Summary Data vs Actual?

Discussion in 'Segmented Turnings Forum' started by W6JHB / Jim, Apr 9, 2019.

  1. W6JHB / Jim

    W6JHB / Jim PRO Member

    Just wondering how “exact” the dimensions in the summary tend to be, as far as outer/inner diameter. Here’s why I ask:

    I just finished cutting the segments for a fairly simple closed segment bowl. I set the board widths and segment edge lengths on my saw using a digital caliper. The rings came out almost perfect - needing very, very minimal sanding of each half ring. One particular ring is listed on the summary sheet with 4.9 and 2.0 inches outside / inside diameters. What I wound up with was 5.5 and 2.5 inches. I re-measured the segment dimensions and they are spot-on with the specs on the summary sheet.

    Is this normal, and should I be concerned?
     
  2. mfisher

    mfisher Super Moderator Staff Member

    How many segments per ring are you dealing with?

    I also have had rings come out larger and smaller than the summary plans. When I really look at the segments some were spot on and others were either slightly larger or smaller in segment length. Also the amount of glue in the joints can change the diameter.

    In WTP I did built 2 rings based on what you reported. That is ring 1 has a OD/ID of 4.9 and 2.0. The second rings has a OD/ID of 5.5 and 2.5".
    12 Segments Ring 1 SL= 1.31
    12 Segments Ring 2 SL= 1.47
    Difference in SL between the two rings is 0.16"

    18 Segments Ring 1 SL=0.86
    18 Segments Ring 2 SL=0.97
    Difference in SL = 0.11"

    36 Segments Ring 1 SL=0.43
    36 Segments Ring 2 SL=0.48
    Difference in SL=0.5

    As you can see the more segment per ring the more accurate the segments need to be to make the ring the diameter you planned.

    My guess is some of your segments were slightly larger in SL. I have used digital calipers to set up and check segments. Yet not all of my segments will come out the same.

    Ray Allen always suggested to add 0.125 to 0.25 in OD with in increased board width. You can always turn down to the planned diameter. This also helps you if your rings are not exactly centered on the vessel.

    I generally produce two summary lists. One that is designed for build. The second is to profile (snapped to profile). The second list is what I use when I am turning the vessel and checking the profile diameters.
     
    Last edited: Apr 9, 2019
  3. W6JHB / Jim

    W6JHB / Jim PRO Member


    ——————>
    Sorry, I should have mentioned - all rings are 24 segments. Interestingly, on this bowl, the only ring that the dimensions are off that much is this one, the smallest of the bunch. What may have hosed it is that I told the program to optimize the SL. Perhaps that rounding made the difference in the diameters. I did the optimize because I thought I was going to have to set the SL with a steel ruler - I had forgotten that I had the digital calipers. Perhaps had I not told it to “round off” on the SL that ring may have been as exact as the others. At any rate, I’m just glad that the ring came out larger - smaller might have been a problem with glue-up and turning!
     
  4. mfisher

    mfisher Super Moderator Staff Member

    I always build larger and wider. It is easy to turn down to the dimensions we really want.
     
  5. Jeff Miller

    Jeff Miller Moderator

    I tend to get too stingy with the wood and learned long ago to make the ring larger and just turn to the finish size. I also optimize the measurements to save time.
     

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