Editor's Note: This is the first of a year-long series of articles on design for manufacturability. We recommend you read the series to refresh your own knowledge and share the articles with younger engineers. Our author, Deborah Munro, D.Eng., begins by sharing why this topic is important today.
Something strange has happened to engineering education. At the same time that designs are becoming more and more technically complex, offering of hands-on machining experiences are disappearing. By the 1990s, high school shop classes had all but disappeared. In today’s college curriculums, it is rare to find a required manufacturing course. At my prior university in Oregon, even summer jobs in the campus machine shop were eliminated to both control costs and ensure student safety.
An entire generation of highly educated engineers who were never taught the basics of how to machine their own designs.
This may seem inconsequential, but it isn’t. An engineer who doesn’t understand machining will make design decisions that greatly complicate or prohibit the fabrication of a part. They will dimension and tolerance drawings in ways that are not conducive to machining, and they will frustrate the often overworked machinists with their ignorance. The result is a loss of productivity and suboptimal designs.
Universities are not going to reintroduce hands-on manufacturing courses or any other practical labs, because the costs to run multiple small lab sections are prohibitive and the liability risks too high. Thus, this series of articles for 2019 will provide engineers with an introduction to the principles of operation of all the major machining tools, along with an in-depth look at the strengths and limitations of each tool in order to give insights that will improve engineering designs and drawings.
In this article, I examine the drill press. Going forward, topics I’ll cover include mills/lathes, inspection equipment, reverse engineering, wire/block EDM and more.
The Drill Press
A drill press is used to create a hole in a piece of metal, wood, plastic or other material. Usually, a tool bit is mounted into a chuck and tightened into place with a chuck key. The tool bit rotates at high speed and is driven downward into the surface of the part. For metal and CNC machining, the part is held with a machine vise; otherwise, it can be handheld.
Drill presses are often employed when there’s a need for holes, especially tapped holes. Although an end mill also works well, use of a drill press is cheaper and frees up time on other equipment, allowing work to be done in parallel.
For a drill press, the face of the tool bit and its sides must be able to cut material. These cutting surfaces are called flutes and they often helically spiral around the cylindrical surface of the tool bit. The larger the number of flutes, the smoother the finish of the resulting cut hole. The main types of tool bits are:
- Centering bit: Small diameter bit that creates a divot in a flat surface to guide the drill bit and keep it from wandering on the surface of a part
- Drill bit: Long, helically fluted bit with a pointed tip for rough hole drilling
- Reamer: Straight-fluted bit used at high speeds to remove minimal amounts of material and give a hole a smooth finish and high tolerance diameter
- Countersinks and counterbores: Specialty bits of various shapes that create recesses for screw and bolt heads
- Tap: Screw threaded bit that cuts threads into a reamed hole
Drilling occurs in three or four steps. First, a locating center is cut. Then, the holes are drilled. This is followed by reaming and any countersinks or counterbores, and finally the holes are tapped (if desired). It is important to remember that holes are first drilled and reamed to their shank diameter and then tapped to create the thread pitch.
A key aspect about drilling is speed. The larger the diameter of the drill bit, the higher the speeds at which the outside flutes will travel. Conversely, very small diameter drill bits have very low speeds. Thus, the RPM of the drill press chuck must be adjusted to be optimal for the diameter.
A second key consideration is chip clearance. As a drill pierces through material, it creates chips and generates heat due to friction between the tool bit and the material. Thus, it is usually preferable to “peck” at the cut, which serves to
both help clear chips and reduce the heat load. For hard metals like steel, a liquid coolant is also necessary; soft metals and wood can be machined without coolant or with air.
In an engineering drawing, create holes as either a rectangular or circular array, and always dimension from an outside corner of the part, not the centerline, and not from a location that will be machined away. However, the rest of the holes should be dimensioned from the first hole, not the edge of the part, as the critical dimensions for later assembly are the holes. Machinists zero the material on one corner and program (or hand machine) relative to this baseline. Therefore, it’s imperative that they can always relocate the part the same way, even if they need to remove the part or flip it over to machine other features. Circular arrays should always be dimensioned across the diameter, with an angular reference to the first hole location and then an angle for the pattern.
In summary, the main purpose of a drill press is to machine holes. It is a multi-step process that begins with center drilling. Holes are machined as linear, rectangular or circular arrays. The first hole is machined relative to a corner or the center of cylindrical parts, and the rest of the holes are machined relative to the first hole. Keeping these rules in mind will improve your engineering drawings and make the machinists’ job easier.
Deborah Munro, D.Eng, is the President of Munro Medical, a biomedical research and consulting firm. She is Senior Lecturer in Mechanical Engineering and Lead for Minor in Medical Engineering at University of Canterbury in New Zealand. Dr. Munro has worked in orthopedics for almost 20 years and holds numerous patents, mostly in the area of spinal fusion. She taught mechanical and biomedical engineering at the University of Portland, where she also founded a Master’s in Biomedical Engineering program. She can be reached by email.