Machine Shop Trade Secrets. James Harvey

using dedicated tools can greatly improve your speed.

      A simple example is a screwdriver.

       You could buy a screwdriver with interchangeable tips. The idea is that with the one screwdriver handle, you could cover all of your screwdriver needs. I suppose this is true if you want to fiddle around with the tips, changing them, dropping them, and, yes, losing them. However, it’s faster to grab a tool that is ready to go than to keep changing tips.

       Another example is having an assortment of dedicated air tools. I have a drawer full of cheap die grinders. Each is mounted with different cutters or abrasives. If I need to cut off a pin, I can be cutting within seconds, instead of fiddling around with wrenches, collets, and Murphy’s little surprises.

Figure 1–1 Full-width cuts are the fastest way to clean up a surface and also make parts look better.

       4.Make all your parts the same.

      This suggestion is a great way to expedite just about any job. If you’re making multiple parts, then by making all the parts the same, you’ll always know exactly where you stand with the job.

      If your parts are all over the map dimensionally, you will constantly be re-measuring them, dealing with “special cases” and hassling to get consistency and control over a job.

      5.In a milling machine, when practical, use a large enough cutter to cut across the entire surface of the part in one pass. (see Fig. 1-1)

      This step provides an effective way to save time and also to make parts look better.

      Taking several passes with a small diameter cutter to clean up a surface is usually a waste of time. When practical, it is much faster to cut an entire surface in one pass. This is especially true when milling with a manual machine. With a CNC machine, it is not as important because CNC machines execute with greater speed and efficency.

      6.Turn the shanks of your larger drill bits down to common collet sizes. (See Fig. 1-2)

      I dislike cranking the knee of my mill up and down to accommodate a drill chuck. If you turn the shanks of your larger drill bits to a common collet side, you can avoid that hassle. You’ll be able to use standard size collets to hold your modified drills, without having to use a drill chuck. You can also do this with reamers and other cutters.

Common Fractional Collet Sizes
1/8"	1/2"
1/4"	9/16"
5/16"	5/8"
3/8"	3/4"
7/16"	7/8"

Figure 1–2 The shanks on these drill bits have been turned down to common collet sizes so they can be mounted in the spindle without having to use a drill chuck.

      7.Use stub drills (see Figure 1-3).

      Anytime you can drill a hole without first center drilling, you simply save whatever time it would have taken to do that center drilling. Normally, a high percentage of holes in parts are simply clearance holes used for bolting parts together. Clearance holes are usually anywhere from .015" to .030" larger than bolt diameter.

      If you know you’re going to be drilling clearance holes or other non-critical holes, you can use stub drills without center drilling. A stub drill that has been properly web thinned will cut with little pressure (see Figure 7-4). It will produce a surprisingly accurate hole. You can either buy stub drills already made or you can make them by cutting off standard-length drill bits and regrinding the tips.

      Even if the stub drill runs out a little bit as you start a hole, you’ll probably have enough tolerance on a clearance hole so that it won’t matter. If the hole is deep, or has to be located precisely, it is best to center drill first to maintain accurate location.

Figure 1–3 Stub drills can be used to drill holes without first center drilling.

       8.Use chip color to determine speed, feed, and depth of cut in ferrous materials.

      Most machinists in small shops using conventional equipment-set feeds, speeds, and depths of cut based on feel and experience. With a little practice, a newcomer can soon get the hang of these.

      The best rule of thumb regarding this subject is the tan chip rule: If you want your cutter to last, then your chips should come off the workpiece no darker than a light tan in color if you are using highspeed steel or cobalt cutters, and brown if you are using carbide cutters.

      If you push a cut much beyond 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.

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