From Bump Rolling to Modern Rolling Techniques – Thread Rolling

The earliest form of producing a rolled thread came from a technique called bump rolling. This utilized a single roll that pressed the form into the material with side pressure. While it was the original approach to rolling threads it certainly earned the craft the nick name of “black art”.  It took painstaking effort to achieve set up success and was helped by the wide use of soft leaded steels. Set ups were best done “When the moon was in the seventh house and Jupiter aligned with Mars”. Despite its perils it could be productive and most certainly became the catalyst for developing more robust attachments and machines.

Further development brought about robust mechanical devices that could be pneumatic or purely cam driven. Few of the early devices could be used easily on modern CNC equipment but remain in use today on multi spindle automatic machines. Pneumatic devices require dry filtered air for best performance. Pneumatic delivery systems during high humidity days are notorious for compressing water out of the air and fouling the operation of the tool resulting in sketchy tool performance.  The pure mechanical devices are unencumbered by humidity.

Today’s materials with the removal of lead from most manufacturing have created a new set of demands on the tools and machines that frequently demand rolled threads for quality and economy. Virtually all machine applications have one or more tool selections available to achieve an economical end result. Review your application with us. We can recommend one or more solutions to your specific needs.

Thread Rolling Styles

Cold Forming (rolling) threads in metal has been used in the United States for about 65 years. Originally developed in Germany the technology migrated to the US in the 1950’s. Initially used on multi spindle screw machines the tooling styles have been altered to accommodate use on virtually all of today’s existing machine styles. Multi Spindle, Manual, Rotary Transfer and CNC Turning and Milling Centers.

Three styles of tooling have been developed for very specific applications. Axial rolling uses either a two-roll concept or 3 and 5 roll concepts. This style can create unlimited thread lengths by producing one thread pitch at a time. Process time is about 1 inch of thread length per second of cycle time. The process this style supports leaves 2 or more partially finished threads at the end of the thread journal. An undercut in the part design that can mask the unfinished thread will allow gauging to a shoulder with a full thread form.

Parts designed with less than a 2-pitch clearance to a shoulder can be rolled with a Tangential Style attachment. The Tangential tool is a 2-roll side entry tool. It can be used on multi spindle and CNC applications where the machine has mechanical control of tool movement. The Tangential process can roll within 1/2 pitch length to a shoulder with a full thread. Thread length is limited to the width of the roll and the complete thread form is rolled at once. Cycle time for this concept is typically 2-3 seconds for a complete thread.

A third and unusual method is available in the form of a Radial attachment. A Radial tool is a three-roll approach that has the same length limitations as Tangential tooling. The rolls are ground eccentrically and when at rest clear the part to be rolled. There is no regard for feed rates. When in position the head is tripped, and the rolls make 1 revolution to achieve a finished thread. Typical cycle time is 2 tenths of a second. The tool can mount on any machine that can rotate the tool or the work piece. It has become the tool of choice in high volume machining. It runs on any machine and frequently used in automotive production on CNC machines due to its short cycle time.

In any application where time and cost are relevant we have a thread rolling approach that will efficiently reduce both. Reviewing a drawing of your part will determine the style most suited to your specific needs.

Thread Rolling Works with Both High and Low Volume Jobs

Rolling threads on various materials became a necessity in high volume manufacturing in the US following its discovery in Germany after the second world war.

The most dramatic effect was in the production of spark plugs. In the US alone more than 10 million spark plugs were being produced daily by several manufacturers. Leaded fuels were necessary to lubricate engine valves but subsequently fowled spark plugs limiting their useful life to about 10,000 miles. When lead was outlawed in motor fuels in the 1970’s hardened valve seats became the standard and without lead deteriorating spark plug life increased more than 10-fold. This reduced product demand equally.

Thread rolling migrated from multi spindle screw machines to modern CNC machines for the very same reasons. Cold forming of threads became equally important in lower volume production from a time and cost consideration. Threads can be formed typically at 1 inch of thread per second of cycle time. A cold formed thread has greater yield strength and surface quality than a cut thread. Many materials that are difficult to cut are easy to roll. Time cycles and part volume create significant value in throughput cost. This value impacts on the final cost of the finished product. In a job shop atmosphere, it can be the strategic difference between winning the quote or losing the work to a lower bidder. Rolled threads can reduce the effects of corrosion improving life cycles in many applications. The relative ease of applying this technology to today’s metal working machines makes it even more important as a value-added component of the modern machine shop.

We can evaluate your part, your process and your machine when applying the application of thread rolling to your operation. Thread rolling was once conceived as a “black art” but is truly a straight forward application driven technology easily applied in any modern machine shop. Many military and aviation components demand rolled threads for strength and surface quality. Let us help with your application.  The cost implications can be the leverage your company needs to improve profitability.

