# Material with a tight bend radius?



## sploo (20 Aug 2015)

More a materials rather than a design question...

I'm thinking of building a cyclone that's similar to one of Bill Pentz's designs, and would end up with a cylinder of about 60cm height and 50cm diameter. The cone section would be about 70cm tall, obviously 50cm diameter at the top (to match the cylinder) but only 20cm diameter at the bottom.

ClearVue make the cyclones from 3mm PETG plastic, but they do claim to thermoform, even though I understand PETG can take severe bends even at room temperature. Having never worked with it I'm nervous I might not be successful.

Galvanised steel sheet is the next choice, but again I'm not a metal worker.

Based on Table 1-2 in http://rgl.faa.gov/Regulatory_and_Guida ... r%2001.pdf, a ~2.5mm bendable ply should be OK for the cylinder (maybe laminating two layers for strength) but the cone is going to be a problem.

I could go crazy and cut a massive stack of rings on the CNC machine, but that would be really ugly (and timeconsuming). I could perhaps laminate several sheets of wood veneer, but that seems like hard work - especially without a 1:1 former.

Anyone got experience with PETG plastic, or could offer other suggestions for materials?


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## Beau (20 Aug 2015)

Kerf some ply?


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## beech1948 (20 Aug 2015)

Sploo,

Have a look at a web site called Cyclone Central. A UKL site set up by a member here.


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## sploo (20 Aug 2015)

Beau":1l6ycnfo said:


> Kerf some ply?


That's an idea. I suppose with sufficiently angled kerfs I could coax a sheet into a cone - especially if I wetted the uncut ply layer in order to try to make it less likely to crack.



beech1948":1l6ycnfo said:


> Have a look at a web site called Cyclone Central. A UKL site set up by a member here.


I wasn't aware of them - thanks. Unfortunately their largest model is too small for my uses. A cyclone that tiny would result in way too much static pressure to be used for the whole workshop. Pentz advises that a 16" diameter cyclone really needs a 5hp motor and a 17" impeller to overcome the losses. An 18" cyclone is good with a 5hp motor and smaller impeller, and a 20 or 22" should work with a 3hp motor. I also need 6" inlets.

However, Peter Parfitt's review is interesting as it shows just how flexible the plastic is. He mentions "Makrolon", and Google provides: http://www.par-group.co.uk/engineering- ... -makrolon/

I'm not sure what thickness they're currently using (maybe 2mm) but ClearVue use 1/8" (3mm) PETG for the 18" diameter units, so it all seems appropriate in terms of stiffness once made, but sufficient flexibility to form.

I think I'll have to investigate further!


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## beech1948 (21 Aug 2015)

beech1948":1fxknyqx said:


> Have a look at a web site called Cyclone Central. A UKL site set up by a member here.


I wasn't aware of them - thanks. Unfortunately their largest model is too small for my uses. A cyclone that tiny would result in way too much static pressure to be used for the whole workshop. Pentz advises that a 16" diameter cyclone really needs a 5hp motor and a 17" impeller to overcome the losses. An 18" cyclone is good with a 5hp motor and smaller impeller, and a 20 or 22" should work with a 3hp motor. I also need 6" inlets.

OK. So where in the UK or the EU are you going to find an impeller of 17"+ for sale. Some 2 yrs ago I looked exhaustively for such an impeller and could find nothing across the EU except a company in Poland who in the end would not sell to me because they were uncertain of my credentials and competence. I am certain I could build the cyclone cone and straight tube from metal, ply, fiberglass etc etc, 3 to 5 hp motors are easy to find but the impeller is a problem.

I even looked at importing one from the US, or rather a kit of parts inc motor and impeller, but the P&P costs + import tax+VAT+handling charges made it uneconomic.

Looking forward to seeing your progress.


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## sploo (21 Aug 2015)

beech1948":2okvl0ry said:


> OK. So where in the UK or the EU are you going to find an impeller of 17"+ for sale. Some 2 yrs ago I looked exhaustively for such an impeller and could find nothing across the EU except a company in Poland who in the end would not sell to me because they were uncertain of my credentials and competence. I am certain I could build the cyclone cone and straight tube from metal, ply, fiberglass etc etc, 3 to 5 hp motors are easy to find but the impeller is a problem.
> 
> I even looked at importing one from the US, or rather a kit of parts inc motor and impeller, but the P&P costs + import tax+VAT+handling charges made it uneconomic.
> 
> Looking forward to seeing your progress.


Absolutely - I have no plans to require a 5hp motor and a 17" impeller! I think the impeller in my Fox extractor is approx 13" (with a 3hp motor) so it's probably just about OK for a 20" cyclone. I spoke with Bill Pentz and he said you could get away with shortening the cone ratio (a 20" unit with a 1.64 ratio is very tall) as long as you accept you won't get such good fine dust separation. That's OK for me as I'm venting outside.

