In attaching the search coil for a metal detector to the handle, the easiest method would be by means of a bolt at the center of the pancake coil to the handle. Given that the detector might be used to detect small metallic objects such as coins, I wonder whether or not the bolt might diminish sensitivity of the detector to such objects.

I can understand this question in two ways: either that the bolt makes an unvarying change of the inductance of the search coil and therefore won't affect the change of inductance when some metallic object is nearby the coil, or that the bolt will in fact somewhat reduce sensitivity to a small metallic object as the discovered object is metal in addition to the metal of the bolt and will therefore only produce a change of inductance proportional to the increase of metal in the vicinity of the search coil.

I'm leaning towards not using screws or bolts to attach it to the handle, but would feel silly doing that if using bolts or screws won't affect the sensitivity.

 

In attaching the search coil for a metal detector to the handle, the easiest method would be by means of a bolt at the center of the pancake coil to the handle. Given that the detector might be used to detect small metallic objects such as coins, I wonder whether or not the bolt might diminish sensitivity of the detector to such objects.

I can understand this question in two ways: either that the bolt makes an unvarying change of the inductance of the search coil and therefore won't affect the change of inductance when some metallic object is nearby the coil, or that the bolt will in fact somewhat reduce sensitivity to a small metallic object as the discovered object is metal in addition to the metal of the bolt and will therefore only produce a change of inductance proportional to the increase of metal in the vicinity of the search coil.

I'm leaning towards not using screws or bolts to attach it to the handle, but would feel silly doing that if using bolts or screws won't affect the sensitivity.

You are, in general right. A static bolt won't matter but, it depends on the frequency (higher frequencies will be attenuated via eddie current losses of the highly conductive bolt. High quality (low loss) inductor cores are made of laminated sheets or plates with non-conductive coatings or tapes between, or ferrite ceramics (magnetic but non-conductive, or metal powders with thin oxide coatings yo limit conductivity which limits eddie current losses (as heat).
Also, the bolt will be an inductor in the traditional sense of the word, it will "induce" the magnetic field into a concentrated area (a field concentrator). You'll have a more uniform field without the iron.
That said, very directional detectors are known with large metal rods in them. They are typically used to find iron stakes to mark property lines (corner irons).
good luck.

 

hi Pete,
Mabe a possible option, non-metallic bolts

E
https://www.mcmaster.com/nonmetallic-bolts/

 

Glue, high-bond tapes, wooden pegs, threaded wooden dowels, dovetails, mortise & tenons - so many options.

 

Hello,

This guy has a description for making a flat coil:
http://chemelec.com/Projects/Metal-1a/Coil-Jig/Coil-Jig.htm

The link was found half way down this page with a lot of projects:
http://chemelec.com/Projects/Projects.htm

Bertus

 

For mine I used a SCH40 plastic pipe for the handle and used the appropriate pipe cement to join/weld it to the coil platform.

 

Hello,

This guy has a description for making a flat coil:
http://chemelec.com/Projects/Metal-1a/Coil-Jig/Coil-Jig.htm

The link was found half way down this page with a lot of projects:
http://chemelec.com/Projects/Projects.htm

Bertus

That guy has a very fancy setup for winding coils. What I did was to cut four discs with a plunge router from 1/8 inch thick hardboard (masonite), two of the discs 4 inches in diameter, and the other two 6 inch diameter. The two four inch diameter discs glued together give me a coil length of 1/4 inch and the 6 inch discs above and below keep the coil in place on the inner diameter.

The coil was wound by attaching the coil form on a wooden disc driven by a gear motor with wood screws.

The coil is not removed from the form, and the form provides a means for attaching the coil to the handle.

The only possible difficulty doing it this way is that a Faraday shield is not possible. So far I've found the shield to be not necessary.

 

Okay, well that looks like one undecided, Mr. Salts, two opposed to using metallic attaching hardware, ericgibbs and MaxHeadRoom, and one abstention, Bertus.