Fisher F75 aluminum shaft tube size/specs??

I'm contemplating making a custom shaft setup for an F75, so I'm trying to determine the specifications of the original aluminium tubes (upper S-bend and middle section). They are obviously Imperial 7/8" outer diameter, but the wall thickness is not so clear. I measure it as between 0.050 and 0.055 inches (I only have a set of vernier calipers at the moment). As this is U.S-sourced tube, would I be correct in assuming this would be 16 AWG (American Wire Gauge) = 0.050 inch? I had a look on US eBay for guidance, it appears 0.050" and 0.058" seem common sizes for tubes? I'm in the U.K, and our Imperial tubing is sold with SWG (Standard Wire Gauge) wall thicknesses, the nearest being 18 SWG = 0.048 inch. Do any of you modders have any insight?
The diameter of tube that plugs into this tube seems to be 3/4" (ie. the swaged upper end of the middle shaft, and the lower composite shaft), so the bore of the outer tube has to clear this dimension. My two main choices of wall thicknesses here in the U.K are 16 SWG ( 0.064" ) and 18 SWG ( 0.048" ). But 16 SWG would be an interference fit, so unless I did a bit of lathe work, that size is unsuitable. The 18 SWG wall thickness tube seems my best option.
Edit: I've found a UK supplier listing the 0.058" wall tube, describing it as 'telescoping', ie. to slide smoothly over 3/4" tube. This might be a better bet....
[www.antennaengineering.co.uk]
It's possible the Fisher tubing is intentionally a little 'sloppier' than it could be, to allow for mud/sand etc contamination, so maybe the 'telescoping' 0.058" tube isn't used? I'll try and borrow a 'points' micrometer, to get an accurate measure of my F75's tube wall size.

Also, would you know what alloy 'grade' the aluminium would be, eg. 2014 ? 6061 ? I would assume fairly high grade is needed, I recall there have been a few cases of the upper stem fatigue-fracturing, just in front of the handgrip (at the point where the holes are for the handgrip fixing bolt).



Edited 10 time(s). Last edit at 09/05/2013 12:28PM by Pimento.

I don't know what to make of the lack of response to my request, but, regardless, I carried on with my own investigation. A points micrometer showed the Fishers 7/8" tube to be 0.053" wall thickness. It seems odd that they would have a non-standard size, but I'm pretty certain it's not 0.050" - I have managed to obtain a short length of 7/8" tube with a measured wall thickness of 0.049" (It's likely to be nominal 18 SWG, 0.048" ), and that is a slacker fit on 3/4" tube than the Fisher's. I also obtained a sample of 1.00" tube with the 'telescoping' wall-thickness of 0.058". Inserting a tube of 7/8" inside it gave a fit that was firmer than the Fishers. These two comparisons back up the micrometer measurements.

However, despite the odd Fisher size, I think anyone considering home-building or modifying shafts should choose the 0.058" wall thickness tube that is specifically designed for a 'telescopic' fit. It seems to be used by antenna manufacturers predominantly. My first post listed a U.K supplier of small quantity orders, and here's one U.S supplier, DX Engineering:
[www.dxengineering.com]
The specific 7/8" tube being available in two lengths, 3 and 6 foot:
[www.dxengineering.com]
[www.dxengineering.com]
The clearance on the 'telescoping' tubes works out as 0.009" (0.23mm), so there's still a little play, but with a 2 inch (51mm) tube overlap, and a cam-lock, you should get a rigid-feeling joint.
Notice how both the Antenna Engineering and the DX Engineering listings describe the alloy composition as type 6063. This is fairly high grade, and is likely to match or exceed the original manufacturers specification in terms of strength.



Edited 4 time(s). Last edit at 09/11/2013 02:46PM by Pimento.

Pimento: I'm glad you figured out the tube dimentions. I've been thinking a lot on a straight shaft for my CZ6 and wasn't sure what to use. I read somewhere that someone has used an "elbow crutch" as a straight shaft. It's the crutch that has an arm cuff instead of the usual underarm style. I can find the underarm style around here at Goodwill stores and such, but not the elbow crutch. I would assume they are strong aluminium.

Re: elbow crutches. Some of my 'tube collection' includes elbow crutch parts. Unfortunately, they are not all made equal....one I had was made from seamed tube (sheet metal, rolled up and welded) so it's anyones guess what grade metal it is, it might not be all that round, and the telescoping is going to be poor unless you can clean up the weld inside, probably not too difficult for a 2 inch overlap joint. Also, they seem to be 1.00" and 7/8" tubes (even here in Europe). The 7/8" tube has nothing fitting inside it, so the choice of wall thickness is open, it could be 0.050", 1/16th", or something else. The 1" diam 0.058" wall tube mentioned in my second post was a crutch upper, looked a good quality drawn type.
If you are contemplating purchasing/obtaining a crutch, it's worth pulling off the rubber foot, just to see what the tube is like. And of course the lengths vary, one for a woman may have tubes too short to be useful.
Given the low prices for the proper tube (in the U.S anyway) I would avoid 'other' sources, unless they were obtained free.



Edited 1 time(s). Last edit at 08/28/2013 01:51PM by Pimento.

Medical crutches are not one size fits all rel diameter and a bit heavier than standard aluminum used for metal detecting rods..There are several commercial setups you might look into but again your call....

As I would need to remove/fit the Fishers' camlocks as part of my project, I had to work out a removal method. I've posted it on a UK forum, link below. The text should be understandable without the pics, but you have to register (free) with the forum to see the pics. I presume replacement camlocks should be easy to obtain from US Tek / Fisher dealers, at a price somewhat less than the 9.95 Pounds (15.50 US dollars) they cost here in the UK.
[www.metaldetectingforum.co.uk]



Edited 1 time(s). Last edit at 09/11/2013 02:42PM by Pimento.

I've bought a 72 inch (183cm) length of 0.058" wall 7/8" tube from Antenna Engineering. The points micrometer reads it as 0.057", so all OK there. I tried the 3/4" swaged end of the F75's middle shaft in it, and the fit was fine, a little less play than with the standard upper S-tube, but as there is only a 2 inch overlap, you can still feel some wobble. More interesting was the fit of the moulded carbon/plastic lower rod. This was much more snug, it was OK for 2 or three inches insertion (which is all I really intended using, as I'm tall) but if you start putting it in more, like 6 inches (15cm), the small clearance starts to show. Perhaps tiny bends in the tube become apparent? It will go in, but not as fuss-free as with the original 0.053" Fisher tube. I guess the moulded lower tube is just not as precisely made as die-drawn metal tubes, just 5 thousandth of an inch difference would be noticeable. If an easy fit of a moulded lower shaft was important to you, maybe choosing 0.050" wall tube might suit you better.
I also found the spring-locating pip scored up the inside of the tube easily. The aluminium cold-welded to the stainless button, making a little pip which then scored more when the shaft was removed. Not ideal, so I significantly weakened the pips by unbending the 30 degree bend they have on the non-pip length. This seemed to reduce the effect, maybe a spot of light oil, like silicone oil / grease would help here? I wonder whether brass pips would not do this?