Cast Resin Parts

[Vic]

Hey Guys, A machinist I'm not but I'm just thinking about something (a rare event)!

In order to cast a gear using an RTV mold you'll have to have a pattern to make the mold from. Unless you copy an already existing gear you'll have to "one off" a gear to make the pattern so you'll have the proper dimensions and tolerences. Seems to me that if you are already set up for the "one off" then you could just go ahead and machine whatever quanity you needed.

The other alternative would be to make "rough" castings and then machine them into the proper size but that might require some pretty sophisticated equiptment.

Just some random thoughts

 

[jon-monon]

If the part can not be cast, or if you just want the simplest means to an end, you might check McMaster-Carr for small parts such as gears. They don't hand out catalogs as freely as they used to, but I believe they sell to the public now. If not, just use your road name! The on-line catalog is fine.

 

[Bill Stone]

You're right, Vic, once you're set up for machining some gears, you might as well knock out all you'll ever want.

There would be no purpose in casting gear blanks to machine (at least in non-production quantities) as they would be better cut from a preferred material, brass, bronze, steel, or nylon, all of which can be purchased as bar stock.

Cast gears would be justified if you haven't the machine (or inclination) for the job, and have an existing gear that you can copy.

Speaking of gears --- facts unknown to many model railroaders: Worm and gear drive trains, as used in model locos, are the least efficient, and the most subject to wear, of all common gear systems. This is because the gear teeth are sliding across one another rather than rolling into mesh and away. Certainly the only reason they're used in model railroading is they are a convenient way to get a right-angle drive, and have the advantage of providing fairly large reduction ratios in a single (read that cheap) stage. Another disadvantage (?) is that they do not coast.

Bill S

 

[Rusty Stumps]

A few notes on casting:

If you do use low melt metals, pre-heat the mold to about 200 degrees. The cold mold draws too much heat out of the molten metal and it solidifies before it gets down into the mold.

I take old or bad molds and cut them up into small pieces. Cubes around 1/4" is best. Then when I'm pouring a mold I wait till the mold is about half full and then I pour in a good amount of this small scrap into the mold on top of the liquid. It sinks in. Then I just top off the mold. It makes your new molding materials go a lot further and you see no difference in the mold. DON'T put the pieces in your mixing container.

I'd like to find one of those old sausage grinders, the cast metal kind that bolts to the table edge. It has a auger that forces the meat against the cutters. This should work great for grinding up old mold pieces.

 

[jon-monon]

better cut from a preferred material, brass, bronze, steel, or nylon, all of which can be purchased as bar stock.


Bill S [/B]

Or as gear stock, which I believe would be a common method of making small spur gears. You just buy the appropriate gear stock and lop off slices at the thickness desired. I would think with brass or plastic, you could do this without a lathe too.

Oh, yes, another small parts supplier, Stock Drive Products has _everything_! As I recall, they are not nice to deal with, but they now have on-line ordering, so one might not have to to buy. They sell through distributors which is how I've always purchased from them.

Hey, at work they toss this stuff every so-on for self imposed shelf life, because the manufacturer has no shelf life for it. They call it potting compound and use it to fill the gaps around 100 Watt heating rods in an aluminium block. It gets hard as rock and looks like bake-light (sp?). Tuff stuff. So, I thot I'd grab a couple of quarts last time around. I was thinking about molding a knob for my MG or something. I just set it aside and forgot about it until reading this thread. So I got it out and read the label. The proper name for it is rat-ta-tat-tat (that's the drum roll, not the name): casting compound. So, being a tight (_!_) I bought a $2 calk gun tube of silly-cone and gooped it on some parts that I had sprayed with WD-40. The tube said 24 hr. cure, which it isn't, cuz this AM was 36 hrs and it still soft on the inside, very close to the part. But anywho, after it does finally cure, I hope to be casting some fine parts for no investment!

 

[Bill Stone]

..... as in "investment casting"?

Okay, what kinda MG? Over the course of several years, many, many years ago, I was into Abingdon's products.

'46 TC, '48 TC, '49 TC, '52 TD, and a '54 TF (not in that order).
Not to mention a bug-eye Sprite, and an Austin America econobox.

Bill S

 

[Vic]

Hi Guys, While we are on the subject of machining I need to ask a question.

A couple of years ago I received from an estate a small metal turning lathe. It is a manual lathe and has no powered feed but it might be useful in at least fabricating HO parts.

But I don't know "squat" about these things. I'd really love to learn how to use it. Can any of you reccomend any books/instructions and etc on the subject?

I'd post a pic of it tonight but my camera battery is dead. Let me know if a pic would help

PS: Bill S. and Jon....The only auto accident I ever had was in a 1953 TD.... when I was 17... The spindle on the right front wheel broke...the whole "shebang" came off and me and the TD ended up in a ditch full of water. Insurance company graciously paid for repairs and I kept it until 1985 when I sold it to pay off some business debts. Gee I wish I still had that car!

 

[jon-monon]

Nothing cool like you guys had, jest a ratty old 77 MGB. A newby compared to a TD. If I had a TD, I would dedicate my life to it!

To me, using a lathe is easy, I just toss whatever I want machined at our machinist and it machines itself! I'll ask him. Perhaps you can get a manual for it if you don't have one. For consumer level stuff like Seras/Crapsman, they often have some instruction on use...

 

[shamus]

Hey Bill, thats a great way to multiply some items and is cost effective.

Vic, any ideas on casting "Stone chimnies" around 5" tall with a base of 3/4" by 1/2"? If so, I would like to buy 4 from you if possible. Never tried casting in my life.

