Gallery, Projects and General > How do I??
How do I.... accurately measure a tilted hole?
seadog:
Joules, a dowel with a centre pip is fine for hole which are perpendicular. However, if you try and use that technique here to spot through the existing plate onto the new one, you'll end up with a centre mark that's out of position. That said, you could then use simple trig. to work back from the position to find the correct point.
sparky961:
I think it's ironic how more than a few people have mentioned "trig" to figure out the actual measurement, but no one has described the method. That's like saying "Oh, that's easy - just use calculus". Well, ok... not quite the same scale of obfuscation.
Crap. Now you've got me thinking about how to figure out how to measure your part.
What comes to mind is this:
Set up as I've already described so that you're aligned with the axis of the hole. Set up your indicator for minimal travel when you drop it in the hole so that you know where "0" is (eg. less than one full rotation). Establish your datum by zeroing the DRO. Now you raise the indicator clear of the hole using the quill or knee. The knee can be used only if you're tilting the part not the head. The advantage being that you may be able to use a scale or dial to measure the vertical offset as well as the horizontal offset I'm about to describe.
Move the long axis close to another well-defined feature. Touch off the indicator by carefully sweeping it back and forth until you get the same reading you had in the hole. By the looks of it the raised threaded mounting bosses might be a good candidate. You can get an accurate measurement of the height, diameter of the top edge, and location.
You'll want to be right on the corner with the maximum diameter of the indicator tip for the most accurate results. Move up and down, closer and farther until you're happy you've found the right spot. Your DRO will now read the distance between the edge of the hole and the edge of your chosen feature. Now you use this dimension in your CAD program using a line coincident with the edge of the feature and parallel to the axis of the hole.
Here's a quick little mock-up that may help with any confusion in my description. I've exaggerated the angle for the sake of clarity.
You might be able to come up with something that's easier to pick up with the indicator than what I've suggested. Something mounted on the table or bolted into a hole perhaps. Maybe even something set at the same angle as the hole so that it's vertical. You'll probably have to do this separately for the X and Y axis.
If you can do all of the above, your accuracy is limited only to how well you can measure the rest of the features.
.... not that it matters anyway. ;) Who knows... maybe there's a much easier way but I can get my head around this one. Unlike calculus.
seadog:
Maybe I'm missing something but I think it is a very easy calculation to make provided you can measure the angle and the thickness of the existing cover accurately.
If the angle is 6 degrees and the thickness is, say, T, then the offset is T x tan6
sparky961:
--- Quote from: seadog on November 23, 2015, 05:38:09 PM ---Maybe I'm missing something but I think it is a very easy calculation to make provided you can measure the angle and the thickness of the existing cover accurately.
If the angle is 6 degrees and the thickness is, say, T, then the offset is T x tan6
--- End quote ---
I certainly can appreciate a simple solution over one that's complex... but offset from what? Looking at the pictures, there's no edge of the hole that's on the same plane as the top or bottom of the cover. This makes an accurate measurement to the edge of the hole very difficult. You could extend the surface of the hole with something but you're introducing a lot of measurement error to an already tricky task.
There's been no discussion of accuracy required but I'm assuming that if the hole needs to be in line for a shaft and gear to line up with it's mate then you'd need to be closer than a degree or two and a 32nd or so. That's about all the accuracy you could expect to get using simple handheld measuring tools in this situation.
Besides, it's much more important mechanically to get the hole on the same axis as the original. Who cares where that hole really is until you start sending out your drawing for quantity pricing quotes.
Apologies if this sounds a bit confrontational.... I'm late for supper and perhaps getting a bit hangry.
seadog:
I admit my reply omitted certain bits of information which were in my head :Doh:
Using the transfer punch, which someone suggested making, the centre point of the inclined hole is transferred to the top of the new plate. The new plate is positioned vertically under the original with mounting holes lined up. The centre mark produced will be offset to one edge by an amount that is the "opposite" side of a right angled triangle. The thickness of the original plate is the "adjacent" side. The measured angle we're currently assuming is 6º. Therefore, for a right angled triangle opposite/adjacent =tan 6º. This can be rewritten as Opposite=Adjacent x tan 6º
The value for the Opposite is the distance that the the centre mark must be relocated towards the middle of the new plate which then becomes the point that drilling/machining starts. This does assume that the bore of the hole is square to one of the edges. If not it becomes a little more complicated.
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