Inconsistent fit - R8 collets

[Brian26]

I have a Grizzly 9975 milling machine. From the first day, some R8 collets will fit easily into the spindle, and others will go in only about 3/4 of the way - then stop. I have done some measuring and the end of one R8 collet (which will not fit) is .9490. Another R8 collet that does fit measures .9475 (same caliper used). These measurements are of the cylindrical part of the collet near the top- not the taper. Has anyone else had this experience and are there any suggestions as to how I might proceed? Even the larger one seems to be within specifications -- I see both .9500" and .9490 listed as the diameter of the top of these collets, depending on where I look.

 

[PeterT]

It could be the OD but another common issue is the vertical slot that engages the set screw in your spindle (assuming you are running one). If that is milled shallow or your set screw was protruding inward a bit too far, that might be the hang-up source. I did a post with pics on my 935 mill which had the screw protruding in too far from factory.

 

[Brian26]

Thanks Peter - that is helpful. Tomorrow I will carefully measure for any relevant dimensions for the slots in the R8 collets I have already tried. Some of those work, and others do not. For the sake of clarity, let me say that all of the collets go into the spindle way past the first encounter with the locating pin. The problem seems to be when there is about an inch to go to seat the collet properly, prior to screwing in the drawbar. But, there are differences in those slots, so I will check it out . Once more - thanks.

 

[Brian26]

Today I read some more about this issue, and discovered a link where someone thought the problem might be the slots - but wasn't. Their solution was to reduce the diameter of the R8 collets that would not fit using abrasive in a lathe. I tried this and it worked! So, my problem is a slightly smaller diameter receiver hole in the spindle. Rather than try to enlarge that, I thought it best to make each of the collets I want to use slightly smaller so they will fit. That way. the spindle bore is not going to be too large. Hard to fix that if it happened. Thanks for getting in on this Peter.

 

[PeterT]

It may well be. I think the upper end of the collet is supposed to be a sliding fit in the spindle but the angled seat is where the main concentric alignment occurs. But if your collet OD was a little big, or the spindle ID was a little small, or either were just a bit eccentric, that would be enough to cause friction at this close tolerance level. You cant do anything about the spindle without disassembly/replacement so I guess collets it is. It would be a PITA to do this to your other tooling though where you have more money tied up. Then technically could be loose on another mill spindle. Sometimes a burr can get trapped & gouge the surface. Possibly could lap the area but that's getting a bit more specialized & permanent. You could check your collet ODs against published data, they do have a defined tolerance range.

 

[Brian26]

The frustrating part about that idea is that I could not find a definitive statement as to the diameter of the end part of the R8 collet. One says .9500, another says .9490. I think I also saw one at .9485! That's the great thing about standards - you have such a choice! I realize that by reducing the OD of these collets I run the risk that they will be a trifle loose in another machine - but I am not reducing them below what several of my collets already measure - so there's that.

 

[PeterT]

Yes I seem to recall checking the same thing. Darn internet. I would trust Machineries handbook