This work was inspired by CNC jobs that need part flipping but the CNC have no edge detection. That's an expensive feature.
The holy grail of part flipping is :
- Not zeroing X,Y,Z (reference) more than one time and \
- Knowing where you are (precision <= 5 thou) after you flip the part.
Hopefully this method will achieve that.
It's an upgrade from the famous custom jig with dowels and through holes in the stock so you match the same holes after you flip the part.
Step 1: Find the Interval You Need to Machine (stock Size)
In my case, I have a 21 inch part to machine from a "stick" of 27 inches of wood. The ShopBot bed has holes in intervals of 3.5" so I'll work with a 7 points distance between the 2 bolts.
This sounds easy and quick but is also very important -- this will dictate the limits that you need to use in your CAM to machine your part. Mind that the cutter will move a bit away from the stock size you define ( I defined 22").
Give it as much clearance as you can. You don't want to accidentally run the cutter into the bolts. It will be likely the end of it. Hopefully not the last thing you'll do in your shop. To be conservative I used brass bolts so if the carbide bit runs into it for some reason, I'm still ok.
Before running your cut, dry-run and pay a lot of attention with the boundaries close to the bolts. Ensure that it's passing far from the bolts while Z is still several inches above your stock or the bolts.
You have been warned.
Step 2: The "template Stick"
This is a fun part of the job, because once you're done, you don't need to think anymore for any cut in similar dimensions.
Basically we'll do the same holes that we'll dig into our stock, in a template stick, so that we can use it as a reference for all other parts.
I got a scrap stick made of 1/2" plywood that was long enough (26") and pretty much dug the holes where the centers match the center of the bolt holes in the CNC bed. The goal here is a as-precise fit as you can. Getting it right first is nearly impossible so I'll show you a trick to correct it later.
For now, just dig the holes and hope for the best. You have a better chance if you start with drill bit sizes a bit smaller than your bolt, and as you go up, try the bolt in the hole.
The most important part is to ensure that both bolts have a snug fit to the almost precise holes (see picture).
Step 3: Try the Template Stick on the Bed
A good way to know if you did your job well with the hole center precision is to actually try to bolt the template to the CNC bed. You'll realize then if you'll be able to just drill the holes in your stock and go home, or if we will need the corrections often needed in the next step.
Encouragement hint : in the third time I did this, I didn't need any correction. Over time and experience you learn how to compensate for your environment specifics and accuracy increases :-)
Step 4: Correcting Imprecise Holes
If you're here, is because when you tried to bolt your template stick to the bed, you didn't manage to match the bolts with the slots precisely. That's ok.
The bad news is - indexing and references are all about precision.
The good news is - this process is somewhat forgiving. As long as we have these 2 constraints satisfied we should be good :
- The first bolt, will fit anyway and should be a snug fit to ensure that the part reference will be exactly that ponit
- The second bolt, is just an angular reference. So if we don't get it right at first, we can carve a small slot sideways, which will allow us to find the bolt socket on the table (see cover image).
Think of it as a reference system in a circle, as long as we know the central point and the angle of the "entity" or "segment", we know exactly where it is.
The way I did it, was to pretty much get my drill and force it gently in the diagonal in the direction of the longer side of the stick. I know, that's likely hard to visualize so here's an illustration of the slot as the cover image to hopefully help.
Step 5: Prepare Stock for Final Drilling
Align the template stick to your stock, start drawing the circles that will guide your drill bit. For extra precision I like to draw a crosshair to find the actual center of the hole and aim the drill bit there.
The logic is - if your template has been successfully bolted to the bed, do the exact same thing to the stock, and you shall be happy.
Step 6: Drill!
Title says it all!
Step 7: Setup and Machine!
A.k.a. - go home!
You successfully held your work on the machine's bed with a precise reference, now when you flip your part, you just need to worry about keeping the same origin as the first job, and happy machining!
I hope you enjoyed this instructable.