Perspective drawing


By Frank W. Harth

No surface grinders are on the market in the handy size Mr Harth has worked out here. His design has improvements which assure the experimental engineer of getting an excellent precision tool.

Nothing will take the place of a small surface grinder for experimental engineering work. For pieces too small to be faced in a shaper or planer, or too large to be conveniently handled in a lathe, or for work where greater accuracy in finish is needed, the small surface grinder is the answer.

Some years ago I designed a grinder very similar to the one shown here. After studying the results builders had with the first design, I altered it in several places to take the last bugs out of it, and the result is presented here. With it such work as is now brought delicately to surface with a file can be handled. Steam engine cylinder blocks, locomotive side rods, valve faces on steam chests -- all can be done to the queen's taste with this tool. What is more, the very making of the grinder itself will be sufficient fun for many.

Most of the work on the castings can be done on the average 9" lathe. The bed and possibly the parts shown in Figs. 10 and 11, also the motor carriage in Fig. 14, will probably have to be taken to an outside machine shop for planing, but the rest of it will be pie.

Only small cuts can be taken with this machine, for the motors which are adaptable to it do not have much power. But this should not really matter, since patience is one of the long points of every modeler. Motors used are of the small 1/8 to 1/6 h.p. type, or may be of the sewing machine type. Possibly a handee grinder could be adapted in the vertical carriage. Details of the grinder itself are purposely left out of this design, for no two modelers will adapt the job in the same way. Merely bear in mind that a number of motors are available at a cost of five to six dollare, and that abrasive wheels of nearly every size are standard items in every good hardware store.

Drawing 2

A pattern is made for the guide rod supports A, as shown on the three projection drawing, or assembly. This is shown, detailed, in Fig. 4. The boss surfaces A must be on a line B. (Fig 4.) The holes C, Fig.4, must have centers which exactly parallel the supports, D. A parallel reamer is used to true and smooth the holes. Fig.5 shows the guide rods. These are cold rolled steel turned to 3/4" diameter, threaded at the ends and polished. (Grind round if you can.) Lock washers should be used to sset up the nuts on the ends when assembling.

The grinding table is shown in detail just abovr Fig. 5. This should be labeled Fig. 6.

The feet through which the holes E-E go can be cast. A Pattern is required for the table top itself, which is of cast iron. Rember to use shrink rule allowing 1/8" in 12" when making this pattern. These feet are shown, detailed, in Fig. 7. If the table and feet are cast in one block, it is impossible to get them to line up smoothly without access to a big jig and a large drill press. Hence, made separately, the alignment can be controlled easily. The guide hole E-E should be reamed for a "goose grease" fit-- that is they should slide easily, but with absolutely no play, for the tendency of the rack feed on the table is to lift the table up. Any play spoils accuracy.

The gear and rack and pinion can be had from any gear supply house, as they are standard. The rack should be of brass. It is held in place with 8-32 screws.

The detail of the clamp-bolt holes is shown in Fig. 8. These may be spotted as desired on the table. The hole centers are marked, drilled, and then are filed out between walls. A counter boring drill is then used to cut the shoulder. It is diagrammed in Fig. 9.

Stuff the oil holes in the feet of the table with wick to keep out chips. The wick is saturated with oil, which then lubricates the guide rods.

The motor carriage is carried on similar guide rods, the ends of which are shown in Figs 10 and 11.

The tightening nuts for the guide rods are not used on the supporting end, (Fig.11) which abuts the grinding table. The nuts are omitted to give clearance to the table and any castings that may be clamped to it. Instead, a tapered nose screw is set in as shown in Fig. 12. This locks the rods positively.

The guide rod, Fig. 13, is made up like the other guide rods, and threaded 20 threads per inch.

Drawing 2

Fig. 14 details the carriage mount. A pattern is made for this part, leaving the hole in the web to prevent distortion when cooling the casting. The feet for this bed are constructed just as for the table. Here again, nice fitting will be required of the holes and guide rods. The motor bracket guides can be bronze or steel, fastened as shown in Fig. 16. The bearing plate, Fig. 15, is of bronze. It acts as a bearing for the motor control screw. The screw for the bed as well as the motor are standard gear works products, and can be bought at any gear supply or hardware store.

Motor Carriage

The motor carriage is moved by the threaded screw shown in Fig. 18. It's application can be observed in the assembly drawing on the opening page of this article.

No. 20 U.S. threads will give a very close regulation of the cut. Bronze spring bearing at the bottom of the screw is shown in Fig. 19. This prevents end-play, makes a smoother working feed. The washer is grooved to feed oil to the screw end. The thrust of the screw is taken up by the large sized washer shown in Fig. 22. This same spring should be used behind the hand wheels.

The serrations in this washer are made with a hacksaw or jewelers saw. They give spring to the dished metal.

Fig. 20 shows a section as well as an external view of the wheels. Use headless set screws to fasten them to the shafts. The grinding table handwheel should be larger, to be more comfortable in use. This is shown in Fig. 21.

The supporting bearings for the rack and pinion gear are shown in Figs. 23 and 24. These, of bronze, eliminate need for special bearings for the gear shafts. Three gears are used in the pinion train -- one being an idler thrown in so that the table has the same direction of travel as the hand wheel. If need be, the bed can be chipped out to accommodate the main pinion gear. The driving gear and the transfer gear should be of the same size -- The idler merely reverses rotation. The bearing for the hand wheel shaft is detailed in Fig. 25.

Motor Mount

Drawing 3

The motor shown in the perspective has been mounted in this manner for two reasons: first, it allows the actual cutting load to be transferred to a shaft separate from the motor armature shaft. This allows the wheel arbor to be mounted in bearings of your own devising which will be more accurate, and in line with the work close up to the carriage. Secondly, it allows greater rigidity. The closer the wheel is to the bulk of the motor, the better cutting can be done.

Gear the arbor with bevel gears, using a 1:1 ratio. Motors are available at low figures which are of about the bulk shown, of sufficiently high speed (6,000 to 10,000 r.p.m.)

Surfacing table plates, engine heads, and a wide variety of frogs's hair work can be done when one owns a tool like this, and as has been said, the fun of merely building the thing is justification enough to tackle it.

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