View Full Version : Spindle recommendations for a lathe toolpost grinder?

12-18-2016, 08:30 PM

I want to add a toolpost grinder to my lathe. Eventually I would like to CNC it for making things like involutes and cycloids, or one-off cutting tools.

My lathe is just a little 7x14. Preferably I would like to have the motor located remotely, via flexible shaft or pneumatic. It would make positioning of the compound slide easier.

Problem is, I don't have a clue about different manufacturers of grinder spindles, what they are good or not good for, and was hoping for some advice. Any wheel attached to it would probably get no bigger than 50-75mm/2-3". I realise that to work with tiny grinding bits, it will also need to operate at very high RPM


12-18-2016, 10:26 PM
those little air die grinders are pretty popular or mounting a dremel/ flex shaft is another option. Didn't think a whole lot of my dremel tool poster grinding experience (lots of chatter and bounce) but it's better than nothing in a pinch.

Other than that a small ER11 or ER16 powered spindle from China is your best bet for a complete unit. They sell them for CNC routers.

12-18-2016, 10:42 PM
Was the dremel the source of the chatter? I've only owned one dremel tool (dremel stylus), and haven't had a great experience. I had to return it, because the on/off button failed after about a week or two. It was nicely aligned, spun at max speed with no significant vibration. However, the replacement unit vibrated horribly at nearly any speed. I sent it back again, they sent me another one just as bad. Again, I sent it back, but this time insisted that Bosch technicians inspect the tool before shipping it to me. I finally got a better one, but it still vibrated at the highest speed settings. I gave up trying again.

It now belongs to my mum who uses it for model ship building. She doesn't use the high speeds so it isn't an issue.

12-19-2016, 12:18 PM
I actually have a black and Decker version which is a step up from a Dremel in that it has a small ball bearing spring the spindle. I don't know if the chatter was from the tool or the crappy undressed wheel I had on it. When I need it again I'll definitely see the wheel on the grinder and see if that helps.

12-19-2016, 12:35 PM
I used a 56,000 RPM pencil grinder on my lathe a few times for grinding work and it never worked well. The spindle actually gave out during a job, it got all wobbly in the housing. I think the ball bearings partially exploded.

You can often find Dumore spindles on eBay. I'd go for those if I was looking to buy a toolpost grinder spindle.

http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR8.TRC1.A0.H0.Xdumore +spindle.TRS0&_nkw=dumore+spindle&_sacat=0

12-19-2016, 01:08 PM
For a small lathe, the Foredom line of flex shaft grinders can be had in 1/3 Hp and variable speed to 15,000 rpm. The hand pieces are ball bearing equipped with collet shaft holders ( up to 1/4"). It would be fairly simple to construct a mount for your tool post.


12-19-2016, 02:46 PM
I do like matts idea of using a cnc spindle unit, seems sensible, good bearings etc, I've had no luck with dremels, Wizards (b&d) or air, poor finish, better than nothing but only just, TP grinders are only made of platinum with solid gold wires over here, judging by the price so I think I'll try a spindle unit
Don't laugh I had better finish using the hub of a bycycle wheel once, not a bad spindle as it turned out

12-19-2016, 03:40 PM
Looking at the motorized spindles on Ebay the issue I see with them is that they only go up to about 12K RPM. Now that'll be fine for 3/4 and larger diameter stones. But it's not enough for smaller sizes to really make them work well. But I see that there are also higher powered water cooled motors that run up to a max speed of 60K RPM. That should do the trick and allow the use of smaller grinding points.

The other thing I see is that these motorized spindles are not all that small at roughly 2 inch diameter by around 5 to 6 inches long. That's going to fill up the room on a 7x14 pretty quickly. LIkely need to pull off the tail stock. Or if you want to grind an item held between centers it may turn out that you need an odd off set mount for the unit.

12-19-2016, 08:00 PM
Hmm, yeah, I haven't really thought much about the space available on the lathe bed. My tailstock is way too high anyway, about 0.3mm, so I have to shim the head before I can use it.

I looked at those water cooled motorised spindles, but yeah, they are fat. A spindle housing (without incorporated motor) that large would hold an enormous stone

boslab, how fast were you running the bicycle spindle? The fastest cyclist on record is only doing around 1200 RPM

RWO, those Foredom ones look pretty nice. Though a max RPM of 12000 is about 15.7m/s (3100 SFPM) surface speed on a one inch wheel

Andre, yeah, a pencil style seems just a little light duty :) No Dumore in my region though

What would be some other way the home shop (CNC) machinist could grind a cycloidal gear surface?

