Finding the center…

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  • #48753
    tmac1956
    Participant
    • Topics Started: 171
    • Total Posts: 1259

    A dumb milling question:

    I’ve looked at a couple of ways to find the center of a round rod piece on my mill. I’ve looked on YouTube and found two ways. One way is to use a wobbler (which david recommended to me), but you have to find each edge in both axis’ and divide the coordinates by two to find the center in each direction. My problem is that in the example, the mill is using digital read outs which makes the measurements very accurate compared to what I can get with the analog measurements on my machine.

    Another way is to use a last word dial (which I have), spin it around the round piece, and adjust each axis until it zeros out. The problem I have with this method is that I don’t have the swivel ball joint attachment that allows one to move the dial as needed to do this.

    Coming from a drafting background, I can find the approximate center using a hermaphrodite caliper and geometric construction – it’s just don’t think its accurate enough.

    I sure do hate to attempt to put digital readouts on this thing, but I can see how that would be a big advantage. I just have so many other things that I need… I know – would you like some cheese with that whine? ;)

    Suggestions?
    Tom

    #54687
    Bob Tascione
    Moderator
    • Topics Started: 38
    • Total Posts: 1559

    Hi Tom,
    I’m not sure of your mills accuracy but from what David said in past posts about it you should be able to use the wobbler and dial in within .001 or better if you take the backlash out of the lead screw. If in doubt you can throw your travel indicator on the mill X & Y axis and dial right in. I would highly recommend getting that swivel for your LastWord as you’ll be doing a lot of indicating of holes, pins etc. once you get moving on that mill. Starrett has them at their web site for under $20. I’ll grab the link to it and will post it here in a few minutes. The LastWord also has a 3 segment 2 jointed arm (I’m certain that’s NOT what it’s called) that comes in real handy too. May want to get that one too if you don’t already have it. That indicator is a good old work horse! Also I thought I read in a past thread post that you had picked up a quill mounted indicator bracket to sweep the head in?? I may have read this somewhere else but if it was you and you have one that will also work for now.

    EDIT: Here’s the link to the swivel Tom,
    http://www.starrett.com/metrology/product-detail/Precision-Measuring-Tools/Precision-Hand-Tools/Indicators-and-Gages/Indicator-Accessories/711ea

    Enjoy!
    Bob

    #54688
    david pierce
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    • Topics Started: 90
    • Total Posts: 1360

    Tom,
    To find the center of a round piece of rod first scribe the approximate center with whatever you have to do it with. This can be a height guage and surface plate of a pair of digital calipers or any other way you can think of. Make sure you have a couple of scratch marks that cross in the center. Next fixture the part in the mill in a vertical position. A V-Block and vice should be ok. Next put the wiggler with the long point into the chuck and center in the wiggler with a piece of wood. Do not use your fingers to do this. Next, move the X and Y axies to the approximate center of your part; you are almost there. Take the wiggler out of the chuck and put your small dovetail dial indicator into the chuck. The indicator holder should have come with the indicator when you purchased it. Adjust the dial indicator so the probe touches the side of the part. Rotate the chuck by hand and adjust the X and Y axies until the needle nulls out. Lock the bed in both directions and recheck the dial. Use a mirror if you have to. Enco has a special centering indicator for this purpose that makes the job a little easier. This will center the part far more accutately than a readout.
    I remember seeing a picture of your small dovetail indicator somewhere. I just scanned through some of the past blogs and can’t find it. I think you should be able to configure it in such a way to have it mounted in the chuck and be able to run it around the outside of your part. I think there are dovetails on the front, back and top of the indicator body. I wish i could find the picture to take a look. I am almost certain you put one of them up here.
    david

    #54689
    tmac1956
    Participant
    • Topics Started: 171
    • Total Posts: 1259

    david:

    I do have the small indicator that you mention. What I don’t have is a ball joint attacment that will allow me to move it far enough out of the vetical to do the adjustment. However, your idea about the mirror just made perfect sense to me. I can move the point far enough out by mounting the dial in the horizontal poisition. Unfortunately, this puts the dial upside down, but I can use the mirror to overcome this.

    Bob posted a link for a ball joint holder and I’ll search Enco for the center finder that you recommend.

    Thanks again!
    Tom

    #54690
    david pierce
    Participant
    • Topics Started: 90
    • Total Posts: 1360

    Tom,
    You do know that the probe can be swiveled into different positions. It is mounted to the indicator with a friction clutch that puts tension on the probe arm with a Belville spring washer. What is the diameter of the shaft you are centering?

    I just looked through the Enco catalog and found some holders. The one I used and still use is called a ZERO SET and I paid $35.00 for it about 30 years ago. It is item number 326-1124. Unfortunately, the price has jumped to $195.61. Enco offers a holder only 625-8822 for $14.32 and a couple of holder and indicator sets 606-4704 for $39.98 and 625-8521 for $27.98. These clamping fixtures are fairly sturdy but are a bit more troublesome to use than the ZERO SET.
    david

    #54691
    david pierce
    Participant
    • Topics Started: 90
    • Total Posts: 1360

    Tom,
    The centering indicators are 607-4910 at $299.98 and an import model 607-4907 at 126.51. The prices sure have gone up since I bought them. In my opinion the import model is fine because neither model can center in a part as accurately as the dial indicator you already have can do. If I had something that was critical to center in I would use a centering indicator to get it close then use the dovetail indicator to tweek in the setting.
    david

    #54692
    tmac1956
    Participant
    • Topics Started: 171
    • Total Posts: 1259

    David:

    The rod diameter is .75″. Unfortunately, I can’t get the arm into the correct position so that it swiviels around the perimeter of the rod. Still, I am going to keep playing with it as its probably just me not having done this operation before.

    As usual…
    Thanks!
    Tom

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tmac1956Finding the center…