Читать книгу Machine Shop Trade Secrets - James Harvey - Страница 8
ОглавлениеAs machinists, how often are we asked to produce hardware that was needed “yesterday?” The answer is “quite often” because shop personnel try to keep squeaky wheels greased. Squeaky wheels come at us from all sides, including:
•Production workers count on machinists to keep lines going.
•Research and development employees count on machinists to keep new product programs on track.
•Maintenance specialists count on machinists for repair parts.
The bottom line is this: when people want parts, they want parts. They don’t want excuses or anything else. That’s one of the beauties of being a machinist. Your responsibilities are clear and simple. If you can get people their blessed parts, they’ll go away.
Most people, including myself, don’t want to work any harder or faster than necessary. At times, though, when the crisis monkey is on us, we have to get the lead out and get going. Crisis machining can actually be fun once in a while. Generally speaking, any glory to be had usually comes from helping someone through a crisis. If nothing else, it can be a nice change of pace. The hours tend to go by quickly when you’re working on a “hot” job.
1.Turn man-time into machine time.
This philosophy is popular for saving time. It goes with the idea that you should never hold back on technology. From a business standpoint, the philosophy makes sense. You can also turn man-time into machine time with thoughtful planning. This is easier said than done. The fast paced environment many of us work in today often doesn’t allow for proper planing.
2.Have lots of tools.
An easy ways to improve speed is to have lots of tools. I like to have and use my own tools—even if the shop is well stocked. Shop tools are never put back in exactly the same place and you’ll never know what condition they’ll be in when you find them.
If you’re just starting out in the trade, I recommend buying a set of tools from a retired machinist. This way, you’ll hit the ground running and you’ll immediately be worth more to your company. Machinist’s tools hold their value well, so be prepared to pay a fair price. You can upgrade later as you see fit.
Avoid borrowing tools over and over. If you use a tool often enough, then either buy one or make one!
Don’t waste your time making easily purchased tools like 1-2-3 blocks and V-blocks. Instead, make custom tools and fixtures that aren’t easily purchased; you can tailor these tools to the machines you use and the type of work you do.
3.Use dedicated tools.
This business of having one tool that does everything isn’t very efficient. You often see these kinds of tools advertised in the back of the mechanic’s magazines or on television infomercials. For the most part, each tool you have—whether it is a hand tool or a power tool—should serve just one purpose. You want the tool to be ready when you need it.
I suppose you could get carried away with this type of thinking. I wouldn’t go so far as to buy a handle for each socket I have. Nevertheless, having and 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.
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 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. It is difficult to hold size with a thin, springy boring bar.
As far as I’m concerned, you can never have too many boring bars. You’ll need a variety of different size boring bars to match the different hole diameters and depths you’ll encounter.
21.Power tap blind holes that are drilled deep enough.
Do it under the right conditions or chances are you’ll be digging out of a broken tap. Many instructors would just say “never power tap blind holes.”
If you drill a tap size hole at least one and a half times as deep as the threads you need, chips will have a place to go and won’t cause binding. Use a spiral point tap so chips get pushed ahead of the tap. A spiral point won’t bind like a plug tap or hand tap. As long as you use a sharp tap with some cutting oil and give the chips a place to go, the tap should cut freely.
If the part design is such that you don’t have enough material to drill a hole at least 1.5 times as deep as the thread, then it is safer either to hand tap to final depth or to use a tap that pulls chips out the top.
By hand tapping, you can gauge the amount of torque you put on the tap. With this approach, you can also clean the chips out as you go.
I’ve had good results with Shark-Line® high performance taps for tapping tough materials. Use Shark-Line® taps with Hangsterfer’s Hard Cut® tapping fluids for best results.
Spiral fluted taps that pull chips out of the tops of holes are weaker than other taps and are best used in aluminum and other easily machined materials.
22.Saw your raw stock about a tenth of an inch larger than finished size.
It’s certainly possible to saw closer than .1", but for me it’s not worth the extra effort. Most saws are not high precision machines. Blades are often in less than ideal condition.
A tenth of an inch gives you a little breathing room if the saw blade runs off a little bit or if your stock isn’t held square to the blade for some reason.
If I have many small parts to make, I’ll try to saw a little closer than .1", maybe to within about 1/16" of finish size. You can usually get away with cutting small parts closer because the saw blade has less chance to run off.
