[ARC5] ARC-5 Screws

[Brian Clarke]

Aha, Mike,

You raise an interesting point. I have been lucky in having access to a 
printed wiring board (PWB) manufacturer's drill press, where you view the 
hole, magnified, from directly above; it has what appears like a sniper's 
graticule. I start with a 1 mm drill = bog-standard 0.040" approx, ie, what 
you normally drill a PWB with, and then follow with a 0.075", ie, 1.875 mm 
drill. If that gets 'sticky', creep up with a 1.5 mm drill (= 0.060"). All 
these drills have very fat shanks and short fluted sections, and run in the 
PWB drill press at about 12,000 to 18,000 RPM and so, are less likely to 
flex and reach their Euler critical buckling load. Always wear safety 
spectacles.

And, where do you get dental picks? Just ask your dentist after your next 
examination - the cost of new vs the hazards associated with autoclaving 
such instruments makes most dentists throw them away after one use. If you 
get stuck, ask me - I may have a few spare.

73 de Brian, VK2GCE.

On Sunday, March 11, 2012 3:27 PM, Mike Hanz said:


> Brian's solution is absolutely correct, of course.  But it raises the
> question of how you can accurately guide a *very* flexible .078" drill
> to just touch the edge of the root diameter throughout the length of the
> thread.  I suggest a perhaps overboard method of doing so at
> http://aafradio.org/garajmahal/C-29.htm but the fundamental key is using
> a #1 center drill to establish the guide hole.  With practice you can
> get within a few thousandths of the center by eye without using the
> center scope shown, especially if you use a head mounted magnifier of
> some sort and view the center point from 90 degrees apart while
> positioning the workpiece.  I try not to use a tap unless all else
> fails...a sharp dental tool will most often allow you to tease the
> remaining thread spiral from the female thread without damaging it.
> Corrosion may make that difficult, however, as Brian alludes.  A tap may
> easily cut off center, at least in my experience.
>
> On 3/10/2012 8:26 PM, Brian Clarke wrote:
>> The standard engineering solution is to drill at the root diameter and 
>> then
>> retap - the tap, if you're lucky, will drive out the remnant of the 
>> previous
>> screw. Sometimes, I've been really lucky, where there is no corrosion, 
>> and
>> high pressure air has cleared the remnant.