Locate hidden screw and nails?

I built a workbench with a four-layer MDF top and did not keep close track of the location of the screws securing each pair of layers together (in addition to the glue), thinking that I could locate them afterwards with a stud finder and ovoid them when drilling holes for bench dogs. I do know that I ensured that screws were placed so that the later ones did not collide with the ones already connecting other layers.
BUT neither the electronic stud finder nor the swiveling-magnet nail finder will locate these screws.
I know that fancy and expensive devices exist that are claimed to be able to find nails in lumber, but is there some simple and cheaper way to do what I need?
Perce
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wrote:

I was thinking along the same line. Maybe one of the magnets out of an old hard drive. They are strong enough to penetrate a short distance.
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On 8/7/14, 12:42 PM, Ralph Mowery wrote:

Or the magnets from a fuel-injected engine throttle body. I have several that are so strong you need a wedge to pry 2 of them apart ! They beat any standard stud-finder.
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On 08/07/14 12:42 pm, Ralph Mowery wrote:

The screws are steel rather than brass.
I thought I still had several magnets from dead hard drives (mostly Western Digital), but I've found only one small pair. Sliding one of these (one half of the pair) along the surface of the workbench, I do find some spots where it wants to stay put and swivel rather than slide further along, so I think it is doing the job. When I have more time, I'll mark the spots and see whether they are at regular intervals: I can't be sure where I drove the screws, but I am reasonably confident that they were equally spaced (or close enough to).
Perce
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On 08/07/2014 07:35 PM, Percival P. Cassidy wrote:

Yep, you found them.
The easy way to find them of course is with a drill bit.
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Lee Valley sells Rare Earth magnets.
However, I'd expect the magnet in a magnetic stud finder IS a rare earth magnet, and if it's not responding to the presence of steel screws, then another rare earth magnet won't either.
I think this is one of those cases where there isn't a good solution to a problem.
--
nestork

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On 8/7/2014 2:22 PM, nestork wrote:

The installers got tired of catching their clothes on the flooring nails and decided to just bang them up flush. I had nails sticking up at various heights under the rug.
Used a harbor freight stud finder
http://www.harborfreight.com/5-in-1-stud-finder-voltage-tester-92375.html
Found 'em no trouble. Used a center punch to bang 'em back down thru the rug weave.
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On 08/07/14 06:48 pm, mike wrote:

I'm not sure why the Hazard Fraught one would work better than the Stanley. Keep in mind that we are talking about screw that were driven in from what is now the underside of the bench top, so only the points of the screws are within 1/2" or more of the surface.
Perce
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On 08/07/2014 04:22 PM, nestork wrote:

The best magnets I've ever used are those taken from SCSI drives.
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On Thu, 07 Aug 2014 17:25:06 -0700, Percival P. Cassidy

you need a 'tiny' metal detector! A small version of those things people scan around over the ground, but only 1/2 inch in diameter. I've got one with a 1/4 inch diameter in my lab. If you want a LOT of details, contact me offline. From memory it can 'see' a nail something like 6 inches away. or was that a foot? can't remember now.
you can make one with some effort. A coil and your soundcard. Power the coil with the soundcard drive while monitoring the 'response' of the coil with the input of your soundcard 'listening'. Then moving the coil around on top the bench will find stainless steel as well as ferro iron screws/nails.
To really add some 'gain' to the system, use two coils one to transmit and one to send. put the transmit under the table and move it around, put the receive on top and move it around at the same time. When you line up on a screw/nail the signal(s) will be greatly enhanced.
[Haven't tried but might work, try using a 'cable locator', one of those $25 thingies from HD, launch the tone into a small coil under the bench, and move the probe around on top at the same time, might really hear it get louder when you line up on a screw/nail.]
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| The screws are steel rather than brass. | | I thought I still had several magnets from dead hard drives (mostly | Western Digital), but I've found only one small pair. Sliding one of | these (one half of the pair) along the surface of the workbench, I do | find some spots where it wants to stay put and swivel rather than slide | further along, so I think it is doing the job. When I have more time, | I'll mark the spots and see whether they are at regular intervals: I | can't be sure where I drove the screws, but I am reasonably confident | that they were equally spaced (or close enough to). | Why not just drill out the holes and take a chance? What have you got to lose other than maybe a cheap drill bit or two? I wonder about using MDF, though. It chips easily on the edges and it's very sensitive to water. I'd be inclined to put something like fir plywood on top. (Or maybe birch plywood if it has to be very smooth.)
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On 08/07/14 09:11 pm, Mayayana wrote:

And some 3/4" holes that are not deep enough to use for bench dogs but will collect sawdust and wood shavings, etc.

It has 1 1/2" solid wood all around to protect the edges. MDF was recommended specifically because of its smoothness and flatness. (It's a woodsmithshop.com design.)
Perce
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Perce,
Look at Mayana's solution again ad think about it. There are very few screws in the wood so, most likely, you won't hit any screws. If you do hit a screw get a metal bit and finish drilling the hole. Losing a screw is not a tragedy and you can certainly off-set and put in another screw. You seem to be looking for a difficult solution to a simple problem.
Dave M.
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| It has 1 1/2" solid wood all around to protect the edges. MDF was | recommended specifically because of its smoothness and flatness. (It's a | woodsmithshop.com design.) |
I'm surprised. It won't be smooth and flat if you spill your coffee on it. It will swell up. It is very nice in terms of compressive strength and smoothness, but really not good for much other than a substrate for laminate.
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On Thursday, August 7, 2014 10:45:11 AM UTC-4, Percival P. Cassidy wrote:

Get one of these spherical rare earth magnets and roll it around in the area. It will stop when it finds a nail. I have a "solid" desktop and it works on that. Also finds the nails in my hardwood floors. http://www.heavenlyswords.com/product.php?productid2298
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If Perce's swiveling-magnet nail finder won't find them, they may be brass screws. Either way, the metal detector approach may be the best way to find them.
You can build a simple oscillator circuit that operates near the frequency of a local AM broadcast station. You would use the oscillator's coil as a probe to detect metallic objects, assuming it is an air-core coil.
You would tune the oscillator to produce a whistle when listening to the station on an AM radio. When the coil gets close to a metal object, the whistle pitch will change. Use the lowest pitch you can hear for best sensitivity.
I have not tried the sound card approach, so do not know how sensitive it may be. It should be easy to try with two coils of wire, if you know how to generate and detect a tone.
Fred
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There's lot's of free software to run the soundcard so all is 'kind of' free.
if using two coils, might wrap on 16 penny nails, makes for VERY cheap core material, and has the right shape.
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