Technology 307 Cold Heading 101 by Thomas Doppke History Early bolt manufacture was a labor intensive, slow process. Handmade, usually forged of wrought iron, and costly. Hot forged, most early bolts were shaped by hammering a head from the end of an iron rod. The invention of the screw cutting lathe in the 17th century improved the quality and made for a more reproducible product. But still, bolts were a costly and hard to procure commodity. The Industrial Revolution increased the demand for items a thousand fold greater than could be manufactured by the slow, hand crafted, and often cottage industry based industrial supply market. This gave rise to the invention and manufacture of machines; machines to make textiles by the yard, locomotives and trains to move product and people, ships, machines to make armaments, and a multitude of must have items for the growing middle class. Many machines and machines are held together with bolts-lots of bolts! The capability of lathe manufacture of bolts began to lag behind demand greatly. Although steel, which became widely available in the 17th century, was known to be malleable in lower carbon versions, it wasn t until the mid-1800 that it became the metal of bolt making choice. The screw lathe of Jesse Ramsden (1777) and the precision of Henry Maudslay s (1797) paved the way for increased production. But still not enough-micah Rugg reported the enormous volume of 500 pieces per day in 1840!! However, by the end of the 19th century screw machines were not fast enough. There is still uncertainty about who and when the cold heading process was invented and utilized. Many companies kept their processes secret. Some data shows about mid 1800 (1850-1860) for at least one company and for certain the process was in wide use by 1900. By 1905 there were over 500 hundred factories making bolts and nuts in the United States. By the time World War II had ended, cold heading was the major method of bolt manufacture. Processes There are two general methods of bolt manufacture today. Screw machining and cold heading. Each has some advantages and of course, some disadvantages. Starting with a short explanation of each: screw machine parts are produced by the removal of material from a piece of stock, leaving the bolt behind. Like a famous sculptor once said when asked how he could carve such a beautiful statue from a block of stone, he replied I just remove all the material that isn t part of the statue! As can be seen below, screw machine production generates a lot of scrap, in many cases as much as 70% of the basic piece is cut away. Cold heading produces a part with little or no scrap. The cold headed part often has a finish quality and smooth surface, negating the need for secondary handling. Screw machine parts require secondary polishing in many cases. Screw machines run slower, the exact time is dependent upon the part geometry of course, but a medium size bolt configuration (a simple shape) would be manufactured at a rate of about 100 per hour. Cold headers would produce hundreds more than that in the same period, some machines running near a thousand per hour (size dependent of course). However, cold headers are limited to fairly standard shapes, certain features are not possible with standard tooling.
308 Technology Iron and Steel Cast Iron Copper Brass-Cartridge Brass-Free Machining Gold, Silver, and Alloys* Platinum, Palladium, Tantalum, and Alloys# Nickel and Alloys Titanium and Alloys Silicon, Tungsten, Rhenium Cobalt and Alloys Rhodium Stainless Steel Metal Suitability for Cold Heading Pure iron Yes. Steel-depends upon grade and hardness. Free Machining grade-no, sulfide stringers promote cracking No- Too brittle Excellent Fair. Exact alloy selection required for best results Poor-Additive materials promote cracking Excellent Most are headable Pure Ni-Yes. Alloys with 20% elongation-yes Pure Ti-Yes. Alloys with high ductility-yes. Others-No No- Too brittle No-Most are too brittle. No-Most are too brittle Yes- if HQ (heading quality) is ordered. Notes: To be headable the metal must be in a soft condition. Some alloys such as exotics are not softenable as are many high corrosion nickel alloys. * Many metals are hard to machine and tear easily (gold, silver, etc.) and their use in cosmetic areas and areas of medical necessity (implants) requires a smooth finish that is obtainable by cold heading. Also the cost of scrap is very high with these metals. #Cold heading is preferred for expensive metals over machining because of the small to no scrap generated. A few examples of what are not possible are shown below. Screw machine can make about any basic design but may require secondary operations for some features. Economics plays a great part winning the selection of the manufacturing process. Beside the cost advantage of volume (screw machines are more economical for low volumes due to
Technology 309 the other factors mentioned here), the initial cost of the equipment, tooling (a set of cold heading tools may run from US$5,000 to 25,000), part set up time (about 1 hour or so vs 8 hours or more for a cold headed part) all play a part in selecting what is used to make the part. Of course, the availability of the equipment is also a major consideration. With the use of many exotic materials in today s market place a general thought is that the metal s elongation must be at least 20%. Since the parts are formed by the action of a large force deforming the metal blank by pushing it into a die cavity a high yield-to tensile strength ratio allows more cold working without the possibility of part fracture. Cold Heading-The General Process The first step in cold heading is to ensure that the wire that the part is to be made of is correct. By that it is meant that the diameter must be controlled. Too little volume (small diameter) in the heading die will not allow the part to be fully formed, too much (oversize diameter) will cause excessive flash and can even break the die. Most wire coils are shipped with a lubricant coating to assist die drawing. The wire is pretreated to soften it to maximum malleability. Commonly the wire is drawn down 3-7% to the correct diameter at a wire draw station before the blank cutoff process. The wire is fed through a cutoff machine which cuts a blank to a predetermined length. This length is equal exactly to the amount of material needed to form a fully shaped part. While steel is malleable it has limits. Usually a second blow is required to produce the final shape. This blank goes into a header (the various types and operations will be explained below). In its simplest operation the blank is struck with force great enough force to deform the metal which takes the shape of the punch striking it. The illustration below shows a simplified version of such an operation. Many times you will hear the process under discussion being called a 2 blow, 1 die (like the illustration) or some other combination (3 die 5 blow, etc.). This refers to the fact that the first blow can only upset the metal so far and a second blow is needed to fully form the feature. Now that the basics of cold heading are firmly in mind, lets discuss the complexities. The shape of the part and
