The “Fast 8s” Lead Screw Upgrade

Another title for this post could be “Don’t Fear the Rebuild”.  It just would need more cowbell.  The new lead screw came in about a month ago.  However, between the fear of taking my machine apart and the fact that something would certainly have to be changed in the software, I put it off.  It should have been done sooner as the upgrade sped things up significantly.  For a comparison, this post shows how the axes performed when I first started the machine up (keep in mind all the axes should move at the same rate in that one).

What made me quit procrastinating is that I decided to add a sacrificial MDF plate on top of the stock bed so everything could be milled flat.  There was an unacceptable amount of run-out from one end of the x-axis to the other on my machine, and this has been suggested as a possible solution (more on that in an upcoming post).  Since everything had the potential to be thrown off anyway, upgrading the lead screw at this point seemed only logical.  I’m proud to say that I got both things done in around an hour and a half.

Keep in mind that I have a ZTW 7×12 model, so if you have the 12×12 things may be a bit different.

The process was pretty easy, something like this:

  1. Loose any excess weight on the Z-axis.  Like a Dremel tool or other spindle that you’ve attached to it.
  2. Loosen and remove one of the uprights including the nut (with two set screws) holding the lead screw on it.
  3. Remove this piece then slide the Z-axis off with Mach 3 or whatever control software you’re using.
  4. Take the original lead screw off of the X-axis and replace it with the new one, tightening down the set screws on the shaft coupler.
  5. Remove the old one and install the new Anti-backlash assembly with the same nuts.  You may need to drill these holes out just a bit to get it to fit correctly.  This doesn’t seem to be critical to any dimension, so it seems like a good solution.
  6. Roll it back onto the X-axis and replace the upright.  Tighten all screws back.  Measure upright position with respect to the back edge to make sure it matches the other one.

Autoprogram your axis with this button

So there’s the issue of getting the software correct.  This was actually pretty easy.  The pitch of the stock lead screw is 1.25mm and the new one is 8mm.  Therefore the linear distance the spindle travels with the new screw is 6.4 times as much as the original (works out to be 5080 steps per inch) for the same motor rotation.  Program this new value under config > motor tuning and setup.

If you’re not confident that you’ve got the right value, Mach3 can actually tune the steps per unit value for you!  Select the settings tab (alt +6).  The button for this is in the bottom left of the screen.  I’d recommend this as a good check for your calculated value, but not the primary way to set your machine.

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Mach3 handled everything very nicely.  Even with a 7 x 12 model and only one of the new lead screws, everything seems to be sped up quite a bit.  The software handles calculating the different feed rates of each motor and coordinating them so you don’t have to worry about that.  Definitely a great upgrade, and it should keep me cutting much faster!  Highly recommended.

Update 3/4/2012: One axis is good, but I’ve now upgraded the X, Y, and Z axes to the F8 leadscrew.  In this post, Check out the results and compare it to the old screws!


  1. Swapping the Z-axis for a “fast 8s” lead screw | JCOPRO.NET - pingback on February 22, 2012 at 5:38 am
  2. Milling Your CNC Router to Make it Flat | JCOPRO.NET - pingback on May 14, 2012 at 10:02 pm
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