Case Trimming Fixture

Completed 2/5/04

The tedium of trimming many hundreds of 223AI fireformed cases led me to build this simple fixture to greatly speed up the task. Since I already had a small, ancient, Craftsman lathe, I decided to build this fixture that allows a case to be trimmed by an end mill held in the lathe chuck in less than five seconds. Total cost was about $15.00 and a fun afternoon in the shop.

The idea was to build a cantilevered arm that supports a form fitted guide or collet, exactly fitting the fired cases to be trimmed. The case will index of the shoulder of the fired case. The arm is attached to the compound of the lathe, and adjusted so that a case fully inserted into the holder will be exactly coaxial to the spindle bore. A simple end mill held in the lathe chuck is used to do the cutting. The precise amount of trim is very easy to set, since the compound travel adjustment is calibrated. One a case is just kissed with the cutter, the OAL is measured, and the additional amount of trim is dialed into the compound.

The major problem was how to support the holder rigidly, and still have it adjust it easily and quickly over all the degrees of freedom necessary to get perfect alignment. To this end, a crude but effective ball joint holder was built using 1 ¼ inch iron pipe fittings, a round plastic ball drawer handle, and other pieces of hardware.

Rectangular steel tubing was used to build the arm, with a simple T bolt method of attachment to the compound tool holder slot. The ball joint uses the plastic spherical handle captured between a 1 ¼ inch pipe cap and plug. The plug is brazed to the arm, and a simple insert cut from a PVC pipe fitting is used in the cup to provide a non-marring positive lock on the ball. The pipe cap is turned on the lathe to provide a chamfered clearance hole in the cap top to allow a bolt threaded into the ball handle to protrude. The bolt has a stainless hose clamp brazed to the head, and is used to hold the form fitted case bushing.

The case bushing is made using five minute epoxy, mold release agent, and a short piece of ¾ inch copper pipe. After a case is coated with mold release (I used Brownell’s aerosol release spray) , it is set base down into a pad of modeling clay, and the copper pipe piece carefully centered around the exposed case. Some five minute epoxy is then mixed and poured into the void between the case and the pipe, and allowed to cure. Once cured, the case extracts easily, and the rough ends of the pipe-epoxy can be faced in the lathe.

The result is a bushing that exactly fits the fired cases, and allows the case neck to slightly protrude past the epoxy for easy trimming.

Once the fixture is assembled and mounted on the lathe compound, the process of alignment and trim length adjustment is started.

Using a bullet minus 0.005” diameter mandrel in the lathe chuck, the fixture is adjusted to allow a case fitted into the bushing to easily slide on and off the mandrel as the carriage is traversed. It is easy to tell when the case is misaligned, as the case will not rotate freely in the bushing, or the fixture and case will bind on the mandrel as the carriage is traversed. Once the cross slide and ball joint adjustments are correct, the pipe cap is tightened to lock the ball in place. The carriage is traversed away from the chuck, and the alignment mandrel removed. An end mill is inserted in the lathe chuck at this time.

Setting the trimming depth of cut is now a very simple process. The carriage is adjusted to bring the neck of a case inserted into the bushing to a point where it is almost touching the end mill. The carriage lock is then engaged, and case that needs trimming is inserted into the bushing by hand. Set the lathe to turn at a moderate rate of rotation. Slowly adjust the compound (set for a 0 degree angle of travel) until the cutter just kisses the case neck. Measure the case length, and calculate the additional depth of cut needed to achieve the desired length. Dial this additional length into the compound, and you are ready to trim cases.

I was able to trim over 400 cases in less than an hour using this fixture. The trimmed length never varied over a couple of dozen randomly picked cases by more than 0.0005”. This is equal to the accuracy of my hand operated Wilson case trimmer, and is much, much faster. New bushings can be made for any case in a matter of minutes. I found that rotating the case about a half turn during the cutting operation made for a slightly cleaner cut, and also eliminated any slant due to residual misalignment of the bushing and the lathe axis of rotation. A two flute end mill is shown in the above photos and works just fine, but I had slightly better results (smoother cuts) with a four flute mill. The cuts are as clean or cleaner than the Wilson trimmer, bright and shiny with almost no burr.