in , , , ,

High Precision Bores and Finish Grinding

Working for an international OEM supplier of the world’s most accurate precision rifle barrels. 

Brief: 

  1. Outer diameter clamping to grind bores in high precision bolt carriers for modern sport rifles.  
  2. Inner diameter clamping for finish grinding the outer diameter. 

Challenge: 

The two challenges are:

  1. Accommodating the extended part length
  2. Completing a second finish grinding step without damaging the first
Sport Rifle Bolt Carrier Grinding Group
Sport Rifle Bolt Carrier Grinding Group

Northfield Solution: 

The first operation involves grinding a bore to finish size. This bore houses the critical components that allow the firearm to function properly. 

We built a Northfield Model 620 Sliding Jaw Chuck with an extended air cylinder housing. This accommodates the long part length. The hardened and ground top jaws clamp on the rough outer diameter of the customer’s part.  

Custom Northfield 620 Air Chuck
Custom Northfield 620 Air Chuck

During loading, the part is set down on a hardened and ground axial locator, ensuring an accurate datum location each time.  

Secondly, this customer also requires work holding for finish grinding the outer diameter of the part. that they clamped on using the model 620 sliding jaw chuck in the first step.  

Custom Northfield Collet Chuck
Custom Northfield Collet Chuck

We achieved this with a custom internal grip collet chuck with a collet diameter of .5232in. This chuck clamps on the finished inner diameter while the other end is supported with a center. The operation grinds the whole outer diameter of the part, producing an accurate O.D. dimension. This ground diameter is one of the most critical dimensions required for the firearm to function reliably.  

 

Do you have a multi-step machining process that requires high precision work holding? Then get in touch.  

in , ,

Northfield Chucks Help Dana Corp Stay On The Road To Success

Dana Corporation

Manufactures drive shaft components for the big three auto makers. At Northfield, we’re proud of the part we play in helping them solve their production challenges.

8000rpm_Diaphragm_Chuck

 

Challenge – Getting a grip on the situation

Dana needed a chuck to be mounted on a balancing machine holding a drive shaft on one end, gripping the component called a slip yoke. These drive shafts, for the Daimler Chrysler Corporation, were complicated by several factors.

  1. The shape of the part – long and thin –
  2. The grip force required to hold a 5ft. long drive shaft
  3. The accuracy 1/10,000 T.I.R.
  4. The speed – as high as 8,000 rpm – during the balancing process.

Normally, balancing operations are performed at only 900 rpm, but Dana needs to balance this part at simulated road conditions. Since the shaft will deflect (bend) at high speeds, this is the state they want it balanced at. For production cars, drive shafts typically rotate up to 3,000 to 5,500 rpm. This component, used in Daimler Chrysler’s high-performance stock racing cars hit 200 mph with their drive shafts rotating at 8,000 rpm.

Solution – Sometimes, two chucks are better than one.

To solve the problem, Northfield engineers decided to use a diaphragm chuck, rather than a sliding jaw chuck. Before making the final decision we conducted several tests using solid models on our CAD systems. This ensured that our diaphragm chuck wouldn’t lose grip force at these high rpms.

Our solution consists of two separate diaphragm chucks, with two separate sets of jaws, mounted one in front of the other. This  accommodates the long, thin shape of the part. With one set of jaws gripping near the front of the piece and the other gripping near the rear.

Results

The result is accuracy of a 1/10,000 T.I.R. and repeatability to 50 millionths of an inch and extreme rigidity.

The chuck also uses a male spline driver mounted in the center of the chuck. This helps drive the shaft on start up and stop, and guide the shaft into the jaws during loading. These drivers are .002in. undersize of the internal spline and must not influence the centering of jaws. The spline driver is a gauge quality piece of tooling supplied by Michigan Spline Gauge Co.. Providing a concentricity of less than 50 millionths with the chuck.

In Dana’s production car area, the chuck runs the same component. In the stock car application it is used to run a several different components. Parts and tooling can be switched and still maintain an accuracy of less than a 1/10,000 T.I.R.

Finally…

This chuck was designed to be air opened with a self-contained clamping system to meet Dana’s safety standards. It uses the spring pressure of the diaphragms for clamping that will hold the part in place if air pressure and power are lost during the balancing operation.