Wednesday, February 22, 2017

Ignore The Surface Grinding Process And You Lose Money

The surface grinding process is usually the beginning and the end of a plastic injection mold making project. Ignore this and you will probably lose money on the job. You either “pay now, or pay later”, so it makes sense to do things right the first time.

A good foundation is essential for top quality in any application, especially in mold making, where the tolerances may be as little as .0001 in. A little error here and a little error there adds up to become a nightmare. For this reason it is essential to have the best injection mold making training you can find.

Surface Grinding Process
Porter Precision Grinding

Take care of the basics, such as surface grinding, and the rest of the job is much easier.

Injection mold components and the surface grinding process

Magnetic chuck

Before anything can be precision ground the magnetic chuck must be absolutely flat. This is not that difficult, yet most chucks are not truly flat. Because it takes a special technique and a lot of patience to properly grind the chuck, many operators just give up and live with a low spot in the middle or in the front.

Everything ground on that chuck will be slightly off, out of square, tapered and subsequently difficult to work with. Take the time to do it right and you will be rewarded later on.

Precision surface grinding accessories

In the same way that the grinding chuck is the foundation for everything, so the accessories are equally important. If your magnetic squaring block, precision grinding vise, angle plate, V-block or spin fixture are out of square, you have an uphill battle that can become quite frustrating.

You must have a master square of some type to check and confirm that your tooling truly is square. There are ways to check for squareness without a master, but life is much easier if you have something like a 6×6 in. magnetic squaring block to use for grooming your accessories.

Granite surface plate

A quality granite surface plate is another item that must be correct if you expect the components you inspect to be correct. It is very frustrating to deal with a surface plate that has low spots or is damaged from misuse.

Keep the surface plate clean and protected from dust, grit and chips. Watching toolmakers slide their part onto a dirty surface plate and expecting good measurements is a bit odd. Cleanliness is essential for accurate surface grinding.

Surface grinding process for tool steel

Most injection molds are made of either pre-hardened tool steel or through hardened tool steel. Some prototypes are made of aluminum or even special composite materials. Knowing how to grind various materials is invaluable in the grinding process.

Surface Grinding Process
Precision surface grinding

The basic rules are quite simple and, if they are followed, the entire process becomes faster and more accurate.

Rough, semi-finish and finish. It is surprising how many people don’t actually follow this process, or any procedure at all. They just start grinding with no plan, other than to get the job done.

Roughing means just that. Here is a step-by-step plan for the surface grinding process:

  • Suppose you have .015 stock to remove after heat treat and you are grinding a cavity block that is 8x6x2 in. Realize that there are only 6 sides and only 3 basic dimensions to consider. De-clutter your brain by minimizing the information you really need to length, width and height.
  • Stone off the workpiece on at least 3 sides. Pick several holes or details that must be accurately located. Figure out the location of these details from each side and determine how much you need to approximately remove to bring in the dimension relative to the outside dimensions.
  • Assuming that you must remove equal amounts off of all 6 sides to bring the cavity block to size, while maintaining the locations of the datum holes or details, you start roughing. If you have .0075 in. to remove, take .005 in your first pass, with a slow cross feed. Many toolmakers are too cautious and start with .0005 or less per pass. Why? You are roughing, not semi-finishing.
  • Redress the wheel, take another pass on the block and check your locations. If you now have .002 left on the first side, remove .001, let it cool, then .0005, then .0002, then .0002, and finally .0001.
  • This is classic roughing, semi-finishing and finishing. It saves a great deal of time and work. There is no need to continually inspect the part because you already determined how much to take off. With this method you can proceed with confidence that the part will be the correct size and the details will fall into place.
  • Once 2 sides, preferably the largest, flattest ones; very likely the top and bottom, are done you can begin the squaring process.
  • Once you have 3 sides square, follow the same method above for roughing, semi-finishing and finishing.

Surface grinding non-ferrous metals and graphite

Grinding aluminum, copper, brass and graphite all require some special techniques. The non-ferrous metals cannot be dry ground, so don’t even try. With some coolant, wax, or kerosene you can grind to your heart’s content though.

Graphite is easy to grind, both dry and wet. About the only time I’ve ever used coolant was on a Ded-Tru grinder set up to do round electrodes. Otherwise, all you need is a very good dust collector and an open grit wheel, such as a 46G or H aluminum oxide.

Learn more about surface grinding process

 

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