Why Consider Machining?

What happens if you don’t want you use any of the above processes?  Say you want to just make a single part, or a limited number of parts.  Are you really going to manufacturing dies and complex processes just to make a single piece?  Absolutely not.  Many of the above processes only make sense if you’re making many, many components (like thousands or hundreds of thousands) so that the equipment can be amortized.  For parts that are of limited production, or that need to be relatively flawless, or are extremely complex, there’s a saviour: machining.  Machining is probably the most versatile manufacturing method.  You can machine pretty much anything.  Now, just because it’s ideal for custom parts and one-offs,  that’s not to say that machining can’t be used for high production runs.  It’s just that it tends to be more costly.  In recent years, machining operations have improved in speed, and advancements in material science have resulted in harder cutting tools to be made, allowing for harder materials to be machined.

Basics of Machining

The idea behind machining is probably the easiest to understand: start with a chunk of material, and cut away what you don’t need until you have the exact geometry that you want.  This is a particular type of manufacturing called subtractive manufacturing, meaning that material is taken away, instead of added (those processes are called ‘additive’ manufacturing).  If you’ve ever carved anything out of wood, or soapstone, that is basically machining.  It can be classified in a number of ways, some of which we’ll get into below.

There are certain machining operations that are designed to remove significant amounts of material to produce the geometry desired – such as turning, boring, milling, and lathe operations – and there also exists some machining operations that are more specifically for imparting an excellent surface finish.  This isn’t usually necessary, because the surface finish is usually quite acceptable without these specialized operations.  The big three operations are turning, drilling/boring, and milling.  Let’s discuss each in turn.  But first, let’s just go over the very basics of the cutting tools, because we need cutting tools obviously to remove material.  The two basics types are single point tools and multiple cutting edge tools, and they are as simple as they sound.  WIth a single point tool, only one edge cuts the material at a time.  These are commonly used in turning and boring operations (discussed below).  Multiple cutting edge tools have, well, multiple cutting edges, like a drill bit.  Indeed, multiple cutting point tools are usually used with rotational motion – like drilling.

Turning

Lathes are commonly used to machine material.  A lathe turns the workpiece about its axis, and removes material from the piece as it spins.  This is used all the time in woodworking and pottery as well.  It’s very efficient and precise, and you can produce pieces with complex geometry and high precision.  Since the workpiece is being rotated, it will typically have symmetry about that same axis.  Let’s think of the process of making a baseball bat, which are made on lathes.  Imagine starting with a rectangular chunk of wood, and fixing it into the lathe.  Then you rotate this piece of wood, and remove material as it spins, giving you symmetry.  You can then move the cutting tool along the bat, moving it in and out to give you that variable diameter that a baseball has – the handle is thinner than the end of the bat, and it transitions nicely between the two regions.  At the very end of the bat is the knob, which gives the batter a good grip on the bat and prevents it from flying out of their hands while swinging.  

A lathe operation can be classified by the number of dimensions.  With one dimensional turning, the lathe cuts into the workpiece and gives the workpiece a constant cross section or diameter.  In other words, it’s not adjustable towards the workpiece, but it can slide along the workpiece.  With two dimensional turning, the cutting tool can be moved in and out towards the workpiece, allowing for variable cross sections to be produced, and can be slide along the workpiece as well.  Two dimensional turning would be necessary to create a baseball bat, since it has a variable cross sectional profile.  Besides turning, lathes can be used for other machining operations, such as creating threads, and cutting workpieces, even drilling.  With turning, there are a few parameters that can be considered: rotational speed (either the workpiece rotates and the cutting tool is fixed, or the cutting tool is fixed and the workpiece rotates), the feed rate of the workpiece (basically how quickly you cut the piece).

Boring

Boring is another machining operations that can be performed.  Interestingly enough, boring is kind of like the internal counterpart to turning – whereas turning removes material on the outside of a part (external), boring removes material on the inside (internal).  Overall, they are very similar processes.  Indeed, sometimes boring is called internal turning.  Another definition –  it’s the enlargement of a hole that’s already been drilled, or a hole that’s a result of a casting.  The barrel of a gun is bored.  Why bore?  Well, boring is generally good for three things: it’s good to make a hole the exact right size.  Drill bits usually just come in standard sizes (⅜ inch, ½ inch, etc), and so that’s one reason to bore instead of drill. Generally speaking, boring is very accurate, and give the desired finish.  The second reason: bored holes tend to be very straight.  Drills don’t always produce equally straight holes, especially if they’re long.  The drill bit can deflect a bit due to the forces of drilling, and even just inconsistencies in the material and the drill bit – and you’ll end up with a hole that’s not entirely to your specifications, in certain situations.  In those situations, boring is an option.  It is likely that the hole will first be drill, or partially drilled, and then bored in order to create the proper dimension hole.  This is even more true with a hole resulting from a casting process.  You’ll likely need to bore the cast hole if you need a precision hole.  Finally, with boring operations, you can make nice concentric holes.  Ideally, you don’t move the workpiece between boring operations – removing the workpiece and putting it back can introduce error. With boring, you also need to consider vibrations and the tool rigidity, which can affect the rate of material removal and the accuracy of the bore.  The details of such operations won’t be discussed here.

