Holier Than Thou: Precision Holes by Drilling, Boring, and Reaming (2021)

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I am also VERY thankful they transcribed their videos otherwise I would have skipped the site entirely as video is not a format I enjoy. We need more quality content like this that isn't locked up behind some crappy walled garden guarded by commercials and flanked by ads.

2. For long holes, guide drilling with a stubby drill is superior to spotting for orientation, and does just as much for position.

3. Spotting at 75% of the finished diameter isn't usually necessary, it's better on small sizes to pre-chamfer using the spot drill to 130% or spec rather than post-chamfering. Most holes would want to have a chamfer.

4. Optical comparators are useful for profile measurements.

5. Go/no go gages are not well characterized as a cheap and easy way, they are the final say and represent both the state of the art in metrology and the actual geometric definition of the tolerance limits for size and any other geometric control that you care to specify. For metrology of very tight tolerance holes and bores at small sizes, precision ground gage pins are the most expensive way to go and are irreplaceable. For larger bores, 2- and 3- point bore gauges and micrometers are to be seen as a necessary evil. I view 2 point spherical gages or micrometers as superior to 3-point cylindrical mics, because they tell you much more information about the bore shape.

6. The best designers avoid tight tolerance features except where strictly necessary.

7. I would tend to avoid straightness, cylindricity, and circularity, when tolerancing holes, and focus on the more general concepts of size, position, and surface profile.

8. Diameter compensation offsets are usually easier to adjust in the machine control than adjusting the boring tool, and can usually get close enough, especially on a horizontal CNC milling machine.

Its comments on measurement and metrology are particularly strong and well worth listening to.

On that point, I wouldn't use calipers to measure the ID of a precision hole but I'd almost forgotten that scaling error requires the ID jaws of calipers to be a thou or so smaller than the OD ones. Stands to reason when you think about it. Raising that issue shows how useful this video is, it shows that its authors pay careful attention to detail.

Similarly so, their comment in respect of plug gauges and the trouble in differentiating between them during hole measurements due to burrs etc. (I'm a bit obsessive about deburring workpieces properly and I get pissed off when I see others failing to do so - so this comment rang bells with me.)

I'd strongly recommend this video to just about everyone in the machining game.

If I were King for a day, all inch rulers would have at least one decimal scale. 8)