Proceedings of KGCOE-MD2004: Multi-Disciplinary Engineering Design Conference Page 3

MD2004-04005

Rochester Institute of Technology

Fly CutTer Chamfering Machine Project For THE Gleason Works

Vincenzo Mansueto Mechanical Engineering / Matthew Liddick Mechanical Engineering / Julie Wilcox
Mechanical Engineering
Brian Banazwski
Mechanical Engineering / Mark Trotta
Mechanical
Engineering / Greg Baesl
Industrial and Systems Engineering / Philip Raduns
Electrical Engineering

Rochester Institute of Technology

Proceedings of KGCOE-MD2004: Multi-Disciplinary Engineering Design Conference Page 3

Abstract

This paper details the efforts of The Gleason Works senior design team and their attempt to design a stand-alone fly chamfering and deburring CNC machine with corresponding market analysis of the project. The CNC machine created by the Gleason team is meant to maximize throughput, while minimizing company cost. The scope of this project is limited to a top-level design package by use of ProEngineer software with no expected prototype. This design represents the first step for the realization of a new type of CNC machine for The Gleason Works. The desire of Gleason is to ultimately have a design that could be marketed as an individual piece of machinery but this project will only serve as a proof of concept.

introduction

The fly cutter chamfering project is one of many current multidisciplinary research efforts underway through the Kate Gleason College of Engineering focused on the integrating the skills of many engineering disciplines toward a common goal. The Gleason Works manufactures a complete line of machines and tools for the production of bevel and cylindrical gears. During the creation of these gears, sharp edges and burrs are produced. These burrs and sharp edges, usually most prominent at the heel edge, present a problem for several reasons. The first being the risk of operators cutting themselves while handling the parts. Hardening of sharp corners will cause brittle edges, which may break off during heavy loading. Burrs may vaporize during heat treatment and contaminate the chamber which could result in breakage during the lapping process and contamination of the recirculating compound. This would lead to the physical integrity of the piece being compromised. Due to these problems, the sharp edges and burrs must be removed from the gear prior to final production. This process is known as chamfering and deburring.

Background

Gleason currently has two processes for gear chamfering and deburring. The first process is a subsystem on the Phoenix CNC spiral cutting machine. The Phoenix CNC spiral cutting machine’s subsystem is shown in Figure 1. The chamfering and deburring subsystem process is a large electro-mechanical device that sits on a vertical rail. This subsystem is a problem because when it breaks down, it causes the whole gear production process to cease. This results in a costly down time for the customer utilizing the machine. The subsystem is also bulky and awkward in the machine. This is a problem for two reasons: insufficient chip removal due to crowded and complex geometries, and difficulty in operating maintenance.

Figure 1: Chamfering Subsystem for Phoenix II.

The second method The Gleason Works has looked into is a modified GTR 250 VG CNC as shown in Figure 2. The GTR 250 is a stand-alone process, which is too robust and costly for this chamfering application. This machine was designed for precision handling of straight and helical gears, spiral and hypoid gears, and pinions. The GTR 250 also has a large footprint (1.24 m x 2.14 m) to allow for a sizeable machining compartment for inside and outside pointing and rounding, or combinations there in. The system is also only capable of using a single start cutter in conjunction with a limited rotational speed of the work piece, which translates to longer setup and process time. Process time is also increased due to greater travel time of the mechanisms within the large machining compartment. In addition, the vertical work spindle has a hydraulic chucking system, which is bulky and interferes with tooling access to the work piece.

Figure 2: GTR 250

The Gleason Works views both methods as wasted resources. They gave the responsibility of determining a better solution to this problem. The Gleason team decided it would be best to design a complete stand-alone CNC machine rather than adapting the current machine technologies available at Gleason.

Objectives

While the tolerance of the machining process does not need to be precise, the quality of the chamfering and deburring technique must be assured for the proposed design and should include the following critical performance parameters:

· Design team shall use ProEngineer for all drawings.

