Saturday, March 31, 2012

Improving on the Centering Tool

As I have previously mentioned, the centering tool, while better than just measuring with a micrometer, is still in the hundredths of an inch (sometimes many hundredths) accuracy for determining the center of a round rod.  I had occasion to use this again this evening, and I decided to see what I might be doing wrong that could explain this.

One possible explanation is that I am using an awl to draw the radius line, and depending on where your hand is when scratching along the radius marker of the centering tool, the tip of the awl can be either directly below the diagonal marker on the tool, or many hundredths of an inch to the one side or the other, depending on whether the awl is exactly vertical or at an angle.  Unfortunately, because there is nothing that clamps the centering tool onto the rod, and I am somewhat disabled in only having two hands (instead of the far more useful three or four hands), you may not get exactly the same angle with the awl each time.  What this means is that if you draw several radius lines, some of them are going to be more remote from the true radius than others.

One solution to improve the accuracy of my center point was to take the micrometer, measure the maximum diameter of the rod, and move the micrometer to half of that diameter.  Then I put one jaw on the edge of the rod, and pivoted the other side so that its jaw scribed a line through the center of the rod.  By doing this twice, the intersection of these two circles with the radius lines gave me something pretty decent--perhaps as accurate as .01" to .03".  That's better, but not as good as I would like.

I am thinking that what I may want to do is take a piece of aluminum square tube 5" to 6" long, and drill a hole in each leg, then drill and tap into the sides of the centering tool so that I can screw the right angle onto the centering tool.  Then I can drill and tap some 1/4"-20 holes in the square tube walls to lock the rod that I want to center in position.  Then I can put the whole assembly in a vise, and use the awl to scribe the radius lines without having to hold the centering tool on the end of rod with one hand, while holding the rod firmly with the second hand, and scribing with my non-existent third hand.

Or perhaps I will take the next step: start with a square tube large enough to handle the rod, drill and tap the 1/4"-20 holes to hold the rod in position, and machine an end plate perhaps 1/8" thick that looks like this:

I would cut two 45 degree slots that were just wide enough to accept the tip of the awl.  They would be in immediate contact with the rod, so it would be almost impossible to get anything but a directly vertical relationship between the awl point and the rod.  This should make it possible to get a center that is thousandths of an inch accurate, not hundredths, and with very little effort.  

In addition, by having two radius lines, there is no need to rotate the rod to get two crossing radii.  You could still rotate it after drawing these two lines, and quickly scribe two more crossing radii lines, improving accuracy, but it wouldn't be necessary.

Of course, like the Groz centering tool, it would work with any polygon with an even number of sides: squares, rectangles, hexagons, octagons.  I don't have much occasion to need to center pentagons or heptagons, and nonagons and undecagons are, shall we say, not common machining problems, except for the machine shop in Flatland.


  1. Now you know why woodworkers start with a slightly oversized piece of stock - find the approximate center - then turn the outside down to make your approximate center "the center".

    But then again in woodworking measurements are seldom in hundredths and never in thousandths.

    The problem with your centering tool is that it is built for woodworkers. So probably will not have the accuracy a machinist will be looking for.

    You might be better served trying to improve your awl. Replace it with a scratching tool that will consistently mark your workpiece right next to the dividing piece. Maybe grind down one side of the awl tip so that it is no longer round, but half round. Then the tip will slide right along the edge of centering tool.

    Depending on the hardness of the material you are marking you may get by with using a knife blade such as an xacto knife for a scratch.

  2. suggestions:

    1) use a razor blade or utility blade instead of an awl

    2) scribe three lines about 120 degrees apart, and the center is inside the triangle they make.

    3) maybe you can chuck it into a fixture and spin it, scribe a spiral line in from the outside, and find the center that way???

    4) I saw a fixture on youtube under "home machine shop projects" that used a mirrored tool to help find centers - couldn't see how it worked, because I only saw the free part of the vid. You chucked up the tool, looked at the reflection of a center in the tool's mirror, and by checking from a couple of angles you could accurately find the center, I think. Maybe.

  3. David:

    In practice, the same thing happens when using a metalworking lathe. You can only approach perfectly centered, never quite reach it (even if you use the lathe to find it). In this case, I was starting with a 1.624" diameter piece of acetal, and I needed to cut it down to 1.590" diameter.

    The first one I actually did pretty well about getting centered, perhaps .01"-.02", and while turning it down, I could hear that the tool was hitting one side but not the other when I started, but after the first pass, no problem.

    The second was not quite as well centered, and even at the end, I was still obviously not touching one side of the rod, but cutting the other.

    In this particular case, it doesn't matter. If the object is slightly oval, but averages 1.59", that is sufficient for my purposes. This is going inside a 1.60" ID aluminum tube, and it will be held in place by three machine screws. The goal is to have something that is a press fit or friction fit while my son drills through holes in the aluminum tube, into the acetal.

    I do like to make stuff as precisely as I can, but as is usual with machining, how much precision can the customer afford?

  4. When I first got out of college as a young mechanical engineer on my first day of work my mentor took me down to the machine shop. Introduced me to the three machinists working there. Set me on a stool and told me "You will spend this entire week right here. These men are not as formally educated as you are, but they are infinitely more learned that you are, so far. Shut up, pay attention, listen to them and try to learn something."

    I learned a lot from those men. But the one thing that stuck with me my whole career was: "If you bring me a job asking for tolerances in the hundredths I'll have it for you tomorrow. Ask for thousandths you'll get it next week. Ask for ten thousandths, it will be next month."

    It was a gross generalization but the concept is sound, when dealing with time and costs.

    It never ceases to amaze me how many young engineers never manage to grasp the idea that it is not always about purity of design. You also have to consider cost and time to manufacture. Now that I am doing the mentoring, it took me a while but I finally found a couple cranky old machinists willing to help...

    Except that I have to put up with a lot of crap from them ever since they found out I'm a woodworker...