Gallery, Projects and General > How do I??

How do I.... accurately measure a tilted hole?

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AdeV:
Problem: I am making a new "valley plate" for my Jaguar V12. I want to use the distributor drive to give me an accurate engine position reference for the ECU. I can't easily use the original valley plate, because I'm also mounting 12 coils onto it, and headroom is an issue.

Here's the original distributor mounting plate:



The inner hole is 40mm (maybe a shiver less), the recess takes a rubber seal, and the rest of the plate is a reference surface for the bottom of the distributor to sit on.

What's not obvious from the photo is, the plate is at an angle with respect to the mounting face, of 6 degrees. Fortunately it's only off-horizontal in one plane... otherwise I'd be in serious doo-doo... Anyway, this is the underside in case it's useful:



The flat bits are my horizontal reference.

So... what I need to do, is accurately locate the hole in my replacement valley plate, at the same angle, but I'm not sure how to do it...

My thought so far is: Turn a bar to a sliver under 40mm (the original dizzy drive is 39.80mm by the digi calipers, I've not broken out the micrometer yet). Turn a point on the bar, so effectively a fat dead centre.

Next, mark up the plate with some micrometer blue, and clamp to a piece of cardboard. Smear a little micrometer blue on the point of my "dead centre", and push it through the hole, until it contacts the cardboard (leaving, one hopes, a tiny blue dot). Remove & unclamp. Measure from blue dot to edge of casting using a digivern. That should give me the exact centre point of the circle at the base line of the valley plate. When it comes to milling out, I should be able to centre-punch the bottom of the plate, set my angle plate up to 6 degrees, use a pointy centre finder to align the spinde, then using a fat end mill, mill a flat, then centre-drill, twist-drill out to as big as I can get with a twist drill, and finish off with a boring head to give me the final 40mm dimension.

Am I on the right lines there, or is there a better way? Thanks in advance!

efrench:
Does it look like A or B or neither?

Lew_Merrick_PE:
Ade,

Such things have traditionally been accomplished using (old name) Tooling Balls that are now called shoulder construction balls -- here in the U.S., such things are most commonly acquired from Carr-Lane (http://www.carrlane.com/catalog/index.cfm/27825071F0B221118070C1C513E111D081B0006280B1713050245221E0107070F1A3C3B285356425D)

Does this help?

sparky961:
I'm interpreting your question as: "how do I get a hole in exactly the same spot with the same angle regardless of what that angle actually is".  The "measuring" part may not actually be required if I understand correctly what you're trying to do.

This suggestion assumes you have access to a mill with a tilting head.  If not the same can be accomplished with a bit more difficulty by shimming up the work piece as has been discussed in other recent threads.  You'll be using your existing part as a setup fixture.  You need to have some references that are identical between the two (mounting holes, edges, or the like).  Set up some locating pins or stops so that you can positively remove/replace the old and new while maintaining the same position each time.

Mount the old part on the table and set the head at the correct angle to drill/bore the hole.  Make sure there's enough travel in the quill to do all of your operations without moving the knee once you begin!  Take the crank off or lock it to prevent yourself from doing something you'll regret.

Use a DTI in the spindle (or coaxial indicator) to get yourself centered on the hole in question.  The bore is pretty shallow but you should be able to verify the angle by moving the quill up and down as you sweep the indicator around the inside of the bore.  If the mounting flange is on the same axis that makes it much easier because any error will be more obvious when sweeping the indicator on it.  Get yourself as close to "0" as you're happy with - meaning the quill and your hole are on the same axis.

Replace the old part with your new part using the established references and go nuts.

Now that will get you your hole same as the original but it doesn't tell you where it actually "is".  If you need this information (which I suspect you probably don't) then you might be able to expand on the above advice to figure out the actual location too.

AdeV:
Hi efrench - it looks like your diagram A. i.e. the dizzy mounting face is at 6 degrees, and the bore is perpendicular to that.

Lew, unfortunately, not so much in this case, but thanks for the thought! I'm not quite sure how I'd use a tooling ball in this instance...

Sparky - That's the one, that's the method I'm going to use (to make it at least). I actually started to think along the same lines when I read Jeff's answer to the question over on the XJS thread, but your write up cemented it. I'll use an angle plate rather than tilt the head (I hate having to tram it back in afterwards...), and the dizzy mounting flange is nice & big (~95mm dia), so I can put quite a large sweep on it, which should get me accurate enough.

OK, that doesn't give me an accurate position for drawing it on the old CAD package.... but as it will be a mixture of CNC & manual, that's not a problem :)

I would still be interested (academically) on how one goes about accurately locating angled holes, as it's something any of us might need to do in the future...

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