My take on inboard and outboard spiders

[Tozguy]

Fitch, Top notch work and photography. Beautiful!

At the other end of the spectrum we have a 4 cent option for barrel work in a 4 jaw.

In my opinion any approach that will prevent a barrel from slipping during the cut has the potential to induce stress in the barrel. Proof of this is to loosen just the outboard spider screws and see if the barrel drops. If the barrel doesn't drop then any outboard adjustment will bend the barrel. Screw tension should be kept as light as possible during dial in and then snugged up evenly before turning starts to avoid stress in the set up.

 

[wrmiller]

Would a 6-jaw set-tru chuck lessen the risk of uneven stresses in the barrel? I've been considering this as an alternative to the inboard.

 

[Stob]

The biggest worry of stressing a barrel using a spider is holding the barrel hard in jaws, then dialing in the outboard end. If the jaws aren't perfectly in alignment on center, then you will be bending the barrel. Plus, there is no guarantee the OD of the barrel is concentric. This is why a copper wire is used to grip the barrel, to allow the barrel to be aligned without stress. A 6 jaw would grip a barrel more evenly, but to me it's not that hard to use a 4 jaw and pay attention how tight you use each jaw.

 

[dave2176]

Would a 6-jaw set-tru chuck lessen the risk of uneven stresses in the barrel? I've been considering this as an alternative to the inboard.

Doesn't that adjust all the jaws at one time like a 3 jaw instead of the independently adjustable 4 jaw? The bore of the barrel is not perfectly centered so you need to be able to dial it in using a DTI. This thread should show the idea. http://hobby-machinist.com/threads/chambering-and-profiling-a-m4-barrel.36968/ You can see I used strips of aluminum to keep the contact point short in the jaws although I like the wire idea Fitch shows. I used strips of aluminum on the outboard end too so the spider won't mark the barrel. Plus the aluminum has a lot of stick to it meaning it doesn't get slippery like some metals.
Dave

 

[wrmiller]

In a set-tru chuck you can perfectly center anything. I have a 3 jaw set-tru already and am considering a 6 jaw to handle more delicate parts.