CNC Skills: Origin Points — Part Two: Finding and setting the Zero Point

The origin points on your CAD drawings and the material that you’re cutting have to match. Several methods, tools and techniques make the job easier.

In the first post in this CNC Skills series on Origin Points, I emphasized how critical reference positions are for digital woodworkers. When you’re working on a drawing in CAD, the origin point is at the intersection of the X, Y and Z axis. All measurements — positive or negative, begin at that point. By the numbers, that’s X=0,Y=0,and Z=0. It’s from that position that the piece you’re cutting is positioned.

As you’ll soon see, there’s a lot of ways to do this. It’s a good practice to understand and be able to use a number of techniques, even if you end up adopting only a few as your methods of choice. The first challenge is setting the intersection of the X axis and the Y axis on a board in relation to its position in your CAD drawing. We need to establish an origin point on the corner of a board. There are a couple of advantages to using a corner. For one thing, once you’ve found it, it can be used as a reference point to accurately move your origin anywhere else you might want to on the board. Second, if you’ve setup stops on the bed of your CNC so that other boards will register to the same position, then you’ve made it easy to mill multiple boards. For example, you could use four boards and machine four table legs just as easy as one.

Because the wood boards we work with usually have edges and corners, we’ve got a handy physical reference to work with. Just like the intersection of the X and Y axis in your drawing, a corner is just two edges intersecting at 90 degrees. That makes the corner of a board a handy place to set your origin point.

The process is straight forward. Find one edge, center. Find the other edge, center. Reset your machine to X=0, Y=0. Though it sounds simple, it turns out to be an interesting challenge to accurately put the center of your bit or mill over that edge. Fortunately, it’s a well-known problem in the machinist world and a lot of thought has already gone into this. The result is that there are several methods and tools to choose from that deliver varying results. I have several on my list. Let’s have a look.

Manual Methods

Use your eye. Sight along one side of a board and center the bit.

By Eye… 

Look straight along one of the edges. Get close to the corner of the board you’re cutting and line up your eye along the edge and the center of the bit you’re using. Do the same for other axies. With practice, you can get okay at this, but as you’d expect, this is just an approximation and it’s not very accurate. If you have enough waste material left on your board this might be good enough for a one off part. But, if you need to repeat it may not be good enough. Here’s something better…

Using a known diameter bit, bring it to the edge of your material until it just touches. Then raise the bit and move it one-half the diameter of the bit to find the center.

By Touch…

The idea is to move your bit right up to the edge of the board you’re cutting until there’s no gap. This method can work pretty well but it takes some time to make minute movements of your spindle to get it just touching the edge.

Do this one axis at a time. The idea here is to take advantage of a readily known measurement, the bit that’s in your collet. A 1/4 “ or .250” bit for example. You move it along one axis until by eye you’re close to the board. Then by moving .001” or .01mm at a time get close to feel the bit’s just touching the board. You can check by rotating the bit until you feel a tiny resistance. Once there, set the axis to zero. Raise the bit above the board and then move 1/2 the diameter of the bit farther in. In this case .125”. Then reset to zero. In practice, I find that this works pretty well. But, I get even better results if I use a small flashlight behind the bit to see the gap for final adjustments, or alternatively use a piece of paper with a known thickness as a feeler gauge between the bit and edge. One the paper moves when rotating the spindle by hand, you’re there.

Thanks to the machinist world, there are tools designed for this specific job. Most are inexpensive and simple to use.

Mechanical edge finders will move from their center position once it touches and edge. Once it moves, you just need to raise it up and move one-half the distance to find the center.

Use a Mechanical Edge Finder…

Edge finders are specialty tools that you mount in your spindle or router that have a moveable piece held by a spring that shifts slightly once you’re touched an edge. Just like the visual and touch method, these tools also use a known offset based on the diameter of the probe. Often this is .200 inches. Once you find an edge, then move .1 inches and you’ve centered your bit. Edge finders come in mechanical form — where the bottom of the finder jumps out once it touches and edge. And, electronic versions where a LED light goes off once it touches. Unfortunately, the electronic ones only work on metal. To run these make sure your RPM of you spindle is set to the recommended speed. Typically this is 1,000 RPM. This may not be possible if you use a router on your CNC. How well do these probes work? With a little practice, pretty well, actually. Here are a few examples…

Edge finders are not just for edges. They can also be used for finding the center or hole. A task that comes up frequently in digital woodworking. Here’s an example of how to do this…

Wigglers…

There are other types of mechanical edge finders. One of the most interesting to use and watch are wigglers. Their probes spin concentrically. Once they touch an edge they wiggle and spin around dramatically. Just like offset edge finders they work well. Fun to watch in action…

3D sensors…

Much revered in the machinist world are the Haimer 3D sensors. Essentially, they’re super accurate calibrated dial gauges that work in all three dimensions. As you’d expect with this kind of precision German made instrument, they are expensive at $400-600 and up. The results are typically within .002” to .004” accuracy. They are also very versatile for calibration,  tool setup and evaluating part accuracy. On my wish list for fine-tuning a CNC and most of the fixed power tools in my shop. Watch the videos below and you’ll see why.

 

Sometimes you want to set your origin point on the center of a hole rather than on an edge of a board. There’s various methods and other specialty tools available just for that purpose. Some of the above tools can be used and there are mechanical center finders, but you also find centers with an angled dial indicator or a centering microscope.

These tools and methods give you accurate results. There are plenty of times when that level of accuracy is needed so it’s important to have techniques and tools to help you find and set an origin point. But, I’ve found that in working with a CNC day in and day out in a woodworking shop that these processes can be a little slow. For that reason, I regularly use a couple of quick techniques and new tools to find my edges. I’ll get more into that in my next post on Origin Points.

To see the entire series on Orgin Points, click here.

— Tim Celeski