Adventure Cutting Aluminum with my CNC

This is my tale of trying to cut some 6061 aluminum to make an aluminum version of a push button depth stop for my Wen benchtop drill press. 

I bought a 4" x 6" x 3/4" thick piece of 6061 aluminum from Amazon. From this I cut the body of the depth stop.  I also have a 5/8" diameter rod of aluminum I used to make the button for the depth stop from.  

Some web research found recommendations for feed speed (17-24rpm), plunge speed (6"/minute), depth/pass (0.03") and bit RPM (13,000) to use when cutting aluminum.  My CNC uses a variable speed Dewalt router that can be set at the recommended speed. The other variables I can set for each toolpath in the CNC software I use (Aspire from Vectric.com). 

My first try was to cut the perimeter and center hole of the press button body from the 4" x 6" block.  I set up the job to cut the part near one corner of the block. The bit I used was a 3/16" spiral upcut end mill.  Two sides of the cut were 1/2 the bit width from the edge. The other 2 sides were full width cuts through the aluminum block.  I thought all was going fine until about halfway through the aluminum. At that point I started hearing more chatter from the bit as it made the full width cuts through the block.  With 1/8" or so to go the chatter was so bad that I had to stop the cut.  I took the block off the CNC, then cut the part free from the block using my band saw.  I then used a spiral flush trim bit on my router table to trim off the remaining aluminum from the bottom edge. I held the part with a c-clamp as I fed it through the flush trim bit.  The bit speed was also turned down to about 13,000rpm. 

First attempt

With the part free from the CNC I could see the affect of the bit chatter on the revealed side. Measuring it with a digital caliper it was close to 1/32" too small in both X and Y direction. the center hole was also not centered between the sides as it should have been.  The deflection from the full width passes was clear.  

With a night to contemplate what had happened I decided to try again the next day. It had been full width passes and ramping down that my CNC had trouble with. The conventional profile cuts had deflected toward the part.  

For my second attempt I started with a block only 1/8" larger than the final part. I rough cut out the body block from a machined corner of the original block using my bandsaw.  

No perimeter passes would be full width.  To avoid having to ramp into pocket cuts for the center hole I pre-drilled a 1/4"d hole (using my drill press) through where the pocket was to be cut. 

Holding the block in place for the cuts started with a t-track bolt up through that 1/4" hole.  A lock washer under a nylock hex nut held the block down tight to my spoil board. To make sure the CNC cuts didn't spin the block while cutting it I separated the cuts into unique toolpaths for each side. While any side was being cut I clamped a block against the opposite side. All profile toolpaths were climb cuts, and only through 1/16" off each side. Twenty two steps were used to get through the 3/4" thick block. All cuts were climb cuts so that any deflection would be away from the part rather than toward the part. 

Once the perimeter was cut I took it off the CNC.  Now I could clamp the part in my low profile vise to cut the center hole. Starting each pass in the center 1/4" hole I had no need to ramp down between passes.   When done I again measured the part and found it to be within .01" of the intended size. The hole was perfectly centered as it was supposed to be. 

Part with hole in top and side. 

Next came cutting the elliptical hole in the side for the button.  I again used a smaller drill bit to drill an opening in the center. With the part clamped side up in my low profile vise I used profile cuts on-the-line to start in the opening before cutting around the perimeter of the elliptical hole. 22 passes again for roughly .75" of depth.  The elliptical hole measured exactly the intended 1/2" x 5/8". In the bottom of the elliptical hole a smaller round pocket was cut for the conical spring. 

Conical Springs.

With all the holes in the depth stop body complete, my attention turned to making the elliptical button.  The button was cut from a 5/8" diameter aluminum dowel.  With the dowel clamped vertically I did a 3D cut to round off the top end of the button.  Then I milled a flat surface down one side of the dowel.  Clamped flat on my CNC with the flat side up I used a 90 degree V-bit to mark exactly where the first 10.8mm hole would be pocketed. 

On my drill press I drilled a 1/4" diameter hole through the dowel.  This center hole would provide room for a 3/16" bit to drop down for each pass before cutting the perimeter of the hole. No ramping needed. 

I used my M12 tap to thread the 10.8mm, hole.  

Next to and overlapping this threaded hole I cut a 12mm hole so the button could slide over the depth stop post.  Each pass started in the open area before moving out to cut the perimeter of the hole.  No ramping needed. 

Holes done, and all that was left was to cut the elliptical shape from the 5/8" round dowel. 

My first try reminded me that a spinning bit in a router will exploit any weakness in how a part is clamped.  I had clamped the aluminum dowel vertically against my vertical clamping jig and a vertical reference fence.  Quick grip clamps held the dowel in place.  This is how I normally clamp down wood dowels.  

Within a few seconds of contact to the dowel the part shook. A clamp fell off, and the part was twisted away from the bit before I could hit the emergency stop. Clearly my clamping strategy had not been good enough. 

I tried again, only now moving my low profile vise to the vertical jig so I could better clamp the aluminum rod in place. 

With the dowel now held securely I tried the same toolpath again.  This time I slowed it down 50%.  A few minutes later I had an elliptical button on the end of an aluminum dowel.

I cut the button free from the dowel at my bandsaw. I then clamped it into my bench vise to file the rough end down smooth.  

Depth Stop Collar Assembled

Almost surprised that the button slipped cleanly into the elliptical hole of the collar.  This depth stop collar works great on my drill press. The strong conical spring keeps the internal threads pressed hard against the threaded post.  Pressing the button in lets the collar move easily up or down the post. 

Low profile vise:  Plywood Vise You Can Make!

HDPE version of this depth stop: Making a Push Button Depth Stop

Comments and questions are encouraged!

4D