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- Jan 4, 2021
- Messages
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A Shars 0.0005” DTI and an economy set of 1-2-3 blocks shouldn’t set you back more than about $60 and you’ll use them both regularly.
I followed Yuriy’s procedure and got within 0.0005” on all axes.
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Help me decide which of these to set CPI
- Thread starter littleb28
- Start date
I followed Yuriy’s procedure and got within 0.0005” on all axes.
Cool thanks
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- Mar 12, 2022
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I agree. The DTI doesn't have to be any good at 'measuring' for calibrating the DRO - as long as it will 'hit zero' repeatably it will be good enough. I used my cheap DTI and the equally cheap pair of WEN 1-2-3 blocks to calibrate my Touch DRO installation a couple of weeks ago and it seemed to work well.
https://www.amazon.com/WEN-10423-1-Inch-Steel-Hardened-Precision/dp/B0000AY61E/ref=sr_1_1?
It turned out that my scales were 'standard' and used the same calibration # as in the TouchDRO information page. The 1-2-3 & DTI confirmed that.
- Joined
- Apr 4, 2013
- Messages
- 884
On most machines hand wheel method is "good enough". The trick is to use longer distance, so if there is an error, it will be spread over multiple inches. For example, if your lead screw has 0.002" of error, if you calibrate over 10 inches, you end up with calibration that is 0.0002" off per inch. After calibration, I would move to a different spot on the lead screw, set 0, move the axis 10 inches according the DRO and check the dial on the wheel. If it's off, note the error, move to a different spot, check again. If you get readings that are close enough (you decide what that means for you), your DRO is now your reference instrument. The scales are probably as accurate than a set of standard Mitutoyo calipers.
All this said, the worse I've ever seen for glass scales was 4 encoder ticks worth of error per inch. I.e. nominal was 5080, actual was 5084. That amounts to 0.0008" of error per inch (0.0002" per tick roughly). On capacitive scales the worst I've seen was about 5 ticks off (0.00039" per tick). 2-3 off is very common. For many types of work this is more than good enough.
Regards
Yuriy
All this said, the worse I've ever seen for glass scales was 4 encoder ticks worth of error per inch. I.e. nominal was 5080, actual was 5084. That amounts to 0.0008" of error per inch (0.0002" per tick roughly). On capacitive scales the worst I've seen was about 5 ticks off (0.00039" per tick). 2-3 off is very common. For many types of work this is more than good enough.
Regards
Yuriy
- Joined
- Feb 1, 2015
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I wouldn't trust the lead screw pitch to be accurate. Especially on a HF machine. Many of the Chinese machines use a metric lead screw marked with Imperial measure on the assumption that .03937" is equal to .0400". Even if the lead screw has an Imperial pitch , you would be trusting that the manufacturer's machining accuracy is spot on. Given the reputation of Chinese machinery, that is a pretty big trust.
The accuracy of the calibration depends upon the accuracy of your primary standard. It will also depend upon the distance between the calibration points so the greater the better. I used my 6" micrometer as the primary reference standard. Ideally, it would be calibrated with a good gage block which it wasn't so I have to trust that it is accurate. My verification of trust is that it agrees with other measurement devices that I possess.
Even my cheap 1/2/3 blocks are fairly accurate. They typically measure about .0002" under nominal. It should be noted that even name brand 1/2/3 blocks can vary from nominal. They are often sold as slightly over to allow for lapping to final size. Assuming the .0002" difference and using the 3" distance of a 1/2/3 block as a standard, its contribution to error in calibration would be .00007"/1" which is close enough for any hobby class work. (This assumes good technique and care in measurement). I used a .0001" test indicator so repeatability should not be a factor in the calibration. If using a .0005" indicator, one should still be able to repeat to +/- .0001" with care.
My setup was as follows. I clamped a bar to my table parallel to the axis being calibrated, as determined by sweeping the edge with my test indicator. Next, I clamped a 1/2/3 block for a stop at one end of my calibration distance. (I used the 1/2/3 block because it has a reasonably good surface and is reasonably square.) Using my test indicator, I zeroed my DRO at the the stop and then moved the table to slightly more than the calibration distance and inserted my calibration block. Then I used the test indicator to find the surface of my calibration block and noted the DRO reading. Finally, I measured my calibration block and used that number, compared to the DRO reading to determine the scale factor. I used one of my parallels which actually measured slightly longer than 6" for my calibration block but was within the range of the 6" micrometer.
Yuriy's method is basically the same, the difference is that I swept the surface with a test indicator rather than using a square to determine parallelism of the block to the axis. Any error in the angle will show as a cosine error. A 1º error amounts to .0002"/1" error. By sweeping the surface, you will greatly reduce that possibility.
Lacking a test indicator, it is still possible to do the calibration using an edge finder. With care, it is possible to locate an edge to +/- .0001".
When opportunity presents itself, I would suggest preparing a set of calibration standards as long as compatible with your machines and measuring them accurately with a calibrated micrometer. This may be by taking them in to work and measuring them with a NIST traceable calibrated micrometer. Mark them with the measured size and safely store them for future use.
The accuracy of the calibration depends upon the accuracy of your primary standard. It will also depend upon the distance between the calibration points so the greater the better. I used my 6" micrometer as the primary reference standard. Ideally, it would be calibrated with a good gage block which it wasn't so I have to trust that it is accurate. My verification of trust is that it agrees with other measurement devices that I possess.
Even my cheap 1/2/3 blocks are fairly accurate. They typically measure about .0002" under nominal. It should be noted that even name brand 1/2/3 blocks can vary from nominal. They are often sold as slightly over to allow for lapping to final size. Assuming the .0002" difference and using the 3" distance of a 1/2/3 block as a standard, its contribution to error in calibration would be .00007"/1" which is close enough for any hobby class work. (This assumes good technique and care in measurement). I used a .0001" test indicator so repeatability should not be a factor in the calibration. If using a .0005" indicator, one should still be able to repeat to +/- .0001" with care.
My setup was as follows. I clamped a bar to my table parallel to the axis being calibrated, as determined by sweeping the edge with my test indicator. Next, I clamped a 1/2/3 block for a stop at one end of my calibration distance. (I used the 1/2/3 block because it has a reasonably good surface and is reasonably square.) Using my test indicator, I zeroed my DRO at the the stop and then moved the table to slightly more than the calibration distance and inserted my calibration block. Then I used the test indicator to find the surface of my calibration block and noted the DRO reading. Finally, I measured my calibration block and used that number, compared to the DRO reading to determine the scale factor. I used one of my parallels which actually measured slightly longer than 6" for my calibration block but was within the range of the 6" micrometer.
Yuriy's method is basically the same, the difference is that I swept the surface with a test indicator rather than using a square to determine parallelism of the block to the axis. Any error in the angle will show as a cosine error. A 1º error amounts to .0002"/1" error. By sweeping the surface, you will greatly reduce that possibility.
Lacking a test indicator, it is still possible to do the calibration using an edge finder. With care, it is possible to locate an edge to +/- .0001".
When opportunity presents itself, I would suggest preparing a set of calibration standards as long as compatible with your machines and measuring them accurately with a calibrated micrometer. This may be by taking them in to work and measuring them with a NIST traceable calibrated micrometer. Mark them with the measured size and safely store them for future use.
- Joined
- Jul 22, 2013
- Messages
- 130
I used cheap 123 blocks and a cr@ppy Chinese 5 tenths dti. If you are using the touchdro v3 calibration it is easy and accurate to within the limits of a minimill. Make sure your gibs are adjusted correctly and take up the backlash. The glass scales are the bomb.