Accurate hole location

I sometimes need to place holes in precise marked locations, and I wonder if the technique I've used for years is optimum.

Say I want an accurate 1/4 inch hole at a scribed location on a brass clock plate to an positional accuracy of (say) 5 thou. I would start by dot punching the scribed location using an optical centre punch. I would then move to the drilling machine and use a short, stiff, CNC style centre drill to create a centre. I would then use a small drill, say 3/32, to create a pilot hole, and follow this up with a 7/32 drill, and finally a 1/4 reamer (hand reamer, used in the drilling machine). Sometimes I get the accuracy I want, sometimes I don't.

Would it be more accurate to omit the somewhat flexible pilot drill, or is there a completely different sequence which would lead to more consistent results?

Hi Mike,

By the initial sound of it I dont think you have a milling machine, so my thoughts depending on your cash availability, would be to look out for a half decently calibrated dials compound table for your drill.

I would then eliminate the variables from marking out, punching and picking them up and go for indexed pitching from the calibrated hand wheels. Easy to map out your co-ordinates on paper, but you will need an accurate start datum.

Edited By MalcB on 14/03/2017 17:56:21

Mike,

You could clamp a hardened 7/32" drill guide on the exact centre location and drill 7/32" direct without using a centre drill or the 3/32", if securely clamped the guide will give you the accuracy required for the drilled hole so you should heve a good chance of the reamed hole being where required.

Emgee

MalcB

I do have a milling machine equipped with DRO, and I often use it for coordinate drilling. My question related to drilling holes starting from scribed locations, such as those created when planting clock trains.

Emgee

Using a hardened drill guide would be specific to only one size of drill, so not a general method, and how would one go about accurately positioning the guide over the scribe marks?

When I need to pick up scribed marks I use the mill with a wobbler (sticky pin will do) to locate the point of intersection, then Lock the table and zero the DRO. Spot drill straight onto the lines as no need to punch and then stub drill out to reamer size then ream under power, twist drills will do if you don't have stub length.

7/32" sounds a bit small for 1/4" reaming try 15/64"

+1 for 'sticky pin'

Tony

Thanks Jason, that sounds like an improved approach which eliminates the pilot drill, which I had doubts about.

I have a wobbler of the George Thomas variety, but my deteriorating eyesight makes it hard to line it up really accurately with scribe marks. I'll have to try harder with magnification and better lighting

If you want to be posh you can buy/make an optical centre finder, a sort of bent microscope that fits in your milling machine spindle, but you can do better than 0.005" with a sticky pin and a loupe

Thanks for that Duncan. I like the sound of the optical centre finder. I feel another project coming on!

Mike,

Reverting to more traditional horological technique:

With a pair of crossed scribe lines [from the depthing tool] you should be able to feel the intersection with a sharp scriber ... pick-up and deepen that intersection with the scriber, then use a 'spade drill' to open it up to a nice conical depression. ... Then proceed as previously discussed.

• If moving-up to a conventional twist drill, it's important to get the diameter of that depression big enough to contain the chisel edge!
• It's very convenient to make your spade drill from a swivel-topped screwdriver.

MichaelG.

.

Edit: this is the shape of spade drill, 'though of course you don't need carbide for brass clock plates!

https://www.eternaltools.com/carbide-spade-drills

Edited By Michael Gilligan on 14/03/2017 20:01:53

Simple optical finder design here (+ an optical centre punch)

www.model-engineer.co.uk/news/article/an-optical-finder/24734

From MEW issue 1, no less!

Neil

Another thought, I would use s 15/64 drill before reaming (assuming it was a good one that drilled close to size).

Neil

.

In a brass clock plate, I would use a horological [five-sided] cutting broach, not an engineering reamer.

MichaelG.

Here is how we used to drill and ream holes over any distance, usually better than .002" when making airframe jigs in the aerospace sector using drill bushes. You will need two 'drill bars', essentially these are gauge plate 150x 20x 12. With a slot for clamping. In one end is a reamed hole either 12mm or 1/2" depending upon the O.D. of your slip bushes. You pick up your first position using the optical centre finder, clamp the bar in position, then drill and ream to size. The second position is established by using a dowel in the fixed drill bar, then, with a dowel in the second drill bar, position using either a micrometer or inside mic., clamp bar in position, then drill.

I still have all the kit to dispose of after 20 odd years! Roy M.

The one I remember had lenses and graticules and all sorts of other complicated stuff, either ME or MEW. I suspect you could use a USB microscope, but that's the end of my knowledge

Mike

As you have a mill with a DRO why not clamp the work clear of the table surface then use an edge finder to locate the hole by XY table movements to match the hole centre co-ordinates from the 2 edges, lock the XY axis and drill and ream the hole.

Emgee

Mike

Quite a few years ago I had the same problem and thought long and hard about solving it. In the end I made an alignment microscope based on a design by D A G Brown & R A Stephen. The optics were not cheap and the manufacture was interesting (there is a Whitworth based microscope thread). However the resulting instrument can locate to better than 0.001" with ease. There are a few difficulties such as the work piece has to be very well lit and that it can be difficult to get one's head in the right position to look through it. [There are two pictures of the "thing" in one of my albums]

This together with a good DRO gives an order of accuracy better than one usually needs. Therefore it is used only when really needed.

As for the rest of your question I think it is jig boring using a small centre drill to locate/start the drilling of the next hole.

JA

Edited By JA on 14/03/2017 23:24:59

Edited By JA on 14/03/2017 23:28:40

Edited By JA on 14/03/2017 23:30:13

I got a battery operated engraver from TKmax recently

At the time it was a "might be useful" moment but it is good for making a dimple exactly where you want it because it has a tiny rounded diamond tip

Some years back the spinning laser pen was popular. Just a trammel type arrangement with the pen angled down and adjusted in "Z" until aligned by the convergence of the beam, or the described circle. Good for centering over round bars as I recall. ref. https://www.youtube.com/watch?v=q_aa4rANib0

Ok. My overall take is to pick up the scribed mark using a wobbler or optical device on the mill. Use a s spotting drill to create a centre, follow this up with a stiff reaming size drill (no pilot), then ream.

Thanks all

Mike