How to Replace a Collet Aligning Screw on a Bridgeport Milling Machine

Another one of those hot topics on the machinist sites is “How do I change the collet aligning screw in my mill?, or “Do I really need it?”. I’m not going to debate weather you need or don’t need the screw; everybody has a different view on the subject.
My buddy Dave was having trouble getting the collets on his mill in and out, and had decided to replace the dog end set screw in his spindle. I jumped at the chance to help, and get some pics. This would give me a chance to rub elbows with Dave and Don Nichols, who both have forgotten more about machine tools (and their use) than I’ll ever know.
I had anticipated this was going to be a big job, but despite the fact that none of us had tackled the job before, it was a breeze. Even taking our time, the whole affair took about an hour and a half. Credit the Bridgeport Operation and Maintenance Manual (see later blog entry for Free Download), and a whole lot of experience from these two guys with the ease of this repair.

 

First step is to kill the power, and remove the collet (we realized this later) Once the spindle nose is off, and you have to knock the collet out, it might cause the bearing preload to change. We got lucky, and it didn’t tighten up much, but it is worth mentioning. The clearance between the nose cap and the quill should be .003″ before you remove the nose cap.This is what the Bridgeport manual specifies, but I have been told this clearance is not real critical (as long as you have some). After checking the clearance, mark the nose cap with a marker, to aid in lining it up on re-assembly.

Here you can see our index mark, and the set screw in the back of the quill. This comes out next.

Heres Dave taking the locking set screw out. Have a towel or rags on your table to catch any small parts you might drop.

Dave tried a standard pin wrench first (didn’t work), these nuts are fairly tight.

Dave had made up this industrial sized spanner before we got there. It allowed enough torque to snatch the nose cap loose.

have somebody catch the nose cap when it falls off the threads.

Inside you should have an O-ring

Heres a shot of the threads, and the set screw hole. The hole was about 90 degrees off the set screw, and we suspect the spindle cartridge may have been worked on before.

This shot is from the front of the spindle, and you can see the alignment screw LOCK SCREW. This is a shallow 1/8″ allen screw, and is a thru-hole. I mention this so that you don’t push the allen wrench all the way through the lock screw, and engage the 2nd screw. You WILL need to use ball-end allen wrenches, because you don’t have a clear shot at the screw; You will be attacking this at an up-hill angle.

The locking screw on the allen wrench. You can see how shallow it is.

Here is the new dog end collet alignment screw that is under the lock screw.

Heres the old screw. The end is bell shaped, and this was causing a rough time inserting and removing collets from the spindle.

Old and new screw, for comparison.

Heres what you will be dealing with (the upward angle).

Dave whipped up this tool to try and get the screw lined up, so we could get the screw started. After a while we gave up.

You can see the concept, but it didn’t work real well. we ended up just starting by hand (finger tips, really). Skinny finger would help alot.

Once the alignment and lock screw are in, check the fit of your collets or tool holders. Take your time here, and don’t set the screw too deep or it will make tooling changes miserable. Once you are sure of the fit, lock down the locking screw, and check the fit again. The lock screw only has 2 or 3 threads on it, so don’t get too frisky.

At this point you can re-install the nose cap on the quill. If you hit any snags, stop and find out why. You don’t want to force anything. You should be able to screw the nose cap on with minimal resistance (if its clean).

Lock the cap down tight, and check the clearance, which should be .003″ between the quill and the nose cap.

Snug the lock screw in the back of the quill good and tight, and recheck everything for smooth Z travel, collet loading, and spindle rotation. At this point Dave whipped up a new drawbar. The one he had been using broke, which caused the problem with the alignment screw.

Cutting the treads.

Drilling for a roll pin.

Finished drawbar.

At this point, all that was left to do was install a collet, and let it rip. All was smooth, and tool changes will be much more pleasant from here on in.

Don explains the Coriolis effect on Dave’s tapping head.

From start to finish (less the time to make the new drawbar) this job took about an hour and a half. It was done with the quill and spindle in the head, and only 4 parts came off the mill (nose cap and lock screw, and the alignment screw and lock screw). Total parts outlay was less than ten bucks. The only “special tools” were a 1/8″ ball end allen wrench, and a pin (or spanner) wrench. From all the discussion on different machinery forums, I had expected a much more difficult repair. It is well within the skill range of most guys, and not real hateful. I hope this will help some of you guys out there that struggle with tool holder changes. Get a buddy to help; the extra hands will come in handy.

Please have a look at the and let BlueChipMachineShop.com provide your special maintenance and tool needs..

4 replies
  1. Tom Pomeroy
    Tom Pomeroy says:

    Thanks for this post!
    Good stuff..no hype.
    I’m going to to this now after 25 years without one!
    Tp

  2. admin
    admin says:

    Tom,

    Thanks. I’d be interested in knowing how you like/dislike having the screw in there. Let us know how the change works out for you. Might take some getting used to, after all that time without one.

    Mick

  3. CROZ
    CROZ says:

    Great post! Bought my bridgport a few months ago and it has no alignment pin, and i have been dreading this fix since i got it, but not now, thanks guys, you rock!!

  4. admin
    admin says:

    CROZ,

    Good luck, it shouldn’t take more than an hour or two once you scrounge up all the tools. Bear in mind: some guys don’t think you need an alignment pin, or are better off without it. I’m keeping mine, and if it breaks, will probably replace it. Let us all know how you make out.

    Regards,
    Mick

Leave a Reply

Want to join the discussion?
Feel free to contribute!

Leave a Reply