Member postings for Craig Smith 3

Hi Daniel

Okay, you are into it. I'm sure the PE will do the job. Seeing the boat moored like that takes me back to my youth. No sailing anymore, but now I spend a lot less money as well.

You mentioned about the top and bottom bearing/bushing of the rudder stock. Just keep in mind that the bottom one is the fulcrum and takes most of the load, so any slop down there is not good. If either is to be an interference fit, it should be that bottom one. The top one can handle a small amount of tolerance if needed to ease things up.

Once made, let us know how it went. Just as a matter of interest, where is the boat moored in that photo?

Craig.

Hi Daniele,

Like always, a picture says it all. I wondered if you were talking about small bearing inserts like in a dinghy rudder gudgeon or as your picture shows, larger bushing for the rudder shaft. You did say bushes, but best to check.

Hard to be certain, but the way the break has happened it looks like a form of Acetal. Delrin/Acetal tends to be quite hard and boney , if that is even a word (like bone), so it breaks cleaner than a lot of other plastics, which stretch and deform before they and break.

Frank has mentioned it a couple of times and I think he is spot on. UHMWPE (you can see why they use the acronym) is a great alternative. Doesn’t machine as cleanly as Delrin, but it machines well enough. It’s has good dimensional stability in moisture and great resistance to abrasion. And like Frank says, CHEAPER, especially as you have to buy some quite large diameter solid rod.

I used it a bit in the past and I remember one time in particular I used it as end plugs in a spinnaker pole. I made them a neat fit and even after five years in and out of water and UV, the plugs never swelled and always slid in and out without issue.

Hope it goes well.

Craig

Hi Vic

I use all kinds of adhesives every day and so this topic is one I think about regularly. Superglue does well in the freezer and can stay there for years. The freezer is probably the driest place in most people’s homes. Any of the adhesives that go off when moisture creeps in would probably do well in the fridge. Two-part polyurethane casting resin is one of them and I would think polyurethane glues would also fall into that category.

Most paints and adhesives in their unmixed state (Part A to part B. Resin and hardener) aren’t so much temperature sensitive, but instead are moisture sensitive and so the fridge being a dry place (normally, as long as the kids haven’t gone feral with other liquids and there is space) can be a good place to store not all, but for the most part many of the non-volatile stuff from your workshop as long as it’s really really well sealed! I have superglue as well as a two-part acrylic glue used for joining Solid Surface bench tops stored in our fridge.

Usability is another topic. Once out of the fridge they need to come back up to a usable temperature. I once saw a guy in a lab microwave the superglue bottle for about 4 or 5 seconds to quickly get it back to room temperature. Too much may cause some issues I imagine. Epoxy hardener can crystallise when stored for extended periods in the cold, but comes back to life without any detrimental effects once warmed up.

Hi Daniel,

Can you tell me what size is your sailing boat? Dinghy, Trailer Sailer or bigger?

I am a boatbuilder by trade and rigging boats ready to go sailing was my business for many years. I think most of the materials the other guys suggested will do the job and yes Nylon tends to cause issues. Might be able to minimise the issues using Clive’s suggestion, I don't know, but with other stuff readily available and easier to machine, I would go for something else.

Frank is correct, rudder bearings are quite often made from Acetal or Delrin, which is the trade name of one of the common forms of Acetal. It is easy to machine and is hard enough that it can be polished up if required.

Depending on the size required, it might just be faster and easier to by some off the shelf as suggested.

Craig.

Cont...

Instead of using an epoxy Gelcoat, it’s even possible to spray an epoxy undercoat like Dulux Luxepoxy or a two-pack polyurethane directly onto the mould surface. Then add the tie past layer which protects the surface from aggregate attack.

After the epoxy moulding has cured and different epoxies require differing curing processes, I apply a cosmetic fill to the rough B surface as John described using a car filler. Personally I use a mix of a faster curing epoxy resin and glass spheres, which is normally sold under the trade name Q-Cells. I will throw in a bit of Cabosil or talcum powder to toughen it up as the Q-Cell mix on its own can be a bit light and fluffy and when spread can have trapped air bubbles. By adding a bit talc or Cabosil it smooths it out which helps it to spread better.

