The machine table and all sliding surfaces should be cleaned prior to setup or operation. Any nicks or burrs found on the table or work-holding devices should be removed with a honing stone.
Selection of a work-holding device and method will depend on the machining task to be done. Review Unit 4, and if you are clamping directly to the mill table, be sure to follow the rules of good clamping. Use a work stop if necessary to prevent workpiece slippage from cutting pressure.
Mill Vises A mill vise is an accurate and dependable workholding tool.
When milling only on the top of the workpiece, it is not necessary to set up the vise square to the column or parallel to the table. However, if the workpiece has already machined outside surfaces, steps, or grooves, the vise must be precisely aligned to the machine table.
Many mill vises are equipped with keys that fit snugly into the table T-slots (Figure K-44). Once the keys are engaged, the vise is accurately aligned. Before mounting a vise or any other mill fixture, inspect the bottom for small chips, nicks, and burrs. Use a honing stone to remove these and make sure that the tooling and table are clean. Set the vise gently on the table and position it according to the job to be done. Install the hold-down bolts and tighten just enough to permit the vise to be moved by gentle tapping with a lead or other soft hammer. If the mill vise does not have alignment keys, the following procedures may be used to align it parallel or perpendicular to the machine table.
Parallel-to-Table Alignment Step 1 Fasten a dial indicator on a magnetic base and attach to the arbor or overarm (Figure K-45). Be sure that
overarm clamps are tight. Position the indicator to contact the solid jaw of the vise. Preload the indicator about half a revolution.
Step 2 Move the table by hand so that the indicator is positioned at one end of the solid jaw. Set the bezel to zero. Crank the table so that the vise jaw moves past the indicator tip, and note the reading at the opposite end of the jaw.
Step 3 Tap the vise gently with a soft hammer so that half of the total indicated runout is canceled (back toward zero on the indicator). When tapping the vise, move it in such a direction that the solid jaw moves away from the indicator.
Moving the jaw against the indicator tip can damage the delicate indicator by shocking the indicator movement.
Reset the bezel to zero.
Step 4 Crank the table back and observe the indicator reading. If a zero reading is obtained, tighten the hold-down bolts securely and recheck the alignment.
Perpendicular-to-Table Alignment A vise may be aligned at right angles to the table (Figure K-46) by the technique discussed previously. Once again, always indicate on the solid jaw and move the saddle to carry the vise jaw past the indicator. Always recheck after tightening hold-down bolts.
Squaring a Vise to the Column A vise may be aligned by squaring the solid jaw to the column. Two paper strips may be used as feeler gages between the beam of the square and the vise jaw (Figure K-47). This method should not be used when an accurate alignment is required.
Angular Alignment A protractor may be used to set a vise at an angle other than 90 degrees to the table (Figure K-48).
The accuracy of the angle is dependent on the type of tool used and the technique. Errors can be introduced from the angle setting on the protractor and the relative alignment of the vise jaw along the protractor blade. Considerable care must be exercised in making a setup by this technique. Securing the Workpiece The vise will effectively secure a workpiece in most cases. Whenever possible, set up the vise so
that cutting pressure is applied to the solid jaw (Figure K-49).
Avoid applying cutting pressure against the movable jaw. If the workpiece is sufficiently high, it may be seated on the bottom of the vise. If not, parallels may be used to elevate the work to a point where it can be machined. In many cases it will be necessary to apply the cutting pressure parallel to the vise jaws (Figure K-50). Remember that friction between the vise jaws and workpiece holds it in place. The more contact area there is, the better the holding power. When cutting pressure is applied parallel to the vise jaws, there is always a possibility that the part will be pushed from the vise. Therefore, if you must use parallels to elevate the workpiece, raise it only high enough to accomplish the required machining, because this reduces contact area, and the vise may not be able to hold the workpiece safely and securely.
Selecting the Cutter
For a flat surface, use a plain milling cutter wider than the surface to be machined.
The diameter of the milling cutter selected should be as small as practical.
A large-diameter cutter must travel farther than one with a smaller diameter (Figure K-51).
Therefore, a smaller-diameter cutter is more efficient, because time will be saved. Use a sharp cutter to minimize cutting pressure
and to obtain a good surface finish.
Whatever diameter cutter is used, it is important to have sufficient clearance between the arbor supports and the vise or other work-holding fixture (Figure K-52). As material is machined from the workpiece, this clearance is reduced, and it may become necessary to reset the workpiece if the vise and arbor supports will not clear each other before the final cut is taken.
Climb and Conventional Milling
In milling, the direction that the workpiece is being fed either can be in the same direction as cutter rotation or opposed to the direction of cutter rotation (Figure K-53). When the direction of feed is opposed to the rotation direction of the cutter, this is said to be up or conventional milling. When the direction of feed is the same as the rotation direction, this is said to be down or climb milling because the cutter is
attempting to climb onto the workpiece. If any appreciable amount of backlash exists in the table or saddle, the workpiece may be pulled into
the cutter during climb milling.
This can result in a bent arbor, broken cutter, damaged workpiece, and possible injury to the operator. Climb milling should be avoided unless the mill is equipped with adequate backlash control. Remember that any cutter can be operated in an upmilling or down-milling mode depending only on which side of the workpiece the cut is started.
Selecting and Setting Up Mill Arbors
When selecting a mill arbor, use one that has minimum overhang beyond the outer arbor support.
proper arbor, insert the tapered shank into the spindle socket. Be sure that the socket is clean and free from burrs or nicks
(Figure K-54). Large mill arbors are heavy, and you may need help holding them in place until the drawbolt is engaged. Do not let the arbor fall out onto the machine table. Thread the drawbolt into the arbor shank all the way, then draw the arbor into the spindle taper by turning the drawbolt locknut.
Tighten the locknut with a wrench (Figure K-55).
Remove the arbor nut and spacing collars. Place these on a clean surface so that their precision surfaces are not damaged. Position the cutter on the arbor as close to the spindle as the machining task will permit. Place a sufficient number of spacing collars on either side of the cutter to position it correctly. The cutter, spacing collars, and bearing collar should be a smooth sliding fit on the arbor.
A key is generally used to ensure a positive drive between cutter and arbor. However, a milling cutter can be driven without a key. Consult with your instructor and follow instructions regarding the use of keys.
Place the bearing collar on the arbor and locate it as close to the cutter as the machining task will permit. Place the arbor support on the overarm and slide it in until the arbor bearing collar slips through the arbor support bearing. Tighten overarm and support clamps. Tighten the arbor nut only after the support is in place (Figure K-56). Tightening the arbor nut before the support is in place may bend the
arbor. Do not overtighten the arbor nut, and always use a wrench of the correct type and size.
Removing and Storing Mill Arbors
Exercise care in removing the arbor from the spindle. Loosen the drawbolt locknut about one turn (Figure K-57). You may have to tap the drawbolt lightly to release the arbor taper shank from the spindle socket (Figure K-58). Hold the arbor in place or get help while you unscrew the drawbolt from the arbor shank (Figure K-59). Remove the arbor from the machine and store it in an upright position. Long arbors
stored in a flat position may bend.