12-19-2016, 09:45 PM
Hiya Swarfer, I built one with a used ebay Sherline milling spindle and an ebay 24vdc scooter motor. It's mostly completed but I need to finish a few coolant details here: http://bbs.homeshopmachinist.net/threads/69450-Po-man-s-TP-grinder-as-it-sits-now?highlight=TOOLPOST and here for details on what I'm doing for coolant management, post #28 http://bbs.homeshopmachinist.net/threads/69748-Shop-vac-cyclone-question?highlight=TOOLPOST

I'm using a 4" CBN wheel which keeps the speed well within the Sherline's 10K rpm limit. Personally I think it will go faster but not by much. I plan to use 1" alum oxide points as well right at the 10K limit. Probably not fast enough but we'll see how it goes. HTH:)

12-19-2016, 09:55 PM
Gotta watch the speed rating of some of those points.


12-19-2016, 11:32 PM
boslab, how fast were you running the bicycle spindle? The fastest cyclist on record is only doing around 1200 RPM
I'm going to guess about 6000, pedalling furiously, I stuck a pully on one end to an old elecric motor, 1500 rpm, then a smaller pully on the other end, can't remember sizes as the thing was a tinkering exercise with plastic pulleys.

J Tiers
12-20-2016, 01:13 AM
I have some older Dremel units that always seem to work decently as far as finish. I used them to grind chuck jaws to be straight again,and they did OK. If I really want good, I have a TP grinder, but I hate using any grinder on the lathes.

Paul Alciatore
12-20-2016, 03:51 AM
I have made an adapter to mount my Unimat head stock on my SB9 quick change tool post. It is sized to hold the Unimat at center height with the usual fine adjustment via the tool holder. The Unimat spindle is not perfect, but worlds better than any Dremel or other hand held tool.

http://img.photobucket.com/albums/v55/EPAIII/11_OnSB.jpg (http://smg.photobucket.com/user/EPAIII/media/11_OnSB.jpg.html)

I was doing some drilling with it in that photo, but I can mount large abrasive wheels directly on the spindle and small abrasive points in a collet.

I can also mount it directly on the compound with a different adapter and the Unimat column for a greater range of positions.

http://img.photobucket.com/albums/v55/EPAIII/P02.jpg (http://smg.photobucket.com/user/EPAIII/media/P02.jpg.html)

And I will save you the trouble of calling me out: yes, I was milling with a drill chuck and not a very good one at that. All I can say is that it worked. I have since made a collet adapter and have a set of ERs for it.

12-20-2016, 08:54 PM
I have made my first toolpost grinder more than 10 years ago based on a Rotorzip tool. The spindle speed is 30000 RPM and it has a 1/4" collet. Put 1" grinding stone in it and it cuts steel like a butter. To put it on a lathe I have made an adaptor, which clamps on the Rotorzip nose and gets installed in place of a regular tool. Rotorzip is much more robust tool than Dremel and can easily use a 1" stone.

Recently I have bought a Habor Freight die grinder with extended nose, which is even better candidate for a toolpost grinder. It has 25000 RPM no load speed and also has a 1/4" collet. For bigger wheels the speed can be reduced by reducing the power supply voltage (you will loose power as well). I like this design because the grinder spindle is separate from its motor and rides in 2 ball bearings. I have a long term project in my mind to replace the bearings with high accuracy angular contact ball bearings, put a proper preload on them and install labyrinth seals. This will essentually create a real grinding spindle, capable of machining with good accuracy and surface finish. The tool will install on the lathe via an adaptor, similar to Rotorzip one.

12-21-2016, 07:48 AM
make it yourself. its fun.

12-21-2016, 08:41 AM
Like the rotozip idea, the things useless for Sheetrock, it's easier to use a knife or stab saw

12-21-2016, 03:39 PM
I can fully appreciate building one, but recently searching the web for one myself
this popped up for your size lathe; http://littlemachineshop.com/products/product_category.php?category=-1731696149

12-21-2016, 05:24 PM
"I can fully appreciate building one, but recently searching the web for one myself
this popped up for your size lathe; http://littlemachineshop.com/product...ry=-1731696149"

Unfortunately they do not provide any information, such as RPM, is it capable of internal grinding (I doubt that), what are the bearings? To produce good accuracy and good surface finish you need good bearings and dynamically balanced rotating element. To me the internal grinding capability is the most important, I can produce a decent OD by other means.

