So, you’re probably saying to yourself, I just throw in the correct size bit and go to town. Yes, you could do that but there is so much more you should know about the entire class of machining operations to put a hole into a workpiece.

Machining, is a material removal process, it creates features on your parts by cutting away the unwanted material. It requires a machine, a workpiece, typically a fixture, and a cutting tool. You can make holes on a variety of different machines, including a hand held drill, drill press, and even CNC milling or turning machines. Machines like a drill press are what’s considered specialized equipment. They’re designed to ensure you can get a precise angle. The team here at DroidBuilders suggest a drill press as one of your must have shop tools. If you want to expand, consider finding a tapping machine as well but you can do tapping with the drill press.  We’ll do a simple how to on that later.

The operations involved in making holes are usually performed along side many other operations in machining a part. When using a CNC, I tend to mix when I add the holes and if I’m doing it on the drill press it’s usually one of the last operations I perform.

Machined Holes

During the machining process, a hole is a cylindrical feature that is removed from the work-piece by a rotating cutting tool that enters the work-piece axially.  The hole will have the same diameter of the cutting tool and match its geometry.  (that may also include it’s pointy end).  Note, pocketing while similar, is not the same as hole making.  While all machined holes have the same basic form (I mean its a hole right…), they can differ in many ways based on its given application.  There are several features that help determine the hole making operation and the appropriate tool required.

• Hole Diameter: Holes can be machined in a huge variety of diameters.  The cutting tools used for this are available in standard sizes that be incredibly small to huge.  I think the largest hole I’ve cut was around 10″ on the drill press.
• Hole Tolerance: The precision of your holes can be affected by many factors.  The sharpness of the cutting tool, vibration, build up of material, and even heat in some cases.  No matter, the specified tolerance of the hole will determine the method you need to use.
• Hole Depth: You can drill completely through your workpiece (referred to as a through hole) or perhaps you only need a certain depth (blind hole).  If you’re doing a blind hole, make sure you know if the depth listed includes the tip or not. Otherwise you will need to do some drill point calculations to ensure you get the proper depth.  It isn’t hard, but… apologies to my trig teacher, it turns out you were right and I did need to know how to calculate and use Tangent.
• Recessed Top: This is a very common feature to add while machining holes.  This is usually done to accomodate the head of the fastener and allow it to sit flush with the workpiece surface.  There are two different types of recessed holes to know about.  There’s the counterbore which has a cylindrical recess, and the countersink, which has a cone shaped recess.   The type usually depends on your fastener and the look you want to achieve.
• Threads: Allows you to use a threaded fastener with your spiffy new hole.  They are specified by the outer diameter and pitch.  Pitch is a fancy way of saying what the measure of the spacing between threads, its commonly know as the Threads per Inch (TPI) in English standard, or its the distance in millimeters (mm) between the threads in metric.

Operations

Okay, if you are still reading I am impressed.  Most people would have been bored out of there skull by this point.  🙂  So lets talk about the various operations you can do to actually created the awesome hole you need.  Each of these uses a different type of cutting tool.

Drilling - A drill bit enters acially and cuts a bling or through hole with a diameter equal to that of the tool. A drill bit is a multi-point tool and typically has a pointed end. The most common is a twist drill (you probably have lots), but there are also other types such as a center drill, spot drill, or tap drill.
Boring - A boring tool enters the workpiece axially and cuts along the internal surface of an existing hole to enlarge the diameter or to ensure of the precise dimensions. Boring tools are single point cutting and you set the desired diameter using the adjustable boring head. Have a couple, never used them in production. If you ever needed to do a tapered hole though, these do the ticket.
Counterboring - A counterbore tool enters the workpiece and cust along the internal surface of an existing hole to enlarge the top portion. It's typically done after drilling to add the extra space for the head of the fastener, such as a bolt, to sit flush with the work piece surface. Counterbore tools have a pilot on the end to assist guiding the tool straight into the existing hole.
Countersinking - Tools like the coutersink enter the workpiece axially and enlarge the top of an existing hole to a cone-shaped opening. Countersinking is performed after drilling to provide the space for the head of the fastener, like a screw, to sit flush with the workpiece surface. The common angles for countersink hols include 60, 82, 90, 100, 118, 120 degrees. The most common of these is the 82° and 90°.
Reaming - Reamers are used to enlarge an existing hole to the desired diameter. A reamer is a multi-point tool with many flutes. Reaming is used to remove a minimal amount of material from previously drilled holes to provide more accurate diameters and a smoother finish. Personally, I have one reamer that I've only used to clean up foot drive plates to make the holes for the bearings as precise as possible.
Tapping - A tap cuts internal threads into an existing hole. The existing hole should be drilled to the required tap drill size that will accommodate the desired tap. The tap size is selected based on the diameter and pitch of the hole required. There are multiple types of taps available for bottom tap or through tap. Go figure right, make sure you have the right one for the job at hand.

Drill Point Calculations

I alluded to this above but starting out as a hobby machinist there was so much I didn’t even know that I didn’t know about.  If you know me, you know I use a CNC a lot.  One of the things I’ve always hated was if I set my drill depth to the thickness of the material I wouldn’t go all the way through.  Or, just ham handling it I would just put in some ‘extra’ depth to clear it.  That would needlessly mark up my spoil board.  Okay, I get its called a spoil board meaning its replaced, but it is expensive to replace a 10′ x 6′ piece of 3/4″ MDF very frequently.

The drill point is that wee little part between the pointy end of the drill and the full size diameter.  See ‘P’ below:

To start, there are a couple of ways to calculate this number.  You will need to have two pieces of input to do this;

• Diameter of the drill bit
• Angle of the drill bit

From what I’ve noticed, if you buy some drill bits, they are almost always 118 degree bits.  Those are basically the standard ones at the local home improvement store.  Nothing wrong with them at all, they work great for 99% of the usual jobs.

Couple of things to note, use the complicated method if you plan to do counterboring and really want it to be accurate.  I don’t make anything that a life is on the line, so I resort to method 2 since it takes so little time.

The ridiculously complicated but really accurate method of calculation

1. Divide the tip angle in half
2. Subtract that from 90
3. Get the tangent of that result and
4. Divide that in half.
5. Multiply by the diameter of the bit.

Optional 6; pull hair out wondering what you did wrong when it’s off.

The what I do method cause reasons…

Turns out you can just use simple multiplication based on the drill bit angle.

• 60° Angle: Drill Diameter multiplied by 0.866
• 82° Angle: Drill Diameter multiplied by 0.575
• 90° Angle: Drill Diameter multiplied by 0.500
• 118° Angle: Drill Diameter multiplied by 0.300 (Most Common)
• 120° Angle: Drill Diameter multiplied by 0.288
• 135° Angle: Drill Diameter multiplied by 0.207

And just because I wanted to create a calculator;