Machining fiberglass tubes

Hello, I have very little experience in machining, mostly aluminum and wood.

My team is making a ground mount for an antenna on a tall building for which we are using fiberglass tubes. We want to make a telescopic mount using 3 similarly sized fiberglass tubes (1.5in OD, .25in thick).

So my question is;
1) what is the best way to turn a fiberglass tube on a lathe to shave off some mm in thickness?
2) we also need to drill some holes into the tube, any tips?

I have 0 experience with fiberglass machining as I said, so any tips are appreciated.

Thank you.

If you are mounting something on a tall building, it MUST be safe from causing injury if it comes loose and falls off due to wind or earthquakes. Are you adding secondary arresting elements, like steel cables as safeties?

Composite tubes like fiberglass or carbon will lose strength when they're turned, especially if experiencing a bending load. If at all possible when using composite tubes in a mechanical structure, adapt the other elements (sockets, etc.) to fit the stock tubes. Holes, when not too large, are less of an issue but still create noticeable weak areas in the walls of the tubes.

Please tell us more about how you're designing these mounts.

Hi, we are a university student team, however we are working with a professional structural engineer to ensure we take all safety measures into consideration.

That is our current design fro the antenna mount, however the big rod mast will be converted into a telescopic mast.


Like this, most likely going with the 3in option. To make sure the tubes fit into each other smoothly, we just need to turn it down about 1 mm. Another option is to just sand a little bit of the length that will be sticking into another tube down by 1 mm.

IME glass fibre tubes were about the worst material I've ever machined, .........no where near as stiff as you think, so very prone to vibration, absolutely filthy and of course ATE cutters.

As for your telescoping design IME glass fibre on glass fibre tends to stick, so unless you can live with lots of clearance (sloppy joints) I'd look at making the sockets / sliding elements out of a different plastic material like nylon PP, PTFE or PE etc etc (but I'm not making any material recommendations)

why the tie downs are arranged in that way? they should be from the furthest point of the base, not the shortest, and the point where they attach the central mast would preferably be the top collar of the larger tube - it seems it would be near middle of the bottom tube now

don't turn the od, grind it instead, with an appropriate dust extraction

is glass fiber the choice because of the non-magnetic properties?

p.s. loss of strength will be proportional to the reduction in the tube thickness, because the fiber layers are concentric and the fibers have no radial directionality, unless that is some very sophisticated laminate, which I doubt would be the case

I 100% agree with Slimy Ami. Fiberglass is nasty stuff to machine and large operations like turning or grinding down the entire tube length just sounds wrong approach.

Do you have to engineer this yourself or can you use ready-made telescopic fiberglass poles?
https://mgs4u.com/fiberglass-push-up-masts/?v=f0aa03aaca95

40 feet pole is about ~200 usd

I agree with many of the points made by the above folks, but have a few more to add:

1) Why change from a monolithic pole? It's simpler, doesn't add weakening joints, and properly stayed (guy-wired) will be as stiff as a larger, jointed pole.

2) If antenna performance interference is a reason for the lack of multiple levels of guy-wires, upper stays can be a composite/polymer rope. More stay levels is much more stable, and better at handling wind gusts.

3) That large an antenna will be subject to torsional wind loading, you should do an analysis of likely frequency modes of twist and the stress they will induce in the rotator and mast. Does your university have an Aeronautics department? If so, ask them for advise, perhaps in conjunction with the ME department.

4) If you must go to a segmented mast, is it intended to be raised up and down, or is there another reason? As mentioned, there's a lot of friction between ID and OD, that both increases loads and also wear particle generation.

5) My biggest concern with a segmented mast is the chance of contact forces being great enough to induce cracking when the mast sways under heavy winds, especially if not stayed at high enough levels. You've got quasi-point-loads where each segment touches, and with gaps between them the loads increase and "wobble" exacerbates the effect.

Without a specific series of layers of a tougher composite weave (like Kevlar) added, glass and carbon fibers have relatively low shock load tolerance, and will crack abruptly if overloaded. This must be considered during design and construction. Again, why I'd prefer a single-piece mast.

DO NOT Breathe the dust, it can kill you in 20 years...Phil

Phil in Montana said:

DO NOT Breathe the dust, it can kill you in 20 years...Phil

Click to expand...

That means I have cheated death, by at least a few years.

Yeah, it's best not to breath it, but it's not insta-death either.

Look at the ham radio forums as this is what they do.

Come back to determine structural

Sent from my SAMSUNG-SM-G930A using Tapatalk

Drilling fiberglass. Use a stone parallel to the axis of the drill bit and stroke the cutting edge a few times until you have removed the 'hook' producing a small flat. This stops the drill from grabbing and self feeding and making an awful mess. Works great in any soft material, like copper, plastics of all kinds and also on materials that chip easily on the back side.

HetNeedsHelp said:

Hi, we are a university student team, however we are working with a professional structural engineer to ensure we take all safety measures into consideration.
View attachment 295358
That is our current design fro the antenna mount, however the big rod mast will be converted into a telescopic mast.

View attachment 295359
Like this, most likely going with the 3in option. To make sure the tubes fit into each other smoothly, we just need to turn it down about 1 mm. Another option is to just sand a little bit of the length that will be sticking into another tube down by 1 mm.

Click to expand...

Looks like something from "Mini Products, Erie, pa"

EDIT:
OOooh...now made in Canada eh ?
(Scroll down to the vhf/uhf stuff)
Home

garyhlucas said:

Drilling fiberglass. Use a stone parallel to the axis of the drill bit and stroke the cutting edge a few times until you have removed the 'hook' producing a small flat. This stops the drill from grabbing and self feeding and making an awful mess. Works great in any soft material, like copper, plastics of all kinds and also on materials that chip easily on the back side.

Click to expand...

AKA dubbing the edge.

Limy Sami said:

AKA dubbing the edge.

Click to expand...

I thought it happened auto-matic-ally the first second into the hole....