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“Machining Steel” A Lab in Two Parts
Part A Equipment:
Lathe w/ tool post Single point Triangular Carbide Cutter Positive Tool Holders Micrometer CR 1018 Steel Workpiece (1" (25.4 mm) diameter -- Center Drilled)
During this laboratory the student team will produce the part “Adjuster Leg” shown on
the attached sheets. The machining operations required will include longitudinal turning, "chamfering", and face turning (facing) during Part A. The cutting tool should be positioned with an adequate side cutting edge (lead) angle to allow free cutting operations, we will be using both right hand and left hand cutters to perform each operation. Note the tool’s positive rake angle and clearance angles. The chips as they are produced will slide over the rake face away from the work when properly mounted. Also, the tool post/tool holder is mounted so that any problems in the operation tend to eject the tool from the workpiece to minimize damage. After the tool is mounted, with tool tip on center (or very slightly above center), the tool and tool holder should be tightened rigidly to the lathe's tool post.
When performing the longitudinal turning operations, the majority of the laboratory, the student teams most consider the appropriate "Process Plan." Typically, the operation will follow a series of passes. Starting with "Roughing" operations of high feed, large depth of cut (doc) and moderate speed, through semi-finishing to final finishing operations employing small feeds and doc of 0.005 to 0.0075 [0.127 - 0.191 mm] and high(er) speeds. Specific machining recommendations for CR 1018 steel are found in the Machining Data Center Machining Handbook. For each of your group's machining passes calculate the cutting speed and/or RPM rate, metal removal rate (MRR), cutting time and ideal surface finish generated -- formulae in handout shown below. These calculations should be tabulated in your group lab report to be submitted after Part B. During production machining operations, often the processes, particularly the roughing operations, are designed to minimize cutting time by maximizing the consumed horsepower in making a cut. Most metals exhibit a constant Specific Horsepower in units of: HP/in3 (a minor function of MRR). The specific horsepower for 1018 CR steel using sharp cutters is about 1.1HP/in3. Using this value and the MRR's calculated above, determine the power requirements of your actual operations based and compare it to the available motor horsepower of the lathe being used.
During facing operations, setup the operation using a left handed tool holder and employ the power cross feed mechanism. During these operations, consider the effect of the turning diameter on MRR as your tool cuts on the face. Tabulate MRR at full diameter, "half radius," "quarter radius," and the center of the bar.
During each machining pass, observe and describe the chips being produced, include this
information in your lab notebook. Observe and describe the surface finish (Ra) produced in each
operation as well. Observationally, does the finish correspond to your calculated value using the equation below:
Measure and report, employing a dimensioned drawing, the geometry of the part your team actually produced. Keep your finished part, it will be used for Part B. Part B Equipment:
Lathe w/ tool post Single point HSS Cutter 60° Threading Cutter Threading Gage 5/16” 18 NC Tap and Appropriate Drill and driver Threading Micrometer CR 1018 Steel Workpiece
During this laboratory the student team will produce the finished adjuster leg part as seen
in the attached print. The machining operations required will include drilling, tapping, and tread cutting. The cutting tools should be positioned to allow free cutting operations during all operations. Note the tool grind of a positive rake angle and clearance angles on the 60° threading tool. The tools should all be mounted with tool tip on center and the tool and tool holder tightened rigidly to the lathe's tool post and tail stock during processing.
During external threading operations, we will use the “29°” Compound Feed methodology discussed in reference materials authored by DeGarmo,Black & Kosher (pp. 886- 890) to produce the 5/8x18 NF threaded end of the part. Here the student teams will calculate the required compound infeed distances for the 60° point to cut the threads. In addition the teams should monitor and record the chips produced and the effect of chip loading on the process. Refer to chapter 30 in the reference for details on thread geometry and manufacturing techniques.
fY = 8CR
Y is surface deviation
is feedrate, CR cutter nose radius, average roughnessf R
While machining the internal threaded hole, the team will employ a “gun tap” designed to produce the required thread geometry. This tap pushes the chips ahead of the tool to assure cleaner threads during the process. Thus, we need to provide additional length in the drilled hole to accommodate these chips during the tapping operation. The teams will be expected to calculate/specify the proper drill to produce the "pilot hole" before tapping. Special care should be employed during tapping so as not to fracture the tool and scrap your part.
Before beginning the lab, think SAFETY. Metal cutting is a process that can be very dangerous, you must WORK to perform the tasks as safely as possible. Follow the established rules and REMEMBER, don't grab any fresh chips with your bare hands and definitely do not leave the chuck key in the chuck after using it!!!