How do you center a rectangular bar in a four-jaw chuck?

[HMF]

Here is an easy one for you experts:

If I wanted to mount a rectangular (irregular-shaped) bar in a four jaw chuck to drill a series of centered holes in the piece, how would you mount and center the piece in a four-jaw chuck and what measuring tools (i.e. DTI, and base) would I need?


Thanks,


Nelson

 

[Tony Wells]

Nelson, you're throwing me with the "...series of holes..."

There are a couple of methods that come to mind, depending partly on how accurately placed you need the holes.

 

[bfd]

I usually use a button back indicator and hit the corners. if you get "o" on all corners you are as close as possible, unless you find the low spot on the 2 opposite faces rocking the chuck back and forth till you get the same number then switch to the other faces rocking back and forth till you get the same number then back to the other 2 faces and re adjust then to the other two faces again and again til you make no moves this is the most accurate but a royal pain. bill

 

[darkzero]

Here's another clever way, one of those "how come I didn't think of that".

Looks like it would work well enough. He states the rubber band should hold fine but you could always use something stronger if needed. I still haven't tried it yet but the next time I need to indicate something square or rectangular in the 4-jaw I'm definitely going to try this.

 

[mikey]

There are two ways I can think of. One, use a Starrett #65 center tester. This is the one with the gimbal-mounted long rod. It is surprisingly accurate and will get your punch mark on center fairly quickly, probably within a thou or two of dead center.Its useful because your hole may not be in the center of a square part. Sometimes its off by a lot.

Second is to use a simple solid bar with a 60 degree point machined on one end and a 60 degree center drilled hole on the other. Stick the point in the punch mark of the work and use a dead or live center in the tailstock to go into the rear of that shaft. Put a dial indicator against the side of the solid bar and align the work piece until the indicator reads zero all around.

There's actually a third way but it is just a modification of #2. I have a tool that I made around 20 years or so ago, lost it about 18 years ago but its somewhere around here. It is just a spring loaded tool like one of the modern tap followers you can buy but the tolerances on the movable part is looser so it can move side to side. In use, the process is the same as the solid bar above but because the tip can move, it is a bit more accurate. I can't remember where I found the idea for this but think it came from one of the British guys who write books about these useful little tools. Since I lost it, the few times when I needed to center a square or rectangular part, I pulled out the Starrett 65.

Edit: I knew I saw a repro somewhere and here it is: http://tallgrasstools.com/products/center-finder-kit-small-5011. Note - two sizes are available.

 

[Wreck™Wreck]

I do this often, if not weekly.
If finding the center just indicate opposing sides until they zero, if you want an offset in one or both planes then indicate opposing sides with the known offset, when the offset exceeds the indicator travel as often happens use 2 different diameter settings on the X axis hand dial which is greatly simplified with a DRO as there is no need to count revolutions.

3/8" X 1/2" X 44" long rectangle that needed 8-32 tapped holes in each end on center. I use 2" or 3" travel indicators held in the tool post for this purpose. Because of the length of the part I removed the spindle stop rod and ran the part through the rod holder bore which was just large enough without modification.

If the part were larger across the corners I would have had to make a spindle guide. Many hobbyists would consider building a so called "spider" for this purpose which is only needed when turning at very high RPM's.