How is a co-axial indicator used on a lathe?

HMF

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Hi Folks, <stupid newbie question alert>

I know you need a digital indicator (DI) and a digital test indicator (DTI) to align things on a lathe. Apparently, a co-axial indicator is also needed, on a lathe and a mill.

Can someone please tell me and/or show me how the co-axial indicator is used on a lathe? Photos would be appreciated.

Also, what brands do you guys like? I have a couple Mitutoyo 2416 DI's and a Brown and Sharp Bestest DTI.

Thanks, as usual for the help!



Nelson
 
Actually, not only do you not need a DI, but there are some things that cannot practically be indicated with a DI. I will try to post about this with a picture within the next day or so... right now, gotta run...
 
A quick, simple wiggler

If you want to bore a hole in an exact location on a work piece in your lathe, you have a couple of options, depending on the degree of accuracy you need. One of them is to do your layout, make a punch mark to locate the center of the hole, and then use a wiggler to center that punch mark in the 4-jaw chuck.

This link shows a photo of the wiggler that Starrett used to make:

http://bbs.homeshopmachinist.net/showthread.php?t=9079&highlight=starrett+65

...but you can do just as good a job with a much less elegant tool.

In use, you put the short pointed end in the punch mark, and turn the chuck. Any off-centered-ness of the punch mark will be magnified by the ratio of the length of the arms on either side of the universal joint.

The photos following show the 5 minute version I made, which in a pinch could be made using no more than a drill, hacksaw and file. For a u-joint I used a steel ball from a magnetic construction toy. If I hadn't found that I might have made do with a plastic or wooden bead. For the shaft I considered a used bicycle spoke, but decided it would not be rigid enough, and used a 11 in. piece of 3/16 chromed shaft from a stick-it-in-the-lawn-next-to-your-driveway reflector. You do want something that is as straight as possible and which will flex as little as possible. The aluminum shank that clamps in the tool holder was a piece of scrap from my favorite recycler.

I drilled a hole through the ball using the closest drill size I had, the dented the shaft out of round about 1 in. from the end. Slipping a small wrench socket over the shaft and tapping the ball down over the lump made a snug fit that wouldn't slip. It's easy to sharpen the end by spinning against a grinding wheel.

The seats for the ball are simply a shallow cut using a 1/2 in. drill. Of course, if you have a ball-end end mill, go for it. I drilled both the hole for the screw and a small pilot hole for the seats completely through the holder before slicing it. This guaranteed that the seats would be nicely aligned.

Using a nylock nut, tighten the ball in the seat so that there is no play but that it is free to move. Ideally, you want to design it so that the center of the ball will be at the height of the lathe centers, but it will work even if you don't - it just won't be as convenient to use.

Here's the critical part:

ball-2.jpg

What it looks like together:
assembled-2.jpg

An a poor closeup of the seat for the ball:
ball%20seat-2.jpg

You can also indicate an existing hole by putting a ball (larger than the existing hole) on the end rather than a point. It will follow the hole as it rotates off center in the chuck. If the hole has a poor or rough surface, this may be better than using a dial indicator.

If I were going to make a 30 minute version, I would include some provision for adjusting the height. (I don't have a QC tool holder.) I would also not try to get away with using the cut out piece from the bar to make the top part of the ball seat, It works fine, but would look better if the top seat was more nearly parallel to the bottom. If you have a way of turning your own balls, then you will also be able to drill in the exact center of one very easily. I did this one by eye in the drill press.

Looking at the Starrett, I realized that what looks like part of an an oversized watch spring probably is just that, to keep gentle pressure on the tip. A better design would include that also.

Another alternative to drilling/boring to a layout mark, especially if you do have a dial indicator, is to use toolmaker's buttons, but that's another story.
 
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