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- Nov 23, 2014
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- 2,606
There have been a few threads regarding co-axial indicators and what the numbers on the dials equate to in actual inches. Frankly, I had no clue so did a couple of simple set ups on the lathe and collected data. Basically mounted the co-ax in a collet (overkill), locked the spindle with the tilting plane of the indicator flat to the world. Loaded a couple (1.7” and 6”) of indicator rods in the co-ax. Set the lathe compound to parallel with the cross feed. Adjusted the cross feed/compound until the indicator was against the QCTP and swept side to side to check tram of the QCTP to the Z-axis. Once that was verified square it was time for data collection.
I backed off the compound to its limit of travel, turned in a few tenths and zero’d the micrometer dial. Advanced the cross feed until the needle on the co-ax moved, then zero’d out the co-ax. I then started advancing the compound and recorded the number on the co-ax dial face. Data was plugged into Excel and the attached plots made.
I saw on the face a note for ‘axis offset 0.0005” ’ and had no idea what that was. Changed my set up by turning the compound to parallel with the lathe spindle and rotated the co-ax so the QCTP would directly push against the machined bottom surface of the co-ax. Zero’d out the compound micrometer dial and took more data.
The envelope says . . . the travel with an indicator rod is not linear through the full travel of the co-ax so you can’t just throw a multiplying factor at the number on the co-ax dial and generate an actual offset in inches. For example, the first 0.050” of travel with a 1.7” indicating rod showed a dial offset of 19.65 units on the co-ax. The last 0.050” of travel with the same indicating rod showed a dial offset of 24.65 units.
With a 6” indicator rod, the first 0.050” gave a co-ax offset of 6.9 units. Last 0.050” of travel was 9.325 units.
However, as expected the direct push on the housing of the co-ax is linear. Each unit on the co-ax dial indicates the housing has moved up/down 0.0005”. Makes sense since at this point the co-ax is just a dial indicator.
What I can take away from the data is if you use a 1.7” long indicator rod and set a deflection of 1 on the co-ax, you are within about 0.002”. Using the 6” long rod with a deflection of 1 on the co-ax means you are within about 0.009”. So, the shorter the indicator rod, the more accurate you’ll be. Hope this helps answer some questions! I could only attach photos of the plots, send me your email address if you'd like the Excel file (can't attach it to this post).
Bruce
I backed off the compound to its limit of travel, turned in a few tenths and zero’d the micrometer dial. Advanced the cross feed until the needle on the co-ax moved, then zero’d out the co-ax. I then started advancing the compound and recorded the number on the co-ax dial face. Data was plugged into Excel and the attached plots made.
I saw on the face a note for ‘axis offset 0.0005” ’ and had no idea what that was. Changed my set up by turning the compound to parallel with the lathe spindle and rotated the co-ax so the QCTP would directly push against the machined bottom surface of the co-ax. Zero’d out the compound micrometer dial and took more data.
The envelope says . . . the travel with an indicator rod is not linear through the full travel of the co-ax so you can’t just throw a multiplying factor at the number on the co-ax dial and generate an actual offset in inches. For example, the first 0.050” of travel with a 1.7” indicating rod showed a dial offset of 19.65 units on the co-ax. The last 0.050” of travel with the same indicating rod showed a dial offset of 24.65 units.
With a 6” indicator rod, the first 0.050” gave a co-ax offset of 6.9 units. Last 0.050” of travel was 9.325 units.
However, as expected the direct push on the housing of the co-ax is linear. Each unit on the co-ax dial indicates the housing has moved up/down 0.0005”. Makes sense since at this point the co-ax is just a dial indicator.
What I can take away from the data is if you use a 1.7” long indicator rod and set a deflection of 1 on the co-ax, you are within about 0.002”. Using the 6” long rod with a deflection of 1 on the co-ax means you are within about 0.009”. So, the shorter the indicator rod, the more accurate you’ll be. Hope this helps answer some questions! I could only attach photos of the plots, send me your email address if you'd like the Excel file (can't attach it to this post).
Bruce