The Unified National Miniature Thread Series (miniature metric thread) was accepted as a new thread series in 1958 and was intended for general-purpose use in instruments, miniature mechanisms, and for jewelry and watches. The use of this series is recommended for all new products in place of the old standard miniature and the many improvised thread sizes still in existence. We will continue to supply the ongoing demand for the old standard miniature screw threads.
The UNM thread series has fourteen miniature metric thread sizes that are systematically distributed from .30mm to 1.40mm in diameter and supplements the Unified and American Thread Series that begins at .060 inch diameter. The diameter-pitch combinations provide for maximum strength against stripping and for excellent manufacturability and interchangeability. Technical data contained in this site is taken from Fed-Std-H28/5 and ASME B1.10M-1997.
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Newman Tools offers thread cutting tooling in a wide range of sizes for the main two standard miniature thread series.
Miniature Taps
Taps are available in either a special alloy high carbon tool steel or high speed steel. The heat treatment of high carbon, tungsten-vanadium tool steel provides for 25% to 30% greater torsion toughness than high-speed steel. They are effective and efficient even with some tough materials, but for high nickel and chromium alloys high-speed steel is recommended. Our standard taps are a 3 flute plug tap or a roll/form style. To prevent excessive breakage, careful alignment of the tap and the hole is essential. Extreme care must be exercised when using small taps, always rotating forward only a fraction of a turn and then backing off to break and clear the chip.
Special tap coatings are available. Please contact us.
Not Available in Form Taps
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We are currently experimenting with roll dies for miniature threads that form an external thread similar to forming an internal thread with a roll tap. Our initial tests on a CNC chucker and CNC Swiss screw machine have been quite impressive. Benefits include significant time savings over single pointing, a mechanically superior thread, and long tool life. If you have interest in this product and process, please contact us and we will be pleased to help you determine if this technique might be an advantage in your application.
Ring and Plug Gages The FED-STD-H28/5 and ASME B1.10M-1997 provide the dimensions and limits for the Unified National Miniature Thread Series but no gaging system is specified. It is suggested that external threads be checked by optical projection. For this method to be satisfactory, it is important to use magnification of at least 100X’s or greater particularly for the smaller sizes in the series. This method is slow, requires accurate enlarged charts and an optical comparator, and does not lend itself to production. Following the principle of gaging used for larger threads, we have adopted simplified ring and plug working gages which have proven highly acceptable in the industry. The ring gages are threaded with special very accurate taps made to the limits of the external threads specified in the H28 Handbook.
Miniature Thread Ring Gages
The ring gages are made from special alloy high carbon tool steel and heat-treated. The ring gages are not tempered since they are not subject to breakage, allowing for long lasting service. Typically the ring gages are double mounted into an aluminum holder with the No Go member on one end and the Go member on the other end. The size and respective pitch diameters are stamped on the holder. The overall length of the gage set is 1.25 inches and .25 inches in diameter.
Miniature Thread Plug Gages
The Plug gages are essentially very accurate taps (without flutes) which are manufactured to the limits of the internal threads as specified in the H28 Standard. The Plug gages are also made from a special alloy high carbon tool steel, heat treated and tempered. The Plug gages are obviously delicate, and to help reduce breakage they are single mounted into small diameter aluminum handles. The handle is identified with Go or No Go, the size, and the pitch diameter of the gage. The overall length of the gage is approximately 1.56 inches and .12 inches in diameter.
Tapping Tools
Newman Tools supplies various ways to hold and use our precision taps. Both of the tools described below will work well and quickly pay for themselves by reducing tap breakage.
Newman Tools offers a Tapping Tool for use with miniature taps. This tool will help ensure good alignment thus reducing tap breakage and can be used in the turret of a small lathe or mounted in the spindle of a vertical mill or drill press. This tool is available with two spindles 1.0mm and 1.5mm.
The PLAIN or the RELEASING Die Holders can quickly and easily be converted to a pin vise with pin vise collets. These collets are available in two sizes 1.0mm and 1.5mm providing the ability to convert the die holders to a tapping tool holder. The pin vise collets are made with 3 grooves like the acorn dies to provide for the required accuracy and spring action. They are hardened, tempered, and polished and identified as "VISE".
The Die Holder
The die holder is for use with dies only. The die is adjusted by tightening an adjusting nut often referred to as the nose cone thereby closing the dies 3 cutting segments equally. By repeatedly adjusting a small amount and taking a cut and checking with a ring gage, the correct size can be obtained. When the exact size is achieved, the split brass ring located just behind the nose cone should be rotated so the slot is aligned with the reference line on the nose cone. This allows the die to be removed from the holder for cleaning or removal of a broken screw and replaced in correct adjustment without the necessity of making additional trial cuts. Two styles of die holders are available, the PLAIN die holder and the RELEASING die holder.
The Plain die holder measures 1.75 inches with a barrel diameter of .312 inches. The hole in the barrel permits a .180 diameter rod to support the holder in a tailstock for easy setup alignment.
Because these small taps are fragile, it is recommended that 75% thread be adopted only for soft materials and short threaded holes. As the length of the hole increases, the percentage of thread should be decreased. The depth of the hole to be threaded (*) as a function of the major diameter is provided in three groups and each group has a minimum and maximum for tap drill size. The minimum diameter should be used for soft and ductile materials and the maximum drill diameter for harder and more difficult materials. In many cases the hole diameter does not correspond to a numbered drill, so the closest millimeter drill should be selected.
The tap drill sizes shown for form taps are only a recommendation as a starting point. Depending on the material being tapped and the depth of the tapped hole, the hole size may have to be slightly increased or decreased to obtain good thread form and good tap life.
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