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eddief
05-03-2011, 10:30 PM
Chucked up a piece of 1 3/4" OD aluminum tube and turned it (NOT between centers) to see how the lathe leveled up and if there is any bed twist.

I got about .0003" difference over 8" after my finish cut. Would you guys call it good?

Hal
05-03-2011, 10:46 PM
Yea your good to go.

What lathe are you using?

Hal

jackie schmidt
05-03-2011, 11:33 PM
That is VERY good.......jackie

Matt P
05-04-2011, 01:50 AM
Jackie
What is classed as acceptable for gunsmithing ???
Matt P

eddief
05-04-2011, 09:45 AM
Thanks guys.

Hal,

I'm using a Eisen 1440EV. Variable speed is awesome!

Zebra13
05-05-2011, 01:57 PM
Eddie,

It's fine...if you can live with .0000375 taper per inch!

Justin

Butch Lambert
05-05-2011, 08:13 PM
Matt,
It could have .010 taper in 8" and still do a very good chambering job.
Butch

frwillia
05-07-2011, 02:52 PM
Matt,
It could have .010 taper in 8" and still do a very good chambering job.
Butch

Yup.

That lathe is pretty darn level. Neither of mine have ever been that good.

Fitch

kiwi smith
05-07-2011, 10:41 PM
Chucked up a piece of 1 3/4" OD aluminum tube and turned it (NOT between centers) to see how the lathe leveled up and if there is any bed twist.

I got about .0003" difference over 8" after my finish cut. Would you guys call it good?


Hi Eddie,

I notice you mentioned Aluminum tube, and not bar. Also you have mentioned 'cut', rather than ground.

3 tenths seems good, but any metal will have some bow or flex, and when it is being cut I don't think you are getting a true reading of what the headstock alignment actually is. Different types of cutters will also produce a varying surface finish as well, which can influence the ability to measure accurately. Cutters with a more rounded edge will produce a nicer surface finish, but will tend to bow a test bar away more.

I believe alot of factories set-up lathes this way due to the main consideration - time.

If this is the case, then you will actually find that alot of headstocks are set-up deliberately crooked in order to turn a steel test bar of 4 to 6" or more, straight.

To test this theory I set-up my GTR action jig with a Rem action and the 0.705" ground mandrel in it, and dialed the whole thing to run in dead true with 2x 1/10000" Mitutoyo DTI's, 5" apart.

Check, recheck, and check again. Then remove one of the DTI's. Run the remaining DTI along the front edge of the hardened and ground mandrel by advancing the rapid traverse wheel on the apron. On one of my lathes the error/cant over a 4.5" length was 0.0005" (half a thou, or 5 tenths.) On a lathe I purchased recently (Harrison), the error on the same test was 0.0012" (12 tenths!!). By correcting the leveling of the lathe and adjusting the tension on the bolts at the tailstock end of the bed I was able to correct the problem on the Harrison, all with the DTI still reading off the ground test bar for zero runout. It is amazing what amount of variation the bedways will move if not levelled correctly. I think that so many guys must overlook this.

On my other lathe there was adjustment facility on the headstock, to correct the alignment of the spindle bore to the bedways.

What you will find on alot of these lathes that are set-up to turn a testbar true, is that the fly in the ointment is what happens when you come to boring holes? You are not going to be boring a true hole, you will be boring a reverse cone, where the ID 3 or 6 inches in, will be larger than the ID at the end of the workpiece. Don't believe me? Try it some time......

Beware as well, if you are truing female action threads with a lathe set-up with a canted headstock/spindle, as you will be cutting reverse taper female threads, and positive taper barrel threads.
Butch has gone off on a tangent again by mentioning chambering. If you are usuing a 'floating pusher' then you will overcome a crooked spindle, as well as height alignment problems. But the main issue is in the threading phase.

Once I corrected the alignment on my old lathe, I reckon I could 'feel' the difference after correcting the half-thou over 5" factory cant, when final fitting barrel/receiver threads.
Does this explain it clearly enough?

Regards,

Dean.

Butch Lambert
05-07-2011, 11:44 PM
Dean,
Do you want me to start another thread? What is your answer to my question?
Butch

jackie schmidt
05-08-2011, 12:54 AM
Good Grief, My eyes glazed over.

Eddie, your lathe is cutting just fine.........jackie

kiwi smith
05-13-2011, 07:27 PM
Ok, I guess I have assumed that seeing as this is about gunsmithing, and benchrest, and most BR gunsmiths are mainly concerned with the accurate threading and chambering of barrel blanks, I have answered Eddies question with regards as to what would be the best set-up possible for this sort of work.

However, if you are just doing general engineering, or other general purpose work then sure, sounds like your lathe is set-up fine for that, so you're good to go.

However, if the gentlemen out there would care to understand a bit more about where I am coming from in reference to my post above, then for a second opinion I would refer you to Vaughn's 'Rifle Accuracy Facts' book, where he discusses different types of thread fit (Chapter 6). By discussing both theory and practical test application, he showed that a standard 'straight' or 'true' 60 degree type thread on a barrel receiver junction will actually only pull up tight on the first few threads near the recoil lug. In some cases the threads nearest the bolt nose may actually be totally unloaded. Therefore, the whole barrel joint fit will not be fit as secure as what it could possibly be if the threads on the tenon were more evenly loaded.

Having a very slight taper on the threads can help to correct this phenomenon IF it is done the correct way. But, if done with the slight taper the wrong way it can actually make the problem of only loading up only the front few threads much worse. What i was trying to point out is that if you are chambering through the headstock, with all machining steps to be done in one set-up (theoretically best method), then with the way that most lathes come out of the factories the way the threads will likely be cut will be in the format of the worse case scenario, which will exaccerbate the problem of only partial thread loading, no matter how perfectly formed or chatter free you may think your threads are.

With all due respect to those who have posted aready about this, my statement/post above was simply to demonstrate that if your headstock spindle bores' centreline spinning axis does not run exactly true (or where you want it to be) for the best threading (for the purposes of precision gunsmithing/barrel fitting), then you may be not cutting as good a barrel/receiver fit as what you may achieve by taking a different approach.

Dean.