Eliminate milling machine backlash: Splitting a Bridgeport feed screw nut

I love my Bridgeport milling machine.  When I noticed a small jump in my table during climb cutting operations I figured something was wrong.  It turns out my Bridgeport milling machine had about .045″ of backlash in the x-axis.  Typically, backlash is adjusted by tightening the feed nut retaining screw. When I tightened the feed nut retaining screw, I was only able to reduce backlash to .030″.  As the one piece feed nut wears, and backlash grows, the feed nut can either be replaced (a new one costs about $36), or split into two pieces.  This allows the backlash to be adjusted down.

For reference, a parts list of a Bridgeport leadscrew assembly can be found here.

I called H&W Machinery Repair in Fort Wayne, Indiana.  They suggested that I split the nut I had on the machine instead of replacing it so that is what I did.  They also told me, they will often see old machines with backlash in excess of .100″!

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zeroed dro

Measuring backlash is easy with a digital readout (DRO).  I simply push the table in one direction and zero the DRO.

030 backlash

When I push the table in the opposite direction, the backlash is displayed.  In this case .0302″- I’ve read factory spec on a Bridgeport should be around .005″.

lead screw nut retaining screw

I need to remove the two screws (I couldn’t focus on them very well) from the feednut bracket.

power feed

The Power feed needs to be removed.  This is a Bridgeport 6F power feed.

unbolt power feed

I unbolt the power feed unit.

powerfeed is off

And remove it from the machine.

remove left side of the table Next, I removed the four screws that hold the table bearing bracket in place.

leadscrew nut

The entire leadscrew assembly can now be removed.  The bronze longitudinal feed nut needs to be unscrewed from the leadscrew.

orginal leadscrew nut

The is the longitudinal feed nut.  Notice it is one piece and slotted in the center.  Newer Bridgeports use a two piece design.

split w hacksaw

I mount the nut in the lathe and split it along the factory cut into two pieces with a hacksaw.

faced nut

Each cut end is faced on the lathe.

burr in threads

The facing operation leaves a burr in the threads.  You can see it in the photos at the top of each half of the nut.

deburred threads

A quick pass with the deburring tools cleans the threads up.

view inside the saddle

Time to put everything back together.  The longitudinal feed nut screw is threaded on the lead screw.  It must engage the key located in the feednut bracket.


Getting everything to line up isn’t that bad.  Next, the feed nut retaining screw and a washer head screw (that retains the feed nut retaining screw) must be replaced.  This is the hardest part of the repair.  I taped the screws to the end of a screw driver and it worked.  You don’t want to drop the washer head screw inside the machine.  If you do, go buy a 8-32 washer head screw from the hardware store for 11 cents.  Don’t pay $8 on Ebay.

put together

The rest of the machine is put back together and the back lash is adjusted.  I tightened mine up to .003″.

I’m happy with the repair and the machine is ready to work!

milling machine