Ian Robinson
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Reged: 01/29/09
Posts: 1164
Loc: Gateshead.NSW Nth Coast,Austra...
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My OTA weighs 20kg , the PVC tube by itself (316mm OD, 6.5mm wall thickness and 1.253m long) with nothing inside it comes in at 11kg (based on a density of PVC tube = 1400kg/cum.)
There wasn't much gain in lightness if at all in going to a similar size rolled aluminium tube (aluminium density is 2710 kg/cum) and I thought I'd need a sheet thickness not less than 4mm to have a rigid and durable tube , this was why I opted for my OTA based on the PVC water pipe offcut , the price was right too (free !).
I am now dreaming of maybe having a go at making a DIYS carbon fibre tube but don't know what tube thickness would be suitable for a 10" astrograph with a OD = 315mm and similar length as above , so I can use my existing tube rings.
.
I googled carbon fibre epoxy composite tube properties (small tubes unfortunately) and came up with a density of 1540 kg/cum of composite.
So how thick should a suitable carbon fibre epoxy composite 3i5mm ID newtonian tube be ? Would 3 mm be enough ?
How many layers of carbon fibre cloth would this require approximately ?
Orion Optical's AG 10" astrogfaph OTA comes in at 12kg , yes I know it's shorter too (f3.8) , and they use CFC (carbon fibre composite which involves use of a vacuum in the forming process) .
I'd be happy to lighten up my OTA by about 5kg.
Edited by Ian Robinson (09/30/09 02:42 PM)
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PrestonE
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Reged: 04/29/05
Posts: 1170
Loc: Houston,Texas
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Hello Ian,
Depending on the weave and weight of the CF cloth we got about 0.010-0.015 inches of thickness per layer. I'd say from the parts that we made that a tube thickness of between 0.900-0.125 inches should be plenty. Or a tube of 0.090 inches with several reinforcing rings to build the thickness at both ends and where you mounting points will be made with unidirectional CF tape would provide the stiffest tube for the weight.
If you have other question, just ask. 
Best Regards,
Preston
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A few I enjoy,
and a few more in the works ;<)
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paul11
professor emeritus
Reged: 01/30/06
Posts: 576
Loc: Essex UK.
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Hi Ian I have just finished a 250mm x 1200mm C/F tube for a 200mm mirror I have made. This is my first go with C/F and I way over engineered it, I used a single layer of C/F then 3mm of coremat and then 2 more layers of C/F, the finished weight was 7lb .The tube is so strong I can stand on it. I am also going to make a 350mm mirror and a 400mm x 2000mm C/F tube to put it in, for this tube I am going to use a single layer of C/F then a 1mm coremat and then 2 more layers of C/F and I think this will be strong enough. Also I found C/f very easy to work with you just need a good working area. Here are some images of mine as it was being made,,,Paul.
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LX90-10" Canon 350D Toucam 840K Barn-door tracker
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paul11
professor emeritus
Reged: 01/30/06
Posts: 576
Loc: Essex UK.
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no2
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LX90-10" Canon 350D Toucam 840K Barn-door tracker
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paul11
professor emeritus
Reged: 01/30/06
Posts: 576
Loc: Essex UK.
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no3
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LX90-10" Canon 350D Toucam 840K Barn-door tracker
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paul11
professor emeritus
Reged: 01/30/06
Posts: 576
Loc: Essex UK.
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no4
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LX90-10" Canon 350D Toucam 840K Barn-door tracker
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paul11
professor emeritus
Reged: 01/30/06
Posts: 576
Loc: Essex UK.
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no5
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LX90-10" Canon 350D Toucam 840K Barn-door tracker
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paul11
professor emeritus
Reged: 01/30/06
Posts: 576
Loc: Essex UK.
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no6
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LX90-10" Canon 350D Toucam 840K Barn-door tracker
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PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1170
Loc: Houston,Texas
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Great Build Paul...
I had forgotten about Core materials
Very Good post 
Best Regards,
Preston
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A few I enjoy,
and a few more in the works ;<)
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Ian Robinson
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Reged: 01/29/09
Posts: 1164
Loc: Gateshead.NSW Nth Coast,Austra...
