IS-13A

THE FORGOTTEN ART OF SKIVING

Skiving is one of the oldest and most efficient methods for producing certain types of parts on screw machines: Parts that are long and slender, parts with close diameter tolerances and finishes, and parts requiring truly spherical radii. Yet most layout men, set-up men, and operators seem to go to any lengths to avoid skiving.

This is probably due to a lack of knowledge and familiarity with the principles of successful skiving, including applications, tool design and manufacture, and the unavailability (until now) of a tool holder with all the features necessary for simple, dependable operation. Skiving is a basically simple operation. Once a few principles are understood, any screw machine plant should be able to run skiving jobs routinely with few, if any problems.

What is Skiving?

Conventional form tools are mounted so that the formed cutting edge of the tool is on the centerline of the part and cuts radially. Cutting action is determined by the centerline of the part and cuts radially. Cutting action is determined by the combination of radial clearance angle and top rake angle, as shown in Figure 1. Diameters are controlled by advancing the forming tool towards the center of the part.

But difficulty is encountered in forming long parts because the entire form contacts the work piece at one time. Therefore, the smallest diameter is formed to its finished size at the same time as all the other diameters are being formed, causing the part to break off prematurely.

On the other hand, skive tools are mounted so that the formed cutting edge of the tool is advanced into the work piece below center and cuts tangentially. Cutting action is determined by the combination of shear angle and lead angle, as shown in Figure 2. Part diameters are controlled by raising the tool towards the center, and the form is ground along the full length. The cutting edge is obtained by grinding the "shear" angle on the end of the tool and the "lead" angle across the width of the form.

Since only that part of the tool that is in contact with the work piece at any given time is actually cutting, the part is not weakened until that portion of the tool which forms the smallest diameter of the part actually passes below the centerline. Furthermore, as any portion of the tool passes under the centerline, that diameter is completely formed to size and the tool exerts no further cutting pressure on that area of the part. This means that all cutting action takes place in the area from where the tool contacts the part until it passes under center. This area is shown as X-X in Figures 3 and 4. Figures 3A, 3B, and 3C show what happens as the tool is advanced past center.

Part 2 below

The shear angle affects the cutting action. But, unlike the top rake on form tools, it does not affect the part diameter relationship; this angle can be varied until the best possible cutting action is obtained. Too blunt an angle will tend to cut hard and deflect the part, causing dimensional errors and poor finishes. Too steep an angle causes the cutting edge to burn out prematurely, again affecting diameters and finish.

It has been our experience that approximately a 20° shear angle and a 20-30° lead angle are usual starting points: A little experimentation can produce surprisingly different results in finishes, tolerances, cutting action, and cycle times and is well worth the effort.

Once these principles are understood, it is easy to see how the lead and shear angle can be varied to suit the configuration of the part so as not to weaken it prematurely. It's important to keep the lead angle as short as practical, since the steeper the angle, the more throw (and cycle time) is required. See Figures 4A and 4B.

Advantages of Skiving

Since skiving is a freer cutting operation, feed rates can be increased approximately 2-3 times for ferrous and difficult-to-machine materials. This offsets the increased throw time required because of the lead angle. At times it is possible to grind a double angle to reduce throw as shown by dotted lines in Figure 5.

Another advantage of skiving is that, since skive tools cut tangentially instead of radially, step differentials remain unchanged and angles do not have to be corrected, simplifying tool design. Since diametral corrections are not required (as in conventional tools) skiving is about the only way a perfectly spherical radius can be formed on a work piece. Diametral corrections flatten radii into ellipses which are just about impossible to produce with conventional radius dressing equipment.

In addition to being used for long parts, skiving is used successfully for parts requiring close diameter tolerances or finishes. Much closer tolerances can be maintained because part diameters are controlled by raising or lowering the skive tool, as in a shaving operation, instead of feeding the tool against a stop. For the same reason, tool wear does not affect diameter dimensions directly (except through poor cutting action).

Better finishes are also obtained with skiving. As the formed portion of the tool continues to advance under the center of the part, it produces a shaving or burnishing action of the part. Carbide tipped tools are particularly recommended where very good finishes are required, since carbide doesn't gall or weld as high speed steel might.

Although a certain amount of this burnishing action is desirable, too much drag can cause the part to spring or deflect. Therefore, most skive tool holders are built with a 1/4 degree maximum backtaper to prevent excess rubbing as the tool passes under the work piece. However, while eliminating drag, this backtaper causes tapers in the part. In combination with the lead angle, it causes the point at which the tool passes the center of the part to drop away from center, thus increasing part diameter.

The amount of taper produced can be calculated as follows:

Width of skive tool X Tan of lead Angle X Tan of backtaper angle (usually 0 deg. 15') = Taper per side. These tapers have, in the past been overcome by either packing up the holder by this calculated amount or by grinding an offsetting taper on the bottom of the skive tool itself. Both of these methods are hit or miss and a cause of many operating problems. (Tapers are also occasionally caused by deflection of the part, but this can be overcome by use of an end support on the part.)

Another difficulty encountered has been that most holders are not designed to accommodate extra wide tools and, therefore, necessitate the grinding of "T" or "L" shape shanks on the tools. This shortens resharpenable tool life, increasing the cost of the tools. It also causes rigidity problems in many instances. A further difficulty has been that the depth of form in the tool is limited because of a too-small distance between the center of the spindle and the lowest adjustment of the taper wedge in the tool holder.

Somma Skive Tools

With all these problems in mind, Somma introduced a new skive tool holder to fit the Brown & Sharpe and other machines. The most unique feature of this tool is a rocker-type taper wedge that can be adjusted to offset part tapers and locked in that position.

Other features include: Extra wide tool openings to eliminate cutting down of shanks of skive tools; greater distance from center of spindle to lowest adjustment point of taper wedge to accommodate larger part diameters; rugged construction to eliminate chattering; available for rear slide for forward rotation or front slide for those jobs requiring left hand rotation; 1/4° maximum back taper built into the tools to prevent part deflection.

In our many years of supplying tooling for the screw machine industry, we have seen all types of parts skived, such as ball point pencil tips, hypodermic needle hubs, ornamental lame finials, and fireplace andiron parts, as well as ball-type fittings and parts requiring close tolerances or exceptional finishes. We hope that the preceding information will influence you to join the ranks of the companies that are producing unusual parts efficiently and economically by skiving.

Our master catalog lists available skive tools. If further information is needed, please do not hesitate to contact us here in Waterbury.