Speeds for Drilling. When drilling in automatic screw machines designed for high speeds, the best results are generally obtained by giving the drills light feeds and high peripheral velocities. High-speed steel drills are adapted to drilling Norway iron, machine steel, tool steel, etc., and ordinary carbon steel drills are used for brass and similar materials, when the surface speed does not exceed that given in the following: Feeds for Drilling. - Feeds for high-speed and ordinary carbon steel twist drills are given in the table "Feeds for Twist Drills used in Automatic Screw Machines." These feeds are for general work and can be increased in some cases. For general practice, it is more satisfactory to use rather light feeds, as a straighter hole can be produced than when the drill is forced. Drills from 3 inch to 18 inch in diameter are capable of standing the heaviest feeds in proportion to their diameter, and when a hole does not pass through the work, a 1-inch drill has been fed as much as 0.016 inch per revolution when drilling brass. Feeds as heavy as this are not recommended, because concentric holes cannot be produced when the drill is forced to such an extent. Feeds for Twist Drills Used in Automatic Screw Machines Counterboring. As a rule, more trouble is experienced in using counterbores on automatic machines than with any other cutting tool. This is probably due to the fact that counterbores are generally improperly made for the work on which they are to operate. Generally speaking, there are several reasons for the unsuccessful working of counterbores, some of which may be summed up as follows: 1. Too many cutting edges, not allowing enough chip space and also not providing for sufficient lubrication. 2. Too much cutting surface in contact with the work. 3. Insufficient clearance on the periphery of the teeth. 4. Improper location of the cutting edges relative to the center. 5. Improper method of holding the counterbore. 6. Improper grinding of the cutting edges. 7. Too weak a crosssection. 8. The use of a feed and speed in excess of what the tool will stand. For work in automatic machines, where the counterbore cannot be withdrawn when it plugs up with chips and seizes in the work, the tool should not have more than three cutting teeth. The periphery of the teeth should be backed off eccentrically, and the body of the counterbore should taper towards the back. The amount of taper generally varies from 0.020 to 0.040 inch per foot. The relation of the cutting edge to the center has an important bearing on the efficiency of the tool. For deep counterboring, where the difference between the diameter of the teat and the body of the counterbore is great, the cutting edge should never be located ahead of the center; often, if it is located a little behind the center, better results are obtained; but this rule is only general, as the material to a considerable extent governs the location of the cutting edges. It is advisable to have the cutting edge ahead of the center when the counterbore is to be used as a facing tool, or for counterboring brass, provided it is not required to enter the work to a depth greater than its diameter. For general work, the cutting edges should be radial. Straight flutes are suitable for either brass or steel, but for steel, it is better to have the teeth cut spirally, the spiral being sufficient to give a rake of from 10 to 15 degrees. If the difference between the diameter of the pilot and the body of the counterbore is not very great, and if the counterbore must extend into the work to a depth greater than its diameter, the cutting edge should be back of the center, that is, to the rear of the radial line parallel to the cutting face. When the counterbore has to remove considerable material or enter the work to a depth greater than its diameter, it is generally advisable to rough out the hole to the diameter of the body of the counterbore with a three-fluted drill; then the counterbore is used only for squaring up the bottom of the hole. This method is especially advisable when counterboring machine or tool steel. Speeds for Counterbores. The surface speed at which a counterbore can be worked is slightly less than the surface speed used for drilling. The surface speeds given below are recommended for counterbores made from carbon and high-speed steel. Feeds for Counterbores. — The method of holding a counterbore when applying it to the work, and the strength of the cross-section in proportion to the width of the chip being removed, governs, to a considerable extent, the amount of feed. The material being cut and the depth to which the counterbore penetrates into the work also have an important bearing on the rate of feed. These conditions should be taken into consideration when using the feeds given in the table "Feeds for Counterboring in Automatic Screw Machines." These feeds are for counterbores having three cutting edges; if there is but one cutting edge, the feed should be decreased from 40 to 50 per cent, and for two cutting edges, from 15 to 20 per cent. The feeds given in this table apply only when the counterbore penetrates from one-half to three-quarters of its diameter into the work. When the counterbore penetrates to a greater distance, the feed should be decreased from 15 to 25 per cent. It is good practice to always drop the counterbore back after it has penetrated to a depth equal to half its diameter, to remove the chips and to cool and lubricate it. Feeds for Counterboring in Automatic Screw Machines Reaming. When reaming holes in automatic screw machines, it is advisable not to leave any more material to be removed by the reamer than is absolutely necessary. For general work, the following allowances will give good results for reamers ranging in diameter from 1% to 3% inch. For reamers over % inch diameter, a drill 64 inch less in diameter is generally used; this would leave from 0.012 to 0.015 inch to remove, as the drill will cut slightly larger than its nominal size. Various reasons for the inefficient working of reamers are as follows: 1. Chattering, which results when the teeth are evenly spaced. 2. Chips clinging to the teeth, owing to high peripheral velocities and insufficient clearance. 3. Enlarged and tapered holes, due to holding the reamer rigidly instead of "floating." The floating type of holder should always be used when reaming deep holes. There are various methods adopted to prevent reamers from chattering, but the unequal spacing of the teeth has been found the most satisfactory and inexpensive. For machine reamers varying from 1% to 4 inch, three cutting edges are sometimes used, but the difficulty of measuring the diameter limits their use. As a general rule, four and six cutting edges are used on reamers varying from 8 inch to % inch, and eight to twelve cutting edges on reamers varying from % inch to % inch. Speeds for Reaming. - The surface speeds used for reaming should be slightly less than for counterboring, as the reamer generally penetrates to a greater depth and has more cutting edges in contact with the work. When a good supply of lard oil is used, the following surface speeds will be found satisfactory: Feeds for Reaming. - The feeds for reaming, given in the table "Feeds for Reaming in Automatic Screw Machines" will be found suitable when the amount of material removed does not exceed that given in the preceding paragraph on "Reaming." When reaming thin tubing, especially of brass, the feed should be decreased somewhat. -- Speeds for Recessing Tools. - The surface speeds of recessing tools can be slightly greater than those used for counterboring, on account of the light feeds and small amount of cutting surface in contact with the work. As a rule, the following surface speeds can be used with satisfactory results: Box-tool Cutters. - Box-tool cutters are applied to the work either radially, as shown at A and B (see accompanying illustration), or tangentially, as at C and D. Generally, in automatic screw machine practice, the cutter is set radially for turning brass and, when held in this way, the cutting angles are approximately as given in the illustration. Tool A is for roughing and tool B for finishing, the cutting face of the latter being ground parallel for a short distance y equal to approximately one-fifth of the diameter being turned. For steel turning, the cutter should be set tangentially to the work as shown at C and D. The end of tool C should be ground to approximately the following angles: The form of tool shown at C is commonly used for roughing cuts, but will not produce an absolutely square shoulder. For finishing cuts, the tool is ground as shown at D, which produces a square shoulder. The cutting angles for tool D are as follows: While the cutting face on the tool shown at D is straight, it is usually advisable, especially when cutting machine steel and Norway iron, to give more "lip" to the tool as shown by the dotted line h. The cutting edge of a radial cutter for rough turning brass rod is set above the horizontal center-line of the work, an amount |