Tuning your Machine
Adjusting steps per mm
We often hear people say their parts are coming out too big (or too small). This is frequently due to the wrong value in $100, $101, $101 - Steps per millimeter. While you should be able to determine this number simply by knowing the number of steps per revolution of your lead screw and dividing that by the "lead" of your screw (how far it goes in 1 revolution), that isn't always true. (Yes, these terms can be confusing.) Manufacturing errors, temperature differences and wear can cause inaccuracies. But, there is a simple way to know precisely what to tell grbl.
Step 1: Get a ruler and a pointy bit
You will need a good quality metric ruler with clear markings. It should be able to fit on your router's bed for X and Y. Place a pointed bit in your router or spindle collet. A 30 degree V-bit is perfect but 60 or 90 degree will do as well.
Step 2: Start with your X Axis
Jog to the left as far as you can go and lower your spindle so the tip of the bit is slightly more than the thickness of your ruler above the bed. Set your X Axis DRO to zero. Place the ruler so 0 is directly and precisely under the point of the bit and the ruler is exactly parallel to the X Axis. Use a little tape to hold it in place.
Step 3: Jog your X Axis as far as you can
If your ruler is 300mm and you can move your X Axis that far, jog to 300mm. Adjust this to smaller or larger depending on how big your ruler or machine is but we will use 300mm for this example. The farther you can go, the more accurate your measurements will be.
Step 4: Determine your error and adjust your machine
Carefully find the exact location of the V-bit on your ruler. If it is in between marks, call it .5. This is how far it actually went. Now, a little math. Divide the commanded length by the actual length. If it went 304.5mm for 300mm jogged then you will get 300/304.5 or 0.9852. Then, get the number of steps per mm from grbl settings, $100, and multiply it by 0.9852. In this example, $100 is 250. So 250 * 0.9852 is 246.03. Make that your new $100 value and then repeat steps 2 through 4. When you get the same number 2 times in a row, move on to the next axis.
Step 5: Repeat the process for Y and Z.
For Y, it is very much the same. For Z, you won't be able to go as far so you will have to be satisfied with a shorter measurement distance. Also, you probably will not be able to start from 0 on the ruler. I usually pick an edge on the spindle mount rather than using the point of the bit.
Step 6: Plan on doing this every few months.
As you use your machine, it will wear so you should plan on turning it every few months. Also, as the seasons change, your shop may change temperature which will change the dimensions of your machine's metal components.
If you have digital calipers, you can use them in place of a ruler. You need to figure how to clamp it in place and get the spindle mount to push the moveable part. You will probably also have to use a shorter measurement distance. In theory, you can get much more accurate results. However, we tested this on a CNC router and found that our ruler method was pretty accurate, within 0.1% of using calipers.