Thread: Bridgeport Mill spindle bearing replacement

This post is intended to assist a novice, or anyone that has not replaced the spindle bearings on a Bridgeport Mill and I hope it helps. I trawled this site and others before undertaking the job.........and still had many unanswered questions.

I found the discussion on the "right" bearings for the job to be somewhat "anal". The discussion and advice about bearing standards (Abec 1, 3, 7) , and comments about the assembled spindle needing to be reground after bearing reaplacement left me both perlexed and disbelieving, thinking that maybe there were dealers looking for a job? So, I stripped down the spindle to find out myself.

The first problem I encountered was the spindle nose cap was seized. On the underside are 2 pin holes to get a spanner on, but my pin spanner would not budge it, even when hit with a soft hammer (BTW, you have to remove the grub screw at the back of the quill first. It lies about 1 cm above the spindle nose cap). With each hammer tap the spanner would come loose, so I wound up the table to support it with the vice jaws. This worked well and after about 1 hour the spindle nose was free.

I was then able to knock out the spindle from above the machine by tapping on the drawbar (or another bar once it is out of view). It came out quickly and quite easily. There are 3 spindle bearings: one at the top, which has a lock ring; and 2 arranged as a pair with spacers at the bottom. It was clear that the pair at the bottom were shot (which is normally the case I understand). I wanted to check the id on the 2 bottom bearings.........and was glad I did before ordering new ones.

My Bridgeport was made in England by Adcock & Shipley and the bearings were not FAFNIR ones but RHP. The only serial on them was 7207. In my mind were posts I had read on several forum sites about bearing "specials" i.e. off the shelf bearings that had been modified on some Bridgeports, either bearings ground on the interior or exterior ring.......so I went to get my micrometer.

I found the inner ring and outer ring to be identical and 17.00mm thick. I measured the inside and outside dims and found them to be 35.00 and 72.00 mm........and nothing "special" about them. On Ebay, I found an "official" Bridgeport 7207 (207) FAFNIR bearing replacement for 300 USD.......but did not know if they would be right for an UK-made Bridgeport, so I called a bearing supplier in Bolton, UK, and subsequently got a matched pair of 7207 bearings for £50. On arrival, conscious that the spindle stack was preloaded by the locking ring, I wanted to make sure the thickness was exactly 17.00mm, and it was.

I noted the arrangement of the bottom bearing pair being "back-to-back" i.e. the id on the bearings face each other (the outer ring on the bearing is a L-shape and the bottom of the "L" needs to face towards the spacers). I reassembled the spindle in this way, greased and put the top bearing back on and did up the lock ring tight. Then put the spindle back in the head using a soft hammer on the nose. The manual I have states there to be a 3 thou space between the bottom of the quill and the spindle nose cap.......but I could tighten no further and ended up with a 5 thou gap. I put the set screw in the back of the quill just hand tight.

I ran in the bearings at about 500 rpm for a while and tested for any play in the spindle (which had been the sole reason for replacing the bearings) and it was rock solid. BTW, I use micro mills in my work and pre-replacement I was breaking even 1mm mills by the dozen and even 1.5 mm mills. After replacement, I tried a 1.00mm end mill....it cut about 5 times faster and didn't break. I then tried a 0.3 end mill and got the same results. I did not measure for run out (if any) as the previous test is as "small" as it gets in my world. job done! Looking back, it's hard to believe what improvement, if any, would have been got from the 300 dollar bearing pair. I don't suppose they will last any longer than the ones I bought.

Just wondering, what did you measure for runout before and after?
I replaced my bearings and got .0004" runout with new ones. I did not deem it worth a spindle regrind.

Everything running fine this morning and run out is 3 thou. As such does not translate to the slightest significance in my work and likewise not worth the regrind. Maybe the Abec 7 balls would have reduced it by half-a-thou, or maybe not. I am running a 1970 machine and there might be a 3 thou wear factor somewhere on the spindle surface or nose cone. May have been less if I had been able to close up the nose cone to a 3 thou gap but I am not going to worry about it. The surface finish I am getting now is the best I have seen in my shop, period.

The RHP number should be 7207CTRDULP4

Hi all,
I'm a proud owner of a Bridgeport mill with a 2J2 head on it. I don't seam to be having any problems with accuracy ... yet. But I am wondering if there is any past threads, with pictures, of course on checking run-out of my spindle bearings. Or any other preventative maintanance tips that are out there?
Thanks, Ed

I replaced the spindle bearings on my Sharp BP clone and frankly wasn't as anal retentive about measuring spindle nose gaps and preload and all that as some people think I should have been, but it turned out fine.

I had more to do in my case as I retrofitted a 30 taper Quick Change spindle to the machine, which was NOT a drop-in replacement as there are subtle but important changes between the Bridgeport and the Sharp version.

I had to get a new bull gear hub and matching bronze gear (mating to the worm on the hub) and get the new hub ground .002 under to fit the original bull gear. This was necessary as the internal spline pattern of the original hub was too "small" to slide over the splines of the Erickson QC30 spindle splines.

The matching bronze gear was required because the Sharp parts were made with metric profiles.

