I Don't Understand My Drill Bit Chuck

[MontanaAardvark]

I have a Jacobs-style chuck on an R8 arbor for my Grizzly G0704 that is doing something I simply don't understand. The centerline of the holes seems wrong but the bit doesn't seem to wobble as much as it should.

I've seen this issue before but dismissed it as an error on my part, but yesterday I saw it again. The other times, I readjusted the mill to put the part on the centerline and assumed I had gotten the edge zeroed incorrectly.

I'm making some part for my flame eater engine project and my plan was to cut the outline of some parts and then go back and drill holes in specific locations. This way the holes will be on the same coordinate system as the outline of the parts and be exactly in the right place.

Last week, while making the flywheel, I made a holder for a #2 center drill that fits a 1/4" collet and used it to start the holes.



Then I switched from the quarter inch collet to the drill bit for the big holes at the top, with a 9/32 bit in my chuck and it was visibly off center. Even my wife commented on it and she doesn't have the eye for small differences I've developed. If it's visible from a few feet away without a magnifier, it's way off.

I chose the 9/32 bit almost blindly; it's to clear a 1/4" shaft and allow my 3/8" OD bearings a shelf to sit on. So as an experiment, I tried switching to a 5/16 bit in my 5/16 collet and it as visually right on the center. I drilled 5/16 holes. The other four holes (at the bottom) were a size I didn't have a collet for, so I went back to the drill chuck and visually centered the drill bits. It was about .015 off.



So how can a drill bit in a chuck on an R8 arbor not be on the centerline like a part if a collet in the same spindle? Shouldn't it wobble that .015 or nearly .030 total? I don't see how it can be as wrong as it seems to be.

 

[JimDawson]

Just a guess, but it sounds like your head is out of tram. The further you extend the bit tip from the spindle the more error that is introduced. Years ago I had the same problem and I had to call my machinist neighbor over to figure it out, I thought I was going crazy.

 

[MontanaAardvark]

Just a guess, but it sounds like your head is out of tram. The further you extend the bit tip from the spindle the more error that is introduced. Years ago I had the same problem and I had to call my machinist neighbor over to figure it out, I thought I was going crazy.

I thought about that, but then thought that the smaller collets are dead nuts on center with each other so how could the arbor and chuck be different.

It's something to look at and it means doing something, though. Beats sitting around not doing anything.

 

[RollingPin]

I had a heck of a time finding chuckless drill chuck for an R8 arbor. Every one that I tried wobbled like nobody’s business. I finally got a 1 piece chuck and arbor for my G0704.


Sent from my iPad using Tapatalk

 

[ttabbal]

Glacern and PM both sell decently reviewed R8 drill chucks. You could also get a straight shank chuck and hold it in a collet. Or even hold the drill bit in a collet if you have a properly sized collet.

 

[JimDawson]

I thought about that, but then thought that the smaller collets are dead nuts on center with each other so how could the arbor and chuck be different.

It's something to look at and it means doing something, though. Beats sitting around not doing anything.

Even if the chuck has runout, it will still put the tool bit on the center of spindle rotation. But the tool tip might scribe a circle rather than rotation about a point. In other words, the runout has little effect on the hole center. The hole center will still be on the spindle centerline.

 

[Technical Ted]

I agree to check that the head is trammed in properly... This certainly could cause an issue like you are describing.

Ted

 

[MontanaAardvark]

So I just re-checked my tramming and it seems OK. First off, an overview of how I do it. I don't remember where I got this method. I have a Mitutoyo .25"x .0001" dial indicator that's old but lives a sheltered life. This is mounted in a half inch square aluminum bar and doubles as an optical spherometer (I'm a telescope maker and have made mirrors with this). I drilled a hole for a 1/4" brass rod and hold that in my collet in the spindle. Because I only have one of these cool dial indicators, I rotate the spindle 180 degrees by hand and flick the indicator once or twice and make sure it's not stuck somewhere by sticktion. I have to pull the pin up while rotating because it would go into the Tee slots, and then below the table.

Overview shot with the indicator on the side I need the mirror to read.


Here's a closeup of the dial when it's on the left:



and when I rotate it 180 degrees (and, yes, the subdial is still on the 0.15).



That's not quite one tenth (.0001") and the distance between these two spots is over 10" (10-1/4 I think). It's the way I left it the last time I checked (and that time I moved the headstock slightly) back in May of '17.

Would this vary if the head was a foot over the table? I'm not picturing the geometry in my head but I don't have a way to test that available to me.

 

[Technical Ted]

I'm not familiar with that mill. Is it a Grizzly? Does the table/knee move up and down or is the head mounted on a column and that moves the head up and down?

If the later, it sounds like your column is not plumb with your table and spindle. You trammed your spindle in, but if the column is at an angle to the spindle/table then it would shift the center of the spindle to the side when it is raised or lowered.

Ted

 

[MontanaAardvark]

I'm not familiar with that mill. Is it a Grizzly? Does the table/knee move up and down or is the head mounted on a column and that moves the head up and down?

If the later, it sounds like your column is not plumb with your table and spindle. You trammed your spindle in, but if the column is at an angle to the spindle/table then it would shift the center of the spindle to the side when it is raised or lowered.

Ted

That's very possible. It's a Grizzly, and the head goes up and down on the column. At one point I had it completely apart to add my ball screws and mounts and all, so it might be that when I put it together, I assembled it out of alignment.

The biggest machinist square I have is 4". I have a carpenter's square that gets me to 12" and a framing square that's bigger than that.

I think I used the 12" square when I put this back together. How should I do this?