A method that produces an even better machining rifle blank, which practically stays straight through the different machining operations, was to rough-turn the blanks, then subject them to a heat of practically 1,0000 for 4 hr. Production throughout the different operations is materially increased, with practically no straightening required after drilling, reaming, finish-turning or rifling operations. [Illustration: FIG. 24. FIG. 25. FIGS. 24 and 25.--Roof system of cooling quenching oil.] This method was tested out by one of the largest manufacturers and proved to be the best way to eliminate a very expensive finished gun-barrel straightening process. [Illustration: FIG. 26.--Details of the cooler.] The heat-treating required a large amount of cooling oil, and the problem of keeping this at the proper temperature required considerable study. The result was the cooling plant on the roof, as shown in Figs. 24, 25 and 26. The first two illustrations show the plant as it appeared complete. Figure 26 shows how the oil was handled in what is sometimes called the ebulator system. The oil was pumped up from the cooling tanks through the pipe _A_ to the tank _B_. From here it ran down onto the breakers or separators _C_, which break the oil up into fine particles that are caught by the fans _D_. The spray is blown up into the cooling tower _E_, which contains banks of cooling pipes, as can be seen, as well as baffies _F_. The spray collects on the cool pipes and forms drops, which fall on the curved plates _G_ and run back to the oil-storage tank below ground. The water for this cooling was pumped from 10 artesian wells at the rate of 60 gal. per minute and cooled 90 gal. of oil per minute, lowering the temperature from 130 or 140 to 100 deg.F. The water as it came from the wells averaged around 52 deg.F. The motor was of a 7-1/2-hp. variable-speed type with a range of from 700 to 1,200 r.p.m., which could be varied to suit the amount of oil to be cooled. The plant handled 300 gal. of oil per minute. CHAPTER VI ANNEALING There is no mystery or secret about the proper annealing of different steels, but in order to secure the best results it is absolutely necessary for the operator to know the kind of steel which is to be annealed. The annealing of steel is primarily done for one of three specific purposes: To soften for machining purposes; to change the physical properties, largely to increase ductility; or to release strains caused by rolling or forging.