Centrifugal Casting in Metal Molds of Replaceable Cutting Elements for Sleeper-Cutting Saws

G. V. Simonov, K. A. Leikhtling, L. S. Smorgon

(Presented by Professor, Doctor A. N. Dobrovidov)

Circular-saw machines used in sleeper sawing belong to the group of equipment operating under heavy-duty conditions. The diameter of the saw disks for this group of machines reaches 1500 mm. Accordingly, cutting speeds are 100–120 m/s. Feed rates may also be brought up to 120 m/min. The cutting path of each tooth during the operation of such a saw between resharpenings is measured in many tens of kilometers. Therefore the operating conditions of sleeper-cutting machines require great attention to questions of preparing and using a high-quality cutting tool.

One of the directions in creating such a tool is the development of circular saws with inserted teeth. The manufacture of the saw disk, the holder, and the cutting element from different grades of steels makes it possible to select a combination of properties that is most optimal both for the given element and for the saw as a whole.

The aim of the present work was to investigate the possibility of obtaining replaceable cutting elements for sleeper-cutting saws from high-alloy and high-speed grades of steels by casting into metal molds by the centrifugal method.

Figure 1 presents the working drawing of the cutting element.

Fig. 1.

The currently existing technology for manufacturing them by investment casting is rather complex and has a number of disadvantages inherent in castings of this kind. In investment casting, considerable decarburization of the blank is observed (up to 0.6 mm per side),

which leads to the need for subsequent cementation. The metal structure obtained when casting by the investment process is fairly coarse-grained and has low mechanical characteristics.

The material used in this case for making the cutting elements is 85KhF steel, which, in terms of its wear resistance, is considerably inferior to high-alloy and high-speed steels. Figure 2 presents a graph of cutting-edge wear (according to V. S. Rybalko) as a function of the amount of material processed; it shows that steels KhVG and 9KhS, and steels of similar chemical composition \(I_2\), \(I_3\), \(I_4\), to which 85KhF steel may also be assigned, have wear resistance 3–4 times lower than high-alloy steels of the Kh12F, Kh12M, \(I_1\) type.

Fig. 2.

For carrying out the investigations, a four-cavity quick-release metal mold was designed and manufactured, making it possible to obtain blanks for four cutting elements in a single casting. Figure 3 shows the general appearance of the metal mold.

As the material for manufacturing the replaceable cutting elements, high-speed steel of the R9 type was adopted. Table 1 gives the chemical composition of individual control heats.

The charge for remelting consisted of small waste from tool production. To increase the carbon content in the steel, and also to compensate for its burn-off, additions were made with an alloy based on high-speed steel R9 with a carbon content of 4.5–5.0%. The carbon content in the steel was varied over wide limits (0.88–1.24%) in order to determine the fillability of the mold, as well as the effect on the properties of the casting.

The steel was melted in a high-frequency melting–casting unit of the A343 type, in an acidic crucible under a layer of slag. The temperature of the metal being poured was maintained within the range 1460–1500°.

Fig. 3.

The metal was poured into a rotating mold mounted on the support plate of a centrifugal machine. To eliminate the possibility of metal burning on, the mold was treated with a refractory coating.

the inner surfaces of the latter were coated with a thin layer of chromium oxide. Pouring was carried out into a cold metal mold.

Table 1

To determine the influence of the number of revolutions on filling the mold with metal, this number was varied in different heats from 0 to 500 per minute.

Immediately after casting, the mold was disassembled and the blanks, at a temperature of 600–700°, were loaded into a muffle electric furnace or else left in air until completely cooled.

Before cutting off the riser, part of the castings was tempered at a temperature of 580° for one hour, and part was loaded for annealing. Annealing was carried out in a muffle electric furnace according to the standard schedule: heating to a temperature of 880°, holding for 2 hours. Slow cooling (30–40° per hour) to a temperature of 740°, holding at this temperature for 4 hours. Cooling together with the furnace.

After the riser portion had been cut off, the unannealed cutting elements were subjected to a second and third tempering from temperatures of 600 and 620°, respectively. The heads of teeth that had been annealed were hardened. Hardening of the tooth heads was carried out on the same high-frequency installation. The quenching medium was oil. After hardening, tempering was carried out at a temperature of 580° for one hour.

Table 2 gives hardness measurements of the cutting elements after various stages of heat treatment.

