"Hollow" screw

I need a screw (a bit over 1/4" thread diameter) with a hole
drilled longitudinally throughout its length.  The hole as
large as feasible without significantly reducing the strength
to unusable (nebulous term) levels.

I realize I will eventually have to contract a casting or machined
parts.

But, am looking for onesy-twosy quantities to demo a prototype.

I thought I could approximate it using a threaded tube (NOT
a pipe nipple).  But, getting premade threaded tubing in that
range seems difficult.

I thought of manually threading a (soft, brass?) pipe of
suitable ID/OD.

Also thought of having one *printed* -- but I'm not sure getting
the threads right would be practical (driven by how fine the
layers are?).  (I think most *plastics* would be too brittle
when torqued in such an application so I'd have to print in
metal)

I also thought of physically removing the core material from
a COTS screw (drill/cut -- possible with a brass screw?).

Any other options?

On Wednesday, January 26, 2022 at 2:24:58 AM UTC-4, Don Y wrote:
> I need a screw (a bit over 1/4" thread diameter) with a hole 
> drilled longitudinally throughout its length. The hole as 
> large as feasible without significantly reducing the strength 
> to unusable (nebulous term) levels. 
> 
> I realize I will eventually have to contract a casting or machined 
> parts. 
> 
> But, am looking for onesy-twosy quantities to demo a prototype. 
> 
> I thought I could approximate it using a threaded tube (NOT 
> a pipe nipple). But, getting premade threaded tubing in that 
> range seems difficult. 
> 
> I thought of manually threading a (soft, brass?) pipe of 
> suitable ID/OD. 
> 
> Also thought of having one *printed* -- but I'm not sure getting 
> the threads right would be practical (driven by how fine the 
> layers are?). (I think most *plastics* would be too brittle 
> when torqued in such an application so I'd have to print in 
> metal) 
> 
> I also thought of physically removing the core material from 
> a COTS screw (drill/cut -- possible with a brass screw?). 
> 
> Any other options?

What exactly is the thread you need?  I don't understand why you think you can't find 1/4 inch threaded pipe.  That is what they use in lamps, it is very common.  Now if your thread is something very oddball, I expect you can still find it if you look around a bit.  Certainly any machine shop can make that for you if they have the die.  

Is there some reason why you didn't provide the thread you need?  That would seem to define the task more than anything else other than possibly the length.   So what thread and what length? 

If you can find a brass screw in the thread you need, why do you think you would not be able to find a threaded tube? 

-- 

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

On 2022-01-26, Don Y <blockedofcourse@foo.invalid> wrote:
> I need a screw (a bit over 1/4" thread diameter) with a hole
> drilled longitudinally throughout its length.  The hole as
> large as feasible without significantly reducing the strength
> to unusable (nebulous term) levels.

braze or solder a nut onto a threaded tube.

OTOH parts like this are used in lighting, mechanical, and plumbing
applications, perhaps you can get what you want off the shelf.



-- 
  Jasen.

On 2022-01-26, Jasen Betts <usenet@revmaps.no-ip.org> wrote:
> On 2022-01-26, Don Y <blockedofcourse@foo.invalid> wrote:
>> I need a screw (a bit over 1/4" thread diameter) with a hole
>> drilled longitudinally throughout its length.  The hole as
>> large as feasible without significantly reducing the strength
>> to unusable (nebulous term) levels.
>
> braze or solder a nut onto a threaded tube.
>
> OTOH parts like this are used in lighting, mechanical, and plumbing
> applications, perhaps you can get what you want off the shelf.
>

also bicycles.


-- 
  Jasen.

On 1/26/2022 12:10 AM, Jasen Betts wrote:
> On 2022-01-26, Don Y <blockedofcourse@foo.invalid> wrote:
>> I need a screw (a bit over 1/4" thread diameter) with a hole
>> drilled longitudinally throughout its length.  The hole as
>> large as feasible without significantly reducing the strength
>> to unusable (nebulous term) levels.
> 
> braze or solder a nut onto a threaded tube.

It's finding the threaded tube (of the right ID/OD) that is
the challenge.

> OTOH parts like this are used in lighting, mechanical, and plumbing
> applications, perhaps you can get what you want off the shelf.

