Adjusting mill spindle bearings

[L98FIERO]

I have a TOS KURIM FNK25A vertical mill built in 1987 that is in fairly good shape but the spindle chatters on any decent cut. The motor is about 4 hp but when taking a .1 deep cut with a 3 inch face mill on 2.5 inch wide cold rolled steel it chatters badly. I have found about .001 radial play in the spindle bearings at the spindle nose and abiut .0006 axial play. I checked this by lightly prying on the spindle, probably about 200 pound of force. The play was from no pressure to having the force applied, not force in both directions.

The spindle on this machine has a double row cylindrical bearing with a tapered bore at the spindle nose and two angular contact bearings above it for axial location.

My question is; how can I adjust the bearings to get rid of the play and therefore the chatter? I know that I have to push the cylindrical bearing at the nose further up the taper to enlarge the inner race and will likely have to grind the spacers in the top end to adjust axial play, but how much? How do I determine proper clearances or preload. I have not taken the spindle apart yet but from previous experience think that the cylindrical bearing has a split inner race. I also understand that every spindle is designed with different clearance/preload to fit what the engineers expect from the machine but is there a 'safe' setting?

LaVerne

[RICHARD ZASTROW]

The easiest way to find out those numbers is to get them from the supplier. (microns usually).

Some of the tech sections in the bearing catalogs give the loading numbers for light, medium and heavy preload on angular contact bearings.

The roller bearings with the tapered inner ring bore usually have spacers for preload ground per measurement at installation. The correct preload can be measured with a pulled spring scale. (a bit more accurate than the scale in your fishing tackle box). SKF supplied us with the correct numbers, might be available on line.

My FAG/Bardon Machine Tool Bearing Catalog has a lot of good info. in it. Also on line.

Dick Z

ADD: www.bardenbearings.com/PDF%20CD/MACHINET.PDF

[L98FIERO]

Thanks Dick, that catalogue has a lot of good information, I'll look it and see if I can get what I need. From your experience what preload would you suggest I use, I suppose it depends on the size of bearing, minimum and maximum spindle speed and a few other things so all I was looking for is 'ball park' numbers like how much axial and radial load is used to determine play/preload on a machine like mine. I may be wrong but I imagine/guess that most spindles in machines that are of the same general specifications would have similar requirements.

Thanks again,
LaVerne

[RICHARD ZASTROW]

The catalog tech. section has preload info. Ex. 1807HE 35mm ac bearing L= light preload 19 N, M=medium preload 76N, H=heavy preload 197N.

There are formulas that will give you the torque on the lock nut required to produce force (preload).

Typically, ac bearings are ground so that when they are pushed together, the inner races press against each other, same for outer races. That will provide the correct amount of preload to the pair.

BTW, it's all in the book.LOL

Dick Z

[L98FIERO]

Thanks Dick;

I'm reading through the Barden catalogue now and the main question I had on the cylindrical bearing is answered very well, "Generally, the bearings should be mounted with zero clearance to a very slight preload". I think this part is important, too.
"Clearance Adjustment in Cylindrical Roller Bearings
Cylindrical roller bearings with tapered bore are mounted with a small clearance, line-to-line fit or slight preload. This can be done to the precision of ±1 μm with the help of an FAG boundary circle measuring device. If such a measuring device is not available, a fairly exact clearance adjustment can be achieved by measuring the axial drive-up distance of the inner ring onto the tapered shaft seating, taper 1:12. This drive-up distance is approximately 15 times larger than the radial expansion affected in this way. Surface smoothing and the elastic behavior of the spindle and the inner ring also have an effect which may vary slightly." Although I can't locate it right now, at one time I had information from an SKF catalogue that even took into account the housing and spindle diameters, what diameter the spindle was bored to and possibly the amount of material around the housing bore.

Considering that _if_ the angular contact bearings are 1807HE, and the spindle is on a turret mill that will from time to time be used for drilling, does it seem reasonable to use the heavy preload at 197 N (44.3 lbf) that can be subject to 601 N (135 lbf) unloading force? These spindles are obviously intended to be used for drilling and a 1/2 inch drill would definitely require that and more force. The maximum spindle speed is 4500 rpm.

Thanks for your help!
LaVerne

[RICHARD ZASTROW]

If the bearings in question are angular contact in opposed pair configuration, they will be properly preloaded when pressed together (no axial clearance between any races).

If you use spacers between them, the spacers must be ground EXACTLY the same dimension (length) parallel and perpendicular to the axial centerline. If not, they will be "cocked" (misaligned).

The bearing arrangement you described is typical. One drawback is I haven't found any sealed precision cylindrical roller bearings. I like to use the "lubed for life" sealed spindle bearings, almost always angular contact arrangements with a floating radial bearing supporting the end opposite from the work end.

Remember, the more precise the bearing, the more precise the location and alignment must be.

Dick Z

[L98FIERO]

I'm going to just reorder the equivalent replacement bearings so at this point the problem should be solved as far as the spindle goes however I still have some questions.

It appears from the information in the catalogue that there are different factory preloaded pairs, is that the case?

If that is not the case or you just wanted to change it, when the load is placed on the spindle to check for movement is it checked both ways, i.e., no load to + direction and then no load to - direction and the movement added or is it from no load to load only?

LaVerne

[RICHARD ZASTROW]

The listing of preload, L light, M medium and H heavy. These are built into the pair. When specifying the bearing by number, the preload is specified in the order description. This is usually included in the complete part number.

Your present bearings should have a number that includes the preload.

You'll need the manufacturer and complete number to get the exact replacement.

My guess is they will have a heavy preload.

Dick Z