Drilling

Drilling is the most common way of making standard size holes, especially ones with smaller diameters (like, not a meter, which would be a boring operation).  Sometimes, smaller holes are pre-drilled to guide the larger drill bit and ensure that the hole is relatively true. Often, the inside side wall of the hole will be left with helical marks from the burnishing of the drill bit.  This surface can be improved by the additional operation of reaming.  Something to keep in mind in very sensitive areas is that drilling can introduce stresses in the material and cause local disturbances, which may promote corrosion and fatigue failure (crack propagation).  In most causes this won’t be an issue, but it can be managed with additional finishing operations.  When drilling, the material that has been removed must have somewhere to exit.  Drill bits usually direct any chips, either small pieces or those longer spirals you sometimes see, away from the hole.  The technical term for this is a fluted drill bit.  Methods of drilling can also help with clearing away chips.  Peck drilling is a commonly used method to avoid the buildup of material.  The process is similar to how a woodpecker pecks for food – frequent shallow hits.  In this method, the drill bit is frequently retracted, to provide space for the chips to exit the drilled hole.  This is more generally called ‘backing off’, where the drill is removed either completely or partially to allow the chips to exit.  To prevent drill ‘walk’, where the drill bit wanders off to an unintended area, the drill bit should be perpendicular to the surface being drilled.  If an angled surface is being drill, As well, a certain method called spot drilling (or spotting drill) creates shallow holes that can be used as a guide for longer drill bits to improve the straightness of the hole.  Spotting isn’t necessary if you’re just drill short holes, as they will remain fairly accurate.  When drilling, a special type of fluid is sometimes used to keep everything cool, called cutting fluid.  Cutting fluid also reduces friction – without it, the material might ‘grab’ onto the drill bit, resulting in a less accurate and less smooth hole.  It helps with a number of things: keeps everything cooler to increase the life of the drill bit, improve the finish of the resulting drilled hole, and helps to eject the chips.  Usually the coolant is just sprayed or floods onto the piece and drill bit.

Deep hole drilling occurs when the length of the drilled hole is ten times greater than the diameter of the hole, or more.  So if the hole is an inch in diameter, anything longer than ten inches would be considered deep drilling.  These holes are difficult to drill straight, and specialized equipment may be required.  Vibration especially is of great importance, because excesses vibrations can run the tolerances, cause material removal to drop significantly, or even break the drill bit.

Milling

A more general type of machining is called milling.  As mentioned earlier, the turning machining process really applies to rotational objects, like baseball bat shaped objects – objects that have a symmetrical cross section.  For so called ‘non-rotational’ objects, there is milling.  With milling, the workpiece is fed into the cutting tool along some specified path to produce the desired geometry.  It’s called an intermittent cutting process, because there is a definite beginning and end to the cut process, whereas with something like turning, either the cutting tool or the workpiece continuously rotates.  There are two major categories of milling that we’ll mention here: face milling (also known as end milling) or plain milling.  The difference between the two is mainly how the cutting tool is oriented with respect to the surface that is being milled.  If the tool comes in perpendicularly to the surface, it is face milling.  If it is parallel to the surface, it is considered plain milling.  You might hear of something called five-axis milling.  This refers to a milling operation where the workpiece can be moved continuously in all three cartesian directions – back and forth, up and down, and side to side.  Additionally, the tool can rotate about two orthogonal axes.  In this way, the workpiece and tool can be positioned with extreme flexibility and precision.  Five-axis milling allows quite a bit of freedom to mill the piece pretty much any way you want, including some pretty nice spherical surfaces.  The surface finish mostly depends on how deep you are cutting and how quickly.  Deeper, quick milling operations will leave a rougher surface compared to slower surface processes which barely take off any material.  Usually, the maximum material  removal rate is utilized, so that more parts can be processed, saving time and therefore cost.  Additional surface operations may be performed to achieve a better surface finish.

Design for Machining

We know that when it comes to castings and forgings, we need to consider the actual manufacturing of the part in the design – i.e. avoiding sharp corners, including proper draft angles, etc.  The same actually goes for machining, somewhat surprisingly, because in general, machining is very versatile and can handle pretty much any design.  However, there are things we can do to improve the machining process, and hopefully save some cost.  Costs savings are especially important because machining operations tend to be expensive when compared to techniques like castings and forgings – at least for larger production runs.  One of the most obvious considerations when machining a piece from a solid chunk of material (called billet) is to consider the shape of the billet and the shape of the part.  The minimum billet size will need to be at least as large as the largest dimensions of your part.  Think of it like this: you are creating a part by removing a bunch of material, and so the final piece must ‘fit’ inside the original billet.  To avoid wasting material, you should try to make effective use of that billet.  If you’re making something with large dimensions but most of the features are thin and long (as an extreme example), you’re going to end up with a ridiculous amount of machining and material waste.  If this is not possible, then the billet should be selected with dimensions as close to the final product as possible.  As far as tolerances go, they will usually be quite good with more or less any machining operation.  Still, some operations will leave better tolerances than others, and it will also depend on your tools.  In general, excellent surface finishes and better tolerances can be achieved with some extra machining work, like reaming, which we’ll discuss shortly.  A general rule of thumb is that large and flat parts (like sheets), as well as extremely thin parts, and long and narrow parts, are not ideal geometries to machine, even though it may be technical feasible.

