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You probably won't get anything from this tutorial if you're already familiar with the operation of a vertical mill with a digital readout (DRO). For those just starting to use machine tools, or thinking about starting, here's an example of how to apply some handy addition and subtraction and some DRO magic to cut pieces to exact dimensions without having to position the bed or the piece exactly (or bust out your dial indicator).

Generally, we'll be:
 - Cleaning an edge of the piece
 - Measuring its exact dimensions in-place
 - Setting the zero on our DRO to measure out the rest of the cut
 - Cutting, and finally
 - Verifying that what we cut was correct

If you don't have a floor-standing mill with a DRO lying around your garage - head to your friendly neighborhood Tech Shop (http://techshop.ws) where I did the work for this Instructable.

Word to the wise: be very respectful of powered machinery whenever you are around it. There's no need to fear machine tools, but you would do better to have a healthy level of humility when operating them as their power can cause startling amounts of damage quickly if used improperly.

Step 1: How Big Is It? How Big Do You Want It?

This piece of steel was lying in the scrap bin at the Tech Shop, and was almost perfect for the bike lock holder I'm putting together. I need it to be 1.70" long, and its 0.110" longer than that to start with. No problem! That's what cutting is for...

First step is to clamp it down with the long edge sticking far enough out to cut to size. Because this piece is so thin, I used parallel bars to hold it up near the top of the clamp. Make sure to give whatever you put on the parallel bars a healthy tap with an impact hammer after tightening the clamp to seat it correctly.

Step 2: Cut One Edge Smooth

Instead of figuring out the exact position of the cutting head relative to the clamp, then figuring out the exact position of the piece relative to the clamp, I'm just going to cut a little material away. When I'm done with the cut, as long as I don't move the X-axis of the bed, I will know the distance between the tool's cutting face and (now smooth) face of my piece will be zero.

When I started, I knew I had 0.110" to cut off my piece, so I'll make sure this first cut doesn't eat away more material than I need to. Here roughly 0.020" was enough to get me the flat face I wanted.

Step 3: Re-measure the Piece Without Moving the Bed's X-axis, Set the DRO Origin

Now, without moving the X-axis of my mill bed, I re-measured the length of my piece. Looks like it's still 0.086" too long, which means I took (0.110" - 0.086") = 0.024" away with the first cut. So far so good...

Now I'll set the zero position on the mill's digital readout to be exactly 0.086" to the left (X=0.0860) of where it is now - which means if I make my last finishing cut with the head at the origin (X=0.0000) my piece should be exactly 1.700" long.

Step 4: Cut in Passes Until DRO Reads 0.000

Because this is steel, and I wanted a nice finish, I didn't want to hog out all 0.086" in one go. I ended up making several passes, with the last pass taking the least amount of material for a nice finish. For a full treatment on feed speeds and spindle RPM in various materials, consult your copy of "Machinery's Handbook". For prototype-quality work around the shop, a good rule of thumb is "if it sounds horrible when cutting, change something" (speed, clamp tightness, RPM, lubricant, or a combination of all four).

After the last pass, pull the cutting head away and measure the piece again. If you've done everything right, you should be spot on your mark.

In this case, 1.700" exactly makes me a happy maker.

You'll want to give your piece a good going-over with a file to remove any burrs from the edges before continuing... metal splinters really aren't that fun.

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