View Full Version : Looking at used Bridgeports this week

05-01-2006, 08:28 PM
I've got a fairly decent sense of the obvious places to check for wear, back lash, etc. but any tips and tricks on the non-obvious?

Also checked prices on Bridgeport parts and scared hell outta myself. Just by that price list alone I'll know what I won't want to replace!

John Stevenson
05-01-2006, 09:34 PM
Release the 4 bolts on the front of the head and tip it both ways.
Get someone to support it in case the worm wheels has teeth missing.
Common fault.
Another is check the quill power feed in al ranges and the trips.
There are really weak points on a bridgy, in fact the book says don't feed with a cutter greater than 3/8" which is a bit of a joke.
Check the knee feed whist watching the screw, too much play means the shear pins are in a bad way.

Check play in the middle of the table and at both end travels in case some gorger has tightened the screws and gibs up in the most worn part, if so the bed will be really tight at the ends.


05-02-2006, 09:44 AM
Thank you sir!

A major problem I foresee is these machines are in a dealer and fairly certain they aren't under power.
Unless there's some sort of guarantee I'll just be looking at a large amount of cast iron with a power cable attached. Tell 'em as much too.

John, I'm impressed.
One response from you and the other 100+ cleared off!

At your service!

05-02-2006, 02:35 PM
Here's some notes I wrote a while back for checking out BP's. Keep in mind that the one in question had a lot of backlash.

Paul T.

You can work with a machine with 0.050" of backlash and a DRO but its going to be a pain in the rear. That's an extra half a turn of the handle every time you reverse position. Its possible that the nut may be adjusted or reworked to get rid of some of the backlash, but unless you are getting a great deal on the machine I'd keep looking.

When checking the backlash, check it both at the extremes and in the center, this will show you how much the lead screw is worn. Personally I wouldn't buy a used machine with any more than 0.020" backlash on both axises unless you enjoy repairing machine tools or working on sloppy ones.

Also take an indicator with a magnetic base, put in on the table and indicate on the inside taper of the spindle. Rotate the spindle slowly by hand. You should see less than 0.001" of total runout, and I like it to be way less. Any more and the spindle is either bent from a crash or worn on the inside taper. Spindles can be reground, but figure $300. to $500. to do this.

Run the machine and listen to it carefully, and as suggested above, make a cut if you can. Listen to any abnormal noise from the spindle bearings, which are at the very bottom of the spindle. If its a vari-speed machine, run it through the speed range and listen for any rattles, which shows wear in the vari-speed. This is fixable, but for non-Bridgeport machines parts can be a hassle.

Engage the quill power feed and see if it seems to work. Unfortunately the power feed units on these type of machines are often abused and have worn out clutches. Not to bad to replace on a real Bridgeport, a possible hassle getting parts on a copy machine.

See how much slop is in the quill handle as you switch from down to up motion of the quill (put light drag on it with the lock). Too much slop here makes the machine a pain to use and can cause chatter when drilling. You can get around that by putting a little drag on the lock, but again it makes the machine more a pain to use.

You can also put the indicator in the quill, indicate on the table surface and get an idea of how much the ways on the machine are worn by moving the table back and forth in both directions.

Typically when you find the right machine (used in a model shop or prototype shop, not production) you can tell right off the bat by looking at it that its going to pass all these tests. Those are the machines to find. I learned this the hard way, the first mill I bought as a younger man had been used in production and was pretty beat but I didn't know how to check it back then. I've still got it and use it for "2nd op" jobs but on my new mill I knew what to look for and got a good one.

Good luck-

Paul T.

05-02-2006, 06:04 PM
I found several things in my search that may be less than obvious until you disassemble things later. I will share them here.

Mine had *huge* backlash in the y axis screw. I presumed a *very* worn feed nut, but began to wonder given the amount. The adjustment screw that provides force on the feed nut to adjust backlash turned out to be completely missing and the nut was free to float in its bore.

Worn feed nuts are relatively cheap. Often all that is needed is to complete the "split" in the nut to allow for more adjustment. Worn lead screws are not, and a worn one will not allow for minimal backlash while still allowing full travel.

A *slight* jingle in the head with the back gear engaged is normal. Metal to metal contact in cog engagement causes this and it typically goes away as some resistance is applied at the spindle. lots of this means worn teeth on these mating parts.

My knee to saddle fit turned out to be less than ideal. Run the saddle to the middle (usually the point of worst wear) and grab the table at the ends and attempt to "cock" it one way or the other and feel for movement. Mine was reasonably tight all the way and felt good in spite of wear in the middle. This turned out to be because the gib had Turcite applied (a teflon/bronze product) and it would "squish" a bit at the far ends of travel where it otherwise would have bound. When I removed the gib, I found the turcite and discovered that it was falling off....quite possibly from being squished at the ends. Run with just the cast iron gib (as standard) adjusted for good fit in the middle, it will not make it all the way out before binding.

Chrome ways are often sold as being a big plus, but remember that the underside of the saddle that rubs on these is not. Soft on hard just changes where the wear occurs. Scraping chrome ways would be a real treat if needed.

Some wear on the quill is normal...especially where the quill lock tends to rub a vertical groove. You should not be able to wiggle the quill in its bore however. This is to be a tight *lapped* fit. Wear one out and you are looking at a very expensive fix.

A one shot oiler is a good sign that someone could easily keep it oiled. If it is empty or the oil looks bad, it may have been sitting a long time...and may not have been used. Oil passages plug and then something runs dry. Pull the oiler one stroke upon first arrival as over time, you will see oil run out from the surfaces served if it works. No oiler, make sure someone has not been using grease on the *oil* zerks.

Good luck!
Paul Carpenter