Question: Optical Alignment

I thought that CNC toolpaths were determined automatically by the CAM aspect of the CAD software?

In one case I know of, the vertical axis is aligned with the horizontal plane by optical means (laser interferometer, for example), as well as the two horizontal axes. Five axis machines include angular motion of the head in two planes, one of which must be aligned with one horizontal axis, and the other must be aligned with the other horizontal axis, also done optically. Physical measuring devices, such as squares, may still be used, but I wouldn't bet on it.

The volume of the machine tool would be mapped out with a laser.That data would be input into the controller for volumemetric compensation. Mike

I have a modest home shop for experimental work that I do. Here is what I use. It may sound primitive, but it is more accurate than most mills can hold and it is easy to set up. The process of aligning the head's vertical axis to the table is called tramming. What I do is clean the table of all debris and wipe it clean. I like to use plate glass, but you need to be careful with glass since it can easily cut you. Lay the glass on the table. To determine the alignment I have a horizontal bar that holds a mechanical dial gauge at one end. The other end fastens to a vertical shaft that is inserted into a collet in the head of the mill. You lower the ram on the mill's head until the dial gauge touches the surface of the glass. Next, manually rotate the mill's spindle and observe the dial gauge at the four orthogonal points on the glass. My mill provides adjustment bolts to fine tune the alignment of the two vertical axis. I keep making adjustments until I am satisfied with the result. The longer the horizontal bar, and the circular sweep of the gauge, the more accurate you can be. I never adjust the mill's head angle once I have it set, which is the first day I get the mill.. It is too much of a PIA to do so I always adjust the work piece to get the desired angle cut I need.

The next step is registering the center of the mill's spindle to some reference point on the XY table. The best tool I like for this is a simple edge finder. The edge finder is a precision 1/2" shaft with another 1/2" puck that is attached by an internal spring. You can manually offset the puck laterally and then turn on the mill at low RPM. Using the table, you walk the puck into the edge of the work, or in my case I use a Kurt vise. When the wobbling puck is in perfect alignment with the work's edge the puck and its shaft will be perfectly in line. You can see this easily with the eye. That is how I get the Y axis on my vise. The X axis has a fence that I register to. When done, I move the mill XY table to the -.25" position for both axis and push the zero button. I am now registered where I want to be. I can get to better than 0.0005" accuracy. If I want to be really fussy I can do even better. It takes about 2 minutes to do this operation. Most of the time is spent inserting the collet into the spindle and chucking up the edge finder. There are electronic versions of this, but I don't like them and they are no more accurate.

To find the center of a hole I use another tool made by Blake called a center finder. It is simply a special dial gauge with a finger that is offset from the center. You chuck up the center finder and manually align the mill's spindle as close as you can by eye. When you turn on the machine at low speed the finger of the center finder rotates inside the hole (or if it is a solid rod you put the finger on the outside edge of the work). You then adjust the XT table until the mechanical needle on the Blake gauge stops wobbling. You can get better than 0.0001" accuracy this way.

There are other tools that you can use. One is an optical center finder. It is a microscope with two cross hairs. It chucks into the mill's spindle and you need to align the instrument by selecting a spot and rotating the spindle to check and adjust the cross hairs to the spot. You can get to better than 0.0001" with a cheap one and more expensive ones are even more accurate.

A wiggler is another tool. It is simple a pointed needle that is magnetically or mechanically captured to a shaft. The shaft is chucked into the mill and turned on. You tap the point until is spins concentrically. Now you have a pointer that is directly aligned with the center of the mill's shaft.

Vertical height adjustment can be done with a continuity meter if both parts are conductive (end mill and the work), but I do it by feel and get within 0.001" easily.

There are other more expensive tools, but they don't offer that much more accuracy. Some have mentioned lasers, but you need to be working with very high precision levels to make it worth the effort. Some are gimmicks. One is a laser pointer that mounts in the mill's spindle and is supposed to allow you to find the exact center of the shaft over the work. The problem is that the beam has a width to it and there is no easy way to get the exact center of the beam. I can get 10 to 50 times the accuracy with the methods described above compared to this laser pointer.

Most of the best methods to align a mill are the same methods used 50 to 80 years ago. They work astoundingly well and are extremely reliable. The big boon for machining has been the digital readouts and the ability to CNC machines (computer automation). The readouts are getting pretty sophisticated these days and are essential ingredients today. The readouts allow you to have multiple zero positions so you can place a vise or several vises on a table and have a unique reference point for each one and switch between them with a press of a button.

I hope this helps answer your question. If you have a specific machine in mind you should also talk to a machinist who is familiar with that machine and the degree of accuracy you need to work to. Measurement systems is one thing, but the machine needs to be able to hold that tolerance to be meaningful.

Good luck!

Marv

I should add that mechanical dial gauges are much better than digital gauges! I can visually resolve the slightest movement of the needle between the tick marks on its scale far better than a digital number, which is usually no better than +/- 1 digit. Quality dial gauges can easily read better than 0.0001" and in some cases down to 0.00001".

That being said, I live by the XY digital readouts on my mill and lathe!!! However, both instruments have their place and each has their strengths and weaknesses.

This was a really nice post.

BTW, the AMSE B89 standard may also be helpful here.