· Design team shall not be expected to submit a prototype.

· Design team shall submit a proposal and a feasibility assessment of the design to Gleason management.

· Chamfer/Deburr

· Operation must not roll burr on tooth flank.

· Results must be visually smooth from tooth flank to root.

· Ease of Setup

· Worn tooling must be able to be replaced within two minutes.

· A new job must be able to set up within 15 minutes.

· Training should take no more than two days.

· The final cost of the system should be between $100k and $130k.

· Process shall not require use of a machining coolant/lubricant.

· Shall be capable of reaching the toe and heel of pinions and gears.

· Maintenance and Cleaning

· User should not have to clean chips from system.

· Machine should prompt user as to routine maintenance schedule.

Design Process

The critical performance parameters described in the previous section originated through discussions with the customer. These parameters then became design inputs for a number of concepts.

Data Acquisition

A test was conducted on April 26, 2004 at The Gleason Works to determine the minimum and maximum amount of torque that the chamfering motor sees during a typical and hypothetically heavy cutting cycle of a six-inch, carbon-steel, beveled ring gear. A continuous data acquisition instrument provided by Gleason was used to record the voltage output of the motor as it runs throughout the test individual cycles. The testing conditions of chamfering cycle on the Phoenix CNC machine are: a motor speed of 1,500 RPM, 41-tooth ring gear, four revolutions to complete the cutting part of the cycle using a four-start chamfering tool, 10V- full scale continuous printout recorded at 100 mm/s, and using a variable chamfer depth. Trial #1 uses a 0.75mm chamfer depth, and Trials #3, 4, and 5 use a depth of 1.25mm. The voltage output is then converted to an equivalent torque measurement. A second static test was used to determine the conversion from voltage to torque by a straight line approximation formula. As seen in Figure 3, Trial #1 has a maximum torque of 4.83 N-m at about 1.8 seconds into the cycle, and Trials #4 and #5 have an average max torque of about 10.62 N-m at about the same time frame, the third pass of the chamfering tool. The final pass does not take a full slice of material because it is a cleaning pass. From this data, the size of the motor to power the chamfering tool on this project’s unit can be determined, and must be able to function under the 10.62 N-m maximum load. Static hand tests set at 10.73 N-m torque that take a 1.25mm chamfer confirm the #4 and #5 trial tests.

Figure 3: Chamfering Torque Measurement Results. Note: Trial #2 was a time trial and not a torque trial and in Trial #3 the zero data points are due to the data acquisition running off the instrument scale.

Mechanical Design

Figure 4: Final Top-level Drawing of the Designed Stand-alone CNC Gear Chamfering/Deburring Machine.

The complete machine design is represented in Figure 4. The most important aspect of the design is its ability to adjust the pitch between ± 20 degrees which was the requirement set forth by the customer and is represented in Figure 5. This is a function that is not available using the Phoenix on machine setup which is limited to fifteen degrees in the positive direction. The motor and cutter are mounted in an assembly that is adjusted by a ball screw. It was determined that using a 5 mm pitch would allow for rapid adjustments and accuracy. The customer requirement for this application is a linear accuracy of 50 to 100 microns.

(a)

(b)

Figure 5: Cutter Design with Specified Pitch Axis. (a) Initial cutter position. (b) Cutter is now tilted at an angle due to pitch axis movement.

