06 MISC. PUBLICATION 679, U. S. DEPT. OF AGRICULTURE 



the principal finishing operations applicable to methacrylate plastic cast- 

 ings are given here in outline based on the data furnished in certain fa- 

 bricating manuals. 



Machining 



Minor irregularities can in most cases be taken care of by leveling them 

 off with a smooth- cut file or by applying coarse- or medium-grit sand- 

 paper. When larger sections are to be removed it is more satisfactory to 

 cut them off with a saw. Saws of almost any type may be used — either 

 hand or power operated — but better results are usually obtained with one 

 that has 8 or more teeth per inch with little or no set, In sawing, it is im- 

 portant to avoid too great pressure, which might result in overheating and 

 gumming, and to keep the saw-cut straight so that heat from undue side 

 pressure will not cause binding or sticking. 



Power-driven saws should be provided with suitable devices to protect 

 the operator against personal injury and in using such saws caution must 

 be exercised to avoid damaging the plastic in which the specimens are 

 embedded. 



It is generally necessary to use a lathe for truing the round surfaces of 

 cylindrical castings. In such instances the sanding and polishing, which 

 follow the machining operation, can be carried out conveniently on the 

 blocks while they are still set up in the lathe. Either a wood-turning lathe 

 or a metal-turning or engine lathe may be used for machining methacry- 

 late castings, but the metal type is generally preferable because it per- 

 mits more accurate work. 



Coolants may be used if desired to aid in eliminating excessive fric- 

 tional heating in the various machining operations required in trimming 

 and surfacing the plastic castings. No fluids that contain solvents or 

 other detrimental ingredients should be used, and in most cases soapy 

 water, or even plain water, will be found to be satisfactorily effective. 



Sanding 



After all the faces of the block requiring resurfacing have been properly 

 trued, they should be sanded or ashed until they are free from noticeable 

 scratches and depressions. For the sanding process it is best to use wet- 

 or-dry type of sandpaper, and to apply ample water during the several 

 stages of sanding to minimize scratching from loose grit. 



It is usually advisable to start with moderately coarse paper (240 grit 

 or equivalent) and then change to successively finer paper as the coarser 

 scratches are sanded out. Intermediate grits of 320 and of 400 should thus 

 be used, followed finally by 500 or preferably 600. until the work has a 

 satiny surface free from visible scratches. This operation may be carried 

 out entirely on a power driven sander if desired, but the most satisfactory 

 results are to be obtained by applying the final stages by hand. In all 

 hand sanding it is important to have the sandpaper laid out on a practi- 

 cally true surface and held securely in position to prevent slippage. A sheet 

 of flat pressed window glass, or preferably plate glass, is very good for 

 this purpose, and springy strips of metal attached to a wooden base sup- 

 porting the glass plate can be made to serve as clamps for holding the 

 sandpaper in place. 



