Need a bit of help ?

[Tigman]

I've got a bit of tube 355.6mm in dia I've found the circumference using C = pi × dia
Circumference is 1117.29 mm

So my problem is I want to make a strap to wrap around with 3 holes in to act as a drilling guide , like a big jubilee clip !

If I get a length of flat material that wraps around and drill 3 pilot holes at 1/3 of the circumference ( 372.43 mm ) this should work !
I've marked a bit of plastic and tryed it , doesn't work

Any thoughts would be most helpful
Fag packet drawing

 

[Redsailor]

The two ends of the plastic should also add up to 372.43, so you have 1117.29 / 3

 

[indykid]

How thick is your plastic you're wrapping round? It will bend around the centre of its thickness so it's this radius/diameter you need to consider. If it's 2mm thick, the effective diameter is 357.6mm making the circumference 1123.4mm

A simpler method would be to make a slightly longer piece of something, plastic, whatever, wrap it round the tube, cut it to length then mark out the thirds on the strip. Also at that diameter and with your given technique, do you really need to be working to 10 microns? If so, you'll need to reconsider your technique.

 

[Tigman]

I figured that , when the plastic is bent around the tube , the inside circumference of my plastic would be the same as the circumference of the tube, not working to any real tolerance but need to supply this strip for others to drill tube on site , they will drill a larger hole than is required 2mm larger but need to be near the mark , just doesn't workout when I'm doing it here !

 

[addjunkie]

I figured that , when the plastic is bent around the tube , the inside circumference of my plastic would be the same as the circumference of the tube, not working to any real tolerance but need to supply this strip for others to drill tube on site , they will drill a larger hole than is required 2mm larger but need to be near the mark , just doesn't workout when I'm doing it here !

Use a sewing tape measure…

https://www.amazon.co.uk/Measure-Suitable-Measuring-Sewing-Inches/dp/B08465QB2T/ref=asc_df_B08465QB2T?mcid=540582cefea43cc3b81a62c501084304&tag=googshopuk-21&linkCode=df0&hvadid=697266291530&hvpos=&hvnetw=g&hvrand=13525901450492556958&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=1006794&hvtargid=pla-879435163406&hvocijid=13525901450492556958-B08465QB2T-&hvexpln=0&gad_source=1&th=1

Or

https://www.screwfix.com/p/komelon-stick-flat-2m-tape-measure/163GM?gclsrc=aw.ds&gad_source=1&gad_campaignid=21550393591&gbraid=0AAAAAD8IdPx6B8l8VsTUeilRnJtzRwx1H&gclid=EAIaIQobChMI24SVo4fmkwMVoZFQBh1UuTXTEAQYBiABEgJH2_D_BwE

 

[Agroshield]

Any thoughts would be most helpful

Could you say how it is not working out?

Do the ends of the strip not butt togther? Do they overlap?

Cut a piece of paper 372mm long and use it as confirmation of the plastic strip.

I do not want to complicate matters, but does the clock position of the holes in the tube matter? Do they have to be in some relation to some other feature of the tube (and perhaps avoid the seam in it)?

Murphy guarantees your site worker will align the template onto the seam and then have problems drilling the hole.

 

[Tigman]

That would work if I had the tube to measure , I only have measurements and a plate cut to a circle to measure against !

 

[bricol]

Can't immediately see why that wouldn't work - do the ends of your bit of plastic meet up when wrapped around? And have you really marked it out at 372.4? Mis-measurement, especially if I'm doing it, would be my first check.

 

[Tigman]

I have got it now I think , put the plate on the rotary table marked off the degrees 120 ° and have bent plastic around to mark holes , I'm within a millimeter so should be good enough, just got to find a decent bit of material to use !

 

[Tigman]

Can't immediately see why that wouldn't work - do the ends of your bit of plastic meet up when wrapped around? And have you really marked it out at 372.4? Mis-measurement, especially if I'm doing it, would be my first check.

I marked the plastic then wrapped around this sheet circle that I have and it came out 5 to 10mm out , checked measurements of course ,but not having it , I've cracked it now should be OK thanks

 

[bricol]

I marked the plastic then wrapped around this sheet circle that I have and it came out 5 to 10mm out , checked measurements of course ,but not having it , I've cracked it now should be OK thanks

Checked the plate for dia and circumference? No good if your go/no go gauge isn't correct.

 

[indykid]

I figured that , when the plastic is bent around the tube , the inside circumference of my plastic would be the same as the circumference of the tube, not working to any real tolerance but need to supply this strip for others to drill tube on site , they will drill a larger hole than is required 2mm larger but need to be near the mark , just doesn't workout when I'm doing it here !

I don't think you understood my point.

The inside of the plastic will have the same circumference as the tube, but the outside of the plastic will have a circumference 2*pi*its thickness greater than the inside of the very same piece of plastic. The inside surface compresses, the outside surface stretches - only the centre line remains the same length as when you flatten it out. That's why you need to use the centreline of the plastic for your calculation, not the circumference of the surface of the tube.

Using your rotary table, you've circumvented the calculation, but measuring between your holes now, I would have money on them not being 372.4mm apart.

Crucially, whatever material you use for your more robust version needs to be the same thickness as the plastic, or you need to do the rotary table bit with your more robust material.

 

[Misterg]

I figured that , when the plastic is bent around the tube , the inside circumference of my plastic would be the same as the circumference of the tube,

True when it's bent, but not when it's laid flat. Unless the strip has zero thickness, the inside face will shrink and the outside face will stretch as you wrap it around something. To mark it out when flat, you need to work on the circumference at ~1/2 the strip thickness* on top of the tube radius. (Which is ~ pi x the strip thickness longer than the circumference of the tube.)

e.g.: If you had a 3mm thick strip, the length needed to wrap around your 355.6mm diameter tube would be 1126.6mm (or pi x 3mm longer than the circumference of the tube - about 9.5mm).

*Probably not exactly half, but pretty close.

 

[Tigman]

Right I think I get the idea now , when I drilled the first tube I managed to get it right with a practical method no science , I then reversed it and marked a strip through the tube to give a template , this time I have no tube but have to supply a template for the lads on site , I will apply your method tomorrow !

Many thanks for all the input