Conventional, or manual, milling machines are primarily used to machine flat and angled surfaces by feeding a workpiece into a rotating cutting tool to remove material. They are also commonly used to position work more accurately for the same type of holemaking operations than can be accomplished with a drill press. By combining these operations, components can be machined to countless desired shapes.
Speeds and Feeds for Milling Operations
Calculating spindle RPM for milling operations is the same as calculating RPM for drill press operations. Use the standard formula RPM = 3.82 X CS / D where CS = cutting speed in surface feet per minute and D = diameter of the cutting tool. Milling machines also use IPM (inches per minute) for power quill feed settings. To calculate IPM, the following formula is used: IPM = FPT X N X RPM, where FPT = feed per tooth, N = number of teeth, or flutes, of the cutting tool, and RPM = spindle RPM.
Before machining, I always write some notes on the drawing to make it more understanding and easier for me to make a plan.
Squaring a workpiece means machining the sides of a workpiece perpendicular and parallel to each other. Before beginning, take a few minutes to check the alignment of the machine head and the milling vise. If the head is not trammed and the vise is not aligned, it may be impossible to create a "square" block. A small workpiece like this can be machined using the face of an endmill. I always face off the workpiece before do other operations. The end can be machined by peripheral milling using an endmill. Use the quill to position the endmill vertically. Remember to bring the quill stop against the micrometer adjusting nut and lock the quill. Set depth of cut using the X-axis and then lock it in place to prevent movement during the milling pass. Set a "0" on the DRO to establish a reference position, then machine roughing passes within about 0.010" to 0.020" of final size using DRO to set cut depth. Next, I create a flat reference surface which will then be used position the work as other surfaces are machined. Using the largest surface will minimize any setup errors when machining other sides of the block. If a smaller surface is used to orient the block for cutting a larger surface, any setup error will be multiplied. The less the quill is extended, the more rigid the machine setup. Lock the saddle before milling. If the surface is rougher than desired, decrease the feed rate until an acceptable finish is reached. Spindle speed may also be increased about 10%-20% when taking light finishing cuts to improve surface finish. Remember to check for parallelism between sides by measuring near the four corners. Whenever possible, measure the workpiece while it is still mounted in the vise. This keeps the machined surface on the same plane, which eliminates setup errors that can be caused by repositioning the work.
- Square the block.
- Coat the block with layout fluid and do the layout base on the drawing above. We use these layout lines as reference.
- We start to cut along the 3.00 length first. Use an edge finder to find a reference edge. When using a collet it is important not to over-tighten or the hollow edge finder shank can be damaged. Bring down the quill to set cut depth and lock. Remember feeding across or longitudinal is more accurate than using the quill handle or the knee. So cutting the vertical surfaces is easy but not the horizontal surfaces. We need to add an extra 0.050 to each of the horizontal surfaces. For example, when we cut the first horizontal step to 1.80, we add an extra 0.050 so we cut it only to 1.850 instead. Once we finished all the vertical steps, we remove the workpiece from the vise and turn it to the other side 2.00 width. Now, these horizontal steps become vertical steps. We will use an edge finder and DRO to finish mill all of these steps.
Holemaking operations are performed on the vertical milling machine. The work can be precisely moved using the table and saddle movements to align the work with the spindle. The digital readout (DRO) can also be used to create precise spacing between hole locations or between edges and hole locations. It is a good practice to lock both saddle and the table before creating holes to ensure that neither moves during the actual machining operation.
When the location of a hole from a reference edge is more critical, an edge finder can be used to very accurately find a reference edge. Carefully move the table to bring the edge finder in contact with the edge of the workpiece. Continue to slowly move the table. Notice that the tip will begin to run more true as the workpiece pushed the tip into alignment with the shank. When the tip "kicks," the position of the center of the edge finder (and the machine spindle) is one-half of the edge finders tip diameter from the edge of the workpiece. Every time you remove the workpiece from the workholding device, make sure that you know the reference '0" position from the edge of the workpiece so you can reestablish the coordinate locations from the same edge. Follow these steps to machine holemaking operations:
- Square the block.
- Locate the first hole with edge finder.
- A center drill or spot drill is used to create a more positive starting point for a twist drill. Retract the quill frequently to clear chips from the point.
- After spotting, the next step in the process is drill a pilot hole. A pilot hole is often drilled before the larger drill bit is used. Drill pilot holes on both sides for 2.0 and 3.0.
- Drill 0.375 and 0.438 holes
- Finally, use a countersink 60-degree to chamfer the opening of holes.
A pocket is an internal part feature machined into the surface of a workpiece. Pocket location and size can be controlled by using the DRO to monitor table and saddle movements. Follow these steps to machine a pocket:
- Square the block.
- Determine endmill diameter. I use endmill diameter 0.5 for this project.
- Create a coordinate map. The drawing above shows the X- and Y- axis coordinates of the center point locations of the cutting tool. For example, the center point of the pocket will be 0 for Y-axis. The endmill will cut 0.5 diameter. But we need to create 0.540 pocket. So we need to move the saddle 0.020 away from the column along the Y-axis to 0.480 and 0.020 toward from the column along the Y-axis to 0.520. Also, the radius of the endmill must be taken into consideration when calculating these coordinates. So when we mill the length of the pocket, we move the table from left to right along the X-axis. Use an edge finder to establish the reference "0" position from the edge of the workpiece. We will start to mill the length of the pocket from X-axis coordinate 1.25 which equals 1 + 0.25 (the radius of the endmill) to 3.75 which equals 4 - 0.25 as it is shown in the drawing above.
- Mill the pocket with 0.5 diameter which is the same size as the endmill diameter. Establish the length of the pocket 3.0.
- Mill the width of the pocket using Y-axis coordinate 0.480 and 0.520.
- One thing about milling the pocket is that the bottom of the pocket will be smaller than the top. So you need to flip the part and finish the backside of the pocket. Every time you remove the workpiece from the workholding device, make sure that you know the reference "0" position from the edge of the workpiece so you can reestablish the coordinate locations from the same edge.
- For milling the 15 degree angle, we use a fixture to position our workpiece at the desired angle for angular milling operations.