The cutter assembly is mounted using two Setco hardened way slides; these provide both rigidity and precision. This system allows for a wide range of gears and pinions to be accommodated all the way up to a 100 mm pinion and a 250 mm ring gear. To allow for such wide range of parts the cutter slides move 580 mm in the horizontal direction and 280 mm in the vertical direction, separate of the cutter assembly is the work piece table. For this design a Kitagawa chuck was selected for both its price the ability to allow the easy mounting of pinions. A custom spindle was designed for mounting the chuck. The spindle is driven by the work piece motor via a timing belt arrangement. This setup provides a mechanical fuse in the system should there ever be an impact. The work piece table moves on linear bearings and has a 650 mm travel. This allows the operator to load the chuck well away from the danger of the cutter. Each of the axes was designed to have a maximum travel time of two seconds. To limit the maximum momentum of the system all calculation incorporated a half second of run up, a second at constant speed, and a half second deceleration. The cutter torque was critical to the machine setup because in previous models the motor used was much larger than it needed to be. Using the testing that occurred at The Gleason works and the customer defined safety factor of two, it was determined that a motor torque of 20 N-m was needed.

(1)

To confirm the testing results the cutting loads were estimated using (1). This equation approximates the cut to be a triangle. It was found that the calculations for the load were valid until the depth of cut was increased, because at that point, the triangle assumption becomes invalid because the tooth geometry is too complex. Also it is very difficult to accurately measure the chamfer thickness. Overall, the analytical approach showed that the real world testing had yielded feasible results.

Stock division is also an important part of the overall setup of the machine also. When a part is inserted in the chuck, it is necessary to align it so the tool can properly cut the piece. This alignment is performed by the stock divider pictured in Figure 6, which comes down in-between the teeth of the part. On a traditional stock divider, the operator can use too much force and mar the gear surface. This new design allows the height to be set so it will stop between the teeth without coming in contact with extreme force.

Figure 6: Stock Divider Drawing

Electrical Design

With the desire for open-loop control within our design by Gleason, it was determined a servo motion control system would be used in this conceptual design. The servo feedback network consists of a controller, drive/amplifier, motor, load, and some feedback elements that can be mounted on the motor and/or load. With regards to Gleason’s purchasing preference with GE Fanuc, all electrical components were chosen from that vendor.

The three linear axes (X, Y, Z) of the design provide the moving mechanisms for the cutter and workpiece assemblies. The movement of these axes requires crude accuracy in the 50-100 µm range; a servo system with position feedback was determined with regards to appropriate torque requirements as determined by the mechanical assembly of the design. The servo motors and corresponding drives were chosen based on drive peak current requirements; absolute encoding and optional braking were chosen options on the servo motors based on the customers’ requirements.

The three rotary axes (cutter, workpiece, pitch) and their components were again chosen based on accuracy and load requirements. Gleason stated that accuracy to within ±1.0º would be sufficient for this design. The pitch axis required the simplest components due to its passive operation during the machine cycle. Therefore no optional absolute encoding and braking is necessary for the servo motor of this axis. The speeds of the cutter and workpiece axes dictated the use of servo motors rather than spindle motors due to the fact that the base speed of the two motors would be too low for spindle implementation. With the selection of servo motors for these three axes, drives were selected with respect to motor peak current requirements.

The CNC controller for this application needed to meet Gleason’s desire for customization along with the ability to coordinate three axes simultaneously and six axes in total. The ability to customize the machine controller was controlled by the programming executor and was chosen through GE Fanuc as well.

In conclusion, the electrical system for this system was determined based primary on economic constraints and accuracy requirements. The relatively crude accuracy requirements given by Gleason allowed the price for the electrical system to be very reasonable with regard to total design cost.

Market Analysis

By looking at the costs associated with the Phoenix 275 HC CNC machine, with and without the on-machine chamfering tool, the benefits of having a less expensive stand-alone chamfering machine can be found. The cost of the Phoenix, on average, is $675,000. The on-machine tool costs four to five thousands dollars extra. It has been assumed that the machine is running for eighty hours a week, for fifty-two weeks out of the year, and for five years. By taking the cost of the machine and dividing that by the hours of operation, and then adding the man hour costs, which were assumed to be forty dollars an hour, a total cost for the machines was determined. The cycle times for the machine were found for a specific gear. The time to cut the gear was 93 seconds, and the time to cut the gear and chamfer it was 113 seconds.