Neck turning Q (again… sorry!)

[bow shot]

Ok, I'd like someone to clarify this neck thickness thing:

After turning to a certain neck-wall thickness, what degree of variation on one, individual case should I expect?

ie.: I just turned some brass that originally measured 0.0130" - 0.0145". My target dimension was 0.0125". After turning (3 passed), I find my average, single case varies 0.0125" - 0.0130" (delta of 0.0005").

Is that variance, per each individual case normal, or to be expected… or "not good enough"?

Laupua 6.5-284 brass
new K&M expander
new K&M mandrel
K&M cutter of unknown age
faint imperial wax
drill-turning @ 2 rps.

 

[watercam]

First and foremost, are you measuring with a micrometer with a ball end? If you are using a standard caliper you are probably in the right neighborhood. Seat a bullet and measure across the full width of your neck and do the math - subtract the diameter of the bullet and divide by 2. If it is not what you expected, make adjustments. Measuring accuracy is paramount and the right tool is necessary if you are trimming to a chamber neck clearance. The larger the measurement the better, ie the full loaded neck diameter.

 

[jlow]

I assume you are measuring with a ball micrometer? If you do a proper job, you should be able to get consistency around the neck in the 0.0002-0.0003” range, 0.0005” seems a bit much but not horrible.

I assuming that you first size the complete neck down (did you – this is important?) and then expanded with the proper (paired) expander, and then use the proper (paired) turning mandrel, and your cutter is sharp (I see your list so you are OK there). At 2 rps, that is not fast and so perhaps you are taking off too much in a single session. When you do this, the neck will heat up and hot necks will expand causing you to cut more than you normally would assuming that. I find that it is best to take one or more modest bites first and then finished off with a small bite.

I think you are close but if you want that last bit of smooth cut, take that last turn with a very minimal cut, that should get you there.

 

[bow shot]

Q: "First and foremost, are you measuring with a micrometer with a ball end?"
A: Yup, a Starret 0.00001" with ball end

Q: ...did you do everything right?
A: "yes"...I know I'm giving the short answer there, but that's because:

I'm just curious if that delta is normal, good or bad... maybe the big dogs only see .0001"?

 

[artbosco]

5/10,000ths is absolutely nothing to worry about or lose sleep over. It's so miniscule that you'd need a high powered microscope to see it.

 

[jlow]

Read my reply here:

http://forum.accurateshooter.com/index.php?topic=3844505.15

 

[bow shot]

That's a good one JLOW…

Guess I should review my methods and variables...

 

[bow shot]

Did a method and variables review last night. Result: found my micrometer was not repeating well because the thimble tension was varying. Fixed that. Now getting much more consistent readings, varying no more than 0.0002".

Thanks for the help folks!

There was good, good, food for thought in that link you provided JLOW, thanks very much for that.

 

[jlow]

I am happy to be of help.

interesting to hear of your thimble tension problem as I recently had a similar problem. Seems the micrometer I had recently brought must have been sitting on the shelve for a long time as the lubricant had congealed/hardened and was giving me similar problems. Send it in for warranty service and it is now working perfectly.

 

[BoydAllen]

Back in the day, I had a college course in basic machine shop, from an experienced teacher. One of the things that he demonstrated was how micrometers work and are adjusted. Jump forward several decades, a friend gave me an electronic mic., that also reads manually, I think because it was not reading well enough so that he would feel right about selling it on Ebay. After fiddling with it some, I figured out that the thimble had been backed out, quite a ways. It had been sold as surplus from a large aircraft manufacturer, and some employee was probably tired of it and made sure that it needed replacing. Anyway, I was able to readjust it, and reset the zero, and it works just fine. I also have some gauge blocks that have been precision verified to check it at various thicknesses. Knowing how tools work, and how they may be adjusted can come in handy.

 

[jlow]

Being handy is always the best way. I can fix most things except the really complicated stuff where it is best to know your limits. What is key in most cases is actually understanding how and why things work, cook book is always a poor idea. In my cases since it was an expensive micrometer under warranty, I had no thoughts of trying to fix it.

 

[bow shot]

Aye!!