Machine Shop Trade Secrets. James Harvey

vise on something that is a few thousandths wider than the parts.

      That way, the parts will locate accurately, yet will still slide in and out of the jaws easily.

      Gauge blocks work well for this purpose. As an added benefit, the gauge blocks hold the parallels in place.

      You’ll also need to set up some kind of stop for locating the parts. Using stub drill in combination with a slide fixture is a great way to make good time on conventional drilling jobs.

       11.Rough ugly.

      I believe there is some truth to the idea that “roughing is where you make your money.” You can’t do much roughing if there is little material to rough off. The most efficient roughing takes place in a saw. Within reason, try to remove as much material as is possible or practical with a saw.

      Roughing is sort of a behind-the-scenes operation, where you get to do it as fast and ugly as you want. Roughing is one operation where you get a chance to erase your tracks later on. Take advantage of this situation.

Figure 1–5 A slide fixture allows parts to be changed without loosening and tightening the vise.

       12.Work your way up to a heavy roughing cut.

      When I look for an aggressive cut in a conventional machine, I like to feed the tool by hand first before I use the auto feed so that I can “feel” the cut. If you encounter an excessive amount of noise, backpressure, vibration, or resistance when test feeding, then you may have to adjust the cut somewhat.

      By locking the knee of your Bridgeport, you can increase the system’s rigidity, which allows for more aggressive roughing.

       13.Avoid using a single flute fly cutter to rough with.

      A single flute fly cuter is best used as a finishing tool. From the standpoint of quickly removing stock, you’re better off using a multi-tooth cutter of some sort, like a corncob cutter of a multi-tooth insert cutter.

      In my opinion, it’s hard to beat a cobalt corncob cutter for heavy roughing because of the abuse they can take.

      I like the round insert face cutters for light roughing and finishing—they hold up well. Furthermore, when the inserts get dull, they can be rotated to expose fresh cutting edges (see Figure 1-6).

       14.Try to rough as close to final size as practical.

      For finishing tools to stay sharp, avoid working them too hard. With a conventional machine, leaving 0.10" to .030" stock for finishing works well. With a CNC machine, you can get away with leaving less because of the machine’s consistent accuracy.

Figure 1–6 Face milling can be accomplished with various types of cutters. The insert cutter on the left uses round inserts, which can be rotated to expose fresh cutting edges. For rough milling, it’s hard to beat a short, beefy corncob-type cutter on the right.

       15.Work your machine hard when roughing, but do it the right way.

      You want to make your machine groan, not beg for mercy. Increasing your feed is generally the best way to remove stock quickly. By keeping your depth of cut and spindle speeds moderate, you may be able to increase the feed to get things moving.

      Increasing the depth of cut also works, but puts a lot of pressure on the cutter and machine components. Instead, put load on the motor. If you can hear the motor bog down when a large diameter cutter enters the material, you may be confident you’re working the machine hard without abusing it.

       16.Place your hand on a milling machine table to gauge the pressure of a cut.

      Placing your hand on a machine table when the machine is cutting allows you to feel how much the table is deflecting under load. If you can only mildly feel the cut through the table, the machine is likely working below its capacity.

      This test also works on CNC milling machines where it is sometimes difficult to gauge the amount of pressure you’re placing on the machine or cutter.

       17.Make parts with as few setups as possible.

      One way to minimize setups is to finish right after you rough. Ideally you want to rough in a surface, then immediately finish it without removing the part or changing cutters. After you finish the surface, you can break the setup to prepare for the next cut. Thus, you don’t have to repeat setups.

      If possible, try to avoid roughing in all the surfaces, then finishing all the surfaces, or else you’ll make many moves and setups twice—it can’t always be done. On parts that warp easily—such as thin or hogged out parts—you may have to rough in all the surfaces first. When you go back to finish the part, you then cut out any warp that may have occurred during the roughing operations.

      18.Use air mist to prolong the life of your cutter and increase stock removal rates. (See Fig. 1-7)

      A little air/water mist helps cool and preserve cutters. Many machinists use air/mist sprayers. I usually don’t simply because I find them to be too much hassle. If you follow the tan chip rule mentioned in suggestion #10, a mist sprayer is not necessary.

Figure 1–7 A mist sprayer keeps the end mill cool.

       19.Go as fast as you dare in aluminum and other easily machined materials.

      It is difficult to wear out a cutter in aluminum, especially with a conventional machine.

      Machining aluminum is like driving on the autobahn. You can basically go as fast as safety and common sense dictate. But don’t go so fast that you end up crashing. When conditions warrant (e.g., you have a rigid setup and a lot of stock to remove), you can put the pedal to the metal.

      You can even use the rapid traverse feature on your conventional mill to increase the feed rate. The rapid traverse rate on the mill I use is somewhere around 60 inches a minute. Make sure you run the spindle fast enough to maintain a reasonable chip load in the .010" to .015" range.

      In a CNC machine, you can literally fly through aluminum if your setup is rigid and you have a lot of stock to remove.

      If you’re not using a fast feed, there is no advantage to running a spindle at warp speed. In fact, it is usually a disadvantage because there is a tendency for things to start chattering when spindle speeds are too high.

       I’ve used a 500-inch per minute feed rate with a 1-inch diameter 3- flute end mill turning at 10,000 RPM. That’s as fast as the machine would go. If you do the math, you’ll see those parameters produce a chip load of about .016". But there is no reason to run that fast if there is little stock to remove and your cuts are short.

       20.Bore holes with a mill like you would with a lathe.

      For some reason, many machinists use very slow spindle speeds when boring holes with a boring head. They look like they’re stirring taffy.

      There is usually no need to run a boring head at such slow spindle speeds unless the boring bar is flimsy and prone to chatter. You can usually use the “tan chip rule” for setting feeds and speeds, just like you would if you were boring a part in a lathe.

      Use short, stout boring bars when you can. A short, stout boring bar will help eliminate chatter and won’t spring away as easily as a long thin boring bar.