US2972287A - Milling of metals subject to galling - Google Patents

Description

' Feb. 21, 1961 s. E. RUSINOFF 2,972,237

MILLING OF METALS SUBJECT To GALLING Filed April 24, 1957 INVENTORz MUEL E. RUSINOFF ATT'YS MILLING F METALS SUBJECT TO GALLING Samuel E. Rusinolf, Chicago, Ill., assignor to Walter G. See, Crown Point, Ind.

Filed Apr. 24, 1957, Ser. No. 654,772

1 Claim. (Cl. 90-11) This invention, in general, relates to milling of difilcultly machinable metals subject to galling. More particularly, the invention pertains to peripheral milling procedures wherein the maximum rate of milling of metals subject to galling is substantially higher than that attainable according to practices heretofore known.

Titanium and zirconium and alloys thereof are relatively new as standard materials of construction, and titanium is more and more becoming accepted by industry as a standard material of construction. The tonnage of titanium and alloys thereof going into machine parts is increasing rapidly. The milling of titanium and zirconium, or alloys containing these metals, introduces special problems because of their unique characteristics, and the general procedures employed for milling various steels and other ferrous metals have not proven to be satisfactory with respect to the machining of zirconium and titanium or alloys thereof which are subject to galling.

It has been heretofore recommended in the machining of titanium that the carbon content of the machined titanium be kept at a level below about 0.20%. Above this level hard carbides are formed and reduce tool life markedly. The employment of generous depth of cuts to prevent riding of the tools on the work and excessive tool wear were further recommended because 'of titaniums galling tendency and its rate of work hardening. Also, it was suggested that adequate cooling of the tools be provided because of undue localized heating due to the low thermal conductivity of the titanium, and the low.

thermal conductivity, combined with the galling or adhen'ng tendencies of the machined titanium to stick on the tools, the workpiece or both, led to the recommendation of the use of 'slow cutter speeds and heavy cuts. Also, scaling occurs by the formation of titanium oxide and titanium nitride on the surface of the titanium metal to be machined. Titanium oxide and titanium nitride are among the hardest materials known and should be removed by either chemical or mechanical means prior to machining. While I am in substantial agreement with the foregoing recommendations and precautions, I have discovered that titanium can be milled to a satisfactory finish at much higher speeds than heretofore practiced or known by following the procedures hereinafter outlined.

Zirconium and alloys of zirconium, like titanium, are metals of unusual properties, and zirconium and its alloys similarly have a severe tendency to adhere or gall on the tools, workpiece or both. Although the work hardening of zirconium is low as compared to that of titanium, zirconium and its alloys are definitely abrasive toward cutting tools. This abrasive quality has been attributed to the tendency of zirconium to alloy readily at temperatures developed in machining-actually dissolving the tool material. Again, like titanium, zirconium and its alloys, particularly when hot worked, have a tendency to form a tightly adherent encrustation or scale of zirconium oxide or zirconium nitride, both of which are highly abrasive. Zirconium and its alloys, like tiferrous metals.

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tanium, .have a low'modulus of elasticity-accounting for the tendency of the workpiece to deflect during machiming-and the workpiece of these metals should be backed up to prevent deflection.

The machining of titanium, zirconium and their alloys introduces one additional problemthe finely-divided metal is highly pyrophoric and presents a serious fire hazard. For this reason it has been recommended that shallow cuts and fine feeds be avoided except at very slow'speed. In general, zirconium and its alloys are not quite as ditficult to machine as titanium and its alloys, and the general machining characteristics of zirconium and its alloys have been placed between those of aluminum and titanium.

The milling of titanium and zirconium, as Well as alloys of these metals, with standard peripheral milling cutters having relatively few cutting teeth or blades equally spaced around the periphery of a cylindrical holder can be used to successfully mill these metals only if the peripheral speed of the rotating cutter, often designated surface feet per minute (s.f.m.), is kept below an extremely low maximum, of which 65' s.f.m. is representative for a finished surface and about 160 s.f.m for a semi-finished surface. In contrast with these prior artpractices, the rate of revolution of the cutter can be raised in accordance with the practices of the instant invention, to give values approaching 4,000 s.f.m. by the use of milling cutters having certain predetermined characternique hereinafter outlined.

Heat-resistant nickel alloys containing about 25% orv more nickel and little or no iron are further examples of metals which are difiicult to mill vbecause of their galling tendencies. Examples of these alloys are Monel metal; Illium; Inconel; Hastelloy A, B, C, and D; and Waspalloy.. The latter is an alloy of about 50-60% Ni, 12-15% Co, 18-21% Cr, 3.55%,Mo, a maximum of 2% Fe, 1l.5% Al, 2. -3.25% Ti, and minor amounts of C, Si, Mn, S, Cu, B, and Zr.

