WHEN drilling holes in the lathe it is necessary to withdraw the drill at frequent intervals to ensure that the swarf is cleared from the drill flutes. The smaller the drill, the more important this becomes.  Furthermore, for very small drills, and here I am referring to drills  of 1/16in. diameter and less, the normal screw feed of the tailstock lacks the sensitivity required to properly control the operation. 

The attachment shown in Figure 1 is designed to provide both the sensitivity required together with the ability to withdraw the drill quickly at frequent intervals. The drawings provide details of the attachment shown in the photograph but the dimensions can easily be modified to suit the materials available and the machine on which it is to be used. 

The only essential features are accuracy of concentricity of the body, ram and chuck and the fit of the ram in the body.  A size 0 Jacob chuck is specified. These chucks are expensive (about £50) but do not be tempted to substitute a cheap imitation. The Jacob chuck is a high quality tool and will hold a number 80 drill (0.3 mm) accurately.

Construction is straightforward, involving simple turning, milling and drilling. The only feature which might present any problems is producing accurate tapers. 

Figure 2 shows a simple method of setting the top slide to produce an accurate Morse taper.  Ensure that the DTI stylus is bearing on the setting device exactly on the lathe centre height (i.e. it is following the same path as the lathe tool will follow during the machining operation). Some slight adjustment to the top slide may still be necessary to obtain a perfect fit so check the taper as machining proceeds using engineer’s blue and an appropriate Morse taper sleeve. 

The number 0 Jacob taper is a little more awkward.  The top slide must be adjusted by trial and error until the required setting is achieved, again using engineer’s blue and the actual chuck to check progress. The taper is actually 0.59145 inch per foot, corresponding to a half included angle of 1.412 degrees. 

These numbers are not much help in practice, however. Setting the top slide over to 1 ½ degrees provides a good starting point.  Turn the taper on the end of a length of 5/16in. diameter silver steel.  When you are satisfied with the result the rest of the ram can be finished.

The final picture, Figure 3, shows a number 77 drill being used to produce an injector cone.  Theoretically, the lathe should be running at many thousands of r.p.m. to achieve the recommended cutting speed for the material.  In fact it is running at about 600 r.p.m.  This is perfectly satisfactory provided no attempt is made to rush the job. 

Start with a size S1 centre drill.  Peck away at the hole, a few thou feed at a time and an accurate true running hole will be achieved. If the drill does show signs of wandering off course, stop and face off the end of the material and start again.  If you are making injector cones the lever feed device is ideal for presenting the taper D-bit reamers. The number 0 Jacob chuck holds tooling up to 5/32in. diameter, which is the size silver steel usually recommended for making these reamers.

By Artisan