Collet aligning screw replacement

The collet aligning screw on my J-Head round arm mill has been sheared off. Before I attack the spindle to remove it, it would be nice to know what to expect. I have never dug into one of these but I have the manual with drawings to go by. It looks like unscrewing the nosepiece is the first thing to do, then.... punt? Any help out there?

Before unscrewing the nose ring check to see if there is a setscrew on the backside of the quill. If there is you must remove it first. Then unscrew the nose ring. There should be a jam screw and also the guide pin in the threaded hole. These can be removed without taking out the spindle.

Dan

Thanks Dan, you have saved me a lot of trouble. About what I would expect from a Waukesha man. My oldest boy was born in Waukesha. I used to live just into New Berlin, on Downie Road straight up the hill from the Greenfield curve. Neighbor was a machine rebuilder. I wish he were still around.

This has been covered before on several forums, (including the "better off without it" view) but here is what the manual says, plus my explanation in place of reference to the drawing the manual has.

Note this refers to a J and 2J head.

1 Use felt pen, mark reference line on quill and nose cap.

2 Remove set screw (the little grub screw which prevents the nose cap come unscrewed).

3 Unscrew nose cap.

4 Remove lock screw and then collet aligning screw.

5 Replace collet aligning screw, insert R8 collet and check that the dog on the end of the screw does not foul on the bottom of the guide slot.

6 Replace lock screw (ie in the same threaded hole as the collet aligning screw).

7 Replace nose cap, check felt pen markings for correct alignment.

8 Replace set screw (the one that locks the nose cap) do not overtighten as this will cause distortion.

9 Check gap between nose cap and bottom face of quill, it should be .003" or .08mm.

I used the first 8 directions in the list with sucess. But did not replace the sheared off key. Is number 9 with a .003" clearence a bearing preload or what?
Walt

The reason for the gap is because you want the cap to push against the outer race of the lower bearing and not bottom out against the quill body. If this happened the bearings would be free to float up and down.


Dan

As far as I can see, the nose cap contacts the outer race of the lower angular contact bearing (as Dan says). This keeps the spindle assembly from falling out of the quill.

The nose cap does not affect bearing preload, in fact there is no way of altering preload on these spindles.

The two angular contact bearings at the lower end of the spindle are placed back to back, (like car wheel bearings) and seperated by two spacers, one seperating the inner races, one seperating the outer races (unlike car wheel bearings which have no spacer for the inner races).

When the nut is tightened at the top end of the spindle assembly, everything comes up hard on its spacers, preload is automatically set.

I am not exactly sure how the preload is achieved. I suspect the spacers are ground to the same length, and the bearings have a slight height difference between inner and outer races. Alternatively, the bearings may have the same height on inner and outer races, and the spacer rings have slightly different lengths (though this second idea doesn't sound right to me). I must check next time I have one apart....

ps, re. leaving out the alignment screw - I can't recommend this idea on a well-kept machine. It seems to me that at least some alignment screws are damaged by the tool holder trying to turn in the spindle. It is important that this never happens, the spindle bore must be kept perfect as possible.

Peter,


There is a way to alter bearing preload. I've done it several times.

I've worked on many import mills that have less than optimal spindle bearing preload. I've seen some that have developed play in the bearings after only a couple years service.

The way I've overcome the problem has been to grind the inner spacer shorter. This will increase the preload on the bearings.

I had a customer that was complaining of chatter when using an inserted carbide endmill. After checking the spindle bearing preload I determined it was lighter than other mills I have checked. My solution was to grind the inner spacer a small amount which increased the bearing preload and the chatter was eliminated.

One has to know how much is enough and how much is too much, as the bearings can heat and be destroyed if there is too much preload.

The best solution is new bearings from a good manufacturer. The spacers are then ground to the same height.

Dan