Tap Strength

Roll Form Tapping can increase the tensile strength of an internal thread up to twice the tensile strength of a cut thread. Carbide taps will decrease the cycle time required to thread by at least a factor of two.

Fette offers a carbide nib form tap assembled on a steel shank to take advantage of the speed potential while preserving overall tap strength to tangential forces. The improved cycle times are sweetened by tap life that can be more than 10 times the life of a steel tap.

Machining Aluminum Can Be Most Cost Effective With PCD Inserts

Machining Aluminum can be most cost effective with PCD inserts, however, insert cost is very high. Most Aluminum is milled and turned with carbide inserts honed for sharpness and polished to aid in chip flow. The best of both worlds is a development by LMT Fette in Germany using a coating process defined as Plasma CVD. The coating is applied to a honed insert and increases insert life while maintaining the edge integrity. As an added benefit Milling speeds can be as high as 6000sfm and turning speeds to 10,000 sfm. Speeds significantly improve throughput and insert life can be 10 times longer than uncoated inserts. Insert cost is comparable to uncoated polished inserts.

Cold Thread Forming (Thread Rolling) Often Can Efficiently Improve Machinability.

The mechanical characteristics of many materials can make them difficult to thread using traditional cutting technologies. Chip control and work hardening can be an obstacle to efficient machining.  Cold thread forming (thread rolling) often can be used to efficiently improve machinability.

Thread rolling is especially efficient in many Nickel, Monel, Inconel, Titanium, Silicon Bronze, Copper, Brass, Copper alloys and Aluminum alloys. Typically, these materials can be threaded faster and at lower cost through rolling. The chip-less cold formed method adds strength and improves thread performance and is amendable to today’s modern production equipment. Identifying a material grade can quickly determine if rolling is a benefit both in thread cost and thread quality.

Most common thread forms UNC, UNF, UNEF, M, MF, MEF, Acme, Stub Acme and Whitworth are several thread types that are good candidates for rolling. Other forms are also capable. We can quickly identify your specific need and determine capability.

Performance of A Thread Rolling Tool

The long-term performance of a thread rolling tool is directly related to wear of the spindles, bearings, backing plate and face plate of the rolling head. Wear of any or all these components will manifest initially in pronounced taper of the finished thread. Older style heads were equipped with needle bearings while newer style heads utilized carbide bushings. The carbide helped to maintain parallel form for longer life cycle than the needles. Correcting taper generally requires replacing one or more of the contributing components. In a pinch, the straightness is achieved through preparation of a counter tapered part prep. This can squeeze additional life out of the tool without the time and cost of immediate repair or part replacement.

We inspect, evaluate and quote repair cost and time when tool performance becomes suspect and the inspection is scheduled regarding production needs.

Application of Fette EW Radial Threading Head on Hydromat Machines And Similar Rotary Transfer Machines

Roll threading with a threading head on a Hydromat transfer machine required modification of the operating station of the machine to include an operating trigger mechanism to trip the threading attachment once it was in the correct position to thread.

The latest offering from LMT Fette is a unique EW style tool that uses the part as the trip mechanism. A rotating mandril inside the head is engaged by the part and collapses the tool on itself through a 4 millimeter air gap. This compression trips the tool activates the thread rolls and resets itself once the toll retracts. The mandril accurately positions the thread location and prevents any unwanted forward movement of the part from its gripping jaws. No special mechanism is required of the machine operating station. This eliminates the need for station modifications that typically add $20,000 to the machine cost. If no part is in position to thread no compression takes place and the head is not tripped removing potential for false tripping.

Sintered carbide was discovered in the early 1920’s

What you want/need to know

Sintered carbide was discovered in the early 1920’s in Essen Germany as a resultant of a furnace explosion at the Krupp Iron and Steel Works. It was developed and played an important role in German military production during WW2. The Allied nations acquired carbide knowledge following the fall of the Hitler regime. While all carbide products offered today are sourced from the limited suppliers of the carbide powders. The recipe, tool design and coatings vary from manufacturer to manufacturer and have improved dramatically since 1950. Modern carbide products have been designed to run at speeds and feeds that will shorten cycle times and improve tool life. The biggest mistake made with carbide application is ignoring the information provided by the manufacturer. Adhering to the suggested feeds and speeds provided by each manufacturer guarantees maximum tool performance. Surface speed is the most important followed by feed rate. Depth of cut has the least impact on tool life. Ignoring the manufacturers recommendations will seriously degrade tool life and product output.