12-21-2016, 06:42 PM
I'll add a few thoughts of my own here, possibly a little offbeat since this has been discussed here before. The main issue I have is with the bearings. First off, the ideal bearing would be an angular contact pair with some preload, possible close together, with a suitable rear bearing. All would be precision made to ABEC 27 or whatever the eyebrow-raising grade is these days. The better the more expensive, etc, and the contruction of the spindle would also be of exacting design and of some specific alloy. If something in this arena is desired, then I'd suggest a bought spindle- and it won't be cheap to say the least. But if we stick with more normal bearings, then high speed tools like routers, dremels, etc having shaft diameters ranging from 1/4 inch to perhaps 5/8 inch would be the pool to choose from. Either make your own, or modify an existing tool.

The larger the shaft diameter, the more rigid the spindle is going to be, and the larger the bearings will need to be. The tradeoff is rpm. The faster you turn it, the more energy as heat is going to be generated and lost. If you have a pre-loaded front bearing pair, then more energy is lost there. Seals waste more energy than shields, so you can choose shields only, or one seal and one shield, or one seal and no shields, one seal on each bearing and one shield on each- you get the picture. I've seen router bearings with one seal and one shield, but usually they have shields only. This allows a higher speed with less heating, but is subject to intrusion of dirt and expulsion of the bearing lube. It seems that a good lifetime can be had using shields only, but nothing stops you from adding additional shielding in the form of a non-contact labyrinth type of construction. You can also forgo the bearing grease and use oil instead, as long as you include a method of adding oil often, and clearing away the inevitable ooze-out.

Leaving that aside for the moment, a larger bearing will have a lower rpm rating in general. Hand held dremel type things don't have large bearings, so they will be deficient in rigidity although they might be capable of operating at 35000 rpm. You might need to stay with the smaller diameter bearings if you are grinding small diameter bores, but otherwise a larger shaft with larger bearings will give you better results. Two things here- with small bearings and lots of stick-out of the tool you will have flex regardless of how tight the bearings are. A larger shaft can stick out further before the flex becomes problematic. And secondly, there are two sizes of shank in common use, 1/8 inch as in dremels, and 1/4 inch as in typical routers. You can mount an 1/8 shank in a 1/4 inch collet using an adapter, but you can't go the other way. You do have the option to use a 1/2 inch router shaft, and if the collet end isn't too large to fit the work you'd be doing, then that will offer the most rigidity, the longest 'nose' before flex becomes an issue, the ability to accept all shanks up to 1/2 inch, but probably a reduced top end rpm. Personally, I have built and experimented with toolpost machines with 1/8 inch and 1/4 inch capable shafts- the only use I have for the smaller one is where I simply can't fit the larger one into the set-up. Small hole internal grinding is about it, but then the result is dubious anyway because of the lack of stability at the business end of the tool. My next tool post machine is going to use a 1/2 inch router spindle. I will get around the stability problem by custom making a tapered shank to carry the actual tool if I need the reach, etc.

Something else I thought of- and the actual reason I'm posting this- the spindle shaft has a certain amount of weight inherent in it. If it's not part of an armature, that weight is minimal. If you are going to be using an external motor to drive it, you do not have the weight of an armature surrounding the spindle. Consequently you don't have the stabilizing effect that that mass would add to it. Perhaps a spindle design could include as much weight as you can fit within the small confines of the housing. Either add a steel sleeve to an existing spindle shaft, or if you make your own shaft from a blank of suitable steel, then start with a larger diameter piece and leave as much meat on it as you can. The extra weight of the spindle would help to stabilize the rpm at the end of the flex cable driving it, besides allowing for a better finish from the tool being used. This is conjecture on my part, but it makes sense to me. The only issue I see with leaving the spindle heavy is if its resonance point corresponds to a resonance in the lathe. I can see that being an issue. At any rate, having full control of the rpm though an electronic control would give you the ability to select a 'quiet' rpm to run at.

I'm not going to discuss the obvious issues of making your own spindle at this time. Suffice to say that if it's not well-machined for least run-out or balance, then it likely won't be satisfactory.

12-21-2016, 08:07 PM
DICKEYBIRD - that is a nice project you have going there :) One of the things I want to do is internal grinding of roulette curves, so I will need small bits and high speeds for that. Cheers for the tip on the vortex cone, those things look great for general dust collection.

polaraligned - yeah, T=r*F, the reactive cutting force would ramp up as the diameter ramps down

boslab - thanks for the extra details

J Tiers - Why is it that you don't like grinding on a lathe? Is it because of abrasive dust getting all over precision mating surfaces? Or do you just find it sucks?

