Cutting threads

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Nuts, bolts, screws, and studs are used for holding machine parts together or in place. All of these devices depend on threads. A thread is defined as a spiral ridge, uniform in size and shape, placed regularly on the outside or inside of a cylinder.

Threads can be cut by machine or by hand. Special machines are used for quantity production of threaded machine parts. For general requirements in a machine shop, a screw-cutting lathe is employed.

By using taps and dies, threading can be done by hand. This is the means commonly used when the work cannot be brought to the machine, as when threading holes in a steel doorway to receive the screws that will hold the door hinges. In this section on cutting threads, we shall consider only the use of taps and dies.

It is not absolutely necessary for the mechanic to know all the terms and definitions of threading, but for reference the more important ones are included here.

Thread forms

The three commonly used forms of screw threads are: the American National, the acme, and the square.

The American National is most common in motor vehicle assembly. The external, or male, thread is on the outside of a machine part, as outside a pipe or rod end, or on a bolt or screw. The internal, or female, thread is on the inside of a machine part, as inside a pipe coupling or bolt nut.

Screw parts and measurements

Major diameter, sometimes called outside diameter, is the greatest diameter of a bolt or nut. It reaches from the top, or crest, of the threads on one side directly across the screw to the crest of the thread ridges on the other side. It is the diameter by which a screw, bolt, or nut is specified.

Minor diameter, sometimes called root diameter, is the smallest diameter of a bolt or nut. It reaches from the bottom, or root, of the threads on one side directly across to the root of the threads on the other side.

Pitch is the shortest distance from a point on the thread of a screw to a similar point on the next thread. Pitch in inches equals 1 divided by the number of threads per inch. Thus, if there are 4 threads per inch, pitch is 1/4".

Pitch diameter is the diameter of an imaginary cylinder which, if placed between the major and minor diameters of the screw, would so cut the threads that the thickness (not depth) of each thread at the point of cutting would equal the space between each pair of threads at the place of cutting.

Lead is the distance parallel to the length of the screw that a screw moves when given one complete turn. On a single-thread screw, the lead and pitch are the same; on a double-thread screw, the lead is twice the pitch; on a triple-thread screw the lead is three times the pitch, and so on.

Thread angle is the angle formed by the sides of the threads. It is 60° for the American National Screw; 29° for the acme thread.

Crest or flat is the top surf ace of a thread joining its two sides. Root is the opposite of crest; it is the bottom surface joining the sides of two threads next to each other.

Side is the surface of a thread between the crest and root. Axis is an imaginary line running in the center for the length of a screw.

Depth of thread is the distance between the crest and root of a thread measured perpendicular to the axis. It is half the difference between the major and minor diameters.

Internal threading

Threads on the walls of drilled holes are made with a tap and tap wrench. Taps are made of highly tempered brittle tool steel, with square ends, or shanks, threaded and fluted for the distance of the threading. They look like screws that have three or four evenly spaced grooves cut across the threads along the length of the tap. These grooves or flutes make up the cutting edges. By permitting the metal that was removed to come out, they prevent binding. They reduce friction by presenting less surface than an unfluted screw would. And they permit lubrication to reach all parts of the tap and hole.

All taps have square shanks to permit the use of a wrench for turning them to cut the thread. Different tap wrenches are used for small and large taps. Each is adjustable to fit all taps in its clans. Generally on the shank of each tap is stamped the manufacturer's name, diameter size of screw, number of teeth per inch, and the type or form of thread.

There are three kinds of taps: taper, plug, and bottoming. The taper tap gradually and uniformly decreases in diameter from 1/3 to 1/2 the length of the threaded distance, the diameter being smallest at the beginning of the threading. A taper tap is used to start all internal threading. However, since its upper part has full threads, it can be used to complete the job if it can be run through the work, such as sheet metal.

The plug tap gradually and uniformly decreases in diameter, like the taper tap, but for only one-fourth its threaded length. It is never used to start a thread. After the taper tap starts the thread, it is backed out; then the threading is continued with the plug tap. The plug tap is used on holes that do not go through the work (blind holes) but where there is no need to thread to the bottom.

The bottoming tap has the same diameter throughout. It is used to run threads to the bottom of a blind hole. The taper tap is used to begin the threading and then backed out. Then the plug tap is employed until it reaches the end, and it is backed out. Finally the bottoming tap is employed to complete the job. Naturally, the hole must be cleared of all cuttings before the bottoming tap reaches the bottom. If the material is soft, like brass and topper, the plug-tap operation may be omitted. In this case the threading consists of two steps.

Hole sizes

Taps cannot drill holes. For a tap to be used, a hole of the proper size must first be made. If the hole is too small, the extra pressure and friction of tightness may be so great that the tap will break because of the increased force needed to turn it. If the hole is too large, the threading will be very shallow. This will result in a weak union with the part that is to be screwed into the threaded hole. However, it should be understood that the hole drilled must be smaller in diameter than the required bolt or screw diameter.

