Sinclair NT-4000 Neck Turning?

[jeffwhip]

This is my "first rodeo" at neck turning. I just got done turning some necks on my 280 AI Nosler brass over the weekend. The question I have is that most of the brass came out very uniform. The measurement middle-way up the neck is .0125, but when I take a measurement just below the neck, my measurement is .0130.

1. So my question for you guys, does this .0005 disparity really matter? Or is this close enough for govt work?

2. I do have some brass that are measuring .0120 on one side and .0125 on the other because I have my cutter set at .0125. Is that good enough?

3. Do any of you cull the brass if there is a certain disparity in the neck thickness or do you organize your brass according to overall weight?

4. What criteria should I use to cull brass?

Thanks in advance!

[Mikecr]

Originally Posted by jeffwhip

What criteria should I use to cull brass?

First, thickness variance(as measure at same datum on necks).
Second, H20 capacity of fully fireformed brass.

You didn't asy what your chamber neck is, or what your brass measures before turning.

[jeffwhip]

I have a 280 AI tight neck gun stamped .311. Before turning, I measured a few rounds and noticed a min of .0125 so I decided to turn them as little as possible. There are obviously some rounds that are .0120 because there has been no brass removed in these areas. Do I cull these? Thank you so much for the help as this is my first adventure into long range shooting.

[Mikecr]

I wouldn't worry about .0005 variance with a few. You've improved things enough, and there is clearance.
There is just not going to be any gain in further worry about it.

[SBruce]

Hello Jeff,

I'm not sure how you're getting some .013 measurments when the cutter/turner is set for .0125.? Are you using a ball or case neck micrometer? Calibers can give you false readings due to the flat/square edge of the blades.

Did you chamfer/deburr the case mouths prior to turning? That seems to help some too.

[jeffwhip]

Ok,

Here is my exact measurements. I'm measuring my cases with a nice Sinclair/Starrett ball micrometer.

.311 = 280 AI Chamber
- .284 (Bullet diameter)
.026
divided by 2
.013 neck thinkness (maximum)

When I measure the thickness of the neck wall with my Sinclair/Starrett ball micrometer, most of the cases measure anywhere from .0125 - .0130 (before cutting) up to the base of the neck. There are some cases however, that measure .0120 in places. So I set my cutter to cut .0125. This leaves me with a couple of problems:

1. The cases that are only .0120 on one side are not being cut at all so now I have some cases that are .0120 on one side and .0125 on the other.

2. Even though I have the 40 deg carbide cutter to match the 40 deg shoulder on my 280 AI case, when I cut so that there is a slight cut up into the shoulder (as depicted in the Sinclair manual), I am left with a case that measures .0130 right up against the shoulder and .0125 throughout the rest of the neck.

It almost seems as though the 40 deg cutter is not even and is off by .0005" on the angled end against the shoulder. I don't dare go into the shoulder any more. So:

1. Has anyone seen this before?
2. Should I call Sinclair and have them send me another 40 deg cutter?
3. Should I cull out all the brass that measure less than .0125 or does this really matter?

Thanks

[Mikecr]

Did you COW fireform these cases? Curious if your shoulder angles could be too high.
I wonder if you have a standard 30deg cutter(3300) instead of the 40deg(3340).
It would have been nice if Sinclair had stamped a # on the cutters.

You basically have a donut with this that could come into play if you seat bullet bearing there.