Carbon tube for 10" OO mirror

[Black_hole]

Your doing a great job, I can't wait to see the finished article. What kind of time frame are you giving yourself?? 

Edited September 19, 2019 by Black_hole

[Robert72]

Hello, thanks for the question.  The time frame is open, as my sessions are somewhat limited.  I would like to get it complete this year, but don't know if this is realistic. We shall see.

[Robert72]

Some better pics of the inside as promised.  Here you can have a better look at the inside surface finish.  I have worked out why the split lines are a bit gappy, it is because the tube needs to be put into slightly more compression than the bolts alone can provide.  The moulds are pulling in, figure of eight style, so the flanges are not entirely parallel. Not sure why this is, I will take this into consideration for future moulds. Some wooden compression frames should compress it back, and act as a supporting structure for the upcoming stand.

Side view.  Not too interesting, but gives an idea of the flanges......

 

 

This one gives a better view of the inside overall. Finish not too bad.....

 

Here is an idea of the, now male, scratches.  Fairly distinctive and will pass through to the final I think, however I did a bit of sample buffing, and I am fairly confident that these will buff out and polish up without too much effort (where have we heard that before!).  Just shows how faithfully the gelcoat picks up the master......

 

Note the voids in the flange.  Not too bothered about these at all.  There were only a couple of small voids in the important surfaces.  Note the light patch below the flange with minimal gelcoat coverage, light shines through this.  Must have missed it a bit?.......

 

One of the small holes filled with gelcoat, will need a bit of rubbing down......

 

I was a bit troubled when I initially pulled the mould, but now that I have had a chance to clean, inspect, and do a bit of exploratory work I feel a lot better about moving forward.  Things are looking good for the eventual tube production I think.  Next step is to finish up the mould (fill holes and buff and polish).  After that it is a simple matter of making a simple timber compression frame and infusion stand.  And then after that I can finally move onto infusion, really looking forward to it.

As usual, any questions or comments, feel free to post.

[Rusted]

Looking good. External plywood bracing would be really useful to solve the figure-of-8 problem.
GRP moulds are usually heavily reinforced and glassed over to maintain stiffness during shrinkage and final removal of the item.
The problem then is flexing the mould enough to release the item.
Which is, presumably, why they often use compressed air for separation after previously using the same vents for vacuum forming CF.
In your case you could have temporary formers in a framework bolted to the outside of the mould.
Have the formers in two units so they can be easily separated at the parting plane.

Edit: I meant to add that removing male scratches is far easier than female. With the latter you have to reduce the entire surface.
With male scratches they are raised above the general surface so only the "scratches" need to be removed.
The problem is that any male projections will be carried over to the finished article. i.e: female scratches and dents must be filled.
Or the whole surface must be reduced. A bit like producing optics. A tiny scratch needs the whole surface to be worn away.

Edited October 1, 2019 by Rusted
Additional info.

[Robert72]

Thanks for your continuing support Rusty, and all the others enjoying the posts.  Yes, external formers are the answer.  I think I may try a carbon/epoxy mould next time, perhaps with foam cored structural elements?

I know all about whole surface reduction from that pesky tube pattern! Grrrr! That took ages.  Being optimistic, I think I may be able to eliminate these scratches and do a basic cut and polish in a weekend.

End threaded Al extrusion and box section/channel compression pieces are going to be the answer to compressing the mould properly round/shut prior to layup I think.  Looking forward to continuing at the weekend, I will get more pics up then.

[Robert72]

So the flattening has commenced. Started with 400 grit wet (wet, circular motion, 3 sessions), followed by 800 (wet, circular motion, 2 sessions), followed by 1200 (wet, circular motion, 2 sessions).  Attempted polish with Topfinish2 and medium/soft blue pad in DeWalt polisher, but ended up with deep scratches.  Also have attempted NW1 cutting compound with medium/hard orange pad, still no cigar.

I have ordered 1500 and 200 grit, I will start again at 800 grit and go through 1200, 1500 then 2000. And then probably NW1.

Any other suggestions? You can clearly see the scratches in the pictures, the phone camera picks them up really well. In the third picture, you can see how rough the surface is in the light section. One problem is that the entire mould is concave, but my pads are flat.  Are there hemispherice pads available anywhere? I have posted this question on the TalkComposites forum also, to get the advice of as many people as possible.

Edited October 6, 2019 by Robert72
Text removal

[Rusted]

I would suggest using PVC pipe as a backing for glued-on abrasive paper but you are using power not manual labour.
Are you using a lambswool polishing mop?

NW1 sounds ideal but your technique or tools may be the problem. T-Cut used to get a good press in car circles but that was years ago.
I rubbed an entire GRP kit car down to the gel coat through multiple layers of paint once but I wasn't looking for a polish. I just used an orbital sander.

When polishing metal I used multilayer, cotton disks used on edge like a grinding wheel.
These are as old as the hills for power polishing for brass, chrome and metal plating.
The power and high speed would stiffen the disks into a solid mass. The curvature would ideally suit your needs used longitudinally.
Polish was applied from special wax blocks. But you'd need expert advice on what to use on epoxy resin.
One false move and you could be through to the core material.

Edited October 6, 2019 by Rusted

[Robert72]

Thanks for the reply Rusty.  I am using sponge pads, as they have reasonable thickness - I have to hold the polisher at an angle inside there as you could imagine to get a reasonable surface contact.

I have one of those polishing kits for my bench grinder with the polishing bars.  I can't really remember what the spindle is like, but I am sure it will only fit a bench grinder without some kind of adapter for the polisher.

But yes, you may be right about the orientation and the polishing head.  I will continue to search for a disk that has the mop on the circumference.

 

[Alan White]

I take it that the sponge pad is very soft and not abrasive?

