Help! Runout from neck turning lathe

[hkfan45]

Hi,

I am using the Becigneul Case Turning Motor. See http://riflemansjournal.blogspot.com/2012/01/good-stuff-becigneul-case-turning-motor.html

I seem to be experiencing additional runout after I neck turn. My procedure is as follows:

1. Take once fired lapua brass from my rifle, FL size down (no expander ball)
2. Use sinclair expander mandrel
3. Check runout at neck (usually 0.0015)
4. Lube inside of neck and turn using the powered lathe and the 21st century case holder attached to the lathe.
5. Check runout is now 0.0025 - 0.003.

So, it seems that neck turning adds at least 0.001 of runout. Why is this? Am I doing something wrong? Is there a certain technique I'm missing when guiding the neck turning arbor onto the case?

Thanks.

 

[Keith Glasscock]

How heavy of a cut are you taking? Cutting too hard can cause problems.

 

[hkfan45]

from 0.015 to maybe 0.0137

 

[hkfan45]

How much are you cutting per pass?
Are you getting heat?

If your getting heat:
- your either cutting to much per pass
- not using sufficient lube
- the tuner mandrel and expander mandrel are not in good relationship (to tight)

Donovan

The arbor is VERY tight on the case neck. I'm using both the 21st century expander and the Sinclair, but even with lube, it's extremely tight. It's not that hot though. In one pass I'm taking it from 0.015 to 0.0137.

 

[hkfan45]

Could I not be holding the cutter correctly as I guide the arbor onto the spinning case? Am I supposed to be holding the cutter body tight as I push the case onto the arbor, or loose in my hand to let it "float"?

 

[hkfan45]

any ideas?

 

[jlow]

I don't use the 21st Century cutter but why are you using the Sinclair expander, does not 21st Century have their own? You got to pair the expander with the same manufacturer cutter mandrel or it will be too big or too small, both of with can cause problems.

Taking a cut that is 1.3 thousands is definitely way too big of a bite. To get an even cut, you need at least some reasonable speed and with that big of a bite, there is no way that you are not generating too much heat. If you are in a hurry, at least take the 1 thousands off first and then come back for the 0.3 thousands.

For what it's worth, Lapua brass usually does not require neck turning unless you have a custom gun with a tight chamber.

 

[Alex Wheeler]

How are you measuring runout? Are we talking about thickness variation or neck od to case body od concentricity? If the latter, check it after you fire form and I think you'll find your cases very straight.

 

[jlow]

How are you measuring runout? Are we talking about thickness variation or neck od to case body od concentricity? If the latter, check it after you fire form and I think you'll find your cases very straight.

Good questions but regarding how straight the cases are after fire formed, the caveat is ONLY if you are shooting it in a tight neck, not true for factory chamber, in fact the exact opposite