Matthias Wandel (woodgears) has some articles about creating impellers, so it's possible you could cook one up yourself. I'm not sure if his wooden designs would stand up to a 5hp motor though. Also, my unit is a "material handling impeller" - i.e. it's tough enough to pull debris without damage. Some are only suitable for being after a cyclone (i.e. moving air and fine dust).

Agreed on import costs - anything large/heavy/expensive gets very pricey to bring over here.

I've looked a bit more (and made some calls) regarding the plastics, and it does seem that PETG is probably the way to go. Not sure why Cyclone Central use Makrolon (polycarbonate) but it's clearly suitable. Certainly the info I got leant towards PETG over poly, as opposed to totally against poly, so it's not black and white.

I have found a datasheet for Polycasa PETG, which claims that a 3mm sheet can be cold bent into a 3mm radius with a 90 degree bend! I'm sure there will be stresses, but that certainly sounds like a 100mm radius would be fine. I'm told it'll be OK to cold form PETG, so no fancy thermal gear required.


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## rdesign (21 Aug 2015)

If you r making the cone yourself have a quick look at how to draw the development of a cone. This will help you mark it out on the plastic so it rolls to the right size for u! might be showing you how to suck eggs but if you don't know will make you life a lot easier after words. Technical drawing teacher/ nerd 
https://www.youtube.com/watch?v=HbWW_TQqloM

Regards Richard


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## MusicMan (21 Aug 2015)

+1 for Richard's suggestion. If the surface is developed accurately then making the cone should be easy.

If you do go back to plywood, it could be good to use the boatbuilding technique of butt-joining sheets - drill a set of holes near each edge and stick them together with copper wire. Thin GRP reinforcing tape then makes a good seal. But you will get a smoother surface with plastics, as long as you figure out the edge-to-edge joining 

Keith


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## sploo (26 Aug 2015)

rdesign":2zb6ahoz said:


> If you r making the cone yourself have a quick look at how to draw the development of a cone. This will help you mark it out on the plastic so it rolls to the right size for u! might be showing you how to suck eggs but if you don't know will make you life a lot easier after words. Technical drawing teacher/ nerd
> https://www.youtube.com/watch?v=HbWW_TQqloM
> 
> Regards Richard


Interesting thanks. Pentz has an Excel spreadsheet that gives you all the measurements and angles, so I have the data. But, if I didn't, then I'd just work it out by:

1. Treating the edge of the cone like a triangle - i.e. I know the height and the difference between the large and small diameters (the latter being the base of my triangle); so I can use Pythagoras to calculate the length of the hypotenuse
2. The hypotenuse gives me the radius of a circle (a portion of which will be the segment I need to form the cone)
3. The diameter of the cone I want to create gives me the circumference around the cone, and the outside curve on my segment needs to be that long
4. I know the radius of my segment (the cone hypotenuse), so I can calculate the circumference I'd get if the segment were a whole circle
5. Divide the circumference around the cone by the circumference from my segment and multiply the result by 360 and I'll get the angle of the segment I need to create

The above works if the cone ends at a point (i.e. the diameter of one end is 0). It will work for a cut cone (open at either end) but it's a bit more complex.


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## sploo (26 Aug 2015)

MusicMan":1congkxa said:


> +1 for Richard's suggestion. If the surface is developed accurately then making the cone should be easy.
> 
> If you do go back to plywood, it could be good to use the boatbuilding technique of butt-joining sheets - drill a set of holes near each edge and stick them together with copper wire. Thin GRP reinforcing tape then makes a good seal. But you will get a smoother surface with plastics, as long as you figure out the edge-to-edge joining
> 
> Keith


I wasn't aware of that technique. Though I'd worry that the dust inside a cyclone may damage the reinforcing tape.

The method I'd seen is to just bond a strip of plastic to the outside of the cone (or cylinder) in order to glue it together. Cosmetically not great, but the inside surface would be smooth, so that's all that matters to me.

I've ordered some small samples of polycarbonate and PETG, so hopefully that'll give me enough confidence to go ahead. An 8'x4' sheet of 3mm PETG would just about do what I need, and would be ~£100. Not cheap (so I wouldn't want to screw it up) but acceptable for the job.


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## MusicMan (26 Aug 2015)

Sorry not to be clearer. The copper wire is used to stitch, not stick, and the reinforcing tape as glass-fibre reinforced composite, which can be very smooth. Let us know how you get on with the PETC!

Keith


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## sploo (27 Aug 2015)

MusicMan":1rsvwr0g said:


> Sorry not to be clearer. The copper wire is used to stitch, not stick, and the reinforcing tape as glass-fibre reinforced composite, which can be very smooth. Let us know how you get on with the PETC!
> 
> Keith


Yep - understood you meant stitch with the wire. I just have a picture in my head of the tape being a problem if used inside the cyclone, but I've never used it so it might well be very smooth.


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