310 Technology its structural properties determine some basics of the part s formation. Three forming techniques are usually discussed. Forward Extrusion is the technique used to reduce diameter where, depending upon the required reduction percentage, the material can flow openly or can be trapped in the cavity of lesser diameter. By forcing the metal through the cavity that has a diameter less than the piece itself, the part is elongated as well. Backward Extrusion is the technique used in making hollow shaped holes. The material flows backwards around the penetrating punch. It forms a hole without the action of cutting away metal. And finally, Upset Extrusion is the technique used to form heads, the material being upset at the face of the die. The dies can be open or trapped to upset a particular shape head. Our basic heading operation illustration is the upset extrusion technique. Just as confusing as the techniques used in cold heading is the variety of heading equipment available. The situations vary from large, complex machines, many so large that they have steps, landings, and even office space on/in them. Wire is fed in one end and a finished fastener comes out the other. Needless to say, the price tag is near the million dollar mark. At the other end are smaller machines that each do one function (draw the wire diameter down, cut the blank, form the head, roll the threads, and maybe, point the end, add a washer, and so on). Less in cost and easier to keep production running if a hang up occurs. Since most
Technology 311 companies have several machines, a problem with one machine will not close down operations as would a all-in-one boltmaker failure. The illustration below shows some of the operational types of machines and what they are generally used for. The straight across transfer machine is the standard type used for making regular shaped, headed bolts (like our example). Parts are moved from one station to the next where each gets it hit. The part stays in one holder (die) and the blows are delivered by punches via transfer from one station to the next. Straight transfer machines may have several die stations mounted opposite a number of punches which are mounted on a slide, usually horizontally. The most common setup is a 1 die 2 blow arrangement. Since the steel is not infinitely malleable the first blow shapes the head in a cone like form and the second blow with the same punch (or a second one, feature and size dependent) further deforms to metal to fit the punch cavity, forming the final head shape. Of some confusion is the terminology used; the piece that holds the blank during processing is called the die and the part that impacts the blank is called a punch. The nut former moves the part from machine station to station where each punch operates. Multiple blows and dies require many more transfer pieces than can be accommodated in a single station. Typical parts have varying diameters on both ends, flanges and hex or other formed features, punched holes (nuts). The place to place transfer machines are used for flanged parts, parts with multi-shaped sections (the hexagon shaped car wheel bolt is one familiar example). The selection of what machine to be used is also affected by such items as the machine s cutoff diameter
312 Technology ability (up to 48mm), the capability of the machine to accommodate the part length (from 2mm to greater than 300mm), and the tonnage necessary to form the part (size). Some final remarks about cold heading The question is often asked What is the difference between cold forging and cold heading? Cold heading has, hopefully, been well discussed above. Forging can be divided into hot, warm and cold forging methods. Hot forging is the familiar scene where glowing metal is repeatedly hammered by gigantic drop forge hammers into crankshafts and other heavy objects. Large objects would require a vastly greater amount of force to deform into the required shape than can be delivered by any press made today. Also the question of metal ductility is a factor. Such blows would probably crack and shatter the work piece if the object had not been heated to redhot, malleable condition. The high heat recrystallizes ferrous parts (and oxidizes others). The finished part is of medium to low accuracy, tolerance wise, and is often just a step in a process that requires further secondary work. Oxidized iron scale needs to be removed and usually some finishing machining is also required. Warm forging is a similar process but done at lower temperatures which do not recrystallize ferrous and other metals. Used mostly for aerospace hardware, titanium is a favorite material, parts are run on 3 blow, 2 die and 2 die, 2 blow headers with induction heaters added. Temperatures are in the range of 200-850 degrees C. Its use is almost entirely with exotic metal manufacture. Cold forging is done at room temperature ranges and generally used with softer metals such as aluminum. The metal blank is placed into a die which is attached to an anvil. The metal is forced into the die cavity by the action of a descending hammer which may strike the piece several times rapidly. The process is less expensive than hot forging and produces scale free surfaces. The process is similar to cold heading except that the parts are usually much larger and the forming is down by blows against the metal, forcing it to flow into and conform to the die cavity. Comparison of Screw Machining vs Cold Heading Feature Screw Machine Cold Heading Generates scrap Speed of manufacture Yes (As much as 70% more than Cold Heading) Low (<100/hr medium size) Little or no High (100 times faster than screw machining) Used for volume No Yes Tooling for new part 1-3 days usually to setup a new part Design and setup can take weeks to procure. Dies and punches can run US$5,000 to 25,000 Initial equipment cost Moderate Expensive Part change over(time/labor) Done in < 1 hour (est.) Often a whole shift Part cost (final) Moderate Inexpensive Main Usages Metals Finished Properties Raw Material Used Low volume, special geometry, certain metals Most metals, many special exotic materials More complex shapes As supplied Generally high volume (15,000 +), standard geometry parts Basically steel, few others Higher tensile, smoother finish, tighter tolerances Wire must be pre-heat treated to soften it