Taps for Lead Screw Thread Forms - General Information

About this Section

For customers who wish to manufacture their own special nuts for use on Roton supplied Acme or Hi – lead® screw stock, Roton offers a complete line of taps. This section will alert you to an alternative to tapping your own nuts, explain the benefits of procuring your taps through Roton, give you a guide to submitting a tap inquiry, and provide you with a checklist for successful tapping.

Alternative to Tapping Nuts

Many users consider tapping their own nuts because they cannot find a standard nut that fits their exact specifications. Most often, they do not find a nut with the outer dimensions they require, or they require a nut that is longer or shorter than standard. Before purchasing a tap you should seriously consider using a standard sleeve nut either as an insert for a custom form, or by modifying the walls to suit your purposes. There are several advantages to this. First, it is almost always less costly. Power transmission thread taps are more expensive than nuts, usually many times more. Second, it is much easier to modify the OD of a standard sleeve nut, or turn it down and use it as an insert into a custom form than it is to tap a custom nut. Third, if a standard nut is used as an insert, when the nut wears out, the user only needs to replace an inexpensive nut. If an entire custom nut must be replaced, the replacement cost will be high, and delivery times will be much greater. For those times when a standard nut is not an option, purchasing a tap and tapping your own nuts is the best alternative.

Purchasing Taps

Purchasing your taps through Roton will assure proper fit-up of the screws and nuts.

With over 25 years experience in tapping power transmission threads, Roton engineers are well qualified to provide the optimum tandem or conventional tap for your application. All Acme and Hi-lead® taps are engineered for each specific application guaranteeing improved performance and tool life over imported or domestic “off-the-rack” taps.

To inquire on taps, please provide the following information:

• The complete thread details including nominal size, limits of size or class of fit, pitch or TPI, number of starts, right or left hand, etc.
• Material to be tapped. Avoid using materials that are difficult to machine.
• Length of the thru hole (avoid blind threaded holes, they are very difficult to tap). Thru holes greater than twice the OD of the mating thread should be avoided. If an extra long nut has been specified to improve axial support, consider counter boring a portion of the nut to the OD of the screw thread. (See Figure 12) Extra long thru holes are difficult to tap, may require a special and more expensive tap, and are likely to present fit-up problems.
• Direction of tapping (vertical or horizontal).
• Type of tap feed (lead screw or manual feed).

We recommend using tandem style taps for tapping power transmission thread forms. This style of tap has both a roughing and a finishing section (see Figure 13). This allows the user to tap a nut in a single pass. Tandem Acme and Hi-lead® taps dimensioned on the backside may be used for nuts with a maximum thread length of twice the nominal diameter and for nut materials of brass, bronze, mild steel and cast iron. For longer nuts or nuts made from alloy steel, stainless steel or other alloys with low machinability, special taps may be required. For shorter nuts and nuts made from highly machinable materials, Acme Hand Taps may suffice. Consult with Roton Applications Engineering. ISO Trapezoidal (“metric Acme”) tandem taps are also available with standard or metric shanks and squares.

Advice for Tapping Nuts

For tapping power screw threads, best results are obtained using a vertical, lead screw set-up. This promotes good chip removal and coolant flow. The tap should be passed through the nut after tapping and not backed out by reversing rotation. Engine lathe and turret lathe tapping can be successful with good workmanship, close alignment of work and tap, and attention to proper coolant flow.

Checklist for successful Acme tapping with Tandem style, single pass taps:

• Are the hole in the work piece blank and the tap properly aligned? Check the run out of the work piece bore and tap with dial indicators to assure alignment within a few thousandths.
• Does the tap have a good close fit with the tap holder? Do not use floating type holders or worn, loose fitting solid holders that allow the tap to move off its axis.
• Is the work piece chucked firmly enough to resist rotational and axial tapping forces? Acme thread tapping forces are 3-4 times greater than Unified tapping forces for threads of the same major diameter. Frequently, work pieces need to be backed-up as well as chucked on the outside surface in order to prevent slipping.
• Does the work piece have the proper hole size? An undersized hole can break the tap. Plug gaging each work piece prior to chucking is a good practice.
• Is coolant flow sufficient? Work piece bore should be flooded with coolant during the tapping operation. A tap that exits the work piece dry and covered with chips is an indication that coolant flow is inadequate. Inadequate coolant will result in poor size control and poor tap life.
• If tapping with a used tap, check for dullness or chipped teeth. Use of a dull or chipped tap may break the tap or produce poor threads. Have the tap reground at the first sign of reduced cutting strength. Badly worn taps should be replaced with new taps.
• Has the tap been designed and built to cut the material and the length of hole you are tapping? (Note: Off the shelf taps built for “general purpose” use will not perform as well as engineered taps built for a specific job.)

Warning: Taps can shatter and break. Wear eye protection and use other necessary and adequate safe-guards.