I am trying a new clamping technique I have seen Terry Mayhugh use to machine flat backed parts that require machining all around the outside boarder- temporarily bonding the work piece to a chunk of MDF. My usual way of machining a part like this is to clamp the work piece to the table with some sacrificial material behind it and clamp beyond the machining boundary. I leave tabs connecting the final piece to the work piece so that the part is held during all machining operations. The problem with this is that these tabs require post machining operations to remove them. I usually cut them off with the band saw, then hand file the last remnants of the rough sawn tab. I am not very good at this and usually file some of the surrounding area or do not get a good blend where the tab was. I figured the Cylinder head cover plate would be a good part to experiment with-it is about a 1/4″ thick and 1.75″ X 3.25″.
I first used the fly cutter to get a nice flat surface on the back side of the part, then used 5 minute epoxy to bond the flat side to a block of MDF. I made the MDF block narrower than the work piece so I could use some shims to space the work piece up from the vise jaws and use them to establish my Z axis zero point. I did not clamp the work piece to the MDF, but instead laid a thin film of epoxy on both the MDF and the work piece, them lightly pressed them together to remove all air. Clamping has a tendency to squeeze out the epoxy and I am not interested in maintaining a dimensionally accurate bond as I will be spacing the work piece off the vise, not the MDF.
Using a fly cutter to prepare a flat rear surface
5 minute epoxy to secure MDF to work piece
I am concerned about the use of coolant while machining as MDF acts like a sponge, soaking up water, swelling and losing all dimensional stability. I fear that using even a light misting may cause problems as the machining operations will be about 45 minutes. On the other hand I don’t like running roughing operations where I am removing a far amount of material quickly without coolant; the cutter will load up as the temp of the work piece rises. In the end I rubbed the MDF down with light machine oil and used a small mist of coolant. Let us see what happens.
Work piece clamped in the vise with parallels used to provide proper spacing from the vise
OK, lessons learned. Using both sides of the vise to level a work piece does not work. In my case the clamping jaw is taller than the stationary jaw and the part was machining uneven. I noticed this early so I switched to using parallels under the MDF as is traditionally done. However, due to my caviler bonding process, that is using no clamps, the bottom of the MDF is not representative of the plane of the back of the work piece. Also when I switched to parallels, the work piece was lowered and I did not reset my Z axis zero, so the through holes did not go quite through the part. this can be seen in the last picture below.
Oiling the MDF was not sufficient to prevent water damage, As seen below the outside of the MDF swelled .055″.
MDF absorbed moisture and swelled even though only a light mist was used for cooling
Evidence of the swelling of the MDF can been seen in the final part. The final machining operation used a 1/16″ flat end mill and the part rose with respect to the cutter by a total of .023″ between the commencement of the machining and the end.
.023″ trough due to the MDF swelling and the work piece rising during the machining process.
So where do I go from here? I do like the idea of using MDF as a machining substrate as it is much cheaper than using a piece of sacrificial aluminum, for example. The part I attempted the technique with was relatively small and a little moisture on the MDF and the resultant swelling had an outsized impact on the final result. Do I attempt to seal the MDF somehow? Paint? Is it becoming more trouble than the effort saved? I have further experimentation to do.