Paul Alciatore - that looks pretty versatile, and if it gets the job done to required tolerance, who cares what people think of the methods? :)

mikey553 - This die grinder? http://www.harborfreight.com/electric-die-grinder-with-long-shaft-44141.html
I was looking at a very similar (though more expensive) die grinder by Makita. https://www.bunnings.com.au/makita-400w-die-grinder_p6240031 - and I have a Bunnings gift card we won in a local raffle stuck to my fridge.

Did you gut the unit and make your own housing? I can't see any way to secure and position them otherwise.

dian - Like many here I imagine, I am not exactly short of fun ideas for projects. Rather, we are short on time, resources, knowledge, and skills to get to all of them. An accurate spindle is beyond my skill+equipment.

QSIMDO & mikey553 - ahh, they look like little Sieg Industrial 10131 units, which they don't have an actual page for, but are listed in the optional accessories for their mini lathes.

Grinding attachment
Spindle speed 0-600010%rpm
Wheel size 80*20*10mm
Motor output power 250W
Net/Gross weight 4/5kg

The price is definitely low. Haven't seen them offered in Australia though. And yeah, not suitable for inside grinding or small bits, but interesting nevertheless. I assume they would be of similar quality to their other products.

darryl - sorry, I haven't had enough time to properly consider your post, I will get back to you soon, but I have to go eat :)

12-21-2016, 09:10 PM
darryl - Thanks for this detailed post. Your explanation on spindle designs in particular is very helpful. And thanks for reminding me about thermal considerations and resonance.

I hadn't properly considered the reduced moment of inertia that results from not having an armature. I was just thinking that for a given sized housing, I could use a spindle with the diameter of an armature, increasing my load bearing capacity and spindle momentum, while still having reasonable clearance with smaller wheels. But of course, if I throw a large wheel on, it will have far more momentum than the spindle. Perhaps that is why the TP grinders for larger wheels use a pulley on the back of the spindle, to increase gyroscopic stability, and the belt adding a small counter moment to the cutting forces.

And yeah, I'm definitely not up to maching my own spindle. What did you end up using for you 1/4" one?

12-21-2016, 10:22 PM
How about using an ER16 straight shank collet chuck? Add a couple of bearings, a housing, and a motor and you're ready to roll.

Paul Alciatore
12-21-2016, 11:59 PM
Swarfer, I only said the Unimat spindle beats the pants off a Dremel. And yes, I have tried both.

Grinding chuck jaws with Dremel:

http://img.photobucket.com/albums/v55/EPAIII/ChuckGrind1R.jpg (http://smg.photobucket.com/user/EPAIII/media/ChuckGrind1R.jpg.html)

I like this photo:

http://img.photobucket.com/albums/v55/EPAIII/ChuckGrind9R.jpg (http://smg.photobucket.com/user/EPAIII/media/ChuckGrind9R.jpg.html)

J Tiers
12-22-2016, 01:04 AM
I'm not sure I am 100% on-board with all Darryl said.

With regard to diameter of shank.... The theory of small = deflection is attractive, but should really not be considered too serious for a toolpost grinder. The forces used in grinding, particularly with small shanks, are, or SHOULD BE, small. Therefore there should not BE enough force to get involved with deflection. If the shank is long enough, it might get "whippy", and unstable, but I will assume most folks have reasonable "machine sense" and won't get involved with that.

"Grinding" has a popular image involving pressure and forces to wear away ("grind away at") the work. But in reality, it shouldn't, for any reasonably precise operation. It takes very little pressure to cut well when there are several million cuts per minute taken by the grinding wheel. (10,000 rpm Dremel, and a small wheel with a thousand or more grains on the OD).

It makes sense to use the largest spindle that you can work with. A 3/8" spindle is fairly common with Dumore grinders, for instance. That size could allow you to make, or attach, a collet type adapter for mounted "points", and still use wheels if so desired.

As for bearings, there are some pitfalls and cures. Angular contact are the obvious go-to type, of course. But Dumore, for instance, does not use them in the "non-cartridge" spindles. Instead, they use a normal or deep-groove ball bearing, and preload it just as if it were an angular contact type.

