# Spindle bearing replacement - mill



## precisionworks (Jun 30, 2010)

I knew the spindle bearings were going south, as any milling op (either radial or axial) came with increased noise & vibration. Tooling life suffers, especially for solid carbide end mills which have to be sent out for sharpening. Inserts also wear quickly because of uneven loading of the cutting edge. 

Pulling the quill from the head is a fifteen minute job - remove the clock spring on the left side, the feed handle on the right side, the fine feed casting, and the depth stop. Once all that is done the spindle will fall out of the head, so put some padding on the table or be ready to catch the spindle :nana:

There's a detailed description on this web site: http://www.fignoggle.com/machines/r...ill-drill-spindle-quill-removal-procedure.htm . The upper bearing is a Timken style tapered roller #30206, the lower bearing is a #30207. Regular automotive grade bearings cost about $50 for the pair but are not precision grade. The bearings need to be either ABEC-5 (ISO P5) or ABEC-7 (ISO P4). NSK does list these in ABEC-5 (30206JP5 and 30207JP5). Prices vary depending on your supplier ... the highest quote (for both bearings) was $214.85 :shakehead My regular supplier, Bearing Headquarters, had the lowest price at $142.95 and will deliver them to my shop tomorrow 

Time to clean the bench, put on the surgical gown & gloves, and see if everything goes back together ... photos to follow.


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## 65535 (Jun 30, 2010)

As long as the bearings didn't spin in their respective holes, I hope it goes well. 

Shiny new bearings are always nice.


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## precisionworks (Jun 30, 2010)

NSK tech support is awesome, talked with two engineers so far (about 30 minutes) and will probably speak with another tomorrow. One call from NSK America to NSK Japan to verify a detail on the bearings, that being the "high point" marking, or point of maximum eccentricity. All ISO P4 bearings have that, and the NSK P5's do as well.

The big question mark at this point is determining proper preload. Since the tool manufacturer does not specify a torque value for the preload nut, it's a matter of trial & error, while monitoring the bearing (or bearings) for temp rise. Roughly 50 to 55 degrees C is the number to look for, and quite a few web sources mention that same figure.


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## 65535 (Jul 1, 2010)

So for the temp rise Barry, is that a number that says, the bearings are tight enough and not too tight. I'm wondering if there is a possibility that the number could be lower and the bearings still have good preload just the 50-55C is the number to be sure it's as tight as possible?


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## precisionworks (Jul 1, 2010)

> wondering if there is a possibility that the number could be lower and the bearings still have good preload


From the engineers I've spoken with & the dozens of websites that address tapered roller setup, all agree that inadequate preload will cause little temp rise while excessive preload will cause temps that never stabilize & continue to climb (aka thermal runaway). Tapered rollers work best when there is .0000" axial clearance between the bearings when they are under full load & warmed to operating temp. Which would indicate that they need some minimal end play when cold (ambient temp), and those numbers vary from .0005" to .0050", which is a huge spread.

The last engineer suggested trying a "light preload" and running in the bearings starting at low spindle speed for 30 minutes, next higher speed for 30 minutes, etc., while monitoring the temp at the bearing face with an IR thermometer. Pretty much an all day process, but I have to be in the shop anyway so it should work. He said that the design of tapered rollers is such that correct preload is achieved within a very narrow range of axial play - something under .0010".


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## precisionworks (Jul 2, 2010)

One design flaw in all the small mills is the slotted quill. This allows the manufacturer to install a Morse Taper spindle & use the same quill housing with a MT spindle for a drill. On a mill, this allows chips, dirt, and everything else full access to the lower spindle bearing, shortening the bearing life. Quite a few mill sites mentioned sleeving the quill with an aluminum tube and that's what I did.


Quill with slots:






Steady rest is set up to allow boring the quill, as the inside of the quill was factory bored with a small brick attached to a bamboo stick - inside measurements showed a variation of diameter of about .030. The first boring pass was made at .030" but that did not clean up the roughness, so an additional .031" was taken & that left a uniform bore.











If the quill nose had been larger it would not have fit inside the steady.






Boring out the aluminum sleeve, after turning the outside .005" larger than the quill bore.






Sleeve pressed into place. In addition to friction & compression, the sleeve is also retained with LocTite retaining compound.











