Jacobs Drill Chuck Runout--

B

Bob V

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Hi All, and Happy New Year,

I have an old Van Norman mill with spindle runout under .001.
To tool this up over the past 4 months, I bought a Jacobs chuck, Model 34-33C and i got a new arbor (JT33 to 2M) adaptor to adapt to my spindle taper: Van Norman Style "C".
All of this fits together just fine-- but with a rather long "stack", even without a drill bit in the chuck.

I've read quite a bit on forums about Jacobs chuck runout-- and Jacobs says my chuck should have no more than .004 TIR at half capacity-

.004 seems like a lot of runout-- and doesn't include runout of the spindle and stack of adaptors to the chuck.-- not to mention that twist drills are frequently not that straight (I'm new to this but this is what I've read.)

Anyways, my question is: how do you drill a "straight hole" (where it's supposed to be and the correct diameter) with this kind of setup? .004 plus all the other stacked runouts could really add up-- so what do guys do to get the right size hole in the right place?

--Hope I made this question clear enough! please tell me if it's not clear!
Thanks
Bob
 
What would really help here is apicture of the set-up and the parts. I don't own a Van Norman so without the pics it's hard to help.

"Billy G"
 
When Jacobs mentions the max runout, they're referring to the manufacturing tolerance, not necessarily what yours has. You are right that there might be a compounding of runouts happening but you can also use this to your advantage. I would try indexing the different components one by one to see where you get the best concentricity. Index each component at 90 degree increments and recheck. Use a gauge pin or long dowel pin for your checks. Also check the jaws of the chuck. They may be worn at the tips which is common. A rebuild kit from Jacobs would fix that nicely.
 
Rebuild kit, why, this is a brand new chuck not an old one. If anything get a replacement.

n
"Billy G"
 
Bill Gruby said:
Rebuild kit, why, this is a brand new chuck not an old one. If anything get a replacement.

n
"Billy G"
Click to expand...
I wasn't aware that it was a new chuck. He said that he bought one but didn't say if it was new. Of course if it is new, and doesn't meet the manufacturers tolerances, it should be returned.
 
+1 on checking each component separately for runout, marking the high (or low) spots, and then using the variations to cancel each other out. Put a high spot and a low spot together and they work toward cancelling each other out. It is a simple way to achieve good (or at least better) accuracy while using less than perfectly accurate tooling.
 
Bob Korves said:
+1 on checking each component separately for runout, marking the high (or low) spots, and then using the variations to cancel each other out. Put a high spot and a low spot together and they work toward cancelling each other out. It is a simple way to achieve good (or at least better) accuracy while using less than perfectly accurate tooling.
Click to expand...
I don't actually know how to get the arbor out of the Jacobs chuck--so I can't check the arbor independent of the chuck.
How do I get the arbor out of the chuck?? the drill end of the chuck (with the jaws) is a blind hole--
 
I wouldn't worry about . 004" runout for a drill. If you start the hole with a center punch and use a centering drill, drill bits up to 1/4" will flex enough to keep your hole straight.

Randy
 
Last edited:
You remove a chuck with a Jacobs Chuck wedge
http://www.use-enco.com/CGI/INPDFF?PMPAGE=480&PMCTLG=00
Other wise you have to drill through the bottom of the chuck and drive the arbor out with a drift.

You have brought up an important point about drills. The short story is that drills do not make accurate holes.
If you want to drill in a particular spot, then start with a spotting drill. A spotting drill will carve a vee bottomed hole that is centered under the spindle axis, and unlike a center drill, the vee will engage the outside of the drill bit and make the drill start with no chatter in the correct spot.
If you want a hole a certain size, then you have to ream or bore.
 
Bob V said:
Anyways, my question is: how do you drill a "straight hole" (where it's supposed to be and the correct diameter) with this kind of setup? .004 plus all the other stacked runouts could really add up-- so what do guys do to get the right size hole in the right place?
Click to expand...

Joshua has it.....If you need a very accurate hole you do not drill it.

I am not familiar with your mill, but for the most accurately sized holes:
-keep the extra stuff between the spindle and the tool to a minimum; each adds play
-don't use a drill chuck, perhaps collets if your spindle accepts them
-don't use a drill bit (a bad sharpening can throw them off wildly)
-use a large diameter(stiff) new end mill (be sure it is an "end-cutting" cutter!)
-if possible ream to final size

It could also be done with a boring head on the mill or boring bar in the lathe.

As for "on location":
-on the mill I use an edge finder and the dials on the feeds to set location (a DRO would be even nicer!)
-search "tool makers buttons" to find another accurate way to position holes
-a co-ax indicator or even a wiggler can help get on centre.
-I use a centre punch like this to mark accurately to my scribed lines: http://www.leevalley.com/en/wood/page.aspx?p=45502&cat=1,180,42311

However, with all that said, normally for the accuracy I need a drill is fine.
-brino
 
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