By GCH Machinery on Friday, 25 August 2017
Category: Uncategorized

How to Correctly Design an Internal Grind Process

This month’s guest blogger is Brian Dechene, GCH Machinery’s technical development director ID/OD grinding specialist. Brian can be contacted at brian@gchmachinery.com.

So, you've been assigned the task of buying an internal (ID) grinder for a critical part that you have never produced.  While intimidating, it’s by no means an impossible assignment. With the help of the information in this blog, you can develop common ground with your supplier, and you’ll have the tools to work with him or her to get the right machine for your job.

Before your supplier even thinks about recommending a machine, he or she will ask for a finished part drawing.  As we’ll detail below, the outside dimensions of the part, the surfaces to be ground, the tolerance datums of those surfaces, the part material, and the stock to be ground are critical factors in determining the right machine for your application.

As its name implies, an ID grinder is used to grind the interior of parts such as valves, shafts, fuel injectors, lash adjusters, gears and ball and roller bearings.  Depending on the complexity of the part, your supplier will suggest the best workholding system to use. In most cases, the part is clamped to allow it to rotate around the centerline of the ground surface. Clamping can be done on a face, by a bore, using an expending gripper or an outside diameter. For low-volume ID applications, the work area is designed to allow safe access for hand loading. For high-volume jobs, a robot or dedicated automatic loader can be used, allowing an operator to run multiple machines simultaneously.

With your drawing in hand, your supplier will take a careful look at your part print to evaluate your part dimensions. Internal grinders typically are designed to process a specific range of part sizes. Determining which range your part fits into narrows the number of appropriate machines. 

Next, still using your drawing, your supplier will isolate the areas to be ground and the features that are the tolerance datums. (A datum is a reference point from which measurements are made on the part.)  Ideally, the datums are the surfaces that will be the locating surfaces for the chuck. If the datums are not a workhold surface it may be necessary to add in-process gauging to hold the tolerances. Your supplier can offer design suggestions that will improve the grinding process.

The next area of focus is the surface to be ground. First, the obvious question:  Can the grinding wheel be presented to the surface that will be ground?  Also bear in mind that sufficient clearance is needed for the wheel, the quill and the spindle.  A CNC grinding machine can be programmed to have a path with complex movements controlled by a computer. The feed direction can be in an X axis or a Z axis, or both.  Non-CNC grinders have simple left/right and in/out movements.  Generally, the profile of the ground surface has to be dressed into the wheel, which is then ground into the part. The options for dressing include single-point tools and a variety of diamond rolls. The factors to consider for selecting the dress include tolerances and surface finish; both are detailed in the part drawing. Now it’s time to look at the material callout. The material determines which types of grinding wheel and coolant will be used. Some types of workholding systems require the part to be fashioned with a magnetic material. Soft materials such as brass or bronze require tooling that will not damage the clamping surfaces.

The last factor to examine is how much material (stock) will be ground.  Because grinding is often one of the last operations in part production, earlier processes may have increased the amount of material to be ground. Roundness, concentricity and tolerance stack-ups all add to the point at which the grinding wheel will first touch the part.  The result is simple.  More stock means longer cycle time and potential quality issues. Experienced suppliers will have the experience to offer process suggestions that can lower costs and improve quality.

GCH Machinery is a leading resource for companies seeking to improve their ID grinding operations. Looking to incorporate your ID grinder into a cutting-edge grinding cell?  Need advice on an ID grinding process?  Contact GCH Machinery.

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