Simple center finder for small holes

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Feb 17, 2008
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OK. We have a part and need to locate it in the mill by "picking up" (locating the center) of a hole. If the hole is 1/8 inch (3mm) or larger it is no problem using a DTI, but when the hole is too small for this it starts to become e problem. In the past I have used a wiggler and centered it in the hole by eye. I could get to a couple of thou this way bun as my eyesight has become worse over the years I made up this simple center finder.

There are two parts to this tester and you may already have one of them. First a continuity tester. If you have a digital multi meter it probably has a diode test function with a beeper. It can be used. My multi meter normally resides in my electronics room which is at the furthest point possible from the shop in my house. I found that i used this tester often enough that I quickly got tired of the walk so I made up the simple continuity tester to reside in the shop. It is just a battery box that holds 3 AAA batteries and an on/off switch. A 3 to 28 volt active beeper was glued to the end of the box and two 1 foot test leads brought out. I used a mini alligator clip was nstalled on the lead to connect to the probe and a full size one to connect to the vice or work piece. I used the lightest most flexible test lead wire I had to prevent the wire to the probe from deflecting it. Every thing goes in series. Only precaution is the buzzer is polarity sensitive so you have to observe that.

The probe is just a piece of stiff steel about 3 inches long. I made mine from hobby shop music wire but other straight springy steel such as stainless steel can be used. Hobby shop music wire is fairly straight and stiff although there may be some distortion on on the end for about 1/2 an inchs of a new piece where it was cut to length during manufacture. I cut my piece off with a rotary hand tool with an abrasive cut off wheel. Both ends were squared upl on the bench grinder and any burrs were removed. I made two probes. One out of 3/32 diameter wire and another out of 5/32 diameter. The 5/32 diameter in the photo was a scrap piece and is about 3/4 inch shorter that I would like but is quite usable. Just takes a little more pressure in operation. More on that later.

The insulator can be any hard plastic that machines well and bee glued to.. It needs to be hard so it will not distort when clamping in a drill chuck. PTFE (Teflon) and Delrin are both too soft and difficult to glue to. I used Corian that was a left over scrap from making spark plugs. It was about 1/4 inch diameter and I cut it to 3/4 inch length. It was drilled to be a sliding fit on the steel probe. I did not want a press fit that might creep over time from the induced stresses.

Back to the steel probe. I chucked it in a collet with about 3/4 of an inch exposed. It needs to run true so if not using a collet you may need to adjust things to get it running true. Put a point on the end with about a 30 or 40 degree included angle. Music wire cuts nicely with a carbide tool but i just filed the point on with a fine flat file. As long you don't get in a hurry that will produce a very concentric point. I polished up the tapered area with an India stone slip and slightly rounded the point, say about 0.003 to 005 radius. That is just so I don't mark if I miss the hole when using the tool. I checked that point was concentric with a DFI. I could not see any run out when checking about the middle of the taper. If it had not been concentric I would have cleaned it up with a cutting tool.

The probe wire was reversed in the collet with about 1-1/4 inch exposed. After cleaning the end of the wire the insulator was glued on with 5 minute epoxy. A cup of coffee and a quick check of what was going on in the forum and the glue was set firmly. The insulator was turned to 3/16 diameter. I used 3/16 diameter as my go to precision drill chuck is 3/16 but yu can use anything that is convenient for your tooling. To check the the probe was mounted in the lathe with a 3/16 collet and the taper was was checked with a DTI. I could see the needle on the DTI wiggle a bit so the run out, probably about 0.0003 TIR. Far more accurate than I needed.

Operation is simple and quick. The probe is chucked in the spindle with a little of the insulator sticking out. The large alligator clip is connected to the work piece or vice. With the continuity turn on things are checked by touching the work piece with the small alligator clip and listening for a beep. The small clip is connected to the probe near the insulator as the probe is very stiff at that point so the clip and wire does not deflect the probe. The machine is adjusted so the probe will enter the hole. The probe is lowere or the tabled raised until contact is made and you get a beep. By lightly pressing around the sides of the probe it is easy to see which way the work piece needs to be moved so there is no beep. Repeat this a time or two and a point is reached where when then there is no beep only a light touch on any side of the oprobe produces a beep. With the 3/32 diameter probe this is only about 5 grams force. With the 5/32 probe it is about 25 gram force but if the probe were made as long as it should ha been the force would be less.

As I mostly use 2-56 or smaller screws the 3/32 probe is what I use most of the time. I have only used the the 5/32 probe a few times I have never made a longer one.

Photos show the two probes along with my continuity tester and the 3/32 probe raised out of a 1/32 hole that I used to testing. For testing I put the 1/32 hole in a thin aluminum sheet using a 1/32 end mill with the DRO set at zeros. Jogged the mill off and put the probe in. Using the probe to locate the center of the hole the DRO showed a 0.0003 error. This error would be the sum of the probe and any drill chuck run out. Close enough for me.

Only a couple of precautions that I think of.The work piece has to be conductive so no plastics. Watch out for anodized aluminum as anodize is an insulator. Picking up a threaded hole will also cause a small error but as the error is small it can probably be ignored.

Gail in NM


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