The first thought was that of lamp rod.  But, while claiming to be a
nominal "1/8 inch" diameter, the actual dimensions across the threads
are ~3/8".  Far too fat for my needs.

<https://www.amazon.com/Lamp-All-Thread-Pipe-Steel/dp/B008UWATVK>

If you move to nipples (neglecting the fact that pipe threads are tapered
so even a close nipple isn't truly "square"), then you are faced with
a similar problem; nominal pipe (and, thus, nipple) dimensions understate
the actual diameter.  E.g., a 1/8" NPS pipe is *over* 3/8" OD (0.405")

If you approach it the other way -- taking a "pipe" and threading it
manually, then the thickness of the material comes into play.  E.g.,
a 1/4-20 bolt has a thread depth of ~0.036".  So, the pipe wall would
have to exceed this sufficiently (another nebulous term) to retain
its strength.  (a man can exert a fair bit of torque on a fastener;
ever notice screwdrivers with twisted blades?  Or, small (#0 or #1)
Phillips screwdrivers with missing tips?)

If the pipe wall is too thin, then you are driven to using a finer
thread pitch.  And, risking a sloppy fit or damaged threads, in use.
(as well as complicating its manufacture)

[I found some 6mm hard copper pipe but the wall thickness (0.6 - 0.9mm)
leaves me wondering if I can get a coarse enough thread pitch without
turning the pipe into "spiral cut ham"!]

Lamp rod would be fine if the OD was closer to 1/4 - 5/16".  Which
would probably also increase the thread pitch as the pipe wall
would undoubtedly get thinner (if not, then the ID suffers).

If I start with something like a 5/16 bolt, I can probably get
enough of an ID with careful machining -- if the material is
*soft* (stainless would likely be problematic and need cutting
oil/coolant).  A 1/4-20 bolt is the ideal OD but I don't think
it could be hollowed to yield a sufficiently large ID without
sacrificing the strength of the bolt.  The 3/8" lamp rod is just
too fat to justify the ID it affords.

Don Y <blockedofcourse@foo.invalid> writes:

> I need a screw (a bit over 1/4" thread diameter) with a hole
> drilled longitudinally throughout its length.  The hole as
> large as feasible without significantly reducing the strength
> to unusable (nebulous term) levels.

Drill a hole to the screw ? If you don't have a lathe, you can do this
with a drill press by fixing the screw to the chuck and having the drill
bit stationary. Drill a starting hole with larger drill and through with
a smaller one(s).

Brass is easier to work with, but suitable drills work fine with steel,
too.

--
mikko

Don Y <blockedofcourse@foo.invalid> wrote in news:ssqpff$d5v$1@dont-
email.me:

> I need a screw (a bit over 1/4" thread diameter) with a hole
> drilled longitudinally throughout its length.  The hole as
> large as feasible without significantly reducing the strength
> to unusable (nebulous term) levels.
> 
> I realize I will eventually have to contract a casting or machined
> parts.
> 
> But, am looking for onesy-twosy quantities to demo a prototype.
> 
> I thought I could approximate it using a threaded tube (NOT
> a pipe nipple).  But, getting premade threaded tubing in that
> range seems difficult.
> 
> I thought of manually threading a (soft, brass?) pipe of
> suitable ID/OD.
> 
> Also thought of having one *printed* -- but I'm not sure getting
> the threads right would be practical (driven by how fine the
> layers are?).  (I think most *plastics* would be too brittle
> when torqued in such an application so I'd have to print in
> metal)
> 
> I also thought of physically removing the core material from
> a COTS screw (drill/cut -- possible with a brass screw?).
> 
> Any other options?
> 

The closest you will likely get is lamp parts.  Threaded rod is hard 
steel and not easily drilled, though I am sure there are items out 
there somewhere.  And I am sure that a short length is doable.
Seems like they could take thick wall piping and roll thread it for 
this.  Anyway I found this...

<https://www.antiquelampsupply.com/lamp-parts/lamp-arms-arm-backs-
husks/18-20-24-all-thread-pipe.html>

Rick C <gnuarm.deletethisbit@gmail.com> wrote in
news:13d6d973-5f30-4841-ae0e-924655589b92n@googlegroups.com: 

> What exactly is the thread you need?  I don't understand why you
> think you can't find 1/4 inch threaded pipe.  That is what they
> use in lamps, it is very common. 

