Shop Doc – Concentric Battle

Today’s Machining World Archive: February 2007, Vol.3, Issue 02

Dear Shop Doc,

One of the most difficult applications we encounter on the shop floor is machining short parts on our CNC Swiss-type lathes. Usually there isn’t much material to grip onto with the sub-spindle. The part in question is about .100” long, but we can only grip onto .040” of it. We use a tapered extended nose collet that has a .400” long extension on it. We use all of the same tools for every set-up, but our production numbers are not very consistent. Even with the same operator on the same machine we’ve noticed production quantities vary as much as 50 percent. One of the main reasons for the downtime is due to the operator fidgeting with the subspindle collet. They have a hard time keeping the part concentric with the backworking tools. We’ve even started a preventative maintenance schedule to have the operators remove and clean the collet, but we still aren’t achieving consistent production.

Concentric Battle

Dear Battle,

In the CNC Swiss micro-machining world, detail is king because near perfection is often required. Overlooking even the smallest details can cause significant problems. Never assume your machine tools, work holding tools, cutting tools, bar stock, coolant, machine operators or other production components are flawless. Instituting a PM schedule for maintaining your sub-spindle collets was the proper first step in diagnosing your problem. However, my own past experiences make me think that tooling may actually be your problem.

It’s important to recognize that not every extended nose collet is concentric and the collet sleeve is not always accurate.

I suggest trying a left-handed cut-off tool and a standard pick-off collet. A standard collet will have a better chance of being concentric over multiple machines and be easier to consistently install. Also, because the collet is more accurate, the operators will spend less time fighting the correlation between the main side bore and the sub-spindle side finishing bore. When you order the standard length collet, make sure to ask for “extra precision.”

Two inherent benefits of a standard length collet is that the part is closer to the bearings, which will reduce vibration. The other benefit is that the part has less of a chance to “wobble” due to the chucking mechanics of an extended nose collet. With this fix you should see a considerable boost in your production numbers and more consistency across multiple machines. Also, your tool life will increase. When we implemented these new practices at our facility we attained repeatable concentricity within .0002” or better.

David Cogswell
Regional Manager at Gosiger Inc.

Share this post