The bearings Dumore used in the two grinders of theirs that I have had, have been open types, with the shielding against grit being [art of the housing, and not the bearing. They are both made with oiled bearings, as well, having an internal "tube wick" to supply oil. Grease vs oil is something you need to decide. Shielded and sealed (sealed not recommended) are supplied with "lifetime grease", typically. Open bearings are often supplied with only a preservative grease, and need some means for either greasing or oiling them.

High speed bearings do not need, and do not want, a lot of oil. Oil or grease in excess gets stirred up and created a good deal of frictional heat, which is bad for bearings. There is much to be said for a shielded type with manufacturer supplied grease, as well as a maximum speed spec that you can count on them tolerating as-shipped, grease and all.

Bearings have a spec known as "clearance", which is the looseness, or slop, in the bearing. If you handle a lot of different bearings, you will notice that some are looser than others. Obviously clearance will allow your spindle to "rattle around", cutting slightly deeper and shallower on a random basis, and that would be undesirable if you want precision.

So, a "wave washer", a Belleville spring, or other means are commonly used to set some preload. This is what Dumore does, and is probably a good plan for any lightly loaded spindle such as a toolpost grinder. Atlas lathes, and other machines handling heavier loading, may set preload by a nut that is tightened, or, in higher class machines, by different length spacers that are inserted between the outer races vs the inner races. They also use angular contact bearings, or, in the case of Atlas, tapered roller bearings.

The preload should not be "heavy", because that lowers lifetime. If you look at the detailed bearing specs, they will generally show a number for axial pressure, and may have a graph of axial pressure vs lifetime. A preload way down on that curve is best, it really should not take a lot. Perhaps a couple percent of the axial load limit.

12-22-2016, 03:34 AM
When I'm using my TP grinder on small diameter parts, the finish I get is quite dependent on whether or not I damp the part as I'm grinding. If I don't damp it, it will vibrate to some extent, usually not visibly but enough to affect the grind. The usual result of this is a part that becomes too small in diameter and visually rougher. It follows then, in my mind, that if the grinder spindle can also vibrate, or whip, that the same result could occur. The amount of flex would not be much, but it represents a varying distance between the grinding wheel and the work piece, which translates into a lack of precision.

Even though 10,000 grains are touching the work piece in a sort of random fashion, each time one makes contact it would tend to push the wheel away from the work piece- even though by only a tiny amount. Letting it spark out should be giving you a nice finished result, but you also don't want to have thousands of slightly deeper gouges left on the surface by the previous heavier contact grinding. I don't know- some of this is from actual experience, some is conjecture. Personally I would go with the more rigid spindle every time, and that would be the larger diameter one, other factors being equal.

My 1/4 inch capable grinder uses the shaft from an old Makita trim router. I have also gone partway on a project using the spindle from an air die grinder. I can't say which one would be better, but in all the spindles I've used and saved for future use, there sure doesn't seem to be much meat around the collet end. Mostly they look like they could break off or at least bend if you just looked at them wrong. Looking at it this way, the 1/2 inch shank capable spindle out of a larger router would seem to offer the best strength. The one I saved will be the one in my next tool post machine, unless I take on the task of making my own spindle. So far I've stayed with using the existing spindles to take advantage of the already-machined collet, the nut, and threads. It is a bit daunting to machine all this to a high degree of concentricity.

I did actually build my own spindle once, using a needle roller bearing for the front end and a deep groove ball bearing for the rear, but the front bearing was noisy and I didn't complete that project. I understand now that it's possible to control the radial play of such a bearing by pressing the outer race into a precisely bored hole to shrink the outer race by just the right amount- but I think you'd have to work to a tenth or so in order to remove all play but not have it tight. The intent at the time was to restrain radial play to a very high degree while keeping the bearing OD as small as possible. The housing for this actually fit within the slot on my four-way tool post, but that too was impractical because the fixture bolts on the tool post would eventually have distorted the very thin housing and probably would have destroyed the bearing anyway.

What I really wanted to use was a small-sized tapered roller bearing, but I couldn't find one small enough. And JT you could be right- perhaps there's no need for such a high degree of rigidity in something like this.

12-22-2016, 04:03 AM
I've rattled on quite a bit here, but I also wanted to mention something about my conversion of a dremel shaft to a TP machine. Obviously it takes only a 1/8 shank tool, but apart from that I somehow destroyed the front bearing when the collet tightening nut contacted the work piece at one point. From that very moment the thing runs with an audible knock. The bearings are very small and in my opinion not really up to the task. As a handheld you could have the same problem, but there's going to be enough give in your hand holding to allow the peak bearing pressures to be lower. With something like this rigidly mounted in your tool post, you could encounter the same kind of peak instantaneous pressures that could overwhelm and damage a bearing.

