BOSS Eight Spindle Bearing ID

[Jim Bass]

Hi All,

Have a Bridgeport BOSS Eight that I purchased used a few years ago. The finish on the machined part has always been "rough".

The cutter leaves fine chatter marks on the work piece and a high pitched chatter squeal during cutting. Using a ½” dia. Two flute high helix HSS cutter at 2,000rpm 15ipm, climb milling 6061 alum.

I have tried same cutter in a V2XT and get excellent work piece finish and no chatter at same speeds and feeds.

I can run spindle for several hours with very little temp rise to the spindle housing and the spindle runs quiet.

The yellow seal is marked 207 High Speed Seal I find no other visible markings. I do not want to disassemble the spindle to check other side of bearing or inner race as spindle has zero run out.

I’m wondering if someone replaced the OEM spindle bearings. Can anyone tell by picture if the bearings appear to be OEM?

Thanks, Jim

[polaraligned]

The bearings should be 2MM207WI DUL.
The 2mm and DUL signify a matched ABEC-7 set. The W indicates angular contact bearings.

I have seen JM 207 radial bearings in spindles. They were put in to save a few bucks. They are ABEC-3.

Try measuring the runout if you suspect the bearings. Also, you may need to have the spindle taper reground in a new set of bearings to bring runout to proper levels.

Scott

[NC Cams]

Here is some background to help answer the question.

THe OEM bearings were a hybrid, custom made hodgeporde of pieces parts that were concieved by Fafnir wieh BPT Machines was part of the corporate ownership of Textron - Fafnier was also owned by Textron.

I have the OEM prints at teh shope but the foolloiwng notes are from memory.

THe bearings were originally deep groove ball bearings, 6207 as I recalle, BUT they were modified via preload grinding to turn them into AC's of sort. This was done to retain the seals of the 6207 as 7207A/C's don't come with seals.

They then did stuff lik markeing eccentricity high points of innner and ourter races, adding Grade 10 or 3 balls, and then doing ABEC 7 runout checks. I seem to recall that the outer ring tolearances were ABEC 3. "Seals" on oem were red or balck..

THe factory then selectively fit spindles/housings/etc to match spindle high points and bearing alike to "build a spindle". No wonder BPT went broke.

Even with all this handwork, they still had to ID grind the spindle while the thing was FULLY assembled to remove ID rounout from the spindle ID.

Put a piice of known ROUND rod (piston pin is good known cheap gage, it is easy to get and big enough to read easy) in and check runout. OUr Extrak and V2XT have OEM bearings and o.oooo runout.

I"d suspect you may have some less than OEM grade "servcie" bearings. Many an "Expert" has read the numbers off a bearing and determited what will work - and then are surpriese when they don't.

Even uf you use the Fafnir bearings as spec'd in a prior post, simple R and R will not fix the spindle - you WILL have to pull the head and have an expert spindle shop regrind the ID taper.

I now this to be a fact as I wsa at hardinge/BPT and watched them do JUST taht, spindle after spindle - service and OEM . THis is why the OEM spindles run like thaty do, OEM or service - they'rs simply done right. That's the reason for the sost differentiial too.

[Jim Bass]

Hi NC CAMS,
Thanks for the reply. The seal color info that you supplied was what I was looking to get. I think my suspicions that a past owner of the mill replaced the spindle bearings are correct.

I have checked spindle run out by setting a test indicator against the spindle bore and also with a test indicator against a ground pin in a Lyndex collet style tool holder.

Indicator in spindle bore the run out is zero. Indicator against ground pin, I can just barely see needle movement, I’m guessing maybe .0002 - .0001. The duplex (bottom two) bearings have a light preload.

My guess is that the run out is so good due to the preload, but incorrect bearings for thrust loading lets the cutter chatter. ?????? Is my thought process anywhere near correct?

As of right now the spindle is back in the quill and the mill is running as it has in the past. Breakdown of machine use is 60% hole drilling, 30% as a secondary machine roughing 6061 alum pieces and 10% finish work.

Using the mill for drilling bolt patterns works well, roughing is affected as I cannot run speeds and feeds as fast and heavy as I wish due to tool chatter and lastly finish work is a disappointment due to chatter marks on the finish piece.

I’m not sure what my course of action will be. Cost to repair spindle correctly in a 25 year old CNC with the original control (how long will control continue to function) or sell mill and invest proceeds and repair cost in a newer CNC mill. I’m kind of between a rock and a hard place.

Thanks, Jim Bass

[NC Cams]

For comparison,

Extrak rounout with good OEM bearings - 0.0000 literally 0.0000

Ditto for V2CT.

Runout of 0.0001 to 0.0002 is EXACTLY what the ID tune up grind takes care of.

NObly who makes A/C's uses the same preload "Load" in lb or N. THus "lite" is quite subjective. THe OEM spindle bearings were not for what onew ould call a "HIgh speed" spindle. THus they were more of a heavy light to MEDIUM preload.- Actually they had two for a BPT type mill. THe early ones were 75lb and they wre very quickly uppped to 125lb preload.

ONce could run the risk unloading the LITES under a heavy cut, thus MEDIUMS would be a bit more appropriate..

at this point,you know aht you have, probably a less than appropriately repaired spindle. YOu know what it takes to repair it - proper preload plus OEM procedure. You now make your own call.

The next time someone says "all you gotta do is slap in some new bearings", show them this thread and ask "is this what you want to end up with in return".

BTW, some guys ge lucky and find themselves with low eccentricity spidles - these guys end up being lucky. How lucky do you feel????

[Jim Bass]

How lucky do I feel? With my normal amount of luck, I'll just leave the spindle alone and use it as is.

For now it meets my needs and I fear about the time I put any money into the spindle, the control will die.

When the control dies the mill will be junk to me. Retrofitting a control is something I have no interest in doing.

I enjoy reading your postings, as it gives me a chance to learn more about bearings.

Thanks, Jim Bass

[NC Cams]

Maybe it is time to offer it to a good home and find something a bit better. At least now you now what to look for when you do a run thru - you did run off some parts didn't you???

FIrst think I'd do is a runout check with a ground rod and indicator, in tenths.

SEcond. do a finish cut.

THird, when warm, uses a stethoscope or "screwdriver stethescope" afainst the spindle.

Fourth, if the seller proudly says "and I put in new bearings", ask him if he had the spindle ID regrgound afterwrds.

The above questions will give much better insight into how "dood" a deal is on almost any mill...