Outside diameter of the sleeve is 2.1465" and inside diameter is just over 1.500". The spindle OD is 1.375", so there's plenty of clearance for the spindle to run.


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## gt40 (Jul 2, 2010)

> Steady rest is set up to allow boring the quill, as the inside of the quill was factory bored with a small brick attached to a bamboo stick - inside measurements showed a variation of diameter of about .030. The first boring pass was made at .030" but that did not clean up the roughness, so an additional .031" was taken & that left a uniform bore.


 
Could the part have been cast and then machined on the outside? In any case, your description was awesome...


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## wquiles (Jul 2, 2010)

gt40 said:


> your description was awesome...


+1

and great photos as well


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## precisionworks (Jul 2, 2010)

Time to put the bearing cups into the quill. My arbor press hasn't enough daylight for a part this size so I made a "screw press" from a piece of 1" all thread rod. A disc was turned to the diameter of the cup minus .020" & the nuts squeezed the cups into their bores. (Bottom disc is just visible).






The bottom bearing was slid onto the spindle, which had been lightly greased. A press sleeve was turned to the size of the bearing ID plus .020" - the press sleeve is the part just under the bearing & just above the press plate. A piece of scrap steel separates the arbor press ram from the spindle nose.







Put the spindle into the mill head & ran it for 30 minutes at 375 rpm, temp came up to 95 degrees F. Ran it for another 30 minutes at 500 rpm, temp came up to 107F. Ran another 30 minutes at 900 rpm, temp reached 119F. Preload is pretty close to ideal (or at least it seems that way). Although I started by reading the bearing itself, the hottest spot was the inside of the spindle, just above the R-8 taper. The top bearing consistently read about 10F cooler that the bottom bearing.

The spindle had been binding from time to time & the cause became obvious during reassembly - the screw that presses against the clock spring housing was the culprit. A longer screw was installed along with a spring, which works well. The second spring seemed to work even better.












An indicator was set up to measure axial play in the spindle ... it reads .0000" whether cold or warm.


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## precisionworks (Jul 3, 2010)

> Could the part have been cast and then machined on the outside?


The quill is a cast iron part and the outside is fully machined, as is each end where the bearings are located. The quill ID had been "machined" by some tool which left it looking like the surface of the moon - just slightly more rough. Perhaps not a brick, more like one of those dressing sticks used to true a grinding wheel:






Probably the manufacturer thought that no one would ever see the inside of the quill, much less use it as a surface to receive a press fit sleeve.

The mill got about an hour of run time with the 2" face mill at 770 rpm (about 400 sfpm). The part being milled was the steady rest clamp plate that goes under the lathe bed rails. Primarily end milling with some heavy side milling. Also plunge milled a 1/2" solid carbide tool through some 1/4" steel plate. All the ops were rock solid, with none of the noise and shaking that was present with the old bearings.


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## Tom Anderson (Jul 5, 2010)

Thanks for posting, Barry. 

I love this millwright stuff! :thumbsup:


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## StrikerDown (Jul 5, 2010)

Thanks Barry, I have been anxiously awaiting this since you had the retaining ring work loose. Thanks for posting.

What ABEC rating did you go with, looks like ABEC 5?

You mentioned the high point marking, What do you do with it? 

Does it line up with hi pt on the other bearing?


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## precisionworks (Jul 6, 2010)

The bearings are ABEC-5 (same as ISO P-5). IMO, they are the best value for the money, costing a lot less than ABEC-7 while giving up very little in accuracy. ABEC-3 is also available but has twice the internal runout.



> You mentioned the high point marking, What do you do with it?


All NSK bearings that are ABEC-5 and better, and probably most precision bearings from other suppliers, are marked at the point of greatest eccentricity. The marks for both bearings are aligned so that the spindle has the least runout.

One thing I may do down the road is to send the spindle out & have the R-8 taper touched up. When the taper is ground using the spindle & bearings for support, the taper ends up with TIR in the low millionths, probably half of what it is now (which reads about .00010"). 

I have a few hours on the new bearings & they brought the old mill back to life. Also, the fine downfeed dial was checked against a dial indicator & the fine feed wheel has a 1.00% error at all settings. When the find feed is turned .100", the spindle actually moves down .099", which is not a deal breaker for most ops - but it is worth knowing exactly how much yours moves.