Figure 1–8 Cold cut saws work great for cutting off bar stock. This machine was made by Doringer Mft. Co.
23.Use a cold cut saw for cutting off bar stock. (See Fig. 1-8)
You can really make hay with these saws. They’re especially useful for cutting off bar stock. Yet they have not gained the popularity they deserve, probably because those in charge of purchasing equipment aren’t familiar with them.
Advantages over horizontal band saws are that the blade is rigid and durable, enabling them to cut stock cleanly and close to size. They are also compact and easy to operate.
Although similar in design, these saws work on a different principal than abrasive cutoff saws. Cold cut saw blades turn relatively slowly and cut heavy chip loads, unlike abrasive cutoff saws.
24.When running multiple parts, do one operation at a time in a tool room lathe.
This applies mostly to small parts held in collets. Don’t run a tool room lathe like a turret lathe.
With the collet closer, it’s faster and easier to change parts doing one operation at a time than it is to change tools and settings.
25.Change small lathe parts when using a collet closer without turning off the spindle.
26.Stack parts when you can. (See Figure 1-9)
Stacking parts and machining them all at once can save a substantial amount of time.
However, it can be a double-edged sword. It is more difficult to hold tight tolerances with stacked parts.
Furthermore, setups are usually more involved and time consuming when stacking parts.
If you have to move a stack in order to do other machining operations, try to keep the parts clamped together so they move as one block. In other words, when you can, try to avoid moving the parts in relation to one another.
Figure–19 Stack milling is an effective way to save time. The tradeoff is that setups take more time and tolerances are usually more difficult to hold.
27.Stack parts on edge for drilling and tapping. (See Figure 1-10)
Drilling and tapping a stack of parts on edge can save you time. However, you have to be careful how you do it so that you don’t accumulate error.
Let’s say you have a stack of twenty 1/4" plates that need to be drilled and tapped in the center of their edges. After clamping the stack in the vise, you next have to measure the width of the stack. Then divide that measurement by 20 to get the distance you should move between holes.
Say your overall stack measures 4.94". The real distance between the centers of the plates is 4.94 divided by 20, or .247". That’s how far you would have to move each time to keep the holes centered in the plates. If you moved over .250" each time, your holes would become increasingly off center as you advanced.
Figure 1–10 A stack of parts is drilled and tapped on edge.
28.Consider buying pre-squared blocks for high quantity runs.
There are vendors that specialize in supplying material squared to size so that you don’t have to do the squaring. Having blocks prepared this way is a great way to get a job off to a fast start
29.Use an end mill in the lathe to rough out a flat bottom hole. (See Fig. 1-11)
Using an end mill in a conventional or CNC lathe allows you to rough out a flat bottom hole very close to depth with a square corner.
Any subsequent boring you do to complete the hole will take less cutting than if you used a standard drill bit to rough out the hole. You can hold an end mill in the tailstock chuck.
An end mill fed in with the tailstock will usually cut a hole a little larger than the end mill diameter; be careful that you don’t cut an oversize hole. An end mill used this way will cut with less pressure if you drill a pilot hole first. Use the “tan chip rule” for setting spindle speed.
Figure 1–11 An end mill is used in the tailstock to rough out a counter bore.
30.Keep a box of loose drill bits handy.
If you need to drill an approximate size hold—such as a pilot hole—there is nothing faster than just grabbing and using an approximate size drill from a box of loose bits.
Sample Catalogs for T-nuts
McMaster-Carr
Travers Tool Company
Rutland Tool & Supply
ENCO
31.Keep a large assortment of T-nuts on hand.
Angle plates, rotary tables, drill presses, and other machines and fixtures around the shop are going to have different T-nut slot sizes. Not having the right size T-nut for a piece of tooling can be frustrating when you want to keep a job moving. T-nuts are cheap. Open a catalog and buy yourself a large assortment of T-nuts to avoid getting hung up.
32.Avoid clearing your cutter to go back for another cut.
If you rough off material in steps in a conventional mill or lathe, don’t bother clearing the tool to return for the next cut. If the roughing tool rubs or grooves the part as it is returned to the starting point, so be it. This technique saves you from having to look at your dial settings all the time.
Don’t, however, take finishing cuts over a rough surface with swirls or grooves. Clear the tool when you are within finishing range so the tool cuts with even pressure over a consistent surface for finishing.