It is, therefore, an object of the present invention to provide processes for milling of metals subject to galling at rates of peripheral speed of the cutting tool materially greater than those heretofore attainable.

- Another object of the invention is to provide improved.

in attaining high rates of peripheral milling of metals subject to galling comprises two basic factors, the design of the milling cutter and the technique employed in milling these metals. The invention is described in detail hereinafter in conjunction with the drawing in which:

Fig. 1 is an elevation of a straight-tooth slab mill in cutting relationship with a workpiece; and

Fig. is an enlarged view of a portion of a milling cutter, Illustrating the details of the mounting of the cutter teeth of a preferred embodiment of cutters utilized in the present invention.

Itis important in the practice of the present invention that the peripheral milling cutters be designed so that at leasttwo; cutting teeth-preferably three or four, are in contact with the workpiece at'all times during the cutting thereof. To achieve this function slab mills used in the milling procedures according to the present invention will have a large number of teeth as compared to usual milling cutter design in the machining of steels and other The cutter illustrated in Figure l is an ordinary straight tooth slab mill, often called a plain milling cutter. In

the embodiment illustrated the slab mill comprises a steel blank or blade holder 10 mounted on a driven shaft alloys of aluminum and zinc.

amass? or arbor 12 in rotatable relationship therewith by means of the key and slot connection 14. Cutter teeth 16, which may be made from high speed steels or nonferrous hard materials such as tungsten carbide, titanium carbide, other carbides, ceramic'or oxide, cutting materials such as sinter ed aluminum oxide, borides, nitrides and silicides, are secured in corresponding slots in the blank or holder around the periphery thereof. Monolithic cutters may be used in some instances. The cutter teeth 16 are mounted so as to protrude a slight distance beyond the periphery o f the blank or holder 10 and are ground to give an angular relief surface 18. A small land may be provided, if desired. The relief angle is ordinarily '5-8".

The milling cutters with which the present invention is primarily concerned are peripheral milling cutters opcrating with the milled surface of the workpiece parallel to the cutter axis, such as a slab. mill, and having a large number of teeth distributed about the cylindrical periphery. They are designed with a pitch such that more than one tooth is contacting the work at any given interval during "the milling operation. The pitch" is a linear measure of the distance between corresponding points on the outer edge of adjacent teeth (see Fig. 2). The milling cutters used in the present invention have a fine pitch generally falling between about A; to /2 inch. The teeth may have a helical configuration in which the helix angle does not exceed 45. The helix angle is the maximum angle between a tangent of the helical curve and the longitudinal axis of the milling cutter. The cutting teeth 16 may have a minimum thickness of ,6, and an overhang (see Fig. 2) between about to A2", preferably a to /3" for carbide teeth and other similar teeth which are brittle in nature. The overhang for high speed steel cutting teeth may range between about ,4 to /4, preferably f to A". The tool may be designed with a zero radial rake angle of the cutting teeth, as shown in the drawing, or, if desired, a positive or negative radial rake angle may be provided.

With some types of teeth, e.g., carbide teeth, the use of shims to cushion induced and forced vibrations in the cutter teeth is recommended-particularly where the teeth are somewhat brittle and subject to fracture from the vibrations set up during milling. Referring to Fig. 2, shims 22 of metals having melting points and moduli of elasticity within a predetermined range, hereinafter defined, are fitted snugly between the walls of the slots in holder 10 and the car bide cutter blades 16 and bonded by brazing alloys or other nonferrous filler metals to theblades and the slots. The melting point of the metal shim should be at least 700 Rand usually does not exceed about 2000 F. The range of the modulus of elasticity is from 6X10 to 20x10. Examples of suitable metals within the aforesaid predetermined limits are zinc,

which has a modulus of elasticity of 6X10 and a melting point of 787 F., copper, which has a modulus of elasticity from 13 to 17x19 and a melting point of 1980" F., silver, which has a modulus of elastivity of 10.3 X10 and a melting point around 1700 F, gold, which has a modulus of elasticity of 11.3)(10 and a melting point around 1950*- F., aluminum, which has a modulus of elasticity of 10x 10 and a melting point around 1220 F., and alloys of these metals and other metals having moduli of elasticity within the. aforesaid range and a melting point, of at least 700 F., including brass (alloys of copper and zinc), bronze (alloys of copper and tin), alloys of silver and copper, alloys of aluminum and copper and I Instead of a nonferrous filler metal, the shim itself, particularly in the case of shimmetals of relatively low melting point, may be used to bond the cutter blades in the holder by heating the assembled cutter above the melting point of the shim so long as the component parts of the cutter can Withstand the heat necessary to melt the shim metal suiiiciently to bond it to the holder and cutter blades. The employment of a nonferrous filler metal is to be preferred, how- 4 ever, to obtain the strongest union between the component parts.