  Nope.  Lamps do use threaded pipe, but not that size.

On 1/26/2022 1:44 AM, Mikko OH2HVJ wrote:
> Don Y <blockedofcourse@foo.invalid> writes:
> 
>> I need a screw (a bit over 1/4" thread diameter) with a hole
>> drilled longitudinally throughout its length.  The hole as
>> large as feasible without significantly reducing the strength
>> to unusable (nebulous term) levels.
> 
> Drill a hole to the screw ? If you don't have a lathe, you can do this
> with a drill press by fixing the screw to the chuck and having the drill
> bit stationary. Drill a starting hole with larger drill and through with
> a smaller one(s).

?

Why wouldn't I thread the screw into a tapped block (to support and
reinforce the threads -- as well as act as a heat sink) -- after coating
it with antiseize (as the drill bit's rotation will tend to want
to tighten the screw in the block, possibly making removal more
difficult)?

And, I'd assume move from smaller diameter bits up through larger ones
to minimize the amount of material being removed with each.

This has the added advantage of giving me prototypes with increasing
IDs that I can torque test (i.e., at what point have I removed
TOO MUCH material and lost strength?).  Trying to do this with
ever smaller IDs means having to make (and destroy!) multiple screws
to test (until you find one "strong enough").

> Brass is easier to work with, but suitable drills work fine with steel,
> too.

(polished) Brass will "look pretty" -- enough to deflect attention from
the fact that it's not, e.g., stainless.  (Plastic/nylon/aluminum would
all look "cheap" -- "presentation" is a big part of the issue!)

Two colleagues (email) have independently pointed out that the threads
are the problem.  They are essential to providing "fastening" for a
screw.  But, add to the OD by their very nature.

It was suggested to use fasteners with "flexible" threads -- like the
"blind" fasteners used to hold door panels to a car's door frame:

<https://static.summitracing.com/global/images/prod/xlarge/RNB-45680_OB_xl.jpg>

Here, the fastening mechanism (the "threads" -- though NOT "spiral"!) are
deformed as the fastener is inserted, reducing the actual diameter
of the fastener as it is inserted.  But, return to their uncompressed
state once installed.  I'll have to buy some to see just how much of
a "diameter savings" this affords.

But, it inspired me to think of yet another option; move the "threads"
(fastening mechanism) to the mating piece where it doesn't impact the
OD of the "fastener"!  Spring clips!

<https://www.veckfasteners.com/wp-content/uploads/2015/09/S32.jpg>

So, the "screw" can just be a "tube-with-a-head".  The wall thickness
not having to accommodate the intrusion of any cut threads!  The spring
clip can be stationary and the "post" driven into it's embrace.

The (big) downside to this approach is it makes disassembly difficult
(without damaging the item or marring its appearance).  OTOH, assembly
is a breeze!  :-/

There's got to be something similar that is also disassemble-able...
maybe a *square* (hex?) peg into a 4-pronged (3 or 6?) clip?  Insert
with the flats aligned with the clip's prongs, then twist to engage
the vertices on the prongs??

[I gotta call glen or skip...]

On 26/01/2022 19:07, Don Y wrote:
> 
> If I start with something like a 5/16 bolt, I can probably get
> enough of an ID with careful machining -- if the material is
> *soft* (stainless would likely be problematic and need cutting
> oil/coolant).

I have drilled holes through stainless steel hex-socket-head cap-screws 
(in my case 1.6mm holes through 4mm screws), with no particular 
difficulty up to 20mm depth or so.

You should use a lathe, and cutting oil on the drill. Someone with a 
lathe could do it for you and it is an easy operation unless you want 
the hole to be much more than 10 times as deep as its diameter.

If you are bothered about the remaining wall thickness then you should 
look into obtaining bolts with a different thread. If the part you want 
to obtain is geometrically impossible, no amount of advice on 
manufacturing techniques will help. At least if you drill out a steel or 
stainless steel bolt, it will be about as strong as is possible for the 
given geometry, and with the right grade of steel bolt there remains the 
option of case-hardening and/or heat-treating it if you need a bit more 
strength.