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Sound's very do-able.
Will definitely start looking around for materials locally and start accumulating info and parts.
I've never done any fibre glass work , how do you stop the wet epoxyed sheets from falling off the mandrel ?
Anything special about the mandrel material ?
How long did you allow between coats of epoxy and layers of fibre for setting / curing ?
What would my materials shopping list look like ?
Edited by Ian Robinson (09/30/09 11:16 PM)
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Ian Robinson
Pooh-Bah
Reged: 01/29/09
Posts: 1164
Loc: Gateshead.NSW Nth Coast,Austra...
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Quote:
Hello Ian,
Depending on the weave and weight of the CF cloth we got about 0.010-0.015 inches of thickness per layer. I'd say from the parts that we made that a tube thickness of between 0.900-0.125 inches should be plenty. Or a tube of 0.090 inches with several reinforcing rings to build the thickness at both ends and where you mounting points will be made with unidirectional CF tape would provide the stiffest tube for the weight.
If you have other question, just ask.
Best Regards,
Preston
3.175mm == 0.125 inches
0.09" (I assume this was what you meant ?) == 2.2mm
Assuming a density of 1540 kg/cum ,and a tube thickness of 2.2mm and OD = 315mm and L = 1.253m , this would give me tube mass of 4kg .... WOW !!!
Similarly ,if I built it up to 3mm thick , I'd get a tube of mass 5kg .... , that's still less than half my current tube mass .
Do those masses sound right ?
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JSeay86
super member
Reged: 04/26/09
Posts: 133
Loc: Norman, OK
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I'm rather new to this ATM stuff, but have some C/F layup experience for a Design, Build, Fly R/C airplane team.
We used aluminum mandrels when feasible and waxed them multiple times before laying up the C/F to assist with removal after the layup. I think a smooth mandrel surface is of primary importance.
We did a single layer cylindrical layup (about 1.5" in diameter) and let sit overnight to fully cure. We used a tube sheet of C/F and held everything in place with cheap vice grips after applying the mixed epoxy.
You can also get vacuum bag sheet material and sticky, putty-like material to seal the edges. Then use a mechanical pump to get to low vacuum at least. We used this on parts much smaller than what you mention though.
Heat is also helpful in the curing process.
I am by no means an expert so anyone correct me if I have said anything way off the mark.
Good luck.
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Jared
Orion XX12 Dob
Ten Acre Observatory
Edited by JSeay86 (10/01/09 03:06 AM)
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mak5
member
Reged: 03/09/09
Posts: 22
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Take a look at PrestonE's thread regarding the build of the 20" RC, it will help a lot.
My experience with vacuum consolidation and wet lay-up for tube manufacturing tells me that vacuum should be used only if you are certain that the fabric, peel-ply, perforated film, absorbant fabric and vacuum bag can be wrapped very tightly on the mandrel. If it's not the case, the final part will have a lot of creases. If you use a tube mandrel, it's even more difficult.
The stiffness of the carbon composite depends strongly on the volume ratio of fiber. If you follow what PrestonE did for the baffles of the RC, you will be fine.
A sandwich construction will be better, but you will be forced to use vacuum.
Try always to make the composite symmetric about the mid plane, otherwise you risk to have deformation of the part after curing or when applying heat and the part will have a lot of internal tension that will make it more prone to cracking.
Avoid mixing UD and fabric because this will increase free-edge stress. The holes for the flanges will increase this stress even more.
Avoid the contact of metallic parts and bare carbon fiber because of galvanic corrosion. Seal the holes' edges with epoxy after drilling.
You have a very steep learning curve in front of you. Don't be discouraged, when you will have done everything right, you will see it's worth it.
Try making some smaller tube samples first using fiberglass (it's a lot cheaper) and when you will master the technique, have a go at the big tube.
