Ask them what they'd do it for without the tapping.  Normally I'd do
something like this for free but 40 tapped 4-40 holes in heavy wall
aluminum is a pain.  It would be a miracle if I didn't break the tap off at
least once.  Last time was for a resonant cavity amp (23cm band) and I had
to soak my cavity ring in nitric acid for a month to get the tap out.

At least it's not steel.

-Bob

On Thu, May 18, 2017 at 10:54 AM, cdelect@juno.com wrote:

On 5/18/17 9:54 AM, cdelect@juno.com wrote:

So you're putting 5 holes in each side? in the ends? (i.e. the long axis
of the screw is within the wall?

That's a fairly tricky operation, especially if the tube is standard
extruded tube, because the sides might not be perfectly flat or at right
angles. You're drilling a 0.112 hole in a 0.25" wall, so you've got some
clearance, but I assume you've got a matching cover plate you want to
attach?

How critical is the spacing of the holes?

A good drill press (or vertical mill), and a really good clamping setup,
and "match drill" the cover plate and the tube at the same time.

This is fairly easy to do on a vertical mill, but it can be done using a
drill press.

I would machine and drill the end plates first. Then I would use
hot-glue or double-sided tape to attach one of the end plates to the
tube ends and use the end plate as a drilling guide to drill and tap the
holes in the tube. The critical aspect of drilling the tube holes is
that the end of the tube that is being drilled has to be absolutely
perpendicular to the drill bit.

The drill press can also be used to tap the holes - with the drill press
power off, install the tap and hand turn the spindle while using the
lever arm to gently follow the tap in as it cuts. Since it is aluminum,
hand-tapping the holes without using the drill press should be fairly
easy also.

DaveD

On 5/18/2017 10:54 AM, cdelect@juno.com wrote:

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On 5/18/17 10:03 AM, Bob Darlington wrote:

Very much so.. 40 tapped holes is a lot of holes

Last time was for a resonant cavity amp (23cm band) and I had

Or copper(!)

Price sounds reasonable to me.

Tapping 40 each 4-40's that deep in to a closed hole is the killer. If I
still had my old Bridgeport J and was NC'ed Your looking at say 1 hour to
program, 10 minutes to mount in a vice and get aligned, drill 40 each #44
or #43 holes would be easy, but then slow things down, cross fingers and
start tapping.

Doing it by hand, at 1/2 deep those holes better be close to "perfectly"
aligned.

On Thu, May 18, 2017 at 9:54 AM, cdelect@juno.com wrote:

Tap Magic for Aluminum will be your best friend for this operation.

I've been using it on some extrusion for a 3D printer project. It makes
an amazing amount of difference.

Good luck with you project.

Todd

On 05/18/17 12:18, Pete Lancashire wrote:

People's (mis)perception of machine shop costs always amazes me.  All too
often they equate the fact that they could do it themselves (inaccurately)
on an old drill press and then hand tap with what a real shop will do - all
while that real shop has a $100k - $500k investment in a single machine.

Yes it can be done cheaper, but not profitably.  Its why people end up with
25k pounds of machinery in their garage (nothing wrong with that).  Its
really no different than what you would expect if you took your rubidium
standard into a small, profitable RF shop and asked them to characterize it

• they would charge you going rate.  Which is why many of us end up with
  1000's of sounds of test equipment in the basement or back bedroom - in
  addition to the machine shop in the garage.

Brent

On Thu, May 18, 2017 at 1:27 PM, Todd Caldwell tmcaldwell@pobox.com wrote:

I'm very dubious that you need a tapped 4-40 hole to be threaded any deeper
than 0.25". If you work with the machinist I'm sure you can come up with
some reasonable spec that does not require a bottoming tap and will save
you a lot of money.

I bet you went to 0.25" wall square tubing only because you want to tap the
walls for 4-40. Alternative designs can let you use much thinner material
and a very different flange on the ends, but the costs will likely move
towards welding/brazing rather than machining.

Tim N3QE

On Thu, May 18, 2017 at 12:54 PM, cdelect@juno.com wrote:

I agree, tapping that depth in aluminum is just asking to gall the threads. If you have a decent alloy (6061 7075) you only need 1/8” to hold what ever is save to hold with a #4. If you need more strength then it would be best to use helicoil inserts.

Where are you located? There are a number of hacker spaces across the country / globe. If you are in the US, there is also "The Tech Shop”. If you are in the SF Bay Area I would take a look.

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