33.Don’t bother removing a built-up edge on a lathe tool when roughing.
Your cutting tools may accumulate a built-up edge when roughing. A built-up edge or BUE is a slight accumulation of material from the workpiece that adheres to the cutting edge of your tool.
A tool will still cut with a built-up edge, at least well enough to rough with.
To make smooth, precise finishing cuts, it is best to remove the built-up edge so that chips slide off the tool freely and the edge of the tool does the cutting instead of the built-up edge.
Removing it is often easier said than done. On carbide, if you attempt to just pick it off with a knife or something, you’ll likely pull off a bit of the carbide and ruin the cutting edge. If you attempt to stone or file it off, you may end up rolling the edge over, which is also no good.
You can remove a built-up edge by applying a liberal amount of cutting oil to the tool and then abruptly but intermittently hand feeding the tool into some stock. The build up edge will usually get pushed off by the resultant chip.
Figure 1–12 A deep hole can be drilled with a conventional lathe by sliding the tailstock.
34.Drill deep holes in a conventional lathe quickly by sliding the tailstock. (See Fig. 1-12)
Chip packing becomes an issue when drilling deep holes. The deeper you drill, the harder it is for chips to get out.
Instead of winding the drill bit in and out with the tailstock crank, you can manually push the tailstock in and out along the ways of the lathe.
When the drill bit packs, loosen the tailstock and pull it back to free the chips. Then push it back in until the drill bit bottoms. Re-tighten the tailstock and drill a little more using the crank. Repeat the process until you are to depth.
You’ll be surprised how fast you can drill a deep hole with a conventional lathe using this method.
Always begin drilling deep holes with a standard length drill to get the hole started straight.
Drill as deep as you can with a standard length drill before switching to a longer one.
35.Deburr rough edges with a small angle-head die grinder. (see Fig. 1-13)
Burrs and therefore the task of deburring are the unfortunate by-products of machining. There is no way to completely avoid throwing burrs when machining.
After rough cutting stock to size, you usually have to remove the resultant burr to begin machining. You can remove those and other fairly large burrs by sanding them with a small sanding disk mounted in an angle-head die grinder.
The advantage of this method is that you can do the deburring at your machine between cuts instead of having to file or walk to a disk sander.
There are a few things you can do to minimize burrs and the effort it takes to remove them. One method you can use is to climb mill into material. The use of sharp cutters also minimizes burrs.
Figure 1–13 An angle-head die grinder can be used to remove heavy burrs quickly.
36.To save a little time, avoid turning off a conventional milling machine to change parts.
Common sense is needed here. If the setup is such that you can move the cutting tool beyond the part by at least a few inches, you can usually change parts safely without turning off the spindle. If you measure the part, file the part, or do any operation other than just changing or removing the part, then it is best to turn the spindle off.
Be especially careful around cutters that rotate close to another piece of solid tooling such as a vise. That situation is potentially more dangerous than having a cutter that is just rotating out in the open.
I know of two accidents that happened when machinists did not stay clear of a cutter rotating next to a solid piece of tooling. One machinist used a milling machine to cut screw slots in some small parts with a slitting saw. When he went to change a part, he got his fingers caught in the small opening between the rotating saw blade and the holding fixture. The blade sucked three of his fingers through the small opening. Fortunately, the opening was big enough that he didn’t completely chop off his fingers. The other incident happened in a surface grinder. The rotating grinding wheel was about 1/4" above the magnetic vise when the machinist used his hand to wipe some debris off the magnet. Four of his fingers were ground down as they passed through the opening between the wheel and magnet.
37.Cut the diameter of a lathe part instead of the face to remove material quickly.
If you have a lot of material to remove from the length of a lathe part, it is faster to remove material by taking cuts off the diameter rather than the face of the part.
38.Cut arcs by hand in a conventional milling machine by using a dowel pin as a center pivot. (see Fig. 1-14)
To cut an arc around the end of a part, use a dowel pin as a center pivot and rotate the part around by hand. Mount the dowel pin in your milling machine vise with a V-block. I’ve used this method several times with good results. Be careful. Begin by cutting the protruding corners of the part first so that the cutter does not suddenly jerk the bar and break the cutter.
Figure 1–14 An arc is being machined around the end of a small part by rotating the part on a dowel pin.
