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Sunday, February 14, 2016

Mill Tram

Part of routine maintenance and set up of a milling machine is tramming. This is setting up the machine so that the head and table are in proper alignment. Typically this is done by swinging a dial indicator over the surface of the vice or table and adjusting as needed.

Thankfully, there's a time saving double indicator available to speed up the process.

Edge Technologies came up with this awesome set up. This is the miniature model The two indicators are 3" apart. I believe the standard model is 5". This size is good for my mill, since it is a bench top machine. Though slightly less accurate than a 5" spread, it still can set up your machine to a very high tolerance.

Before using it for the first time, the indicators must be set to matching values. This is done with a single magnetic piece that you place on the machine. You swing one indicator over it and check the reading...

...Then you turn the second indicator over the piece and set the dial to the same value as the first. Do this step carefully, any error here will be carried over into the final reading.
The head of my mill is definitely out of whack. Let's just say I was being overly aggressive with a large tool and caused this myself.  The left indicator reads about .018" and the right side is .026". That's A LOT of error. The actual value of the readings doesn't matter, we're looking for the difference between them.

Adjusting the head is done with three bolts behind the spindle.
After adjusting the head, the values are now the same. The head is now set exactly perpendicular to the table.
This is definitely a big time saver. Making these adjustments is actually a little more difficult on these small machines than on, say, a full size Bridgeport. When doing it the old way, there are plenty of opportunities to bump the indicator and skew the readings one way or the other. Very easy to use, and very accurate.

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