Just wondering if as a Rusted suggests it’s the technique as well as the power?

[Robert72]

Hello Alan.  The sponge pad is what they describe as medium/soft.  I just dont get on with them.  I dont like the "stickiness", they seem to "grip" the material as it were.

I have had much success with mops for polishing Aluminium in the past (hats off to Rusty for that too) so I posted a question to thepolishingshop.co.uk, and got a reply a few minutes later:

"Important to sand to around 1500

Then

 

Stage 1 - Glosswax 16 with a G quality mops (coolair or standard construction)

 

Stage 2- P175 Black with a WDR quality mops (coolair or standard construction)

 

IMPORTANT- Run Coolairs slow ie 1000-3000 rpm max, standard mops can be run faster but would still recommend 1000-3000rpm max. Please dont run at angle grinder speed, mops are rates to only 3000rpm

 

Its a standard process for a gloss finish, good example are bath companies as they are often sanded down then polished with this method.

 

I do have a liquid gelcoat system that would work, it comes down to if you want to use Pad or Mop as I am sure we can get the results on both. I think looking at the shape mop/wheel based would be better.

 

Best Regards Andrew"

 

They also have polishing machine taper adaptors.  I didn't see them initially but they are there.

Will place an order now and them have another crack this weekend.  Also have 1500 and 2000 on the way. Now that all the dusty, dirty work is done the project is now in the conservatory, so no weather worries.

Will keep you updated.

 

[Robert72]

I have buffed up the mould as recommended by Andrew at thepolishingshop, and seems to work well with the mops.  So I have bolted the halves together to rub down the split.  I have attached a picture of the inside.  It shows the splits, and you can see how rough it is at points (this is the worst bit).  Do I try to make the split better, or I am I worrying about nothing?  I just don't like the roughness of it.  I would imagine at worst it will mean buffing a bit of resin off the finished tube, which isn't a deal breaker.  Any recommendations?.....

[bottletopburly]

Are you using gel coat on tube ? If so give the joint two or three coats of gel  finished article will only be good as the plug so if there’s any scratches it will be mirrored in final product , I have to say your doing a splendid job , at work when the moulders do infusion they use a spray glue on dry glass to keep in place .

[Robert72]

Hi bottletop, thanks for the quick reply. No gelcoat, plain carbon! spray adhesive, can affect the outside finish (clouding I think), unless it is used on the inner laminates, or using with gelcoat.  One option is to do a light spray of resin, followed by a light spray of acetone to act as an "adhesive".  However, I am going to experiment with monofilament "webbing" to hold it in place.

[Robert72]

Quick update. Have been buffing down and gelcoating (and again, and again....) the splits to close the gaps. So they are a lot better now. I have included a picture. It looks a bit crap because of the patches of fresh gelcoat, and the buffing slurry that has fallen in the gap.  Looks a lot better here though. The max gap is about 0.7-0.8mm, but now very consistent and quite straight.  I think I am happy with it and will just do a bit more 400 grit before finishing off the polishing....... (Edit: This picture makes it look quite oval. Ignore that, I think it the aspect ratio of this camera in portarait mode that is out).



Edited October 19, 2019 by Robert72

[Alan White]

Looking good now, worth the effort.
How long until you start moulding?

[Robert72]

Moulding will commence in November.  Hoping to get one made up before December. Ideally, I would have the scope complete before Christmas leave, but I wouldn't bet my xmas dinner on it.

[Robert72]

I have got to the point where I really should start considering the design of the finished article.  But I don't really know anything about telescopes, having not owned one before.  To that end, I have put my values in to Newt for the web, to start sanity checking.  Can anyone confirm my "spare focuser in travel"?  I have it set to 14mm, for a parfocal group B TeleVue with a Baader steeltrack focuser with 40mm travel.  Does this sound correct?

Edited October 24, 2019 by Robert72

[Robert72]

Thought I better get some miscellaneous items done while the weather was excellent, so just a bit of an update for this weekend....

On the left, a little infusion stand.  Just enough to raise it off the deck for feed pipe access.  I was going to make something more elaborate, but I think this will do.  The mould should stay upright even with the vacuum bagging items.

On the right, a roundness gauge to check the mould when buttoned up, prior to the layup.........

 

[Rusted]

As we roll into winter.. have you considered the minimum temperature advised for your resins and materials?
Or can you work indoors where there is some heating?

[Robert72]

Yeah, good question.  Pretty much everything is done indoors now.  That's why I wanted to get that stuff done today while I had the chance.

[Robert72]

 

Nice and shiny......

 

I think I am calling it a day with the split lines......

 

Note the gap, until they halfs are pulled in.  This is because of......

The flanges are not parallel, despite the fact that they were moulded against each other.  Probably due to the fact that there was no post cure and other reasons that were discussed in earlier posts.

However, I have added repair washers to provide a bit more closing leverage.....

 

That's better.......

A little bumpy along the splits, but not too bad. A little post buffing will be in order.

 

Infusion gear ordered, but they are out of stock of one of the outer cloths I am after, so i'll have to wait for that.  Next step - make a degassing chamber.

[Robert72]

 

After a quick rummage around in my mum's loft I found this......

 

Anyone got any 20mm clear acrylic lying around?

[Rusted]

That is going to make one hell-of-a Christmas pud!

Can't see the relevance of the 20mm acrylic though?

[Robert72]

1 hour ago, Rusted said:

That is going to make one hell-of-a Christmas pud!

Can't see the relevance of the 20mm acrylic though?

For the degassing chamber lid!  I want to see whats going on in there.

[Rusted]

I thought it might be. Polycarbonate might be safer.

eBay[UK] has stockists for up to 12mm [or more] if you can find anything big enough for that huge bowl!