To turn necks on a lathe, a sacrificial spud must be turned. I turn this spud to match the largest (in diameter) bullet that I may use for the case being turned. Typical match bullets from different form dies (bullet maker to bullet maker) rarely vary more than .0005" in diameter. If necks are turned for .0015" clearance to match a fat bullet, the neck clearance will not exceed .002" on a skinny bullet. I have used tighter clearances in the past but for now have settled out on the above mentioned dimensions. The spud should be turned from free-cutting steel capable of giving a smooth finish with a lathe cutting tool. The spud also should have a strongly tapered point to guide the case. I used some 1141 screw machine stock (cuts smoothly) to make this spud because I have that material in the shop. A good steel for this purpose is 12L14. This leaded bar stock produces a beautiful finish with almost any sort of cutter. I always turn the spud a few “tenths” oversize and bring it to final size with a soft mold-maker stone of fine grit. These stones (with oil) quickly take on the shape of the work and produce a very smooth finish. The stones are available from MSC and Gesswein (they can both be found on the Internet). The spud should be frequently measured during this operation. These measurements will indicate the area and amount to stone. Final finish was with crocus cloth.
Wilson trimming fixture shown in the open position.
The best way to press the case (lubricated inside with Imperial Die Wax) on and pull the case off of the spud is with a loading press case holder. The slotted tailstock adapter was made from a Lee autoprime head. The backside of the head was bored and threaded for a one half-inch shank. This piece could be made from solid material but buying and modifying the Lee part was more time effective.
With the appropriate case holder dropped into the tailstock adapter, the case is run onto the spud using the tailstock handwheel. The cases in this batch were full length sized in a die with the expander rod removed. The progress of the spud in the case neck can be easily observed as the case is pressed on. It looks somewhat like a snake swallowing an egg. A tight fit on the spud insures that the case will stay put during the cut. With this tight fit there is also less chance of any possible inside neck irregularities causing a faulty turning operation. Once the case is fully on the spud, the case holder is removed by rotating it 180 degrees and lifting it up out of the adapter slot. A little wiggling of the tailstock wheel may be necessary to relieve binding on the holder.
To make the cut I used a TPG (triangle positive ground) carbide cutting insert. This cutter insert has a radius that matches the shoulder-neck junction radius of the case being turned. On all inserts I have used, the third digit on the insert description indicates the radius dimension. I use indexable carbide inserts for almost all turning operations in the shop. Carbide can be run at three or four times the surface speed of high-speed steel. The speed advantage saves a great deal of time. Also, if the cutter goes away during a cut, the insert can be indexed to a fresh edge without losing setup by much. Note that the leading edge of the cutter is aligned to slightly more than the shoulder angle of the case. The cutter will contact the shoulder at the junction first. I stop the cut as soon as shoulder contact is observed.