Indexing For Two Sided Machining


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Pretty much everything I know about machining (which isn't too much yet) has been learned from reading and asking questions. However, when it first came to machining two sides of a piece of material, I wasn't sure where or who to ask but I quickly came up with a very simple solution. I don't think this is anything new to many other home machinists but in case it is, this may just help someone else. This time, I took a few pictures to show how I did it.

The challenge in this case was to mill a connecting rod about 3.3438" x .3125" x .1875". The issue was that most of the length of the rod had to be milled down to .125" which meant milling of .03125" from each side. The crankshaft end of the connecting rod had to stay full thickness.


When I drew the part in the Cadd program, I put two points - one at each end of the part. In the drawing at right, the drill point are inside the circles at either end.

I always print and cutout the part I'm going to cut so I have a ready reference for size and placement on the material and mill.


Next, I cut a piece of 1/2" MDF and clamp it down to the mill table. MDF will swell up when it comes in contact with moisture but the whole process is over in and hour or so and this has never been a problem. In fact, I have left the job unfinished overnight and completed the job the next day and the MDF was still in good shape. If you doubt, spray some shellac based primer on it first. It really seals the MDF well and dries very quickly.

Then, the work material is secured with a second pair of clamps. The centerline of the material is marked and I zero the mill on that point.

One of the processes during the machining of the first side is two drill the two .125" index holes at either end. Note that when these are drawn on the Cadd program, they are spaced at equal distances from the center point. Depending on the part, they may be to the left & right or above & below. But, I never need more than two holes.

Next the machining on the first side is carried out. Note the index holes at either end. In this instance, the machining is down towards one end of the material so the index marks don't seem to be symmetrical, but they are.

Once work on side one is complete and the workpiece is removed, the two index holes are exposed. I put in .125" pins in them to locate the material before commencing on the second side.

The material is put in place and clamped down to start on the second side. 

Note, you must think about the process and in which direction the parts are symmetrical. In this case, the two ends of the connecting rod had different sized holes and ends so the material couldn't be flipped left & right. The part was symmetrical top to bottom so it was flipped in that direction.

Before I start machining, I remove the pins.


With the second side complete, here's the result.

You can see that the connecting rod had been reduce in thickness equally on both sides. You'll have to trust me that the ball nose mill cut down the center of the rod is the same on both sides :)

Updated: August 26, 2008