Newbie- Some Spindle Design Questions

[Curtis_F]

Over the past few days I’ve been reading through the custom build posts on the board looking for info on how people have been designing & building their spindle. I pretty much got information overload.

So, I still have some questions, which mainly come from my lack of knowledge of how industrial machine tools are built.

Any help on these will be appreciated. Thanks.

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I’ve read that the most recommended bearing setup in spindles is using two angular contact (AC) bearings (mounted either face-to-face or back-to-back) at the chucking end of the spindle. Then one or two high-radial load ball bearings at the pulley end of the spindle. The bearings are positively held a distance apart from each other by use of spacers or steps in the spindle housing that they are pressed against.

I’ve also seen some posts where all the bearing in the spindle housing are AC bearings.

Is there an advantage to using all AC bearings as opposed to a mix of AC and standard ball bearings?

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In one post it was mentioned that the high-radial load bearings on the pulley end should be a slip-fit on the spindle to allow for heat expansion & contraction in the spindle itself.

For all you guys that have worked on machine spindles, is that an actual practice??

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In the drawings of spindles posted the chucking end of the spindle has a face that is pressed against the inner race of the leading angular contact bearings.

This means that any force applied into the spindle (lathe cutting towards the chuck or drilling / plunge cutting in a mill) will be transferred directly into the inner race of the AC bearings. However, only the press fit of the spindle in the bores of the bearing handles the pull forces generated in a lathe when tool is cutting away from the spindle or in a milling operation with an end mill where the helical teeth of the end mill apply a downward force on the spindle.

Does there need to be something on the spindle shaft (like a threaded on collar) that positively locates the spindle to both sides of the AC bearings or is the one face against one bearing and the press fit of the two AC bearings enough?

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Are there generally seals between the Spindle and Spindle Housing (Head Stock) to prevent debris from getting into the bearings, or is something around a .010” gap enough to let the spindle turn and keep stuff out?

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A question specifically about lathe spindles; Is a VFD capable of being used to single point cut threads or does it require using a Servo for a spindle motor?



Thank you all for your time,

Curtis F.

[RICHARD ZASTROW]

Curtis, I'm not trying avoid typing long dissertations. OK, I lied.

If you "google" the precision bearing manufacturers like FAG/BARDEN, TORRINGTON, SKF etc. and study their tech sections, most of your questions will be answered.

Also, you might do a search on the "Zone" for NC Cams and his contributions on bearing questions. Our departed friend was a bearing engineer that knew and shared much valuable information. A bit irascible at times but a real help.

If you still have questions after perusing those tech sections, we'll be happy to help.

Dick Z

[RICHARD ZASTROW]

Curtis, I have a source of info for you.

Barden Bearings, Machine Tool catalog

www.bardenbearings.com/pdf%20CD/MACHINET.PDF

Dick Z

note: I keep trying to get the URL to work but I guess you'll have to do it the hard way.

[Curtis_F]

Dick,

Thank you for the replies! I had searched Torrington and SKF's websites for information and found their organization greatly lacking. The Barden/FAG catalog you pointed to has some good info and more food for thought.

The note to search for posts from "NC Cams" gave me a whole new slew of posts to read as well...this is going to take awhile.

The end goal is to see if it's viable to make a CNC mini-turret lathe (4" x 10") for machinging special screws, studs, nuts and pins. To turn a brass screw with a #1-72 thread I'm looking at a requirment of around 30,000 rpm, so I need to be serious about the bearing selection and spindle design.

Heat expansion will be a problem, along with oil lubrication, which arn't addressed much on these forums (that I've seen) so I've been having a hard time really pinning down any kind of "common practice" for this stuff.

I have pages of notes for the bed design, the crosslide, tool turret, tail stock, electronics, etc. But high speed bearings and spindles...not so much.


So, really any help is appreciated.


Thanks again,

Curtis F.

[RICHARD ZASTROW]

Curtis; I'm not the electronics guy, my background is mechanical. I doubt you'll be able to thread @ 30,000 rpm. Seems a bit much for accurate control of the tool path. Just me and my lack of expertise in things electronic.lol

Mechanically a 30,000 rpm spindle is possible, but for a lathe spindle extremely unlikely. Consider the balance problems; the centrifugal forces generated; the super precision bearings required; noise and vibration as well as heat/cooling problems.

An old rule of thumb on the bearing max rpm listing is to max the spindle rpm at 10% of the spec. ex.: bearing listed @ 10,000 rpm max., machine normally operated @ 1,000 rpm.

Did you find the tech section in the Barden Machine Tool Bearing catalog? Lotsa info there!!!!

Dick Z

[Curtis_F]

Originally Posted by RICHARD ZASTROW I doubt you'll be able to thread @ 30,000 rpm.

No, NO! Just turning the major diameter of the threads at that speed. The threading operation is much slower.

Mechanically a 30,000 rpm spindle is possible, but for a lathe spindle extremely unlikely. Consider the balance problems; the centrifugal forces generated; the super precision bearings required; noise and vibration as well as heat/cooling problems.

That is precisely why I'm doing this research. And even if I can't reach 30k, I'd like to get as close as I can.

The design I have in my head is based entirely around the 5C collet. The spindle shaft, as sketched up, has a max OD of 2.3" and is less than 7" long so there's very little rotational mass. I may end up with a longer spindle shaft, but I'm trying to avoid going larger in diameter to keep the mass as close as possible to the axis of rotation.