12-22-2016, 05:08 AM
I'm using a Rotozip Rebel as a tool post rotary tool, I do use it for grinding but with a carbide endmill it will mill steel nicely and is great for all sorts of irregular and interrupted slots on odd-ball parts.

- Nick

12-22-2016, 06:01 PM
"mikey553 - This die grinder? http://www.harborfreight.com/electri...aft-44141.html
I was looking at a very similar (though more expensive) die grinder by Makita. https://www.bunnings.com.au/makita-4...inder_p6240031 - and I have a Bunnings gift card we won in a local raffle stuck to my fridge."

Harbor Freight sells two similar die grinders. I could choose which one to buy and I selected 60656 one. It has a regular collet with no threads or hex on it. It appears that the internal construction is different too. You can look at the user feedback at the HF site and see that both grinders have quite horrible ratings. The most of them are due to motor becomes unoperational and grinder overheats in operation. I took mine completely apart and can testify that grinder overheats due to the fact that spindle bearings are assembled with too much preload. There is nothing in the design to adjust the preload and all depends on the parts dimensions and assembler skill. There is not much material in the aluminum spindle housing to work with, but I still hope I will not need to make a new one. I am thinking about machining a portion on the housing OD to create a journal I can use for clamping in the lathe.

I did a similar machining to the Rotozip Solaris tool and it is working very well for clamping. Makita grinder has a rubber cover over the spindle housing, So I cannot tell what is under this cover and how suitable the housing is for clamping. Just don't expect any miracles from Makita - it will not have a real grinding spindle by any means.

I have to make a few remarks about an interesting subject a few of you have touched. It is called rotordynamics. It studies the behavior of rotating assemblies. Our grinding spindles definitely fall into this category. Every rotating assembly has its own critical speeds, at which a small external force (such as unbalance) will cause a very big amplitude of vibration due to resonance. Normally you do not want to operate in this region. You can operate below first critical speed or above it, but not directly at it. Critical speed depends on bearings, distance between them, shaft rigidity and weight, attachments location and weight. We should probably talk only about the 1st critical speed, because the other ones are normally too high to interfere with our projects. With all other things being equal the bigger shaft diameter will make it more rigid and will increase the critical speed, the bigger weight will decrease it.

While the spindle shaft is normally well machined and does not cause a lot of vibration, the attachments (such as grinding wheels or stones) is a different story. The long and slim arbors produce a lot of flexibility and grinding wheels naturally have an unbalance. This creates a potential for the grinding wheel to run above the first critical speed. There is nothing wrong about it as long as you are not running directly on top of it.

I also want to say a few words about bearings. I have ABEC-9 angular contact bearings for my project with 12 mm ID for the front bearing and 10 mm for the rear one. It is overkill, but I have them already. Believe it or not, but even these small bearings are close to speed limit at 28000 RPM. This is what I have measured at no load and full voltage on the motor. So going for a bigger spindle with 1/2" collet most likely will force you to reduce RPM.
Another consideration - is parts quality to accept precision bearings. We are talking about a few microns tolerances for spindle journals and housing bores and alignment between them. I do not know if I can measure the parts that good, leave alone machining them to that level of accuracy. That is why precision spindles cost a lot of money and the ones, that are cheap, are only called "precision", but in reality they are not.
The amount of preload, I am going to put on these bearings, is about 5-10 lbs. That is all what's required since axial forces in operation will be very small. I will use a regular compression spring and one sliding bearing to accomplish that. Sliding fit is needed to compensate for thermal expansion of spindle and housing. I have to admit - this is not my design, I borrowed it from the literature I found online.

J Tiers
12-22-2016, 07:13 PM
Bearings can have different speed limits.

I once saw a grinder head for small mounted points. The spindle was somewhere in the 3/8 to 1/2 inch range, 10-12mm say. The RPM for this grinder head had a limit at 150,000 rpm. I think I can guarantee that the bearings in it were the best grade that could be bought, and possibly selected from among them.

Makes sense, since a point of 1/8" diameter should be going only at about 5000 FPM at that rpm. About equal to a 6" wheel at 3600 rpm.

I suspect nobody would spend that money required to get those, but they exist