39.Cut spherical shapes with a conventional lathe. (See Fig. 1-15)
These tools are easy to set up and work well for quickly and consistently cutting spherical shapes conventionally.
Figure 1–15 These tools make cutting spherical-shapes a breeze with a conventional lathe. This tool was made by Holdridge Mfg. Co.
40.Use short, stubby end mills when possible. (See Fig. 1-16)
Short, stubby end mills don’t deflect as much as longer end mills; as a result, they last longer. End mills with flute lengths about one and a half times the diameter of the end mill, or less, don’t flex much, and can be pushed harder than longer end mills. It’s hard to beat a short, stubby corncob type rougher for removing material quickly.
Figure 1–16 Stubby end mills are rigid and should be used whenever possible.
41.Use a hex cutter for deep arrow slots.
If you have to use a small diameter end mill, be prepared to take your time. End mills under 1/8" in diameter simply can’t be pushed very hard. They’ll break if you do.
I’ve found that when it comes to milling with small diameter end mills, a six-sided single flute cutter can withstand more side pressure than a multi-flute helical end mill of the same diameter.
Hex cutters can be quickly made on a surface grinder or cutter grinder (see Fig. 1-17).
When making carbide cutters like this, it is best to lightly break the vertex of the hex on the opposite side of the cutting edge to reduce the tendency for the cutter to chip out. You can use either a diamond file or diamond grinder to break the edge, as shown in the right-hand photo.
Figure 1–17 Single flute hex cutters are easy to make and can withstand more side load than helical end mills. They work great for cutting O-ring grooves and other features requiring deep, narrow cuts.
42.Measure stock one time only to rough in a feature.
Some machinists waste time by re-measuring after every roughing cut. It’s not necessary. Measure stock carefully one time before you begin roughing to determine how much stock you need to remove, then don’t measure again until you are within finishing range. Either that or cut to a scribe line.
43.Avoid tilting the head of a mill when possible.
When cutting or drilling an angle in a part, either tilt the part or use an angle cutter to do the job. Using a mill with a tilted head is awkward. Furthermore, once you tilt the head, you’re obliged to tram it back in.
44.Take your drawing with you to the stock room.
When searching through stacks of material in the stock room, you don’t want to have to remember what you are looking for. It can be a little confusing at times, especially when you don’t have exactly the stock you need. Save yourself a trip. Take your drawing with you to the stock room.
45.Use a high volume air nozzle.
This subject can be controversial. In some shops, the rule is never to use air to blow chips off machines. Some believe cleaning with compressed air ruins the machine—the air stream forces small chips and debris between the table and ways of the machine, causing binding, scratching, and rapid wear on the ways.
Common sense is needed. If you do use air to blow off chips, don’t aim the air stream directly into the junction of the machine table and ways. Aim or blow the chips away from areas of the machine that may trap chips.
Adjust your cleaning procedure according to the type of material you are cutting. With aluminum or other soft, clean materials, you probably won’t damage anything by using air. With hard, gritty, or abrasive material, it makes sense to use a brush and paper towel to wipe the grit off the machine’s ways. If you use sandpaper or other abrasives in or around your machine, wipe the grit off with a paper towel or rag before moving the table. (See Fig. 1-18)
With that in mind, get yourself an air nozzle that puts out some volume so you can blow chips off quickly.
Figure 1–18 Wipe gritty material from the ways of machines to avoid wearing them out prematurly.