Edited by mak5 (10/01/09 04:14 AM)
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Gene Hunter
sage
Reged: 12/29/07
Posts: 288
Loc: SC
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My experience and I tend to overbuild is that if you use 3k Material then 5 layers is good and stiff. Stiffer is better.
you can get by with less if you have good end rings, but I prefer to lose the end rings and the tube rings and build a good stiff tube. It gets rid of a lot of other issues. you can always go for lighter, but you take the risk of having flex if you don't nail it.
I dont remember the exact thickness but I am thinking around .05" - .08" depending on the fiber material you use. If I get time I will go measure mine.
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Clear Skies
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Ian Robinson
Pooh-Bah
Reged: 01/29/09
Posts: 1164
Loc: Gateshead.NSW Nth Coast,Austra...
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Quote:
Take a look at PrestonE's thread regarding the build of the 20" RC, it will help a lot.
My experience with vacuum consolidation and wet lay-up for tube manufacturing tells me that vacuum should be used only if you are certain that the fabric, peel-ply, perforated film, absorbant fabric and vacuum bag can be wrapped very tightly on the mandrel. If it's not the case, the final part will have a lot of creases. If you use a tube mandrel, it's even more difficult.
The stiffness of the carbon composite depends strongly on the volume ratio of fiber. If you follow what PrestonE did for the baffles of the RC, you will be fine.
A sandwich construction will be better, but you will be forced to use vacuum.
Try always to make the composite symmetric about the mid plane, otherwise you risk to have deformation of the part after curing or when applying heat and the part will have a lot of internal tension that will make it more prone to cracking.
Avoid mixing UD and fabric because this will increase free-edge stress. The holes for the flanges will increase this stress even more.
Avoid the contact of metallic parts and bare carbon fiber because of galvanic corrosion. Seal the holes' edges with epoxy after drilling.
You have a very steep learning curve in front of you. Don't be discouraged, when you will have done everything right, you will see it's worth it.
Try making some smaller tube samples first using fiberglass (it's a lot cheaper) and when you will master the technique, have a go at the big tube.
Doing it as a home DIYS tube , I'd rather avoid complicating things by using a vacuum process, I don't think my old lab vacuum pump is up to it and don't want to have to by a new pump , or the extra consumeables required for vacuum tube forming.
I wasn't aware of the risk of getting galvanic corrosion where the metal bolts that hold the mirror cell and spider (stainless steel) and focusor (brass) contact the drilled holes.
I will make my holes slightly oversized oversized as is my usual practice when I drill bolt holes , allows a little adjustment room , so if I applied a bit of epoxy to the holes , I'd just have to make a little bigger again.
Or
Maybe a bit Singer machine oil on the treads provide sifficient barrier.
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Ian Robinson
Pooh-Bah
Reged: 01/29/09
Posts: 1164
Loc: Gateshead.NSW Nth Coast,Austra...
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Quote:
My experience and I tend to overbuild is that if you use 3k Material then 5 layers is good and stiff. Stiffer is better.
you can get by with less if you have good end rings, but I prefer to lose the end rings and the tube rings and build a good stiff tube. It gets rid of a lot of other issues. you can always go for lighter, but you take the risk of having flex if you don't nail it.
I dont remember the exact thickness but I am thinking around .05" - .08" depending on the fiber material you use. If I get time I will go measure mine.
How does this sound then :
mandrel
then layer 1 = unidirectional 3k CF longitudinal (direction of fibres)
then layer 2 = unidirectional 3k CF wrapped around (direction of fibres)
then 2 layers of plain weave 3k CF
then 2x2 twill weave 3k CF to finish
That's only 5 x 0.225mm = 1.13mm thick .... !?!
I'd feel a lot more confident with 2 layers of unidirectial 3k CF laid as layers 2 and 3 , and an extre layer of plain weave 3k CF before my final layer of 3k CF = 7 layers = 1.6mm thick.
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Ian Robinson
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Reged: 01/29/09
Posts: 1164
Loc: Gateshead.NSW Nth Coast,Austra...
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Someone at IIS http://www.iceinspace.com.au/forum/showpost.php?p=503298&postcount=18 suggested using a Protostar BlackLite tube, said they were very pleased with their's that they used in the 10" f3.8 astrograph as their tube structure.
