A lathe mandrel is a common and useful machine shop tool.
Precision mandrels are finished by cylindrical grinding after they have been machined and heat-treated. Finish grinding the mandrel will provide you with basic experience in cylindrical grinding.
Refer to a working sketch (Figure L-130) to determine the required dimensions. Selecting the Wheel For a hardened steel mandrel, the best abrasive will be aluminum oxide of 60 grit and a J-bond hardness with a somewhat dense structure.
Grinding Machine Setup Step 1 Set up the diamond dresser (Figure L-131).Diamond dressers have a height to match the wheel
centerline. On many machines the dresser is built into the footstock assembly.
Step 2 Dress the wheel using a full flow of cutting fluid. If you cannot obtain continuous fluid coverage, then dress dry.Use a relatively rapid traverse of the diamond, with about .001 in. of infeed per pass, until the wheel is true and sharp
Step 3 Place a parallel test bar between centers (Figure L-133).
Mount a dial indicator on the wheelhead, and set it to zero at one end of the test bar.
Step 4 Move the table 12 in. and read the indicator at the other end of the bar. The indicator should read .003, or a total of .006 in. per foot of taper (Figure L-134).
Step 6 Lubricate the center hole in each end of the mandrel, using a high-pressure lubricant specially prepared for use with centers. Avoid using lubricants that have lead as a component.
Step 7 Place the dog on the mandrel and insert it between the headstock and footstock centers. Move the footstock in so that some tension is on the footstock center (Figure L-136).
Step 9 Set the table stops (Figure L-138) so that the wheel can be traversed some distance beyond the ground surface but have at least in. clearance from contacting the driving dog. If there is adequate clearance on the footstock center, set the stop to permit at least one third of the wheel width to go beyond the surface being ground. This overtravel will ensure that the ground surface of the mandrel gets completely
Mandrel Grinding Procedure Step 1 Move the wheel close to what will become the smaller O.D. of the workpiece. (This is usually the footstock
end.) With the wheel stopped, you may use a paper strip (about .003 in. thick) as a feeler gage to position the wheel
close to the mandrel.
Step 2 Turn on the wheel and headstock spindle. Adjust the work speed to provide 70 to 100 sfpm. Step 3 Start the table traverse. The table traverse rateshould be about one fourth of the wheel width for each revolution of the workpiece. (On unhardened work a more rapid traverse would be suitable, up to half the wheel width.)
If your machine is equipped with a tarry control, set the adjustment for a slight dwell at the end of the traverse, so that the table does not “bounce” on the table stops.
Step 4 Infeed the wheel until it contacts the rotating mandrel. Sparks will begin to show (Figure L-139). Then, start the grinding fluid and infeed about .001 in. per traverse until the surface of the part is cleaned up. At this point, set the infeed dial to read zero. Then, retract the wheel.
Step 5 Stop the machine completely. Measure the mandrel in two places, 4 in. apart (Figures L-140 and L-141), to verify that your taper setting was correctly done. There should be a .002 in. difference in diameter between the larger and smaller diameters. Make an adjustment if necessary. Be certain that the grinding head is retracted from the work if you need to make this adjustment. Step 6 Rough and finish grind the mandrel to final size.
The grinder may have an automatic infeed stop that will stop the infeed when final diameter is reached. On a manual machine, infeed the wheelhead to remove about .001 in.from the diameter per complete traverse cycle. The finishing passes should remove about .0002 in. from the diameter.
Step 7 Check for both taper and diameter periodically while making the finishing passes. The table traverse rate should be reduced to about one eighth the width of the wheel per workpiece revolution. When the final size is reached, allow the work to spark out by traversing through
several cycles without additional infeed (Figure L-142).
Only after careful measurement should the workpiece be removed from between the centers (Figure L-143), as returning the work to the centers to remove small amounts of material is usually not successful, because of minor irregularities that occur.
The smallest amount of grit can cause large differences.