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Drilling of Hole + Tapered Screw Dimension

Drilling of Hole + Tapered Screw Dimension

Drilling of Hole + Tapered Screw Dimension

Hi All,

I have few questions pertaining to drilling, hole size and tapered screw thread.

Please see the picture attached for clearer illustration.

1. If the diameter of my LUG is 21.5 mm, should the drilling size of the hole be bigger or smaller than 21.5? If it is bigger, at what allowance tolerance should be given? In this case, a tapered screw is used. When it is tightened with torque wrench, the gap will open up and flush again the surfaces.

2. Is there any procedure or ASTM standards related to this?

3. What is the proper measurement of the taper screw size? It is not a straight screw and therefore I cant use British metric like M10, M12 ... etc.

4. For copper material, can fatigue mark be seen? I know it can be seen for ferrous material.

Seeking valuable and professional advise.


RE: Drilling of Hole + Tapered Screw Dimension

You have a copper lug (an electrical grounding lug maybe??) with a split (expanding) insert, apparently being tightened into a steel plate of some unknown thickness by the tapered end of a much longer bolt.

No, we cannot give you "ASTM" dimensional requirements from those little bits of data.

First, give the purpose of the assembly, and the original drawings of the entire piece.
Give the dimensions and purpose of the lug, and all dimensions of the tapered bolt.

RE: Drilling of Hole + Tapered Screw Dimension

Hi racookpe1978,

Yes, the copper lug used here allows the current to return to the substation.

Due to the company policy, I cannot disclose the ORIGINAL drawing but I believe the updated document attached should have answered all your questions.

Hope it helps.


RE: Drilling of Hole + Tapered Screw Dimension

The threaded section is a "standard" galvanized M12 thread. The rest of the tapered bolt needs to be custom made, including the 13.3 x 17.3 tapered expansion cone.

Note: You MUST specify the M12 is for galvanized threads, else it will cut for clean threads. Surprised they did not specify the thread pitch, go measure it with any conventional metric thread pitch gage or micrometer - recognizing you are measuring galvanized threads.

RE: Drilling of Hole + Tapered Screw Dimension

What is the tolerance on the 21.5 mm lug diameter? Obviously you want the minimum hole size to be slightly more than that. Must not be very critical as the tapered pin looks to be able to expand the lug at least 3 mm. I suppose they want these things to be reusable and the copper is going to get distorted each time it's bolted up so I would keep the initial clearance small (~0.2 mm). IF it's a one time connection they you could be a little more generous. This is something I've never seen before, very doubtful there is any kind of standard.

All M12 bolts will have the same OD, if it's not specified then the thread will be course (1.75) but it makes no difference.


The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Drilling of Hole + Tapered Screw Dimension

Hi dgallup, racookpe1978,

Thanks for the feedback. I have concern over here and I will summarize in point form.

a) On paper, the hole dimension is 22 mm. On the actual site, it was 21.70mm.
b) The drill bit that was used to drill the hole stated diameter of 22 mm. However, when measured with vernier caliper, it was around 21.70 mm.
c) The LUG connector has diameter of 21.5 mm. With the hole of 21.70 mm, the connector will definitely has difficulties fitting the slot.
d) Should the hole be bigger such that the connector can fit in smoothly and flush against the rail contact using the tapered screw?
e) If the connector was pushed/hammered inside,the contact between the 2 surfaces would have been flushed minimum. I was told they used 75Nm Torque to tighten the bolt with nut.


RE: Drilling of Hole + Tapered Screw Dimension

Sounds like you already know the answer, the hole should be 22 mm. Must be a very worn drill to be 0.3 mm undersize.


The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Drilling of Hole + Tapered Screw Dimension


Is this hole drilled in one-pass? IF SO that could be Your real problem.

Multi-step drills would provide penetration stability and freedom from chatter/squealing for accurate hole drilling. Final diameter drill guided-by a 'pilot' would be a piece of cake to hit the numbers. Also, 'practice on scraped rails to attain the optimum drilling procedures/tooling... and determine true drill-bit wear-rates down to a science... would be very useful... on a repetitive basis.

Are Your drills cobalt-steel or carbide for added toughness and durability? Are Your drill-bits frequently re-sharpened PROPERLY? Are worn-out drill-bits scrapped before they become a nuisance to maintain?

Any significant variability in the rail steel alloying or tempers that would affect hardness, toughness or work-hardening [rail-to-rail steel inconsistencies noted]?

In Your photos I do not see basic corrosion protective measures I would expect... especially considering the electrical energy being transferred thru the jumpers. IE: grit-blasting the rail surfaces/hole to bright metal, installation of the copper-plug/steel mandrel into the drilled hole with conductive sealants... then over-coating both sides for environmental exclusion.

Also, [in aerospace] we ensure bonding/grounding points receive special machining/abrading/cleaning... to bare/bright/smooth/radiused-edges metal... then apply multiple layers of corrosion protection to adjacent/surrounding/faying surfaces that are electrically critical... within the hole, between hardware/parts, etc... and THEN cover-over the entire bond/ground-point installation with sealants to isolate the entire installation from the environment and minor damage that could affect it. We cannot afford to have a high-current grounds fail... or have lightning-surges burn-out holes... or allow flammable fluids to be exposed to raw current from static-ground-lightning sourced arcs/sparks.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: Drilling of Hole + Tapered Screw Dimension

Hmmmn. 7/8 inch ANSI drill would be 21.438 mm. Wonder if that size were used. Hard to measure a manually-drilled-sideways railroad rail to .1 mm dia.

RE: Drilling of Hole + Tapered Screw Dimension

Are the split logs new? Would a squeeze with vise grip pliers or a few taps with a soft face hammer close up the lug so it could be installed in a too-tight hole? Maybe a few swipes with a grinding stone in a cordless drill would deburr the hole and allow lug insertion.

RE: Drilling of Hole + Tapered Screw Dimension

Put a shallow angle chamfer (10deg per side) on the leading edge of your lug OD. This will make it easier to fit into a mating hole that is slightly undersize.

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