Tool time.

[He Who Shall Not Be Named]

Hi Dave, and anyone else who wants to contribute,..

After building Resolution and doing a lot of looking into silicon RTV moulds I have finally dived in the deep end and use RTV as I have wanted to do castings of hydroplanes etc..

i have looked very carefully at all the pics you've put up of your green and Blue Rtv moulds as well as Youtube and have started doing similar for the Resolutions control surfaces. I haven't done the whole spruce and vent set up yet, want to get experience just with the basic castings.

I get the impression from your workshop that silicon moulds are probably the most common form of tooling that you develop for fabricating parts, is this right? Do you still use hard shell (grp) moulds for hulls? I would consider using silicon moulds for larger parts if they give better fidelity but just don't know enough about how wobbly a large mould like that would be and making sure it was supported enough.

what kind of lifecycle do you get out of your silicon moulds? Any particular hints in looking after them or specific things I should keep an eye on whilst making them and or using them?

i see that these moulds are really wobbly. You back them with what looks like a piece of chipboard. Is there the concern that you may have a little give in the silicon mould and it may slightly distort the piece coming out of it?

Out of curiosity you must have made hundreds of these moulds, would that be correct?

the skills you freely impart are fantastic, I just wonder whether more discussion about tooling design and use would complement the other skills that you teach. I would certainly find it beneficial.

thanks Dave,


David H

OK, I'm finally getting around to giving you some useful information, with specific answers to your questions. And some added dope thrown in for good measure.

For large tools, specifically those used to give form to GRP hull parts, I almost exclusively construct glove-mother molds. An inner rubber 'glove that defines the shape and details of the eventual parts surface; and an outer 'mother' mold to support and render ridged the glove mold.





On occasion I will go with a glove-mother mold type tool to render cast resin parts, such as this hollow tail-cone for our 1/60 ALBACORE kit.









Yes, almost all my cast resin, and metal parts are formed within silicon, room temperature curing (RTV) mold making rubber. Today I get my rubber from this source: https://bjbenterprises.com/index.php/silicones/

The TC-5050 for resin casting and metal casting. I use their 'brushable' rubber, TC-5040, for glove molds.

As to the rubber tools utility: I have a metal casting tool that was new nearly 30 years ago, and it's still cooking! Today, using Mann mold/part release spray and a powdering with talc I get about 100 shots out of a typical tool before its trashed.

Yeah, I've built hundreds of tools for resin and metal casting. There's nobody on this planet better at this game than me.

Basic resin casting: It's one thing to introduce resin into the cavities of the mold (the specifics of tool design with all the sprue, vent, and header specifics outlined later), its another to insure all air previously occupying those cavities is displaced completely by the resin. There are four methods of introducing the resin in such a manner as to encourage a complete fill of the tools cavities.

First, is the simple pouring of catalyzed resin into the sprue of the tool and letting gravity do its work. If the resin is fresh, the tool is properly designed, the cavities are of simple geometry and of substantial cross-section, and the correct part-release agents are applied to the face of the flanges and cavities, then this is technique is good enough to achieve well formed parts.



Second, is for tools with cavities of relatively small cross-section, and moderate complexity of geometry. This is pressure casting. Where the filled tool is placed into a pressure pot and subjected to at least one atmosphere of pressure (one Bar, or 15psig, or 30psia). What little bubbles may still displace resin within the cavity are crushed back into the resin solution, insuring a 'pin-hole' free cast part.

[trout]

I hope this will make sense, but you have enough material on your cone that you could put a flat spot or step in it that a nut and washer can secure it down.

[Davidh]

Thank you David. Photos are fantastic.

just a couple of questions.

How do you secure the extension drive shaft? I can see that a hole has been drilled for the motor shaft to slot in. Do you glue it or tap friction fit it in?

So so on this design, the front of the housing secures the lip seal in place and is held by RTV silicon. Dark section at joint is one of the blue lip seals. Once I peel back the paper from the front face of the outer end cap I will drill the motor mount holes, mount motor and push assembly onto motor shaft, (flats ground on motor shaft) and silicon around it.

So so far so good.

thanks once again David.

david h

[He Who Shall Not Be Named]

Hello all,

Some questions for HWSNBN,

I am thinking of redesigning my twin shaft endcap for the Project 661 and so have been looking over lots of your work. I was particularly interested in the series of photos on the most recent Albacore build of Scott’s. You show the assembly of a twin endcap. The photos don’t show how the shaft extension is fixed to the shorter motor shaft. I was really surprised that you only secure the cone shaped seal housing with silicon right over the top of the motor mount bolts?. I am assuming this is strong enough? I have always thought of bolting these extensions onto the main endcap.

Judging by your design the only support of the shaft has after leaving the front bearing of the motor is the seal. Is this correct? I always produced a ‘tunnel’ for the shaft in between the seal and the motor to reduce any water ingress, usually with a tunnel barely bigger than the diameter of the shaft. This has meant that the shaft has had to be dead center or runs the risk of rubbing up against the tunnel, causing heat and more load on the motor. It would seem with your set up that the shaft extension could run with some slop as there is more gap? This being absorbed by the flexing lip of the seal? ( yes I know, you don’t do slop)

I know now you wouldn’t keep this design in production if it didn’t work. I know you shafts don’t leak. I’ve got one of your Subdrivers in the Mike. So I can assume that you place great faith in that seal.. judging from all this I could probably get away with less engineering in my design.

I hope these questions make sense.


I look forward to your guidance.

David H.

I always present the SD drive-shaft with a journal-bearing where is projects from the SD proper. It's not apparent in the shots of the small 2" diameter SD used for the 1/96 ALBACORE, but there is an Oilite bearing tucked in right behind the cup type watertight seal. A cut-away of that bearing-cup assembly presented here:



The design of the bearing-seal assembly originates with Skip Assay. Note that the bearing-seal body is either brass or resin.



