Trimming In Your Mill


Article and Photos By: Paul Wideman

In past articles you have seen us use our Bridgeport mill a ton of times. The Bridgeport and other similar knee mills can be a very valuable asset to any home or professional shop. But this isn’t your grandpappy’s drill press. A mill needs constant attention, maintenance, and trimming.

Aside from lubricating all of the Zerk fittings and oiling the necessary locations, your mill should be trimmed in on a regular basis. Most machine shops require a machine to be checked out at the beginning of each shift. This may be a little more than necessary for the average home garage or smaller bike shop, but we trim ours in every couple days, depending on the amount of use it sees. We also clean and oil daily. There are numerous places to find a proper schedule for maintenance and cleaning. Here I will focus on trimming in the head and vise.


I start by cleaning the table and T-slots with compressed air and then shooting WD-40 on the entire table.


With the table coated in WD-40, I work over the table in a circular motion with a stone. This will eliminate small burrs and bumps, and highlight the larger ones that may need additional work. This type of stone can be sourced from any number of industrial supply catalogs.


Clean off the excess with a rag.


The four simple items needed to trim in the head are a dial indicator such as Starrett’s Last Word, 1/8” collett, 3/4” open end wrench, and a small mirror. You may be tempted to use your drill chuck, but I would not, as the collett is far more accurate than most drill chucks.

On the side of most indicators is a switch that changes the direction of indication.


We want the direction to indicate from back to front, as shown. This way as we swing the spindle around, any high or low spots will show on the indicator.

I like to start in the front, at the 6 o’clock position. I will raise the knee so that my indicator reads at 0. Be sure to leave enough of your drawbar exposed up top that you can turn the spindle.


My starting point is 0. Anything to the left, or counterclockwise, is lower than zero, and everything to the right, or clockwise is higher than zero.


The four studs on the front of the head regulate the head’s movement left and right. I loosen these just a bit so I can adjust.


The three studs on the side of the ram allow the head to rotate front to back. I also loosen these just a little bit.


From left to right you can see that I am a little off zero; the right side high (in the yellow) and the left side low (in the white). After a hard day’s use, our mill may be out a thousandth or two. I loosened it up a little more for the sake of the article.


To adjust the head left and right you turn the worm gear from left to right, or vice versa, depending on the direction needed.


For the adjustment front to back, you will need your mirror.


To adjust the head vertically (front to back) you turn the vertical adjustment worm gear. You will need to go back and forth from left to right a time or two to dial the head in, as each adjustment affects the other. Patience!


To trim in the vice, I start as I did with the table, more WD-40.


And then more stone.


This gives you a good idea of why you should stone your table and vise. The lighter areas I am pointing to were high areas that the stone is working down. I spent a lot more time working these out.


After this I bolt the vise down in the location I want and start the indicator in the middle of the vise, again at zero, and again indicating from back to front.


As I move the table to the left, the right side of the vise indicates that it is closer to me than the middle of the vise.


The left side of the table indicates being farther away.


I will make the necessary adjustments with a deadblow or brass hammer, and check and recheck until it reads zero all the way across.

Now you’re ready to use your mill. It might take you an hour the first time you will knock it out in a few minutes. Trust me, the time it saves not having to rework parts is well worth it.

Until next time, FTW- Paul JFL

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