Shamus

 

[Bill Stone]

Jon & Vic: My last TC, the '46, was my best. It was ser# 1026, and so far as I knew, was the oldest TC in the US. I sold it in '62, with a nearly new factory stage 2 engine, new English glove leather upholstery, and perfect body and paint --- for $1200! About 5 years ago I was talking to a TC owner, and he said that if I had stuck it in a shed and put a tarp over it for all those years, it would now be worth close to $100,000! (Driving a TC was great. 1 1/2 turns lock-to-lock, right hand drive, about 3/4 inch spring travel, and --- with my stage 2 engine --- practically king of the road.)

It's all women's fault: Sports cars distracted me from model railroading, and girls distracted me from cars.

Vic, I highly recommend Doug Briney's book, THE HOME SHOP MACHINIST. It's available on Amazon.com for about $13. I'd hung around and watched machinists before, but had no experience when I got my mill and lathe, This book helped.

By the way, you don't need power feed (for home machining) unless you want to do threads.

Bill S

 

[Vic]

Bill S. and Jon, Here's a pic of the little lathe. I can't identify who made it. The is a logo engraved on it but I can't read it. I did find one mark that said Made In Switzerland. It's calibrated in metric increments and is beautifully made.

 

[Bill Stone]

It's a jeweler's lathe, Vic.

I've seen them before, and have always wished I had one --- if for nothing else, to gaze upon it's beauty.

It will be perfect for turning out small brass parts. You'll love it.

Bill S

 

[jon-monon]

kewl! I would go by the local plastics supplier and see if they have some scrap bar stock you can get cheap or free and just have at it. The biggest thing is to move the tool slowly into the work and just a little, then you can move across to remove material and reduce diameter. Just play with it and you will get the hang of it. If you move the tool intot he work X thousanths, you will remove 2X. Plastics and brass are easy to machine. Plastic is actually a little harder to get accurate. If you move the tool in .002, it will remove a little less than .004, as plastic likes to get out of the way of the tool. A lot of the time you can just use a file to shape the material as it spins. You will have fun with it. I'll be sendign drawings for all the little parts I need. How 'bout a nice brass bell?

 

[Vic]

Thanks Bill, I'll order the book you reccomended tonight. I basically understand how machining is done but I need the "how to" to give it a whirl. Thanks again for your help.

 

[Vic]

Thanks too Jon...I won't be taking orders for awhile though! I got a feeling this is gonna take some "tinkering" to get this done right!!!

 

[Vic]

Bill S. and Jon,

I've finally been able to identify the mfg. of the lathe. It was made by the F. W Derbyshire Co. of Waltham, Mass and not in Switzerland as I orginally posted. The Jacobs type chuck has Switzerland stamped in it and that's where I got that idea.

The company is still in business although they only mfg. CNC machines now. It was orginally cataloged as an "instrument lathe"

I've hooked up with a gentleman in England who is a collector of old lathes. He had some pictures on his website of some Derbyshires that were similar but not exactly the same. The basic design goes back to 1911 but its obvious that this one is much more recent. Hopefully he will be able to give me some more on its history. The company's website gives no information other than a telephone # and address and instructions to contact them thru dealers so I sorta got the impression that they may not be too "hobbyist friendly"

I'll let you know what more I find out.

 

[jon-monon]

Hey Vic, I did talk to my machinist about the lathe and he said just learn by using it. He said you'll learn it's capacity when you try to take off too much. Pretty obvious to keep you fingers back from the tool while it's cutting (that wicked machine could easily take off a fingernail, well at least the tip! ), but also watch loose clothing, long hair and jewelry, if you are so inclined

I don't know if I said it before, one of the easiest ways to work with a lathe, and especially if you want a radius, is just use a file while it's spinning. You should probably put a wooden handle on the file; P-K can't afford another machinist law suit and Paulsen's getting tires of pulling that file tang outa workers palms! You would get a cool Jesus scar though

You will have fun with it.

 

[Vic]

Jon, Thanks for the information. Sure don't want to loose no fingers! Have found out that that projector spring that's on it for a drive belt is something that someone rigged up and is gonna cause problems with the pulley on the drive spindle. It can't be readily disassembled to put on a continious belt so what I'm gonna have to do is get some of that 1/4" belting that you cut to length and run it thru the pulley and then heat the ends to join them together.

Bill S....The book you reccomended arrived Monday....Great...it has just what I wanted to know in it...Thanks!!!!!

 

[philip]

Bill, need more advise.

Bill: I have poured a whole lots of hydrocal and this resin stuff is a whole new ball game. The picture shows one of my miserable attempts with a two- part auto epoxy just to practice with. I have a friend that models O-scale and he has kind of talked me into doing 50 of the shown brackets and 12 of some even smaller ones for some sort of architectural overhang. The two- part epoxy I'm practicing with is like Karo Syrup so gooey it could never flow into the minute crevices. Can this Micro-Mark two- part resin be measured using two syringes.. to conserve the product. I only have two cavity molds. At 10 minutes per demold I''ll be real old before the process is finished.

philip

 

[Bill Stone]

The Micro-Mark resin is fairly thin --- probably not much more viscous than water. I see no reason why you couldn't use syringes to measure it. (And measuring is a snap as it is 1:1.) In the molding I've done so far, I mix in the little paper "bathroom" cups, and have always just made rough guesses on proportions. As I've not had a bad batch yet, I'd guess that the mix isn't too super critical. By the way, the resin bottles are plastic, with dispenser caps, so are easy to use.

If you have the original pattern, why not make six or eight RTV molds. You'd get more parts per pour, and wouldn't be too aged when you're finished.

By the way, the resin is clear when it's liquid (which is nice for looking down into it to see if it covered everything, and cures to an opaque white (nice for inspecting the parts.)

On a fairly complex part like your brackets, you might want to vibrate the mold after it's poured or snap it a few times with your finger to shake any bubbles loose so they'll rise to the top.

Bill S