Precision hole drilling?

I would not back out a reamer, as that will destroy the precision.
Fortunately, most holes in FRC do not require a reamer, save for bolt holes.

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As a trained aerospace hole-driller, spinning the reamer out is going to destroy the hole worse–oversizing. Just pull it straight out.

And using a reamer for a bolt hole? Do you mean a bearing hole? Bolt holes (in FRC) don’t need a reamer, we don’t need quite that much precision.

That said, said aerospace holes did have fasteners through them, and required very precise tolerances. ± .001" as I recall. If you were too small, you did it again. If you were too large, blown hole, major paperwork hassle.

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Yes, I meant bearing hole, not bolt holes.
Fortunately fasteners in FRC don’t require +/.001 clearance, unless you’re using pins for some reason.

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And, one other question: Getting holes in the correct place means marking them in the correct place. Our previous method – a tape measure and sharpie – seems not to be terribly precise. Is there a better way?

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Calipers come in several sizes–I would make sure that you had several 6" and 1 12" calipers if you could. And use a scribe/caliper tip to mark the aluminum; it’s finer than the sharpie tip.

In addition to the suggestion from @EricH, make sure your design/CAD team produces manufacturing drawings for every part. All the features should be dimensioned relative to a sensible datum point.

Do not accept drawings where some features are dimensioned from one point and others are dimensioned from a different point(s). It makes it very easy to mark the holes or cut lines wrong, even in a professional environment (had to fix that).

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With proper equipment, and for a valid reason, I would actually disagree with this particular suggestion. That said, if you don’t have a mill, I would say that it makes sense.

The primary case I come up with is if I need to mount something with a hole pattern around an axle, and I have a mill, I would locate the center of the pattern from the datum, then zero the mill and locate the holes in the pattern. BUT, if I do that, and I need to do more operations, I’m going to either do the other operations first or go back to datum and re-zero.

And the other thing is that it’s a LOT easier to work with if everything is from one point. I’ve coached the CAD crew on my team (by lots of redoing drawings) to try to make sure that all the important dimensions are on there, and all dimensioned from the same point. Sometimes I’ll accept a second datum point, but only if it’s not critical to the part interfacing with other parts, and only if it actually helps the mill crew. (Sure, I can move a part in the vise and not lose position… but that’s a lot more hassle than I like!)

Layout die, a surface plate and height gauge make a huge difference. I picked up a decent surface plate really cheaply because a corner had been broken off.

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If I am reading you correctly, you are talking about using the centre of a shaft as a local datum point for locating holes related to that shaft. I did not mention such cases since I felt that would confuse many readers and the readers with more advanced manufacturing abilities such as you would already know how to use local datum points correctly.

What I hoping to lead readers away from was measurering from one feature to the next, then to the next and so on since one ends up with an accumulation of errors. The other thing I was trying to lead readers away from was dimensioning holes/features from the nearest side since often, the piece is not cut exactly enough, leading to the spacing of the holes being incorrect.

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A wide-tipped permanent marker also works well and is often easier to find at a school without any sort of engineering or metal fabrication classes.

So, right now, I’m just worrying about holes in aluminum tubing. I totally understand “don’t measure one hole from another hole. Measure them both from the end.”

But, a lot of these holes tend to be made near the ends. If, for example, I want to put a bearing hole at 5" and 19" in a 30" piece of tubing, then I have two choices:
(a) From one end, measure 5" in and 25".
(b) Measure 5" in from both ends.

(b) has the problem that if there’s an error in the length, then that error will add to the error on measuring the 5".

But, (a) has the problem of having to measure 19 inches, while most precision measuring tools seem to max out at around 12 inches. And, that means breaking out the tape measure**, and my experience has been that the tape measures (at least those from the big box stores) are really not that precise.

(**I recognize that I’d still have to break out the tape to measure to cut the 24" piece, so maybe (a) is still the right solution.)

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We use a Knee Mill with a vise mounted on the table, ours is a Grizzly. They offer a lot of different models for different price ranges and have indicators on them so you can move the table a known distance.
https://www.grizzly.com/milling-machines

The way I usually put it is “Which dimension is most important to be accurate: from the near end, or from the far end?” For your case, I would say: from the nearest end, MOST LIKELY, due to wheel clearance stuff. (I assume you’re talking drive frame rail.)

Also: I have seen calipers as long as 5’. Once or twice… McMaster sells 'em but they’re half the cost of a regional. (The Caliper Range Extenders are about a tenth of that number but I’m not sure I’d trust those–never used 'em.)

Sure, a knee mill would help out–but they do have a working limit. I think I’ve determined that 1197’s Bridgeport can handle about 26" length (of the top of my head) before we have to reposition. Seeing as we have a CNC router, that makes pieces longer than that number a router job unless there’s something else going on. In that last case, I’d be either doing an on-the-fly repositioning (which takes time and effort) or asking for the references from the other end.

Find some good quality steel rulers of various lengths, preferably with the markings etched into the surface rather than just printed on. They won’t be as precise as the 5 ft long calipers @EricH mentioned but will be easier to use precisely than a tape measure.

The following are a few I found by searching for “machinist ruler” on Amazon.
Starret 24" $30
Powertec 24" $33.48