The Hartness Automatic Die, shown herewith, is supplied in three sizes, viz.: No. 1, for cutting screw threads from 3/32 inch to 1/2 inch in diameter; No. 3, for screw threads from 1/4 inch to 1 1/4 inches in diameter; No. 6, for screw threads from 1 inch to 2 inches in diameter, and No. 9 for screw threads from 1 1/2 to 3 inches diameter.
Right or left-hand chasers are supplied as required for cutting United States Standard, Whitworth Standard, V, Acme and pipe threads; also, the various fine threads in customary use. It was designed expressly for the Plat Turret Lathe, but may be used in any of the existing screw machines or turret lathes by change of shank.
It opens automatically when the travel of its holder or shank is retarded.
The cam for controlling the chasers takes bearing directly over and very close to the cutting strains, hence there is no chance for the chaser to get away from its work by canting or tipping. This insures straight work, which has seldom been done by other forms of automatic dies. The connection between the shank and the body of the die is a double universal joint allowing the die to assume any position required by the work. This connection remains perfectly flexible under the greatest torsional strain of cutting, and provides a
Hartness Automatic Die and Its Parts.
A Group of Hartness Automatic Dies.
compensation for the slight but important change of alignment that takes place in all turret machines as soon as a die begins to cut.
The latch pin which holds the cam in close adjustment is provided with two latch surfaces, one for a roughing cut and the other for a finishing cut. Turning the latch half way around changes it from one to the other without disturbing the principal adjustment for size. With this feature smooth screw threads can be cut when the lead is very coarse. It is seldom used on standard threads below 1 inch in diameter.
Every part of the die is made either from open-hearth or tool steel, the lathe work being done exclusively on the Flat Turret Lathe, and all other operations by special machinery. It is perfectly interchangeable throughout.
The process of forming the chaser teeth is such that the front or working teeth have an ideal cutting clearance, while the back teeth have no clearance, but instead take bearing on the work a trifle back of the face of the chaser, forming substantially a lead nut which rides on the thread produced by the front teeth, thus governing the lead of the screw.
These chaser teeth are formed by special milling machines provided with means for recording to a nicety all angles and positions of approach of work to cutters, so that an absolute knowledge of the clearance and contact of each tooth is possessed. Each chaser is milled separately, insuring a perfect interchangeability.
The milling cutters used are 2 1/2 inches in diameter, regardless of the size of the screw to be cut by the chasers. These cutters are formed in backing-off lathes and possess an ideal clearance. When in use the faces of their cutting teeth are ground frequently, thus maintaining the correct degree of rake and a keen cutting edge; they take a clean cut without any of the burnishing or rubbing action which always accompanies the nobbing or tapping of dies. The importance of this feature is appreciated after the dies have returned from the hardening process. Since the metal in the chaser teeth has been undisturbed by the cutting process, and only the extreme edge hardened, leaving the soft back very near to the edge, no appreciable change of form takes place.
In the process of hardening other dies the compressed or burnished metal - which has been squeezed into shape by the hobbing or tapping action - is quick to assume a more natural position, and this results in a distorted die.
Our method does not depend on the accuracy of the lead screw of a lathe in which hobs, taps and mills for producing dies are made, neither are we affected by the change in hardening of such tools.
All other methods have at least the errors of two hardenings and one lead screw. We correct in the milling machine all errors excepting the final hardening of the chaser, which takes place under such ideal conditions that we cut a practically perfect screw.
The error in lead is less than 1/64 in 18 inches in screws of standard pitch, and when cutting threads of fine pitches, a proportionate accuracy of lead is maintained.
To obtain a full appreciation of the comparative minuteness of this error it is only necessary to measure with a good scale the lead of the best taps on the market, the lead screws of engine lathes and the screws cut by other dies, any of which will show errors from four to ten times as great.
In view of these facts we consider our die practically perfect in its lead-controlling features. If greater accuracy of lead is required than that found in our regular stock chasers, we are prepared to furnish, at a special price, chasers having lead errors not exceeding 3/1000 of an inch in 18 inches.