Additional Machining Considerations

There are some additional considerations with machining, and not necessarily anything to do with the design.  Here, I’ll discuss some general points regarding machinings.

Chips

 The first has to do with what types of chips are produced – i.e. how the metal behaves once it is cut away from the surface.  There are two major categories of chips: continuous and discontinuous.  Continuous chips are like those long pencil shavings that keep going forever and ever, for those of you that actually sharpened pencils back in school (not as common as you might think with mechanical pencils these days).  While continuous chips go hand in hand with a nice surface finish, they are bad because they can tangle in the machine, which may damage the machine, the workpiece, or both, and pose a safety issue.  So continuous chips are broken if necessary.  Short chips can be an indication that the material is hard and brittle, and the chips fracture away from the piece when subjected to the force of the cutting tool.  Very short chips aren’t ideal either, because they could cause tool vibration, leading to excessive wear and a subpar surface.  There’s a couple of ways to deal with chips, by making some slight changes to the cutting tool.  You can add a small feature to the tool so that as the chip forms and begins to lengthen, the geometry of the tool causes it to curl away, eventually resulting in fracture.  There’s a couple of ways to do this, but the end result is the same.

Heat and Cutting Fluids

Machining can involve a large amount of heat due to frictional forces – like up to 1000 degrees in certain regions of the cutting tool and surface.  Heat is generally not a good thing, because it causes excessive wear.  If you want to cut faster (i.e. higher cutting velocity, and higher feed rate), you’re going to end up with more heat.  One way to deal with this is to drench the surface with cutting fluid, which acts as a coolant and draws heat away from the cutting tool and workpiece.  Even better, they act to reduce heat at the source by lowering the friction.  Cutting fluids are usually water or oil based.  One thing to note is that if a tool is continuously heated and cooled to extremes, it may experience thermal fatigue failure – practically exactly what it sounds like.  We want to extend the life of the cutting tool as much as possible, to reduce the cost of tool replacements.  As well, replacements take time, and that’s time that you’re not producing part with your very expensive machine and setup.  It’s also advisable to stop using the cutting tool before it breaks – not when it breaks, since that can cause severe damage to the workpiece, which subsequently may have to be written off.

Materials

Think of what the cutting tool has to deal with – extremely high forces for extended periods of time, for many, many parts.  Ordinary materials just won’t cut it.  It’s difficult because you don’t want the cutting tool to be extremely brittle, because then it would be constantly snapping.  You want it to be tough (click here for a review on toughness, or in one sentence: the ability to absorb energy and deform but not fracture),  But you need it to be a very hard material, and hardness and toughness rarely go hand in hand.  Mild steel is relatively tough, but hard?  No.  Diamond – extremely hard but tough?  Not exactly, and so usually there is some sort of trade-off.  Cutting tools often have inserts, which mean that the actually tip that does the cutting is replaceable, so that if it breaks or wears down then it’s easily replaceable.  Now, you can use a heat treated material (like heat treated steel) to increase the hardness and strength, but there’s a catch: since you used heat to strengthen the material, guess what can also ‘undo’ these properties?  Heat as well.  And where can we find a lot of heat?  During machining operations.  And so a heat-treated cutting tool is a bit of problem, and so it is classified as an unstable cutting tool.  The specific name for these steel tools is high-speed steel (HSS).  They are quite tough, but above around 500 degrees the hardness of these tools will diminish.  Conversely, something like tungsten carbide isn’t heat treated, and so the heat from the machining won’t alter its properties (as much).  But these harder, more stable materials lack toughness, and so are best used in ‘stable’ machining operations where the tool is unlikely to be subjected to bending forces.  Automated machining with a rigid/stable setup/fixture is best for this type of easily breakable cutting tool.  If you’re going to be cutting something by hand, or there are other circumstances where the cutting tool might be subjected to unusual forces, a tougher cutting tool, even if it is unstable, may be more appropriate.

Cutting Speeds

We can define each cutting tool by how fast it can cut – in other words, the cutting speed.  It doesn’t matter what machining operation we’re talking about – it could be a turning or milling operation – it is simply the rate at which the cutting tool cuts removes the material.  This is defined as some distance per minute, with the distance in either feet or meters.  To make it a bit clearer with a non-engineering example, think of how you would cut through a log.  Maybe with a hatchet or a saw.  The cutting speed would be how far you make it into that log within a minute (my guess would be on the saw).  Of course, the cutting speed depends on numerous factors, not limited to the material of the cutter – the material that is being machined also makes a difference, and how far you are willing to ‘push it’ with respect to the life of the cutter.  When using a HSS cutter, speeds of 30 to 40 meters per minute are probable, when cutting mild steels (stainless steels and higher carbon steels will be slower, around 20 to 40 meters per minute).  Comparatively, a carbide tip can achieve cutting speeds of up to 200 mpm.