Making a Fixture PT1
 
A fixture is simply one piece material that is used to hold other material while being machined.  It is not magical, it is not difficult, but if you produce a lot of parts it is required.  Making our parts simply by bolting material to the table would be impossible, but by using a fixture we are able to have huge production gains.

Now of course we put a few more bucks into making a fixture than most of our customers, but you can use similar techniques using extruded 6061 and save quite a bit of money.  Also there is no law saying the fixture has to be so big.  It can easily be just a small portion of the table.

Also, fixture pins are great with fixtures, but not required.  Without them getting the fixture back in the same place is tough, but doable.
 
We have a tackle box with all the necessary items to make a fixture.

In the box you will find:

  • #6-32, 5/16, 3/8 threaded inserts
  • Assortment of 1/4 dowel pins
  • Fixture hold down clamps (more on this later)
  • Fixture Stops
  • All necessary bits and reams
   
For fixture material we use aluminum jig and fixture plate, we had an entire sheet cut into 12" x 36" x 1" slabs.

Since they are all the same size and we know we want the fixture centered in the travels we made these stops.  They fit in the front Tee-slot and are used only when the fixture is getting setup.

   
Pulled up against the stop.
   
The fixture is centered.

Our fixtures are 6" longer than the X axis travel, this gives us 3" on each side clamps and such.
   
Because the fixture is so big, the X axis edges are outside the reach of the machine.  So we manually punched some holes into the table edges so we can clamp these down.
   
The plate is now secure, and pretty much centered.

We are saying centered within .125"  tolerance is not critical until the patterns are cut into the plate.
   
While pinning the table we noted the insert and guide locations.

We drill 2 locating holes.  1/4" pins, undersized 1/4" holes.

Although our table has 10 guide locations we usually use just 2 per fixture.  We choose 2 of the most outside ones.
   
Ream the holes.
   
Continue to the next part Making a Fixture pt2