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## StrikerDown (Jul 6, 2010)

precisionworks said:


> I have a few hours on the new bearings & they brought the old mill back to life.



Just a guess, but it's probably better than new with those bearings.




precisionworks said:


> ...the fine downfeed dial was checked against a dial indicator & the fine feed wheel has a 1.00% error at all settings. When the find feed is turned .100", the spindle actually moves down .099", which is not a deal breaker for most ops - but it is worth knowing exactly how much yours moves.



So at .200" it's actually moving .198"?

This could splain a few things!


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## precisionworks (Jul 7, 2010)

> it's probably better than new with those bearings.


+1

Most likely, the factory supplied bearings are non-precision ABEC-1. Those can be purchased at any auto parts store for use in boat trailer & utility trailer axles - not exactly what you need to support a milling spindle. But cheap is good, right 



> So at .200" it's actually moving .198"?


Exactly. At least it does that consistently. Probably happened because the pitch of the ring & pinion gears is off by just a fraction - I can't figure out how to correct this but it isn't a big deal as long as the correction factor is known.


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## Clark (Jul 18, 2010)

I made a lot of drawings of my mill spindle bearings until I understood the pre load.

I did not have the matched ground pair of Double D bearings on the bottom. I just had regular ball bearings. I replaced the upper bearing with a $3 bearing and inverted the two lower bearings to use their un worn sides.


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## precisionworks (Jul 18, 2010)

Quite a few mill spindles use a precision ground spacer like the one that separates the two lower (angular contact) bearings in your mill. One of the dozen or two articles I read on preload went into detail on spacer preloading, and adding or subtracting .0001" makes quite a difference on AC bearings.



> I replaced the upper bearing with a $3 bearing


On your spindle, as on a Bridgeport mill spindle, the two lower AC bearings do almost all the work under preload, so the upper bearing can be any ABEC-1 electric motor grade bearing. On a mill-drill spindle, both upper & lower bearings work together in preload and both (should be) at least ABEC-5.

Your setup, and Bridgeport's setup, are both better mechanical designs. Neither can be used with a mill drill because of the great difference in quill and spindle design.


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## Davo J (Jul 18, 2010)

Hi,
I have had my mill for a few years now and decided to go through it again. I went through it when I bought it but didn't replace the bearings, just re greased it. The spindle bearings of my HM52 from Australia (picture below) are p6 spec in the manual. Mine has a taper roller at the bottom and a deep groove at the top. I am going to change it over to taper top and bottom as I think taper bearing at the to would be more suited. 
These bearings are large compared to other mills including the Bridgeport being 80 x 50 x 20 bottom and 45 x 75 x 16 top. The spindle is a NT30 taper.
I range the local SKF bearing company and they said there bearings are P6 or above, they don't have any lower. If I wanted a higher spec they would have to get them in as there was none in the country. I was only quoted $14.50 and $11.50 each for the P6 grade.
Dave


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## precisionworks (Jul 18, 2010)

> are p6 spec in the manual


I'm surprised that they didn't use a better class bearing ... ISO P6 = ABEC-3. If you're going to the trouble of replacing the bearings, consider upgrading to at least a P5 or P4 bearing.



> $14.50 and $11.50 each for the P6 grade.


That's really inexpensive ... my guess is that those bearings are the next step down from P6, which is P0 (same as ABEC-1).


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## Davo J (Jul 19, 2010)

precisionworks said:


> I'm surprised that they didn't use a better class bearing ... ISO P6 = ABEC-3. If you're going to the trouble of replacing the bearings, consider upgrading to at least a P5 or P4 bearing.
> 
> That's really inexpensive ... my guess is that those bearings are the next step down from P6, which is P0 (same as ABEC-1).


 
Hi,
Like I said in the above post, any higher than P6 would have to be ordered, as there are none in the country, so they would be expensive. This mill is the next step up from a bench top machine like yours, they would be charging a lot more for the machines if they were fitting P4 grade bearings to them. I see you think they only fit P0 grade to your type of machine, so I am lucky to get P6 in mine.

The grade he quoted on was P6 that were in stock as they don't carry anything less. This is a bearing wholesale place that is near by to me.

Dave


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