Drilling with a spindle and aligning issues on two sided job

[309cnc]

Hi everyone,

I have been doing mostly two sided jobs lately and I have a hard time getting the two sides to line up as accurately as I'd like. My blanks are 1.75 to 2in thick hardwood (Walnut and Cherry) and I am using 3/8 holes and dowels to align things.

The best way to 'make' the holes in the blank and the spoilboard would obviously to drill through the blank into the spoilboard. That way everything would line up perfectly. However, I can't seem to find speeds & feeds to perform this drilling operation. My lowest speed is 5% of 24k rpm = 1200 rpm. Already a little fast for what I am doing but I tried. Result: If I slow the plunge rate down to an almost invisible 'crawl' I don't have to wait very long for it to start smoking. Not good..... If I speed up the plunge rate a little bit it quickly stop the spindle as soon as it grabs the wood fibers. In other words, the spindle appears to have very little torque at that speed.

I have tried brad point as well as regular point drill bits. No luck.

Suggestions?

Here is my workaround (which I am not happy with) so far:

Step 1: After setting my origin where I want it I run a pocket toolpath with a 1/4in endmill to put the necessary holes into the spoilboard about 3/4in deep (about the cutting length of the bit).

Step 2: I then clamp the blank in place bottom side up and run the same toolpath (after obviously setting the Z-height correctly for the bank thickness) and put the same holes into my blank. Also about 3/4in deep

Step 3: After inserting dowels into the holes in the blank I turn it over and align it to the spoilboard by pushing the dowels into the spoilboard.

Step 4: I run the same dowel hole toolpath I ran earlier again and put the holes into the top side of the bank. Again, about 3/4in deep.

Step 5: I machine the top side of my part (various toolpaths with different cutters)

Step 6: I pull the blank from the spoilbard and move the dowels to the other side and mount the blank top side down onto the spolboard.

Step 7: I run the bottom side toolpaths and...... :-(  I find that the top and bottom profile cut are off by about 1-2mm....

Yes, the dowels seem to fit nicely to where I need pliers to pull them out. So probably no play form the dowels.

What I have further done to pinpoint the issue: I took the finished blank over to my drill press and carefully drilled with a 3/8in bit just until I broke through to the other hole. Remember, each side is about 3/4in deep but the blank is around 2in think. In other words, the blank has two blind holes and I continued drilling into one side until it became a through hole. Sure enough.... I can see a small ridge down in the hole indicating that the two holes drilled from opposite sides do NOT line up perfectly.... :-(

So now what? Do I have a small amount play from my dowels after all?

That's the background to my question about drilling with a spindle. How can I overcome the lack of torque at the lowest rpms (which are still a little fast for an ideal drilling operation).....

Any ideas?

[traindriver]

Are the holes offset in the direction that you flip the wood? Such as, offset left to right when you flip the work over like turning a book page? If so, it could be that your width measurement in Vectric isn't exactly the same as the actual width of the wood. In my only foray into the double sided machining realm, I didn't have enough extra stock to put pins in my work, so I had an "A and B" setup. I documented it here axiomprecision.proboards.com/thread/432/fixtures-sided-routing-1?page=1&scrollTo=2315 in the first of two posts about my trials.  One issue I had was that in my first operation, I was cutting the outside profile (that way, I would know exactly how big the piece was after step 1).  I then forgot to take that into account when I located the part for the cut on the back side, resulting in an offset in the features between sides.  Since the pattern was the same on each side, I assumed my part was symmetric and simply cut the same tool path for both sides, but after cutting the outside edge (which I was locating the part off of), the distance to center had changed, making the part act like it wasn't symmetric.  When I finally realized what was going on, I made a second set of tool paths for the back referencing the edges I was locating off of for the side B cuts which were different than the edges I was locating off of for the A side cut.  

[Deleted]

I use precision ground and hardened dowel pins, either 1/8" or 1/4" diameter, 5/8" long and make a hole 0.34" deep in both the spoil board and workpiece relative to the top face. For the 1/8" I use an 0.093" single flute bit and for the 1/4" I use my normal two flute 0.125" bit for both I make the hole a tight press fit that requires pliers to pull the pin out.


Attached are the tool paths for the helical profiles as well as a test file to help you determine the size of the hole to use.

Attachments:

Dowel Pin 250.ToolpathTemplate (6.16 KB)

Dowel Pin 125.ToolpathTemplate (6.19 KB)

dowel pin test.crv (186.5 KB)