How To Grind A Workpiece Square On A Surface Grinder

Nearly every injection mold making component has, at one point, had to be ground square in order to be further machined. Knowing how to square up a workpiece on a surface grinder is very basic, and absolutely essential. The surface grinding process must be followed closely if quality results are expected, there are not many corners to cut.

If the cavity block, for example, is out of square, every other operation done will be affected. The Wire EDM, high speed machining, Sinker EDM, and assembly all depend on the cornerstone work done on the surface grinder.
Most injection mold components are able to be ground on either a manual surface grinder or an automatic CNC wet surface grinder. In either case, the process is similar.

How To Grind A Workpiece Square
Etalon Squareness Checker

Like everything else in a mold making, there are many approaches to a problem, and this is no exception. The trick is to do it quickly and accurately, without endless checking.

3 main ways to square up a workpiece

  • Precision vise
  • Angle plate
  • Magnetic squaring block

Each of these methods works well and are quite interchangeable. Very often, the size of the workpiece determines which method works best. You just wouldn’t want to grind a tiny core block using a magnetic squaring block on a large wet surface grinder, for example. Good mold making training makes life much easier in the shop.

How to square up a block using a precision vise

Supposing that you have a heat-treated piece of S-7 tool steel that has been milled reasonably square, proceed as follows:

  1. Remove any obvious burrs or dings from one of the largest, flattest surfaces. Grind this surface flat.
  2. Grind the opposite side to your desired size. You now have two flat and parallel surfaces.
  3. Clean off one of the longest perpendicular surfaces to lay on the bottom of the grinding vise. It is a good idea to indicate the workpiece once it is mounted in the precision vise. Set the workpiece so that one end is protruding beyond the edge of the vise and the other inside the vise so you can lay it on that side.
  4. Now grind the long side so that it cleans up, set the vise on it’s side and grind the 4th side clean.
  5. Now you can simply use the magnetic chuck to hold your workpiece, or use the vise again, if this works better.
How To Grind A Workpiece Square
Kunz Square Control

Obviously, the precision squaring block, angle plate, and magnetic squaring block must be absolutely square for this process to work reliably.

How to square up a workpiece using an angle plate

  1. Follow steps 1-3 above, but use a C-clamp or Kant Twist clamp to hold the workpiece to the angle plate. This method is rather old school, but still comes in handy once in a while.

Grinding a workpiece square using a magnetic squaring block

  1. Follow steps 1-3 above, only now you use the magnetic block to hold the workpiece. You can also avoid turning the block on it’s side by using the rail to align your block, once you have the long perpendicular side ground clean.
  2. A tip for bigger cavity blocks is to use the magnetic squaring block side rail, but mount a 1-2-3 block on the opposite side. Now run a screw or threaded rod through the 1-2-3 block to push the workpiece against the side rail. This helps overcome gravity and saves your hand from having to push the block against the rail, while turning on the magnet.

Checking for squareness

There are numerous methods for checking squareness, each one has advantages. The surface grinder can do very high quality work, but it requires skill and patience.

  • Use a square cylinder and dial indicator mounted on a special height stand
  • Use a magnetic cylinder mounted to the workpiece and check with an indicator
  • Compare perpendicular sides with a dial indicator mounted in a height stand

For anyone interested, here is a link to the National Bureau Of Standards, which regulates metrology in manufacturing.


  1. I had a discussion with a instructor at a local college in regards the best approach to grinding a precision grinding vise. A class project.

    After milling and heat treat:

    One option was to start with the long side and out side of the vise that would generally sit on the surface grinder magnet in most situation; grinding this surface first and square the rest of the vise to this surface.

    The second option was to start with the side opposite that mentioned above which would be the long and inside of the vise and then square the rest of the vise to this surface.

    Both would work but curious how Hermann Schmidt would/which process/application they apply to their product.

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