Drilling, boring, then reaming is the proper order of operation when machining a hole. This is just one of the fundamentals you will learn in Machining 101. Whether you’re on a manual mill or a cnc milling center, this process will get you the most accurate hole size.
Why can’t I just drill? That is a very good question, if you’re just starting out as a machinist or are in training, you probably won’t know how every kind of tool is going to perform. While a drill, even when spot drilled, can make a nice looking hole, it can’t always hole a tight diameter or circularity tolerance. A standard drill can walk, and that can change the location if it’s a thru-hole. Drills are not always ground perfect, resulting in one lip bigger than the other. This causes the hole to be more egg-shaped and often over-sized.
Want a perfectly circular hole? The boring bar comes next because, unlike a drill, it is sturdy and will follow the same path all the way down the hole. A drill is floating in its holder that causes run-out, but a boring bar is sturdy and will make a circular hole, whether the existing hole is already or not.
The reamer comes last if you want an accurate hole. You should only leave several thousandths left after boring, depending on what material you’re cutting. A reamer is much more precise than a drill, but it will follow the path of the existing hole. This is why you should bore the hole prior to reaming, otherwise the ream will follow the path of the drilled hole, which may not be straight. A bore is accurate, but you can get a better finish with a reamer, and it can still hold tenths for a tolerance if you have a good reamer.
There are a lot of tools used in machining today, so it’s often hard to keep up with the names of all of them, especially if you’re new to this career. Every tool has a specific job, and while a variety of tools may be able to get the job done, some are better than others.
Just so I don’t overload you, I’ll go through a list of the most common tools used in Machine shops, as well as Machine Tech schools. Each tool has a specific purpose, and there are many different kinds of the same tool. The more tools you use, the more knowledge you will get and know what works better for a certain material or operation. However, you must get an understanding of the of machining tools before just using any specific one without knowing what it’s meant to do.
The almighty ‘End Mill’ is one tool that you will use almost every day if you run milling machines. If you need to cut both ends of a part to get it to a certain length, and end mill will side-mill the ends to make a clean and parallel surface. It can cut out pockets, and make square or round features in a part. There is so much that you can do with an end mill on a CNC milling center.
Whether you need to rough out a large solid piece of steel, or you’re just making some finish passes on aluminum, there’s an end mill for each job and every one in between. There are many different kinds of end mills. Here are the main variables you will have to decide when ordering tools: size (diamter), length (flute length), material/coating, roughing/finishing, number of flutes, and more.
Need to drill a hole? How about a few hundred holes? While there may not be quite the selection for drills as end mills due to the fact that you can only do so much with them, there are definitely right and wrong drills for any given job. 118 degree HSS or coated drills are the most common since they work well with most basic materials. However, you may need a drill for a hard stainless job, or perhaps a copper part that requires a deep hole with a close tolerance.
There’s not much else you can do with a tap other than tapping holes. Are you doing a blind or a thru-hole? Is it a metric or a U.S. standard thread? If you’re on a mill, the most popular taps are: cut tap, roll form, spiral point/flute, as well as a thread mill.
Cut taps produce chips because they literally cut a thread into the existing hole. You must use a drill that meets the minimum diameter tolerance for the specific thread you want. They are used on thru-holes because the chips won’t get in the way of the tap.
A roll form tap does not make chips because it forms and pushes the threads into place. It’s great for blind holes because the tap won’t break from chips collecting at the bottom.
A ream is used after a drill or a bore to meet a close tolerance call-out on the blueprint. Cheap drills are far from perfect and can easily make a hole over-sized, which will be rejected if it’s not within print. However, if you drill the hole .010″-.015″ under-size, you can then use the correct size ream to get a much more accurate and consider hole.
However, you should know that a ream will not ‘fix’ a hole. A ream just follows a hole, so if it’s crooked or out of round, the tool will follow that path. This is one reason why you may need to drill the hole then use a boring bar to make it perfectly round and straight before you ream it to size.
That’s it for the basic tools on this article from CNC Machinist Training. Stay tuned for another article that explains more advanced tooling that makes it much easier to do a job…