Author Topic: Drilling at an angle - how to find the centre point?  (Read 474 times)

Offline AdeV

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Drilling at an angle - how to find the centre point?
« on: August 30, 2017, 02:51:04 PM »
I've been asked to make a new set of feet supports for a pedal steel guitar (think xylophone, only waaaaay more complicated. And with strings instead of plates of tuned metal in primary colours...). Each support holds the leg at 12 degrees, to widen the base for extra support. The originals are aluminium, and the threads are tearing out, so I'm re-making them in Mystery Steel.

Anyway, I've honed my block of metal down to the right overall size, and before I cut the L-shape out of it (see pics), I want to drill and counterbore the leg support section; as once I've cut away the excess metal, it'll be a pain in the arse to hold in a vice. I would use my sine bar for this, but it's too wide... and I don't know where the hell it is, so I'll just use one of those digital angle whatsits. Correct to the nearest degree will be plenty close enough...

Anyway... using the original counterbore, I can set the Y axis position easily enough. However, I can't for the life of me figure out how to set the "X" axis centre spot, given that it's all at an angle from the flat. All I can tell you is the top edge of the deep side of the counterbore in the X axis is 0.075" from the edge of the block... I would be able to use an edge finder on the low side (once the block's in the vice), so I guess it's just trig from there?
Cheers!
Ade.
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Or: Zhengzhou, China. An even longer way from anywhere...
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Offline chipenter

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Re: Drilling at an angle - how to find the centre point?
« Reply #1 on: August 30, 2017, 03:38:21 PM »
I would make a wedge to go acros the vice , and use a stop on the vice to set up from the origonal , no trig required .
Jeff

Offline sparky961

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Re: Drilling at an angle - how to find the centre point?
« Reply #2 on: August 30, 2017, 05:32:54 PM »
Set it up so that the angled counterbore is parallel with the table.  Stating it another way, the axis of the hole would be parallel to the quill travel ("Z").  If you can find your sine bar, the judicious use of packing might allow you to use it despite being too wide.  Packing (1-2-3 blocks, or similar) could also be a solution if you really want to cut that "L" shape first, allowing you to still hold it securely for future operations.

Once you get that set up, it looks like you have close to half a circle to sweep an indicator around, which probably gets you close enough for a leg mounting.  You could also extend the counterbore surface to indicate off of by turning a short piece the same diameter and placing it in tight contact with the original.  Alternatively, find something that threads into or fits tightly into the hole and indicate off that.  You could also get it pretty close by inserting something with a cone into the spindle and with a bit of trial and error plunging the quill, find the centre of the hole.

Offline AdeV

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Re: Drilling at an angle - how to find the centre point?
« Reply #3 on: August 31, 2017, 01:25:33 PM »
I think I sussed it.... took some dredging through the old memory banks, a bit of remembered trigonometry, and good old MS Visio to stand in for my rubbish artistic skills...

The picture should explain all:



So, the piece is 1.250" long in total; subtract the 0.075" to get the edge of the 0.950" radius cutout. Using COS(ang)=adjacent/hypotenuse, re-arranging, we get the length of the "adjacent" (i.e. the flat bottom. Knock half the 0.950" diameter off, and we have our "a" to "centre" distance (0.674"). Re-arranging the COS formula again gave me the ae dimension, before I realised I didn't actually need that... as when I put it on the mill & edge-find "a", it'll be set to 12 degrees, thus my red baseline is the new flat... so move the X-axis to 0.674" and I should be bob-on centre  :smart:
Cheers!
Ade.
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Location: Wallasey, Merseyside. A long way from anywhere.
Or: Zhengzhou, China. An even longer way from anywhere...
Skype: adev73

Offline AdeV

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Re: Drilling at an angle - how to find the centre point?
« Reply #4 on: August 31, 2017, 02:40:33 PM »
So.. the acid test, was all that rusty maths up to spec?

First, insert into the vice and set to 12 degrees (after checking the angle gauge was set properly of course!):


Second, edge-find the low corner ("a" in the diagram from the previous post). Then move in 0.674", fiddle with the "Y" axis so it looked about right, then drop the edge finder down the hole. Looks good!:


Finally, dug out my biggest end mill that's less than 1", check either side for centeredness. Looks OK here too!:


I think that qualifies as a success; next stop, making the holes, for that I have to go hunting for my thread gauge....
Cheers!
Ade.
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Location: Wallasey, Merseyside. A long way from anywhere.
Or: Zhengzhou, China. An even longer way from anywhere...
Skype: adev73

Offline paulmo101

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Re: Drilling at an angle - how to find the centre point?
« Reply #5 on: September 01, 2017, 02:32:07 PM »
That looks a great solution - all the best