The OD for the 12" tube they have is 299mm , their wall thickness is 3.6mm , their tube mass = 27g/cm ==> a 1.27m tube will be 3.43kg (that's pretty good !!).
Their modulus of elasticity is given as 1400 PSI.
Being preflocked is a definite plus, that'll save some money as I'd have to buy another lot of self adhesive Protostar flocking material and go through that process again once my CF tube is made , holed and drilled.
Has anyone used a Protostar BlackLite tube for their stand alone tube for a 10in or bigger newtonian here and checked how well it holds collimation without an outer structural tube to reinforce it ?
Proposal : Hybrid CF overlayed BlackLite tube.
Buy a Protostar BlackLite tube
add a layer of 3k unilateral CF (long wise)
then a layer of 3k unilateral CF wrapped with fibres rounding around the tube)
finish off with a later of either plain or 2x2 twill weave 3k CF
with aim of stiffening up the structure of the tube ....
Pros :
**** no need to make a disposable mandrel , the CF will be permanently bonded to the BlackLite tube.
**** mo need to buy another roll of ProtoStar SA Flocking and do the flocking process again.
**** the tube will be significantly lighter than my existing 300mm NB UPVC stormwater pipe offcut as the BlackLite tube weighs 3.4kg and the added 3 layers of epoxy bonded CF cloth will add 1.3kg = 4.7kg for the new tube (UPVC is 11kg).
Cons.
**** my old AstroSystems spider will not fit the BlackLite tube , not a big deal , spiders are cheap.
**** will have to check my tube rings are OK with a 300mm OD tube ,designed for a 316mm OD tube , may need to modify them, no big deal.
**** unshore how well epoxy resin will bond with the outside surface of a BlackLite Tube ? needs to be a good permanent bond.
**** cost of shipping a tube from Protostar to Australia may be prohibitively expensive and a proposal killer.
**** will have redo my OTA design calcs based on the focusor being 8mm closer the OTA centreline (well just shift the location of the mirror cell back 8mm) , no big deal.
Question : anyone tried a hybidised CF coated Protostar BlackLite large diam tube ?
Edited by Ian Robinson (10/07/09 01:48 AM)
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Ian Robinson
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Reged: 01/29/09
Posts: 1164
Loc: Gateshead.NSW Nth Coast,Austra...
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I finally received a response from the only company in Australia who showed any interest in making a 314mm OD x 1.27m long x 1.7mm thick all 3K carbon fibre tube .... they estimated $1000 , so scrub getting them to make my tube , way too expensive.
Protostar are prepared to ship but the shipping of a 300mm OD x 1.27m long BlackLite tube will be murder , they think hundreds of dollars (US) .... so unless a local astrogear retailor decides to start importing multiple ProtoStar Blacklite tubes , a Blacklite tube is not a viable option , nor is making CF reinforced BlackLite tube (for now) .... ashame , it would probably be a great solution .
Postal limitation on length are the big obstacle.
I've asked Vixen Japan how well the New Atlux servos will cope with my existing OTA provided the RA and DEC axes are very close to perfectly balanced.
I'm within the OTA load limit , but still no harm in asking their engineers.
So I'm going to have to source my raw materials ,
Edited by Ian Robinson (10/08/09 07:51 AM)
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Ian Robinson
Pooh-Bah
Reged: 01/29/09
Posts: 1164
Loc: Gateshead.NSW Nth Coast,Austra...
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Paul , what's the white layer you put on the mandrel ?
Edited by Ian Robinson (10/08/09 07:57 AM)
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PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1170
Loc: Houston,Texas
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Hi Ian,
Just make you own CF tube and then worry about the Blackening of the ID...
If you cover the poly on the tube mold with a dacron cloth which is really what peal ply is than you will get a mat finish when you peel it out of the ID of the finished tube...
Then you can experiment with differant flat spray paints to see which one and what technicque works best to reduce reflections at low grazing angles...it took us about 25+ trials to get the right paint and technique...
If you read through my 20inchRC thread I discribe what worked for us in great detail...
Best Regards,
Preston
--------------------
A few I enjoy,
and a few more in the works ;<)
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