Tool Tip: Optimize Your Cutter Angle for Improved Neck-Turning
When neck-turning cases, it’s a good idea to extend the cut slightly below the neck-shoulder junction. This helps keep neck tension more uniform after repeated firings, by preventing a build-up of brass where the neck meets the shoulder. One of our Forum members, Craig from Ireland, a self-declared “neck-turning novice”, was having some problems turning brass for his 20 Tactical cases. He was correctly attempting to continue the cut slightly past the neck-shoulder junction, but he was concerned that brass was being removed too far down the shoulder.
Craig writes: “Everywhere I have read about neck turning, [it says] you need to cut slightly into the neck/shoulder junction to stop doughnutting. I completely understand this but I cant seem to get my neck-turning tool set-up to just touch the neck/shoulder junction. It either just doesn’t touch [the shoulder] or cuts nearly the whole shoulder and that just looks very messy. No matter how I adjust the mandrel to set how far down the neck it cuts, it either doesn’t touch it or it cuts far too much. I think it may relate to the bevel on the cutter in my neck-turning tool…”
Looking at Craig’s pictures, we’d agree that he didn’t need to cut so far down into the shoulder. There is a simple solution for this situation. Craig is using a neck-turning tool with a rather shallow cutter bevel angle. This 20-degree angle is set up as “universal geometry” that will work with any shoulder angle. Unfortunately, as you work the cutter down the neck, a shallow angled-cutter tip such as this will remove brass fairly far down. You only want to extend the cut about 1/32 of an inch past the neck-shoulder junction. This is enough to eliminate brass build-up at the base of the neck that can cause doughnuts to form.
The answer here is simply to use a cutter tip with a wider angle — 30 to 40 degrees. The cutter for the K&M neck-turning tool (above) has a shorter bevel that better matches a 30° shoulder. There is also a 40° tip available. PMA Tool and 21st Century Shooting also offer carbide cutters with a variety of bevel angles to match your case shoulder angle*. WalkerTexasRanger reports: “I went to a 40-degree cutter head just to address this same issue, and I have been much happier with the results. The 40-degree heads are available from Sinclair Int’l for $13 or so.” Forum Member CBonner concurs: “I had the same problem with my 7WSM… The 40-degree cutter was the answer.” Below is Sinclair’s 40° cutter for its NT-1000, NT-1500, and NT-4000 neck-turning tools. Item NT-3140, it sells for $12.95. There is also a 40° cutter for the NT-3000 tool, item NT-3340 ($13.95).
Al Nyhus has another clever solution: “The best way I’ve found to get around this problem is to get an extra shell holder and face it off .020-.025 and then run the cases into the sizing die. This will push the shoulder back .020-.025. Then you neck turn down to the ‘new’ neck/shoulder junction and simply stop there. Fireforming the cases by seating the bullets hard into the lands will blow the shoulder forward and the extra neck length you turned by having the shoulder set back will now be blended perfectly into the shoulder. The results are a case that perfectly fits the chamber and zero donuts.”
* 21st Century sells carbide cutters in: 15, 20, 21.5, 23, 25, 28, 30, 35, 40, and 46 degrees. PMA Tool sells carbide cutters in: 17.5, 20, 23, 25, 28, 30, 35, and 40 degrees.
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Tags: Brass, Carbide, Cutter Angle, Neck Turning Tool, Neck-Turner, Neck-Turning
Rather than guess about excessive thinning at the neck shoulder junction, I find it better to sacrifice a case, and section a turned neck and beyond, down into the shoulder, so that I can see what is taking place. I use a cordless Dremel tool with a reinforced abrasive wheel for this job, making sure to wear a paper mask and glasses to keep the brass particles, (that are sprayed directly at my face as the cut is made), from being inhaled or getting in my eyes.
In my experience, if the cut on the shoulder is even all the way around, and not deep it shouldn’t be a problem. On the other hand, if my necks were as rough as those shown, I would have to investigate as to the cause, or at least do a little power driven clean up with 0000 steel wool.
Sometimes when the cutter is too narrow where it is actually cutting brass, a little hand work can broaden it out and result in a smoother cut. I have improved results with more than one brand of turner that way.
The best way I’ve found to get around this problem is to get an extra shell holder and face it off .020-.025 and then run the cases into the sizing die. This will push the shoulder back .020-.025. Then you neck turn down to the ‘new’ neck/shoulder junction and simply stop there. Fireforming the cases by seating the bullets hard into the lands will blow the shoulder foward and the extra neck length you turned by having the shoulder set back will now be blended perfectly into the shoulder.
The results are a case that perfectly fits the chamber and zero donuts.
How interesting! I just got my first neck trimmer, a Sinclair 4000. This appears to be a great trimmer. It is very easy to set up and the ten thousanths dial make adjustment easy. However I am having the same problem as Craig. Before I can go past the neck towards the shoulder, the Upper tip of the shoulder bevel starts to make contact. I have considered stoning this edge to a different angle. First I tried making the cut slightly deeper. That helped but it not the answer. A 40 degree cutter looks like the way to go.
Al Nyhus’ procedure has been used off and on for years and is tried and proven. Getting the cutter reground to an angle that almost matches the shoulder angle also works well, although I always sectioned a case to make sure that the thickness of the shoulder was still sufficient when setting things up.
The photo of the case neck shows the result of a cutter blade with very little contact area (I think the machinists call it “rike” (sp?)) rather than the smooth finish that is seen when there is a uniform feed rate on a power driven cutter. Some people used to make a cut that was almost to final thickness and then use another turner to make the final cut. That one was often more of a “shaver” than a cutter as it was only going to remove a small amount of material.
I am surprised that no one makes a cutter blade which is set up to handle 30 degree shoulders by now as it is such a common shoulder among accuracy cartridges.
We also make a 25 degree cutter and can do custom angles or radius on request for a small additional fee.
I do a lot of case forming and try to turn the necks at the largest caliber possible (.30 is best) using a bit with a radius as opposed to an angle so it blends in, then reduce the neck down to the desired caliber. Of course, you have to experiment with scrap cases to see what ends up at the target finished neck wall thickness. The bonus is you end up with a neck wall thickness that extends up the shoulder. No cutting anxieties, and the finished product is doughnut-proof.