The advantage of using this mix is that it sands very easily without clogging the sandpaper, and you can even plane it with a hand plane. It doesn’t have the shrink back of a large mass of car fill, which normally uses a polyester resin as its base.

Not using a tie layer and just going with the main mix against the mould is certainly quicker and in all uses less epoxy resin, but it does require more finishing if that is important to you. Also the larger aggregate mix is more likely to crack if knocked than a more resin-rich mix with the finer sand. On smaller parts I will even substitute the sand with silica flour. This is ground up silica (basically sand) looks just like talc, but feels a lot more abrasive to the touch and is a lot cheaper to buy. It can be mixed in with epoxy at a ratio of five parts flour to one-part epoxy and makes a much denser and stronger mix. Another thing I have done to toughen up and strengthen the initial sand/epoxy layer is to add some chopped glass fibres to the mix. These fibres are about 6mm long and make the resulting mix incredibly strong and a lot more resistant to cracking if bumped hard.

Okay, this is quite long. If anyone is interested, I can elaborate a bit more on mould release systems and different epoxies and ways to strengthen, or stiffen (two different things) aggregate parts. If not, I hope this has been of interest to someone that is not that familiar with the process.

Regards,

Craig.

Hi John,

Hey thank you so much for taking the time to answer. I too am a person who enjoys excellence in everything I do, and it’s easy to recognise that trait in you. The quality of work you have shown us all is just fantastic, and I know to achieve that takes full engagement and a lot of focus. Goodness knows how you have made the time to then record it for us all to see. Just amazing.

John, it’s been a great learning experience following your build and reading your posts. Over time I have had need to make many different kinds of moulds and patterns, and like you say, some with draft for easier release and others using a similar technique to the ones you made where the sides are square to the main face and so they are disassembled after curing.

I wonder if there may be some members that could glean a little from my experience with mould making and epoxy resin and apply it to an Epoxy Granit part? I will take the chance and share some of my experience and if anyone finds it of interest, I could elaborate if asked.

I learned a lot about making these kinds of moulds from an old friend who has a large pattern making business. They would box up around the pattern, which normally would create hard sharp corners all round, then after the mould release step was completed, would apply an epoxy gelcoat against the pattern and up the sides of the boxing. Next would come an epoxy paste or “tie” layer. An intermediate layer which was applied with a brush, added as a way of tying the rougher inner core to the outer surface finish. This is really just a mixture of epoxy resin and a thickening agent of some kind. Most likely Fumed silica, which in Australia is sold as Cabosil, in Europe and elsewhere called Aerosil. Not the kind of stuff you would ever want to breath in. Mask is mandatory. I think adding this layer is relevant as an option for Epoxy Granit construction.

This tie layer of epoxy paste is allowed to reach its gel stage where it takes on a comparable hardness to modelling clay, but still remains a sticky, then the bulking mix is added. The mix is usually a ratio of one-part epoxy to four or five parts washed sand. Even with such a high epoxy resin content, and a fine even aggregate, if the tie past wasn’t used, there would be surface imperfections and air bubbles just beneath the gelcoat waiting to cause issues once demoulded. So the dense and smooth tie layer acts as a buffer between the surface finish and the rougher aggregate as well as giving a good resilient interface for the bulk fill to bed into and bond well.

If you would like to get a nice surface finish without filling voids after demoulding, this technique allows for a great finish straight out of the mould, and done well creates a surface that is ready to use as is, or at worst only needs an undercoat after a good sand back. I have made moulds this way and, in an attempt to achieve a nice surface finish out of the mould as well as save on resin, have applied a sand/epoxy layer directly after the tie past, and then mixed a coarser aggregate with a far less percentage of epoxy to fill the larger interior volume. This once again minimises the chance of air voids close to the surface and gives a stronger outer shell less likely to crack when the inevitable knock happens.

To be cont.

Hello John,

I added a comment to your last entry on the "How to build a Mill with Epoxy" on Hackaday forum regarding an alternate supply of epoxy here locally in Oz.

But now can I ask you this.

With the experience you have gleaned from this E/G building process, if you were to do it over, is there anything you would do differently?

Regards,

Craig Smith