An old rule of thumb on the bearing max rpm listing is to max the spindle rpm at 10% of the spec. ex.: bearing listed @ 10,000 rpm max., machine normally operated @ 1,000 rpm.

According to SKF their super precision hybrid angular contact bearings are designed to run at 27,000 to 43,000 rpm depending on bearing materials. Of course they are expen$ive, but they'll be one of the only parts I wouldn't make myself (or have a friend make).

At present it looks like there will be three bearings in the spindle. A matched pair of SKF angular contact bearings on the head end, and a Barden/FAG floating bearing (45k RPM rated) on the drive end.

Did you find the tech section in the Barden Machine Tool Bearing catalog?

Yes! Yes, I did.


Thank you,

Curtis F.

[veteq]

Hey Curtis,

We really mis NCcams over here, he would have made some nice comments on this projects.

I have never seen a 30k lathe spindle, do you really need that kind of rpm?
What is your target for runout etc.
Why not buy a commercial spindle?

regards,

Roy B.

[veteq]

What is diameter off the piece that has to be turned?
If it is say: 0,5 inch and you put your stock material in there
(that isn`t ballanced) it would be hard for the bearings on 30krpm

[Curtis_F]

Roy, Veteg,

I arrived at the goal of 30K based on the following:

The Machinery's Handbook 26th edition says; when cutting Brass with a Carbide cutter the optimal Surface speed is in the 440 to 1170 Feet Per Minute range.

The major diameter of a #0 thread is: .060"
The major diameter of a #2 thread is: .086"
The major diameter of a 1/2" thread is: .500"

Therefore:
Optimal RPM for turning the major diameter of a #0 screw is: 28,000 to 75,000 RPM
Optimal RPM for turning the major diameter of a #2 screw is: 20,000 to 52,000 RPM
Just for comparison;
Optimal RPM for turning the major diameter of a 1/2" screw is: 3,400 to 9,000 RPM

After researching bearings a little I found that the off-the-shelf ones of the size needed to fit around a 5C collet spindle (45mm I.D.) topped out at a little over 30,000 RPM if oil lubrication is used.

Really, the machine would spend most of it's life under 10,000 RPM, but I want to shoot for the head room to be able to hit optimum speeds when I need it.

On the occasions when it would get into the 25,000+ RPM range the material will most likely be less than .19" diameter/hex stock and only sticking an inch or so out of the collet.

And if 30K isn't possible, well that would just make my life easier.


TIR: I could live with a TIR of .001", but with the kinds of speeds I'm looking at I'm going to really have to get it down as much as possible. .00005" would be nice. There will be honing and centerless grinding involved on the spindle parts that's for sure.


Roy,

As to commercially available spindles, I have looked, and Dunham Tool makes some nice ones for the price of a new car and only go to 7,000 rpm.

FAEMAT is another good spindle company, and they go up to almost 90,000 rpm, but you start getting into pressurized oil and air bearings, etc. To pay for one and the supporting equipment I'd have to sell a kidney, half my liver, a lung....


Thanks for the questions guys, it keep me thinking!


Curtis F.

[Curtis_F]

Just wanted to add that the more of NC Cams posts I read, the more saddened I am by his passing. He left some brilliant posts!

...and I'm starting to see that I may be on a fools errand for anything more than 10k rpm. I still have more to a lot more reading to do though!


Thank you all again,

Curtis F.

[veteq]

Hey Curtis,

The specs off your application are more clear to me now, if you have the possibility to use all that hightech grinding equipment i would say go for it.
It would be a hard task but if i had the opportunity i would do the same for sure!!! Make sure you make a nice project log on the forum here.

About that kidney, we should talk, maybe i will buy one in the future!!!

regards,

Roy

[RICHARD ZASTROW]

Curtis, My apologies for this late response to post #6, I had a lightening strike and was knocked out of commission while typing the response.P'd me off.

While reading your post I noticed this "max rpm 47k spec" in SKF precision bearing description.

I have a copy of the "SKF HIGH-PRECISION BEARINGS". In their tech section under "Speed Ratings" to paraphrase;

Valid provided the bearings are lightly preloaded,by means of springs [lathe spindles are normally heavily loaded by means of positive clamping]

Transport heat away from bearings is good. [Difficult without external air or liquid cooling]

When single bearings are adjusted against each other or matched sets are to be used, the speed values must be reduced. [pair of preloaded angular contact bearings]

The higher the contact angle, the greater the axial capacity & lower the speed. The lower the contact angle the greater the radial capacity & speed.

In short, the general bearing requirements for a lathe spindle tend to reduce the max. speed capacity of the bearings. The opposite is true for a high speed engraving spindles.

In reference to your bearing arrangement I'd recommend a pair of back to back medium/heavy preloaded, 25 degree contact angle bearings at the chuck end positively secured to both the spindle shaft and the housing. The rear bearing, a radial bearing secured to the spindle shaft and allowed to "float" axially to compensate for spindle length growth due to heat expansion.

If you intend to operate over 10k rpm, you'll likely need an air/oil mist lube system to avoid overheating. Even a recirculating oil system can get hot at elevated rev's.

Is the 2.3" dia. you mentioned the ID of your bearings? 55 mm? 60 mm?

Dick Z