March 8, 2005
Even with all of the new advances in metal stamping technology, screws and threaded holes are still the trusted staples for metal fastening. Tapping the threaded holes has never been an easy task, however.
In the days when all tapping work was sent to outside tapping services, the problems went out the door with the parts. With the increased use of in-die tappers and in-house secondary operations cells, stampers now are responsible not only for ensuring high-quality parts, but also for creating high-quality threads in those parts.
Two types of taps are used to thread holes: cutting taps and forming taps (see Figure 1).
Two types of taps are used to thread holes: cutting taps (left) and forming taps (right).
Cutting taps create threads by removing material. Proper thread geometry of cut threads can be obtained from a range of hole sizes. The hole size for a cutting tap can be calculated accurately using the formula inFigure 2.
Forming taps create formed threads (also referred to as cold formed threads or roll formed threads) by displacing material without creating chips.
The range of acceptable hole sizes for forming taps is smaller than for cutting taps. Sometimes the difference between a hole that is too small or too large is only a few thousandths of an inch. Variables such as material properties and part geometry have a big influence on the thread percentage of formed threads.
When forming taps are used, the change in material grain flow increases the material strength, creating stronger threads. Speeds for small-diameter taps with fine pitches (20 threads per inch or more) typically can be up to twice as fast as the same size cutting tap under the same conditions.
The hole size for a cutting tap can be calculated accurately using this formula.
Forming taps work best in soft materials that are not abrasive. Typically, materials that produce stringy chips during machining processes can be threaded using forming taps.
High-strength steel, high-carbon steel, and tough stainless steel don't respond well, though, because of intense heat during thread production. The lack of flutes in most forming taps restricts easy flow of coolant to the working area of the tap. This situation of high heat and little coolant can lead to increased tap wear and poor-quality threads.
In addition, the amount of torque and horsepower required to form threads is much greater than with cutting taps of the same size, so larger machines and stronger equipment usually are needed to tap the parts.
Both forming taps and cutting taps come in numerous designs. The geometry, material, and coating of the tap are important features to be considered for each part. A tap of good design and construction that works well for one part may not be effective for another part.
Each tap is marked to indicate its tap limit, which is the amount the tap is larger than the basic pitch diameter. The number following the D on metric taps and following the H on English taps (for example, M6 X 1 D5 and 1/4-20 H5) identifies the tap limit. On English taps, it is 0.0005 in. over basic pitch diameter; on metric taps, it is 0.013 mm.
When selecting a tap for production parts, you need to consider additional secondary processes that may be done to the part after tapping. Parts that require heat treating, plating, or painting require a larger tap limit number to compensate for hole shrinkage or plate or paint buildup in the threads during those processes.
Taps are very delicate tools that are required to do large amounts of work in a very small area. Even minor hindrances can lead to poor-quality threads or worn or broken taps. Ensuring proper tap alignment with the hole will create a good condition for tapping.
If you can't maintain accurate hole locations within the part, "floating" tap holders are recommended. They compensate for tap misalignment, helping to reduce potential tap failure. Ensuring that the parts are lying properly at the tapping position will eliminate the possibility of crooked threads or broken or worn taps.
The best way to ensure high-quality threads is to start with a high-quality hole. The proper hole size is required to create strong threads, and to select the hole diameter, you'll need to calculate the percentage of thread height.
The generally accepted range for thread percentage is from 60 percent to 83-1/3 percent. If the hole size is too large, the threads will be weak. If the hole is too small, excessive tap wear or even tap breakage will occur.
It often is incorrectly assumed that extremely high thread percentages (greater than 83-1/3 percent) are better. Actually, a 100 percent thread takes up to three times the power required to make a 75 percent thread, but with only about a 5 percent gain in strength. More important, a high thread percentage produces excessive wear on taps and the machinery, increases the risk of poor-quality threads, and requires a decrease in tap speed because of increased heat and friction.
Size is not the only hole property that affects thread quality. Hole condition also makes a difference.
Burrs at the top of the hole can hinder the tap from making a clean entrance. A hole entrance that is angled or lopsided can alter the angle at which the tap enters it. This can lead to crooked threads, premature tap wear, and tap breakage.
