Project 9: Fixture Plate

This project teaches the following skills:

How to create a fixture plate.
How to use clamps.
Counterboring.

A fixture plate is a work holding device used to hold thin parts that cannot be clamped in a vise. In prototyping, the plate is usually made of aluminum and is resurfaced and reused. Small fixture plates can be held in a vise and larger plates are bolted or clamped to the table.

Figure A.24 below shows an aluminum plate fastened directly to the machine table with a toe clamp: a low profile clamp that grips the edge of the plate and pushes it against the table. Once the plate is fastened to the table, face mill the plate to ensure it is flat.

Figure A.24: Fixture Plate

Use caution when drilling holes in the fixture plate to not drill into the machine table.

Tips for Fixture Plates

After the plate is secured, machine the face flat to ensure it is parallel with the machine table. If the plate is reused, the face will likely have to be machined again.

It is almost impossible to re-align a fixture plate after it has been removed from the vise or machine table. If you need to reuse the plate to make the same parts, machine all sides flat and square first, and note the datum location on the plate by engraving or stamping letters indicating the datum values on the plate.

Eventually threads in aluminum plates will wear, so use a steel or Helicoil® insert for higher production plates.

Never rely on threads alone to locate a part precisely. Use dowel pins or shoulder bolts to ensure accurate location when machining operations on the opposite side.

Be creative. Use the components above in whatever combination works best for the job. This project shows moving clamps between operations and installing bolts in areas that will be later cut away. Both are techniques that are often used in work-holding.

Figure in Table A.9 shows a fixture plate used to make Project 10: French Curve. This example includes many of the components used in fixture plates.

Clamps hold the plate down. Clamps are versatile, and can be moved between operations to grip the part in different places for different machining paths.

Dowel pins are used to precisely locate the stock or part. To ease pin removal, grind a small flat on the side of the pin. This allows air to fill the hole as the pin is extracted. Without a vent, the pin will be difficult or impossible to remove once installed.

Bolts are used to hold down the part after it is clamped and drilled.

Table A.9: Fixture Plate Machining Parameters
Fixture Plate: Job-1 Setup

Datum: Upper-left corner of fixture plate.
^Op-1 CTR Drill	Center drill all holes.
^Op-2 Drill	Drill .25 holes (3X) for dowel pins. Drill these just deep enough so the dowel pins extend about .25 inches above the plate face.
^Op-3 Drill	Drill the holes for the #6-40 bolts that will be used to hold the part down while the OD is machined.
^Op-4 Drill	Drill the holes for the ¼-20 bolts used to secure the custom clamps.
^Op-5 Tap	Tap the #6-40 holes.
^Op-6 Tap	Tap the ¼-20 holes.