| Suggestions for Working Quickly | ||
| 1. | Turn man-time into machine time. | |
| 2. | Have lots of tools. | |
| 3. | Use dedicated tools. | |
| 4. | Make all your parts the same. | |
| 5. | In a milling machine, when practical, use a large enough cutter to cut across the entire surface of the part in one pass. | |
| 6. | Turn the shanks of your larger drill bits down to common collet sizes. | |
| 7. | Use stub drills. | |
| 8. | Use a speed chuck in a conventional milling machine. | |
| 9. | Use a slide fixture in a conventional mill to drill holes. | |
| 10. | Use chip color to determine speed, feed, and depth of cut in ferrous materials. | |
| 11. | Rough ugly. | |
| 12. | Work your way up to a heavy roughing cut. | |
| 13. | Avoid using a single flute fly cutter to rough with. | |
| 14. | Try to rough as close to final size as practical. | |
| 15. | Work your machine hard when roughing, but do it the right way. | |
| 16. | Place your hand on a milling machine table to gauge the pressure of a cut. | |
| 17. | Make parts with as few setups as possible. | |
| 18. | Use air mist to prolong the life of your cutter and increase stock removal rates. | |
| 19. | Go as fast as you dare in aluminum and other easily machined materials. | |
| 20. | Bore holes with a mill like you would with a lathe. | |
| 21. | Power tap blind holes that are drilled deep enough. | |
| 22. | Saw your raw stock about a tenth of an inch larger than finished size. | |
| 23. | Use a cold cut saw for cutting off bar stock. | |
| 24. | When running multiple parts do one operation at a time in a tool room lathe | |
| 25. | Change small lathe parts when using a collet closer without turning off the spindle. | |
| 26. | Stack parts when you can. | |
| 27. | Stack parts on edge for drilling and tapping. | |
| 28. | Consider buying pre-squared blocks for high quantity runs. | |
| 29. | Use an end mill in the lathe to rough out a flat bottom hole. | |
| 30. | Keep a box of loose drill bits handy. | |
| 31. | Keep a large assortment of T-nuts on hand. | |
| 32. | Avoid clearing your cutter to go back for another cut. | |
| 33. | Don’t bother removing a built-up edge on a lathe tool when roughing. | |
| 34. | Drill deep holes in a conventional lathe quickly by sliding the tailstock. | |
| 35. | Deburr rough edges with a small, angle-head die grinder. | |
| 36. | To save a little time, avoid turning off a conventional milling machine to change parts. | |
| 37. | Cut the diameter of a lathe part instead of the face to remove material quickly. | |
| 38. | Cut arcs by hand in a conventional milling machine by using a dowel pin as a center pivot. | |
| 39. | Cut spherical shapes with a conventional lathe. | |
| 40. | Use short, stubby end mills whenever possible. | |
| 41. | Use a hex cutter for deep arrow slots. | |
| 42. | Measure stock one time only to rough in a feature. | |
| 43. | Avoid tilting the head of a mill when possible. | |
| 44. | Take your drawing with you to the stock room. | |
| 45. | Use a high volume air nozzle. | |
| 46. | Calculate chip load and surface feet per minute. | |
| 47. | Tidy up your desk or bench by handling objects one time only. | |
| 48. | Make notes about machines you don’t often use. |
46.Calculate chip load and surfaces feet per minute.
On conventional machines, I generally set cuts based on feel and intuition. Later I may run numbers to get a quantitative idea of what’s going on.
Two important numbers in machining are chip load (thousandths per tooth) and surface feet per minute, or SFM, which is a measure of the rate at which material slides across a cutting edge. For more in-depth discussion of these formulas, see Chapter 15.
Chip load in a milling machine = Feed Rate / RPM × No. of Flutes
Surface Feet Per Minute (SFM) = RPM × Dia. × .262
47.Tidy up your desk or bench by handling objects one time only.
This suggestion sounds simplistic, but is effective. When tidying up or putting things away, there is a human tendency to pick something up and set it down again somewhere else. Avoid that tendency. Whatever you pick up to put away should be put away the first time you handle the object.
48.Make notes about machines you don’t use often.
Machines you don’t use too often may take some time to get used to after you’ve been off them for a while. Make written notes on the ins and outs of any particular machine so you’ll be able to get back up to speed quickly.
Conclusion
As you can see, there are various approaches to running jobs and setting cuts. Ideally, machinists strive for the most aggressive cut they can get away without quickly ruining cutters or crashing. Often, the limiting factor in terms of aggressive cutting turns out to be the rigidity of the setup and the strength of the cutter. An additional factor to consider is that parts become weaker as they’re machined.
Machinists have to be intuitive stress engineers. They constantly have to make judgments about the aggressiveness of cuts based on the type of material they’re cutting, the rigidity of the machines, the rigidity of setups, and the strength of cutting tools. It’s a skill that comes with practice.
It is unlikely any two machinists would run a job the same way or at the same rate. Most machinists wouldn’t run a job themselves the same way each time. There are simply too many variables and too many different ways of doing things.
Occasionally you’ll hear a machinist say “Why should I work fast? I’m in no hurry. I get paid the same hourly rate regardless of how fast I work.” That may be true, but by learning how to work fast, you’ll have that option when you need it.