Excessive burrs at the bottom of holes can lead to similar quality problems. These burrs have a chance of being dragged back through the hole as the tap reverses, damaging the threads. This is especially common with forming taps. The most common problem is burrs curling into the hole or bending into the threads by contact with other parts. These displaced burrs may hinder the thread gauge or screw during assembly.
Chipped or worn punches in the stamping die can lead to damaged or misshapen holes. Holes that are not round will lead to poorly gauging threads or broken or worn taps. Holes with tapered walls also cause trouble. Holes that taper to a diameter smaller than the recommended size will lead to tight threads and worn taps. Gauging the entire length of the hole with both minimum and maximum pin gauges can ensure the hole isn't tapered.
A common problem, especially in extruded holes, is the presence of excessive taper or roll at the top of the hole, making it bell-mouthed.
A common problem, especially in extruded holes, is the presence of excessive taper or roll at the top of the hole, making it bell-mouthed. When you inspect the hole during the stamping process, make sure that the taper and roll isn't too excessive. The deeper the taper or roll goes into the hole, the fewer threads there will be. This greatly reduces the threads' holding power and creates threads that may not meet strength requirements (see Figure 3).
The surface of the hole wall also affects thread quality. Scoring of the hole walls from damaged punches can lead to rough threads or worn taps. Voids in the hole wall can cause inconsistent thread quality and chipped taps. These voids also can create weak threads.
When tapping parts with long extrusions, be sure to watch for delamination of the material. Delamination inside tapped holes can lead to inconsistent thread quality and damaged taps. Eliminating this condition is especially crucial when roll forming threads.
Part Material. The part material and its properties affect the tapping process. Parts with high carbon content can be unpredictable during tapping. Holes can work-harden if they are overheated during manufacturing from situations such as worn punches, dull drills, insufficient coolant, or long extrusions. Annealing these parts may be the only solution to maintain thread quality and tap life.
The trend to make parts of prehardened steels or tougher alloys, such as stainless steels, has increased the challenge of tapping. Today's steel market also is playing a role in the quality of tapped parts. You might be unable to buy the same quality steel from which you usually make the parts. This fluctuation in material quality from run to run not only makes stamping a challenge, it makes it necessary to adjust your tapping process frequently.
Tapping Speed. Calculating tapping speed is crucial in the tapping process, and it involves numerous variables. One important variable is part material. Some materials such as aluminum can be tapped at speeds of more than 200 surface feet per minute (SFM), while tougher materials such as stainless or high-carbon steels can require tap speeds of 20 SFM or lower.
Proper tap speed is crucial for producing high-quality threads. Use this formula as a starting point to determine tap speed. Adjustments will be necessary to optimize production rates and quality.
Calculate tapping speed using the formula in Figure 4. As production begins, the tap speed can be changed to meet the requirements of the material. On a secondary tapping machine, tapping speed can be adjusted by changing the motor speed or the gears and pulleys. On in-die tappers, stamping rates may need to be reduced to slow down tapping.
Another aspect to be considered when determining tapping speed is the length of the hole to be tapped. As the lead portion of the tap that is doing the work (cutting or forming) gets deeper into the hole, the ability for coolant to reach the working area is diminished. More of the tap surface is in contact with the part in deeper holes. The increased friction creates more heat. Under these conditions of more heat and less coolant, you'll need to lower the tapping speed accordingly.
Coolant. The quality and amount of coolant used in the tapping process also affects thread quality and tap life. Choosing one requires research and test trials, because different parts may require different coolants. For instance, a heavy forming oil might work best for a larger roll formed thread, while a thinly mixed, water-soluble machining coolant might work best for a part requiring a small cutting tap.
Tapping of production parts can be very temperamental, and the process involves numerous variables. These variables often can change without notice with each production run. They can be controlled, whether the parts are being tapped in-die, in-house, or by a tapping service.
With consistent steel quality, a tightly controlled stamping process, and alert observation of the tapping procedure, you can produce high-quality threads repeatedly. You don't need to fear the tapping process if you take the time to understand it.
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