After all this effort, my results are fascinating. I mean that I'm now able
to easily measure just how much runout is actually in many 30 taper
toolholders. It can be alarming. I measure it by mounting the toolholder
in the spindle in both its possible orientations and documenting the runout
in both positions. I've found runout in excess of .0005 in "quality" brands
and as high as .002 in "economy" brands.

But the best of them are dead on and then the true spindle runout figure can be measured. It's right at .0001. I got LUCKY!

The information about b'port using specials (regular commercial bearings with the
*faces* ground to set preload, to convert radial bearings into angular contact
pairs) probably came from me at one time.

Bridgepord *did* do that, but I belive only on the older M heads. The bearings
are spec'd in their manual as "SAE 205" but off the shelf 6205s will not work in that
application.

Jim

Is the nose cap right hand or left hand thread?

Here or in a link from here, I read that you lose 10% of your tool life for every 0.0001" of runout. That seemed sort of shocking, but makes more sense the more I think about it. I use a lot of small end mills and the idea of 0.001" of runout seems totally unacceptable.

Got the spindle out of the quill, actually quite easy.

Just wanted to add this for anyone who may search this later. My machine is a 2001 year model Vectrax GS20F (a big 10X54 5hp Taiwanese clone sold by MSC). It has the larger diameter quill that uses the 7010 bearings. Long story short, the nose cap was LEFT hand thread. I don't know if this applies to all Vectrax machines, but if you have a large quill Taiwanese clone I'd give it a try.

The nose cap was tight, but not ridiculous. I tried the chain wrench suggestion from another thread. However hack it may sound it actually worked quite well and left almost not trace on the nose cap. If you have a tight one that may shear a pin on a spanner wrench I'd give it a try.

Thanks for the good info in this thread, it has made bearing replacement a breeze so far.

I've been rebuilding my 1958 BP for a couple months now. This is a very low budget BP for use in my garage so when it came to replacing the spindle bearings I really didn't want to pay $300 for bearings on a mill that I only spent $350 for to begin with. So I found a "pair" of angular contact bearings on ebay for lowly sum of $40 shipped. For those who cringe at the thought of shortcuts in this area of the rebuild, I hear you and for the most part I agree that it is best not to cut corners here. The bearings arrived as a pair with no instructions and they had no witness marks (so that eliminates one step in the assembly process, right?). These are obviously made in China and they had much more axial play than I thought they should have. During the assembly, I could not get any preload on these bearings. In fact, there was about .007 gap between the outer races and the outer spacer with the inner spacer snug. Thinking that the spindle retaining nut would take up the .007 I assembled the spindle in the quill. I noticed the spindle had .005 end play with the spindle nut tightened to about a .003 gap. So after thinking this over and nearly convincing myself to buy the appropriate bearings and do it right, I took about .009 off the inner spacer and reassembled the bearings on the spindle and it then had some (unknown amount of) preload and it felt good rotationally and axially. So I assembled the spindle in the quill and the .005 end play is gone. I still need to install the motor and wire the VFD so I won't know how well this is going to work for another week or so.

Excellent summary. I'm rebuilding a 2J head - I'll hang on to this for reference. Thank you for taking the time to recount your experience.

The .003" spindle nose cap gap is to insure that the outer bearing races are actually clamped when the nose cap is tight. .005 or as much as .010 only gives a larger area to collect dirt, but will work fine otherwise. Too small or no gap can mean that the outer races probably are not clamped properly, and could be the cause of endplay.

Bill

Has anyone here gone the 5 bearing route offered by C&M Precision?
Appears to be a reasonable price for what you get.
Spindle Re-build Upgrade - Bridgeport J & J2 - Manufacturing - Taper Grinding

I replaced collet stop screw with new one and before i started i noticed nose cap was loose , so i marked it removed set screw and unscrewed it . I then tapped the spindle out replaced screw and lock screw. I then put spindle back up inside and replaced nose cap ,and tightened back to mark or were detent for screw is ! but it is not seated against bearing should it be and how much thanks bob b

You should have some clearance from the quill to the nose cap when the nose cap is tight. See post#13 above. If the cap is tight with no clearance to the quill, the cap needs to be faced off some. I would check to see if you still have bearing preload before facing the cap off. Pull the spindle again, and attempt to twist the outside races of the lower two bearings. One CW and the other CCW. If they turn very easily, the bearings are worn or the spacers between are not the same length so there is no preload.

Bill

I recently purchased a mid 80's Jetmatic Mill. I know, it isn't the highest quality Mill but I got it for less than the scrap price and I figured it would give me something to learn on and I could always scrap it later if it did not turn out to work for my needs.

With that said, I want to do some clean up and maintenance on it including replacing the spindle bearings. The machine has a NMTB 30 quill in it and I was wondering if it might be worth putting an R8 quill in at the same time. The machine did not come with any tool holders and R8 seem to be more available and less expensive. Would this be possible/practical? I have been told that this thing is a "Bridgeport Clone" so some of the parts may be interchangeable.

Thanks,
Greg

Years ago I replaced the bearings on my BP and then sent the spindle and quill to Wells Index for taper grinding. It needed it as one of my employees overloaded a 6" facing mill, and sheared off the locater pin. After assembly the runout of the taper was zero, okay the .0001 needle shook a bit but not a full tic. When collets are installed then are all over, but as good as they can be.