Initial impressions:

So what is this thing? Well, it is a bearing holding a bushing with an endmill.

When we are trimming cases to length, we use what we can measure. For some cartridges, this makes sense. For others, it is nearly impossible.

Our goal is to have a cartridge correctly seated in the chamber with the bullet at the correct distance from the rifling (or just touching), and the distance from the end of the cartridge to the face of the bolt be correct.

Consider a .45 ACP round. The cartridge is designed to position itself in the chamber based on the mouth of the cartridge. When the cartridge is fully seated in the chamber, the mouth of the case will be resting against a feature in the chamber.

The overall length of the case should be such that the base of the case is at a known, correct, distance from the face of the bolt. Using the mouth of the case allows us good control of bullet to rifling distance.

30-30 Winchester, .45 Colt, .303 British, 7.62x54R all position the cartridge according to the front of the case rim.

With a rimmed cartridge, the thickness of the rim is known. This means that the distance from the base of the case to the face of the bolt is the controlled distance.

By correctly seating the bullet and by making sure that the length of the case is correct, we get control the distance the bullet is from the rifling.

In some senses, rimmed cases are the easiest to measure and keep within specifications.

More modern cartridges seat against the shoulder. This gives good control over the distance between the bullet and the rifling, and supposedly, good control of the distance between the base of the case and the face of the bolt.

The issue is that measuring from that hypothetical shoulder location to the mouth and the base is a little difficult.

We address this by “bumping the shoulder”. This is using the reloading press to reform the shoulder at the correct distance from the base of the case. At that point, trimming the case to length based on overall length should give good results.

To trim a case to length, we need to be able to repeatably cut the case to the same length. Thus, we need to have some way of making sure that the cutter is the correct distance from the base of the case.

On a case with a shoulder, we can either measure from the base of the case or from some other known location on the case.

One method used is to have a rod of known length that pokes out of the flash hole. This stop rod keeps the cutter from getting any closer to the face pressed against the base of the case.

When done correctly, this is quick, easy, and very repeatable.

The issue is that each cartridge needs its own stop rod.

Another method is when the cutter is kept from advancing beyond a known location. If I could, I would use my milling machine for this.

The cutter would spin in the spindle, when the quill is brought down it will stop when it hits the quill stop. Very fast, very repeatable.

Most of the trimming devices that have a lathe like spindle that has the cutter works this way. There is a stop collar that keeps the “spindle” from moving to far.

My issue with that style is that it is a little slower and I don’t trust the stops.

Which takes us to the last time. A trimmer that is based on the distance from a known location on the shoulder to the cutter.

Since we’ve positioned and shaped the shoulder correctly in the reloading press, measuring from the shoulder should be consistent.

This is what the UGLY SRT does.

The SRT consists of 5 major components. The first is the cutter. This is just a standard 4 flute 3/8″ endmill. The size is by a mark I eyeball, so it could be anything close.

The next component is the body. This is a simple bit of turning. It has a slip fit for the cutter at one end and is bored to size on the other end.

The cutter slips into the hole for it and is held in place by two set screws.

A baring has been pressed into place at the other end and makes the third major component.

The next major component is a simple disk with a single set screw in it. This is to allow you to adjust the depth of cut.

The final component is the collet. This slips into the bearing. It has a shoulder to press a case against.

To operate, this is simple. Chuck the endmill into a drill. When the drill is running, the endmill will turn and the housing will turn. The collet and inner race of the bearing will not rotate.

Slide a case into the collet and press it into the cutter. When it bottoms out on the shoulder in the collet, you will have cut the case to a length.

This is very consistent.

Adjusting the depth of cut is also simple.

If you want to make the depth of cut less, you make sure that the adjustment disk held in place and that it is firm against the body. You then release the endmill from the housing, slide a feeler gauge of the right size between the body and the adjustment disk. Tighten the set screws to lock the endmill/cutter in place.

Reposition the adjustment disk tight against the housing and lock it there.

This is dirt simple. It is an adjustment method that I will be using in my designs.

To cut more, loosen the adjustment disk, slip the correct feeler gauge between the body and the adjustment disk. Lock the adjustment disk in place. There is now a gap between the adjustment disk and the body. Release the endmill, push the adjustment disk against the housing, lock the endmill in place.

Everything about this is simple, well constructed. The adjustment process is different but easy.

I’ve only tested on a few cases, but I think this might be my end goal for shouldered cases.

The only downside I’ve identified, so far, is that I have to purchase different collets depending on the case I’m trimming. And I’m not sure if I’ll be able to manufacture them from the tooling I have on hand.

More to when I do my next large trim run.

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By awa

Only one rule: Don't be a dick.

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