those point, your cutter will almost certainly start to break down. Once a cutter begins to get dull, the resulting heat and friction tend to accelerate the breakdown.
Cobalt and high-speed behave a little differently than carbide as to when and how they break down. They hold an edge very well up to a certain point. If that point is exceeded, the edge quickly breaks down.
That point for cobalt and high-speed is determined by the combination of speed, feed, and depth of cut that gives you a light tan chip. Once you’ve found this combination, you can make adjustments—depending on what you’re doing. If you’re roughing, you’ll probably use a faster feed and slower spindle to maintain the tan chip color. If you’re finishing, it’ll be the other way around.
Carbide, on the other hand, tends to break down more gradually than high-speed and cobalt. In other words, carbide doesn’t have the abrupt point of failure that the others have.
The “tan chip rule” works fairly well for measuring the aggressiveness of a cut, but it has an exception.
Chips that turn blue some distance after they leave the cutting tool are usually not detrimental to the cutting tool.
Having watched chips come off stock under various conditions for many years, I’ve come to the conclusion there are two separate sources of heat generated while cutting metal in a machine tool.
The first source of heat is the result of friction, as the metal moves across the cutting edge. The second source results from the chip’s metal being deformed as it is forced to flow across the cutting edge.
9.Use a speed chuck in a conventional milling machine. (See Fig. 1-4)
You can change bits with these chucks without turning off the spindle. They work great for quickly changing from a center drill to a drill, which is one of the most common tasks performed on a milling machine.
Figure 1–4 You can change cutters in quick change drill chucks without stopping the spindle. This chuck was made by Wahlstrom Chuck Co.
10.Use a slide fixture in a conventional mill to drill holes. (See Fig. 1-5)
A slide fixture saves vise clamping time. That may not sound like much. But if you have many parts to drill, then the savings becomes apparent.
One way to make this setup is to close down the 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