Table 2

In all, in the first series of experiments 22 heats were made. Of these, two heats were cast in a stationary mold.

Tests of the cutting elements cast by the recommended technology were carried out at SibNIILP (Siberian Scientific Research Institute), in the woodworking laboratory. For testing, [[unclear: text continues on next page]]

an experimental setup was used, the general view of which is shown in Fig. 4. Characteristics of the setup:

a) number of saws . . . . . . . . . . . . 1 unit,
b) diameter of the saws tested . . . . . 300–1200 mm,
c) number of revolutions of the electric motor . . . 1490 per min,
d) number of revolutions of the saw shaft . . . 250–6700 per min,
e) cutting speed . . . . . . . . . . . 36.4–426 m/sec,
f) kerf height . . . . . . . . . . . up to 500 mm,
g) type of feed . . . . . . . . . . . mechanical (carriage with cable),
h) feed speed . . . . . . . . . . . 10–100 m/min.

Fig. 4.

Before the teeth were installed in the saw disk, they were sharpened on the front, rear, and side faces. In the cutting tests the following angular parameters of the tooth were adopted:

front angle . . . . . . . . . . . 32°,
rear angle . . . . . . . . . . . 14°,
sharpening angle . . . . . . . . 44°.

Sharpening was carried out on a special fixture in a universal sharpening machine ZA64 without coolant.

Before the cutting test, the quality of the teeth was checked for manufacturing accuracy. In the present work the results of the measurements are not given. It should only be noted that the values obtained for all indices lay completely within the dimensions of the working drawing and the requirements of the technical specifications.

The interchangeability and reliability of fastening of the cutting elements in the saw disk were also checked. For this purpose, different teeth with one and the same lock were installed in one socket, and one and the same tooth with different locks. The force applied to the handle of the factory wrench was 10–15 kg.

The saw, preassembled with teeth and locks, was tested without load at 1490 revolutions per min. The tests showed,

that there were no cases of displacement of the teeth and locks installed in the sockets of the saw disk. After testing the saw at idle, cutting tests were carried out. The cutting conditions were as follows:

Saw-disk diameter . . . . . . . . . . . . 1200 mm.
Number of revolutions of the electric motor . . . . 1490 per min.
Number of revolutions of the saw shaft . . . . . . 900 and 1490 per min.
Cutting speed . . . . . . . . . . . . . 56.7 and 93.8 m/sec.
Feed per tooth . . . . . . . . . . . . 0.3 to 3.0 mm.
Number of saw teeth . . . . . . . . . . . 36 pcs.
Feed speed . . . . . . . . . . . . . . 16.1 to 60.9 m/min.

The wood being sawn was larch. The moisture content of the wood was 13%. The kerf height was 150 mm.

To obtain maximum dynamic forces during cutting, the tests were carried out both in longitudinal cutting (along the direction of the fibers) and in transverse cutting (across the direction of the fibers). The feed per tooth in longitudinal cutting varied from 0.3 to 3.0 mm. In transverse cutting the feed per tooth varied from 0.3 to 2.0 mm.

No special preparation of the saw disk was carried out before the tests. The disk was installed as delivered from the factory. In the free state the disk had a face runout of 3.64 mm. After the teeth were fitted into the saw disk, the positioning of the teeth by height was checked. The difference in tooth height around the circumference (radial runout) was from −0.25 to +0.35 mm, i.e., the actual feed per tooth differed somewhat from the calculated value.

When installing teeth having the same hardness over the cross section (62–64 HRC), there were cases of chipping of the shoulder. Teeth having a transitional hardness over the cross section (hardened head and tempered shank) were fitted normally.

During the testing of saws under load (during sawing), no cases of breakage of the tooth head or chipping of the cutting edge were observed, either in longitudinal or in transverse cutting. There were also no cases of displacement of the teeth and locks during cutting at any values of cutting speed and feed.

Conclusions

1. Casting in a metal mold with the use of centrifugal casting makes it possible to obtain high-quality cutting elements.

2. Changing the number of revolutions of the centrifugal machine within the range of 150–400 per minute does not substantially affect the quality of the casting.

3. It is necessary to anneal the blanks, followed by hardening of the tooth head on an HFC unit.

4. As the material for manufacturing the cutting elements, high-alloy or high-speed steels should be used.

5. A cutting-edge hardness of 64 HRC does not lead to chipping or fracture of the cutting element.