drilling bolt circle

[wellman]

I am having a problem drilling bolt circles in a few parts. I have a 3 jaw chuck mounted on the table of my VMC. The part is 13.63" tall and the chuck is about 4" tall above the table. The problem I have is that on some parts the bolt circle is offset or shifted. The holes are thru holes and approx 1/16" dia bigger than the bolt so I have some leeway. The chuck jaws are clean and the bottom of the parts are clean so there should be nothing under the part to cause it sit unlevel in the chuck. When setting up the machine I indicate in on a raised register that sticks up above the flange I am drilling the holes in. Any sugestions as to what I am overlooking or doing wrong.

Thanks

 

[BillPSU]

Are you chucking on a small diameter and drilling a flange or is this a large diameter part? Is the end of the part in the chuck faced and parallel with the other face?

3 jaw chucks using hard jaws are notorious for non-repeatability. For precision locating of a part in a 3-jaw chuck, soft jaws which are bored in place while chucking on a spider is the preferred clamping method.

Your current setup is probably creating an angularity with the part from part to part.

To verify if this is the situation. Chuck 4-5 different parts and reindicate each part and see if the part repeats on centerline at the drilling height.

 

[EdDanzer]

Are you really drilling 13.63" deep? if so you could be seeing drill wander

 

[wellman]

I am chucking on a 5.25" diameter flange and drilling a thru hole in a 1/2" thick 7.13 dia flange 13.63" above the face of the jaw.

I realize the chucks repeatibility is not that great but thought i had enough slack in the thru hole to get by. Will the chuck vertical centerline be the only axis for non repeatability or will the face of the jaw vary from flat at different diameters through the stroke?

I have also used a shaft as fixture for these parts. The shaft is threaded into my table and is turned to a diameter about 0.002 smaller than an interior bored diameter that is concentric with the flange I am drilling thru. I have had some similar problems as with the chuck. Any other suggestions as to fixturing?

Thanks again

 

[BillPSU]

It depends on how many you run and what you have available? If its a fairly small quantity and you have a probe you can probe the part each time and adjust the CL. If you don't have a probe mount a test indicator in toolholder and indicate the feature you need each time adjust CL. Run the part on a horizontal machining center locating on the flange. Build a box fixture dropping the part thru the box locating on the correct flange. Lathe with live tooling, turn the flange and drill holes with live tooling. These are a few options and there is many other.

 

[EdDanzer]

It could be a machine problem. What make of machine are you running this part in?

 

[wellman]

I am running a Manford MCV-810 with a Fanuc O-MD control

 

[EdDanzer]

If you have tried 2 different fixture methods with similar results it is probably a combination of tooling, machine and fixture. To eliminate the fixture problem, place a larger OD tube under the flange and pilot around the OD of the flange. This should eliminate fixture deflection.

 

[gbent]

I wouldn't expect very good accuracy from a 3 jaw chuck 10" from the face of the chuck, unless it was a very good chuck. Most chucks I've seen on mills came off lathes because they were worn out, and then found a home on a mill. Your set-up is hard on the chuck too, as the fine chips tend to fill up the scroll.

If this was a short run, I'd indicate the chuck at the top of the jaws, and set that for zero. Then I'd have the first tool out be my indicator holder. I'd insert my indicator, and bump the part around so the top was in the correct location before machining.