Electrically Insulated QCT

(Quick Change Toolpost)

By Chris Krstanovic




After publishing my Mach3 Lathe screen, featuring chuck tool referencing, I have received number of requests to describe construction of my toolpost modification, so I decided to make a quick page to shed some light on it.




To effectively insulate it, it is required to place an insulating sheet under the QCT, as well as to insulate the top mounted bolt from the assembly. See the above and below pictures (click to enlarge).


For bottom sheet I used a piece of 0.031" PC board. If you can't get it, you can use 600-grit sandpaper, or something similar. Important thing to remember is that as the compression load on this item is relatively low (<1000 psi), pretty much anything will work. Just make sure that it is not slippery, so the QCT will be sturdy. Avoid slippery tape.


My setup requires the use of Chicago bolt to attach to my machine. The bolt is insulated in two places: I made an insulating washer from the same PC board material, which I placed under the head. This is quick and dirty, but PC board is made of fiberglass, which is very strong. You can machine or adopt a small plastic washer if you can't get it. In addition to this, the Chicago bolt is wrapped with Capton tape. If you do not have it, any electrical, or even Scotch tape will work. The idea is that the bolt sides do not contact the QCT body. Note: Although PC board may have copper on both sides; they are insulated from each other - i.e. you do not have to grind it off, but make sure the edges are de-burred, as not to make contact.


The compression stress on the washer is more significant (>60000 psi), necessitating the use of a steel washer above it. This spreads the stress across the insulator to something like 3000 psi, which can be handled by just about any plastics. Do not try to use tape to insulate the steel washer. As you tighten the bolt, there will be some slight rotation of the washer, which will quickly damage the tape.


Finally, you will need some means of connecting to the QCT. I am sure everyone will have different way of doing it. I have used a male banana plug in which I tapped 6-32 threads. I did the same to a QCT body, and connected the two with a small set screw. Simple, but strong and effective. It is connected to a pendant. This is handy, as I also use other sensors used for milling etc. By the way, my QCT is wedge type, and it appears to be hardened, so good luck tapping it. Go slow, you only need couple of turns.


I hope this article will be of help to you. You can send me feedback to chris59@novalab.org , which I will be happy to answer if I can.