Before drilling the hole, read the information stamped on the tap: diameter size, number of threads per inch, coarseness of thread, and type. With this information, consult a table, to find the size of drill to use for the particular hole. When a table is not available the following formulas will give approximately the correct size of the hole to drill for tapping: For National Coarse threads:

Size of drill hole is:

  • Diameter of tap in inches — .975/No. of threads per inch
  • For National Fine threads: 1.05/ No. of threads per inch

If the answer gives a drill hole that does not match a standard drill size, the next larger size in the drill table should be selected. (If the next smaller size is chosen, the hole may be too small and cause the tap to break.) These formulas are not based on a full depth thread, but on a 75% thread, which is the depth used for commercial production work. It is only 5% less efficient than full-depth thread, but it is cut much more easily.

Steps in cutting internal threads

  1. If the work is movable, secure it in a vise or clamp.
  2. Make a hole the correct size, not too vide or too small. If the hole is blind (ends inside the material), make sure it is deep enough. If not, the accumulated chips at the bottom will cause the tap to bind and break.
  3. Select the correct size taper tap. Make sure it is sharp, for dull taps break easily. They are not easily sharpened in the shop, for special machines are required for a good job.
  4. Mount the tap securely in the wrench. Be sure it is square with the work. If the chuck of the wrench is worn, it will cause faulty alignment.
  5. Place the taper tap in the hole, as far as it will go. Make sure that it is square with the work. Apply a little lubrication on the end of the tap, as in drilling.
  6. Turn the tap with a slight pressure for three or four revolutions clockwise. That is enough for it to take. After this it will cut its way without pressure. Remember that a tap is made of very brittle steel that has no "give" to it. Therefore, when subjected to side pressure, such as the slightest attempt to bend it, the tap will break.
  7. After the tapping is begun, check the tap in two directions with a square to see that it is perpendicular to the work. If it is not off too much, back it out and retap while gently forcing it in the proper direction. If it is very much off, use a countersink to remove the poor threads or redrill the hole correctly to a larger diameter.
  8. After the threading has been begun with the taper tap, continue with the plug and bottoming taps.
  9. As the tapping proceeds, for every full turn of the tap, back it out for half a turn. This helps clear it of chips and prevents the tap from binding. Aluminum especially should be tapped this way.
  10. Don't back out a tap completely if it can be avoided, for every time it is backed out, small chips land in the thread grooves and cause extra wear on the tap and the threads already cut. This results in a thread that is not accurate. If the hole goes through, the tap should be continued through and out the other end. This avoids backing out.
  11. When finished, clean the taps thoroughly, removing all burs and chips. Immediately put the taps away with the same general precautions taken in storing drills and files.

Removing broken taps

If a tap breaks above the surface of the work, it can be simply removed by grasping it with a pair of pliers or a wrench and using this tool to back out the broken tap. However, if the tap breaks flush with the surface of the work, or below it, it is not so simple to remove. Several methods, though, are available.

Using a small, blunt, cold chisel or a taper punch, gently tap the edge of the tap in a backing-out direction. When it is loosened, a tap extractor can be used. The prongs of the extractor are small enough to fit into the flutes of the broken tap. The collar that holds these prongs is moved down flush to the work and set rigidly in place. Then a tap wrench is used to back out the broken piece.

Instead of using the tap extractor you may back the screw out a few more turns with the cold chisel or taper punch and then remove it as you would remove a tap that breaks above the surface of the work.

Or, you may break the tap further into smaller pieces with a solid punch and hammer. You can then piek out the pieces with a center punch or a magnetized scribe.

External threading

Threads on the outside of a rod or bolt blank are made with a die and stock. The die does the cutting; the stock holds the die and provides the needed leverage. The die may be adjusted and held in the stock by means of a screw place.

Dies are made of hardened tool steel, threaded like a nut but with four evenly spaced grooves cutting across the threads. As with taps, these flutes or grooves provide the cutting edges for removing metal. Also, they provide a path for the metal chips to come out, they reduce friction, and they allow for lubrication. To permit plating the die squarely on the work, the threads on one side of the die are of wider diameter than those on the other side.

Because thread terms and definitions apply to both inside and outside threads, dies agree with taps in diameter, pitch, coarseness of thread, and the like.

A solid die makes threads of just one size. In an adjustable die, however, there is an inset screw which regulates the size of the hole and therefore of the threads cut. An adjustable die is good to use when the material is a little off size or when looser or tighter fits are wanted.

As with taps, the die usually has the following information stamped on one face: manufacturer's name, diameter of screw, threads per inch, and coarseness and type of thread.

Steps in cutting external threads

  1. Cut the required length of rod, keeping the end at right angles to the length.
  2. Remove the hacksaw burs. Slightly bevel the end of the rod.
  3. Mount the work rigidly in a vise.
  4. Select the correct die. Be sure the die is clean of chips from the last job.
  5. Place the guide and die in the stock with the side of the die having the stamped information facing you.
  6. Place the guide over the rod, with the die resting on the end.
  7. Make sure that the stock is at right angles with the rod. This is the position in which it must be kept until the job is complete.
  8. As with the tap, keep the die lubricated; and for Bach full turn clockwise, back it up a half-turn until the job is completed.
  9. If the thread must extend up to a shoulder or some other projection, remove the die, reverse it, replace it, and complete the last few threads.
  10. Clean stock and die and put them away promptly. Pipe threads are cut in a similar fashion. The last few threads taper to provide a tight, leak proof fit.