manual checks are also taken to confirm the accuracy.

Barber Colman 570 programmable controllers enable us to program the furnaces. The controller gives us the same process every time: the same time at heat, diffusion cycle, soak time, and quench time and temperature. They provide repeatability and accuracy.

Monthly checks of the thermocouples in all our furnaces are also performed against a certified thermocouple. This test measures the accuracy of the thermocouples being used.

A regularly scheduled preventive maintenance program is essential in controlling the heat treat process. Even during busy periods we take the time to shut down and check our equipment making sure everything is in good operating condition. The equipment must work properly in order for our controls to work.

We also have certified outside sources such as Pyro-Matics Inc. and Wilson Co. calibrate our equipment. Pyro-Matics Inc. checks, cleans and calibrates our equipment on a quarterly basis, including certifying the accuracy of the furnace temperatures. Once a year they conduct a complete temperature profile, placing a fixture consisting of ten thermocouples inside the furnace. These thermocouples measure the heat at the top, bottom, sides and center of the furnace, making sure there is an even heat pattern.

Wilson Co. calibrates and certifies all our Hardness Testing Instruments. Their calibration standards meet the requirements of MIL-STD-45662. This is done three times a year.

With the combination of these five controls and others, Merit Gear is able to combine its gear manufacturing capabilities with its in-house heat treat to produce precision quality gearing. Having an in-house heat treat makes the difference for us as a gear manufacturer. We are able to work side-by-side controlling the manufacturing and heat treating processes, resulting in quality end products.

This technical information has been contributed by
Gear Specialist and Merit Gear Company

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