Fixtures for two sided routing (Part 1)

[traindriver]

A local trophy shop asks me to do special projects for them from time to time and although it is a paying gig, I do them mainly just to get the experience.  The most recent request was for a 4" x 5" plaque to hold a coin with a hole in it so the coin could be seen from both sides.  The coin is held in place by a piece of acrylic on each side, which means the inset for the acrylic needs to line up on both sides.  How hard could it be, huh?  I've got a cnc router!  Wellll, let me tell you, I made it harder than it should because, in an apparent fit of delirium, I had put a value of  .050 in for the Allowance Offset for the cut out tool path.  The result is that when I flipped the part over to rout the back, the step wasn't center of the hole.  I consulted with a machinist at work, who recommended an A/B load fixturing strategy, but that didn't make a difference.  I finally measured the outside of the finished part and found it was undersized, so I then started looking at my toolpaths and found my error after many hours and scrap mdf test pieces.  When I finally found my mistake, the fixturing suggestion worked great, so I thought I would share it:



 In my V-Carve model, I added three 1/4" holes for the front and back and put them touching the part .  I then set my XY origin and never messed with it again.  First, I put the holes for my fixture in.


Next I put the dowel pins in.  I realized that since my blank is oversized, and I wanted to be sure to rout all sides of it, I didn't need to put the dowel pins on the front side (A side), so I left the dowels out and partly covered the holes with the blank so I knew I'd be taking material off all sides of it.  Note:  parts are held down by double sided tape.


I then put a part that has the front side completely machined from the A side, flipped it over, and located it against the dowel pins on the B side.  




Continued on next post . . . 

[Deleted]

If you have the option, using dowel pins within the wood is a great option instead of just for alignment on an edge especially for two-sided machining. The reason is that edges are not consistent and holes can be made very accurate and then the pins also provide lateral support while machining. I make the holes 0.001" undersize in the spoilboard, clamp the workpiece down and do one side as well as alignment holes, flip the part and do the other side. I pry the part off the spoilboard gently with a putty knife and pull pins with pliers with a vertical motion only so as not to enlarge the holes. I've written more on using dowel pins in other posts. Often you can put the holes in the waste material but sometimes you can also use it as a feature for screw holes. I use 0.250" and 0.125" precision ground and hardened pins with holes milled using 0.125" and 0.093" bits respectively. They're really great for tiling projects larger than your machine too.

[zaxis4]

I do double sided machining using wedge clamping. Once you are set up the positioning is exact with no holes drilled. It also works well for inlays that require several steps with different species and you need to be exact with the V bit each time.

[traindriver]

Aug 22, 2017 10:32:14 GMT -5 zaxis4 said:

I do double sided machining using wedge clamping. Once you are set up the positioning is exact with no holes drilled. It also works well for inlays that require several steps with different species and you need to be exact with the V bit each time.

I've not tried wedge clamping, but I've seen posts of it, and it looks like a useful method, and I thought about milling a shallow pocket in my spoil board and trying edge clamping, but since I had to put an eliptical roundover on the top and bottom edges, as well as completely cut the outside of the shape, I couldn't figure out how to wedge clamp without milling into my clamps.  I never turn down advice, even if I think I already know it, so I would appreciate any advice for using wedge clamps for this kind of thing in case i run into something like this in the future (which I imagine I will eventually).  I will definitely remember the tip for inlays of different species.  

[zaxis4]

I put a piece of .75 MDF on top of the spoil board strips held in place with t-slot clamps. The size of it depends on the size of the job. I made mine the same size as my table. This has a pocket milled down about .125 to get a flat surface and to match my "wedge frame". A wedge frame is made out of MDF, again depending on the job size. The MDF for the frame is .5 in thickness. The example for a frame is shown elsewhere in a post. ("Tips and Tricks"-wedge clamping.) While you are cutting your wedge frame out, wedges are cut also. You draw up your wedge frame in Vcarve. I used an angle of about 4 degrees. After several jobs I have accumulated several sizes of wedge frames so every thing is reusable. This whole system is clamped to the table with t slot clamps. They are completely out of the way of the cutters while using the frame. The workpiece is wedged in the frame and milled. If need be it can be removed and flipped or put back so that 0,0 is always spot on in the same place. Works for me anyway. I did a compass rose inlay that was very delicate work and it indexed perfectly.