The choice of the nonferrous filler metal is largely determined by the temperatures which the cutter blade holder 10 and the cutter blades 16 can withstand in order to secure the melting of the nonferrous filler metal. Typical nonferrous brazing metals are silver alloys, having melting points between about 1125 and 1600 F.; bronze or brass alloys, having melting points in the approximate range of 1600-l900 F. and pure copper with a melting point at about 1980 In the manufacture of inserted-blade milling cutters with carbide blades, silver alloy brazing, often called hard soldering or silver soldering, preferably is used. Silver brazing alloys contain amounts of silver varying from 5-80% and other elements such as copper, zinc, cadmium and phosphorus. Some typical silver brazing alloys are described in Table -1 page 1168, of the Tool Engineers Handbook (1949), The silver alloys are preferred because of their lower melting points. It is to be recognized, however, that in instances where the blade holder 10 and blades 16 can withstand the temperatures necessary to secure brazing, the higher melting bronze or brass alloys or pure copper may be utilized as the brazing compound. In some instances, certain soft solder alloys such as tin-lead alloys, tin-leadeantimony alloys, and silver-leadv alloys may be used. Some of these soft solders are described in Table 79-1, page 1162, of the aforesaid Tool Engineers Handbook. For most purposes, brazing is preferred because of strength factors and a lesser likelihood of melting of the alloy when the cutting tool becomes heated during use.

When, for example, copper shims are employed, the procedure for assembling the milling cutter is as follows. Before the cutter blades 16 and copper shims 22 are placed in the slots, the preferred procedure is to coat with a nonferrous filler metal, preferably having a melting point above 700 F., the contacting surfacesthe walls of the slots, the copper shims 22, and the cutter blades 16. The copper shims, preferably of high purity copper, are then fitted in each slot on each side and at the bottom, as shown in Fig. 1'. it is important that the blades and shims be fitted snugly into the slots with out anyplay between the component elements. The assembled cutter holder or blank with the blades and shims therein is then placed in a furnace at a temperature suflicient to heat the assembly above the melting point of the nonferrous filler metal to secure uniontof the various parts. The union is that of the copper shims with the walls of the slots and also the cutter blades obtained by the melting and subsequent solidification of a nonferrous filler'rnetal at a temperature below the meltingpoint of the component parts.

The blades of the cutter are then ground to provide inclined relief surfaces 13 having a clearance angle preferably between 5 and 7 and slight hands 20, if desired. The same procedure is followed with other shim metals.

The following milling techniques, according to the practices of the instant invention, should be observed. The peripheral milling workpiece feed may be, either conventional (Fig. 1), or climb milling may be employed. The minimum chip load should be 0.002", preferably 0.003. t chip loads, lower than 0.002, there is excessive wear of the cutter teeth. Thechip load preferably is not higher than 0.005; (Chip load is a measure of the linear distance of travel ofthe, workpiece, per tooth of cutter. It is determined by dividing the rate of metal removal by the product of the number of cutter teeth on the milling cutter and the revolutionsper minute of the milling cutter. The rate of metal removal is a measure of the linear movement; of the workpiece past the cutter and is designated in inches per minute (i.p.m.). The depth of cut may vary widely. The. minimum depth of the cut recommended is 0:001. The maximum depth of cut is limited by the'power available in the milling machine. If desired, a coolant such as cutting fluids, particularly sulfurized cutting oils, may be employed.

By following the foregoing recommendations as to milling techniques and cutter design, metals subject to galling can be milled at high cutting tool speed. The surface speed of the cutting teeth can be increased manyfold over practices heretofore employed-reaching values of 4,000 and higher surface feet per minute (s.f.m.). Surface feet per minute is the product of the revolutions per minute of the milling cutter and the circumference around the outer edge of the cutting teeth in feet.

, The invention is hereby claimed as follows:

In a process for machining titanium and similar metals which are subject to galling, the steps of rotating a milling cutter having a pitch between A" to /2 at a peripheral velocity in excess of 100 surface feet per minute, contacting the workpiece of titanium or a similar metal which is subject to galling with at least two teeth of said milling cutter at all times and causing the linear distance of travel of the workpiece per tooth of the milling cutter to be between 0.002" and 0.005".

References Cited in the file of this patent UNITED STATES PATENTS 903,496 Peck Nov. 10, 1908 1,355.511 Edgar Apt. 26, 1932 2,810,189 See et al. Oct. '22, 1957 OTHER REFERENCES worths Scientific Publications, London, in 1956, pp. 99-- Hyper Milling of Steel Forgings Proves 5 to 15 Times Faster, published by Wings, April 1944, 965-967 relied on.

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