Note that the bearing is there to take the lateral, side-to-side shaft loads and not to take any axial or thrust load. In direct drive motors I outfit the motor with an extension, or intermediate drive shaft. That shaft running straight from the motor to the bearing-seal unit set within a resin bearing-seal foundation that is affixed to the wet side face of the motor bulkhead. Note the use of air-dry RTV adhesive to secure these parts together. Any otherwise harmful vibrations are damped out by the flexibility of the assembly.













Point of interest: on those drive-trains where I introduce a gear-train between motor and drive-shaft, there are three journal bearings: one set in the motor-bulkhead end-plate (that mounts the motor); another bearing set within the motor-bulkhead body; and the third bearing part of the bearing-seal unit. The two forward bearings are there to handle the high lateral force produced by the driving pinion gear against the driven spur gear.





It's my practice to isolate the SD drive-train from all thrust loads presented by the propulsor. Those loads applied to a proper thrust bearing at the base of the propeller hub (ahead load) and forward face of the universal coupler attached to the forward end of the propeller shaft (astern load).

David

[Davidh]

Thank you David. Much appreciated...

dave

[He Who Shall Not Be Named]

Anybody, please?

I'm on it. I'm collating now and will put it down in the next couple of days.

David

[Davidh]

Anybody, please?

[Davidh]

The photo of the twin motor mount arrangement.

[Davidh]

Hello all,

Some questions for HWSNBN,

I am thinking of redesigning my twin shaft endcap for the Project 661 and so have been looking over lots of your work. I was particularly interested in the series of photos on the most recent Albacore build of Scott’s. You show the assembly of a twin endcap. The photos don’t show how the shaft extension is fixed to the shorter motor shaft. I was really surprised that you only secure the cone shaped seal housing with silicon right over the top of the motor mount bolts?. I am assuming this is strong enough? I have always thought of bolting these extensions onto the main endcap.








Judging by your design the only support of the shaft has after leaving the front bearing of the motor is the seal. Is this correct? I always produced a ‘tunnel’ for the shaft in between the seal and the motor to reduce any water ingress, usually with a tunnel barely bigger than the diameter of the shaft. This has meant that the shaft has had to be dead center or runs the risk of rubbing up against the tunnel, causing heat and more load on the motor. It would seem with your set up that the shaft extension could run with some slop as there is more gap? This being absorbed by the flexing lip of the seal? ( yes I know, you don’t do slop)

I know now you wouldn’t keep this design in production if it didn’t work. I know you shafts don’t leak. I’ve got one of your Subdrivers in the Mike. So I can assume that you place great faith in that seal.. judging from all this I could probably get away with less engineering in my design.

I hope these questions make sense.


I look forward to your guidance.

David H.

[Scott T]

Is there anyway to cure the gooey spots like vacuum bag it; or is it done catalyzing?
I read " I poured some of the hardener onto the brush and painted it onto the sticky soft spots and applied some heat from the pistol hair dryer. "

[trout]

I would add, make sure your mixing well......Change cups was a tip I got from a demo by Smooth-On. the idea is the sides and areas around the cup surfaces may not get completely mixed even with the best intentions, but pouring the mixture to a new cup eliminates that. Test on small area first. A whole hull can add up in cost, but a bust of David Merriman would be small (or some other subject). I know these are common solutions and I am sure you know these things.....just going back to basics helps me.

[Subculture]

Another method i have seen used is to mix some polyester bodyfiller/putty with resin to thin it down a bit, and use that in place of gelcoat. Haven't tried this personally, I believe that bodyfiller is mainly talc and resin, but they probably stick some other stuff in as well to make it a bit lighter and more elastic- manufacturers don't tend to make public their blends.

[Davidh]

Thanks lads,

I mix up the MEKP and resin first , then add the talc. The talc is recent. As subculture mentioned I should try some of this silica or even an epoxy gel coat. Any other ideas?



David H

[Subculture]

some areas of the gel coat are still GOEY , WHY ?!!

See my answer in post 54, you don't use air inhibited resin in rubber tools. If you want to use polyester resin, then mix up some laminating resin with some colloidal silica to thicken it, that will cure fine. Or you could thicken some epoxy and coat the tool with that.

What tends to happen is that as the resin cures, it shrinks back, and as the rubber tool can give, unlike a hard case tool, the resin is exposed to the air, and it can prevent it curing properly in places.

[He Who Shall Not Be Named]

Hello Subculture and David,

Ok , so I took my large silicon hull mould for the Resolution class and after making sure that the surface was clear of any residue I sprayed down a thin layer of PVA. Then another layer then another. My white mould was now looking decided blue. I mixed up some Polyester resin and grey pigment. I then added a good amount of talc to really thicken it up. After about two days in 32 degree heat from,what's left of summer the gel coat seemed quite hard. Laid up a fine weave. Let dry for another day or so. Then laid up some thicker chopped strand. Hull part now solid.

i just pulled the part from the silicon mould and so far so good, no evidence of the crazing effect that we have been talking about, great, BUT, some areas of the gel coat are still GOEY , WHY ?!! 4 days in hot weather. Not enough?!! Not happy.


any enlightenment would be appreciated. I'm thinking there is an issue With the talc?

regards,

David H

Talc is hydroscopic -- it absorbs water. Was this fresh, recently purchased talc, or has it been sitting on your shelf a few years? Also, did you catalyze the resin before mixing in the talc?

So, you have a viable part-release (the PVA) system. But, you still have a gel-coat issue. Right?

David