Rf-31 Spindle Runout

[mikey]

Evening, Guys.

I have a new-to-me RF-31 that was used as a bowling ball finger hole drilling machine. It was never used to mill anything so it was in decent shape, and the guy only used a 1/2" R8 collet in there and never really took it out of the machine. On initial inspection, the upper spindle bearing was covered in grit and dirt so I changed the spindle bearings to FAG sealed angular contact bearings (I'll do a write up later). These are P5 class bearings, equivalent to ABEC 5, so should be pretty okay for this machine. I thought they would at least improve runout a little bit so I checked.

Runout inside the spindle nose with a Compact 214GA DTI (reads 0.0005") gave me a TIR of 0.002". To me, that's a lot! I got the same reading at 3 different spots inside the R8 taper. I then put a Tormach R8 collet in the spindle and slapped an Albrecht chuck in there. Runout at the base of the arbor was 0.0015" TIR and 0.006" at the end of a 3/8" OD dowel pin, 2" from the nose of the chuck. I got pretty much the same results with a Tormach ER-32 chuck holding the same dowel pin.

Now, I didn't expect a high quality result from a budget mill drill but I did expect much better spindle runout than this. So, my questions:

• What are you guys seeing for spindle runout on your machines?
• Where do you think the runout is coming from? I think it is the spindle, the drive sleeve or perhaps the accuracy of the quill bore where the bearings seat.
  
• I set preload by feel. I would guess somewhere around 20lb/ft of torque or a bit less. Since I used angular contact bearings the preload should not be critical. They run cool and vibration-free, with a temp of only 41 degrees C after 30 minutes at 1800 rpm. However, I am not absolutely certain that this amount of preload is adequate - what do you think?
• Are there other sources of excessive spindle runout I should consider?

I have a Sherline mill that will hold 0.0001" TIR inside the spindle taper and it cuts like a dream. I needed more capacity so I accepted this mill drill but I don't think it will do good work as it is. Your input would be appreciated.

Mike

 

[Bob Korves]

Measure the R8 taper runout directly with a good .0001" reading DTI. You are adding additional uncertainties to the equation by using the collet and pin, and by using the drill chuck. Spindle runout should not exceed .0002", although lots of machines are worse than that. Inspect the inside of the spindle for burs, damage, corrosion, swarf, and for an anti rotation pin that is too tight in the collet groove, too wide or too deep. If you get to the point where you want to regrind the taper I can link you to a very good video on how to do that without a lot of special equipment.

Edit: 20 ft. lbs. of torque preload seems like a lot to me.

 

[mikey]

Thanks Bob. I did measure inside the spindle taper, at three separate places and got 0.002" in all instances. If it was 0.0002" I would be okay with it but not 0.002". It seems to me that this is waaay more than I should be seeing and I need to figure out where its coming from. It may turn out that I'll simply replace the spindle and drive sleeve but I had hoped to make a solid diagnosis before I go replacing parts.

I may have overstated the 20lb/ft - it is probably a good deal less than that.

Yes, I know measuring the pin with the chuck and collet doesn't help. I did that to show what the consequences of that poor runout was.

If I cannot determine what is causing the runout and I decide to try grinding the spindle taper I will definitely take you up on that link offer - thank you!