Dumb question on R8 (i think) spindle

[DavidR8]

I think concentricity is determined by the taper. But that's just my guess.
Did the mill come with any collets?

 

[Ken226]

The concentricity is determined by the taper, but unlike Morse tapers, the R8 has a rather short tapered section. The upper portion of the collet maintains alignment, preventing radial forces on the end-mill from acting as a lever, altering alignment.

Even short collets like ER32 have a longer tapered section than R8. (1.57") for ER32
Vs the tapered section of an R8 (.95")



Without the support of the upper portion of the collet, you can expect more deflection of end-mills under heavier loads. Also, you'd be relying on the tension of the drawbar, and concentricity of the drawbar/collet threads to prevent movement.

The collet threads may not be very concentric to begin with, as the manufacturer never expected those threads to be used to aid in alignment.

If you can chuck up a cut off collet, it would be interesting to see how much runout is indicated at several points along a drill rod.

Anything worth doing, is worth doing right. Replace the spindle, or send it out to be reground.

 

[Ken226]

BTW, pics of your spindle show what looks like deep groove radial ball bearings, which probably suck for a mill spindle. That's likely why there are no threads. Nothing to pre-load.

I think the reason the grizzly replacement is has those threads at the bottom of the splines, is because it is an updated version of yours, designed for tapered roller or AC bearings. That threaded portion is likely for the preload spanner nut/nuts that are needed for the preload adjustment of tapered rollers and/or angular contact bearings.

If... well, when..., you replace the spindle, you'll also be doing a worthwhile upgrade, because you'll need a set of tapered roller or angular contact bearings and the preload nuts as well. You may as well order the bearings and preload nuts from Grizzly when you rebuild the spindle, they'll be necessary.

Here is the one piece preload nut from my G0704 CNC build. The 2 cap screws lock the nut onto the threads.

The 2 empty holes are for a pin wrench.


I used Abec7 rated bearings from NSK in this spindle. It has less than .0001" of radial runout measured in the spindle taper.

If your on a budget, I believe Nachi has some p4 precision angular contact bearings for under 100$ a set.

Do your own research though. I can't be certain they aren't knockoffs or something. Since your mill doesn't run much over 2500rpm, I think the polyamide cage would likely be fine. Polyamide melts at about 450°f. My CNC spindle runs 8000rpm, and doesn't get anywhere near that hot.

Nachi Japan 7207 CYUGL P4 Super Precision Angular Contact Ball Bearing: Amazon.com: Industrial & Scientific

Nachi Japan 7207 CYUGL P4 Super Precision Angular Contact Ball Bearing: Amazon.com: Industrial & Scientific

www.amazon.com

 

[r-mm]

To the best of my ability it seems the bearings on my current spindle and listed in the G0731 parts manual are the same

 

[r-mm]

I think I can set up to bore the spindle. Just need a longer boring bar.

 

[Ken226]

To the best of my ability it seems the bearings on my current spindle and listed in the G0731 parts manual are the same

Yes. 7207 is 35x72x17mm. A fairly common size. Available on amazon from 15$ per set, to 2000$ per set, depending on precision rating.

P4 is equivalent to Abec7. .00015" runout or better, if I remember correctly.

 

[rwm]

I'm worried a boring bar long enough to cut the tail end of the R8 taper will chatter like my ex wife. I suppose a 1" bar might work?
Robert

 

[Ken226]

I think I can set up to bore the spindle. Just need a longer boring bar. View attachment 354395

Not likely. The spindle is almost certainly hardened.. you might be able to cut it with carbide inserts, but in a 3 jaw, you'll very likely cut some axial runout into your new taper. Probably way more than a spindle should have.

If your going to try it yourself (nothing to lose at this point) anyway. Best way would be to put it back together, and rig a toolpost grinder onto your mill table. Grind it in-place, in your mill.

At worst, remove the bearings and use a 4 jaw and steady rest, and a toolpost grinder. (You can try a boring bar. At this point, you got nothing to lose, but expect trouble).

Indicate to the nth degree, off of both bearing journals. Get it indicated to .0001" or better, off both bearing journals, to get rid of all axial and radial runout before touching the taper.

Both of the spindles internal collet bearing surfaces, the taper and the upper portion, need to rotate co-axially with the bearing journals, and with as little radial runout as possible. If you were to send it to a pro shop to be re-ground, most will hold it to a .0001" or less standard.

Personally, I'd replace it and buy good bearings. In a year, you'll be proud of the precision work you can do and won't even miss the money..

 

[r-mm]

To be clear i am not attempting to cut anything to do with the taper. I am only attempting to fix what I presume is galling and scoring at the rear which is preventing a non-shortened collett from being drawn tight.

 

[Ken226]

To be clear i am not attempting to cut anything to do with the taper. I am only attempting to fix what I presume is galling and scoring at the rear which is preventing a non-shortened collett from being drawn tight.

Pull the bearings. A bearing puller is like 15$ at harbor freight. Chuck up the in a 4 jaw, with a steady rest for support.

Get 2 indicators on the bearing journals, and dial them in untill the needles don't even twitch.

Then take the lightest cut possible with your boring bar, test fitting a collet between passes.

A cut that light won't require a 1" boring bar, but if you can get one, it'll surely help.

Your taking a tiny cut, just knocking down the high spots that arose when whatever disaster caused this, occured.