Castings, Castings.....Well uh...

[caddy]

OK guys it is my time to have a reality check. I've found yet another area of machining that I am woefully underspeed.
I know many of you have built small engines from castings. I took the leap and found right away that working to a set of plans is far and away different than just cutting and fitting. Any way, the castings came and I have been poring over the drawings and here is where it began to dawn on me that I was a tad behind the curve.
I need to know how a machinist (can't include myself in that) determines where to start on a rough casting. It appears that a reference point to refer to is mandatory as a place to begin the layout of vitually all other points that are to be machined. Is that true for each plane to be machined? (hor./vert.) It follows that if you don't get the first one right then all the rest are junk as well.If I add to that the differences in each casting then I am even more lost.
This may seem simple but it seems to me that ALL the machining processes must be determined before making any chips.
Tell me gurus of the lathe.......
Thanks
Caddy

[kap pullen]

caddy,

Castings are a different animal to work with.
Many professional machinists have problems with them these days.

What I do is set up to the surfaces that do NOT get machined.

In other words if you are setting up a flywheel, set to the inside of the rim for starters. This way when the engine is in motion all the flywheel surfaces, both cast, and machined will run true.

For cylinder you set to the rough cast outside surface. You want the machined cylinder heads to line up with the rough cast outside of the cylinder without much hand work when the engine is complete.

Drive wheels, brackets, gear boxes, and etc, all have the same requirements.

The areas that you are cutting will be true, daaah.

You will have to take skims to assure the machined areas will clean up and may have to shift things a bit if the casting has a misplaced core or other imperfection.

That was the layout man, and machinists job in days gone by.

A surface gauge and layout equipment was required equipment in the apprentice machinists chest.

Tha layout man was the most experienced, and highest paid machinist
in the shop (usually).

An exception to thet rule was the lead man in charge
of training apprentices, operators, trouble shooting, and saving all sorts of nearly scraped up work by skimming a thou here and there to mantain tolerances.

In dealing with foundryman over the years, they can always tell you what went went wrong with a casting, but seldom had the forthought to prevent the problem ahead of time.

That's my view anyway.

Kap

[ammcoman2]

To follow on from Kap's excellent note, I recently started building a 1/12 scale shaper (PM Research). I started with the main body casting but first made up a base block of aluminum that was dead square. I then mounted the base casting upside down in the vise of my shaper (6" Ammco) using pieces of brass and aluminum to get it aligned so that the outside of the casting was vertical from the side and the front. You have to eyeball this as the draft on the casting precludes using a square (one can use a square though, to get the error on each side the same).

I then machined the base or foot - now you have a reference. Mounting it on the block was next and you can then do all the subsequent operations using the vise of the shaper/mill/lathe, or whatever you are using to do the machining operations, and knowing that all the surfaces are correct.

Good luck with your project.

Geoff

[mklotz]

I've made all the PMR machine tool models and an indispensable tool has been a
small block of aluminum, sized to fit in the milling machine vise in all three
possible orientations. It's festooned with T-slots and tapped holes. In
addition, there is provision for (removable) fences on the sides of the block.

It is, in effect, a set of bolt-on reference surfaces for small castings. In
use, as Geoff describes, the casting is carefully aligned and clamped to the
block and then the accurate block surfaces are used to establish orthogonal
surfaces on the casting. A further feature is the fact that the block is small
enough to clamp in my 8" 4-jaw. If you use a block too large to fit in your
4-jaw, provide the ability to mount a (removable) stud to the block. The stud
I made is used with an assortment of miniature faceplates that are handier than
using the one that came with the lathe.

[JCHannum]

Kap & Geoff are right on in their replies.

To add a bit, what Kap says about the foundryman is true, but the patternmaker's role also plays a large part in the success of the finished casting.

Many engine models are made from patterns that leave little room for machining or chucking. Add to that core shift and other problems from the casting process, and the challenges become greater.

Take a close look at the castings and the drawings. Most projects will have dimensions that must be adhered to, and that determine the relationships of the finished item.

With a model engine, most of the critical dimensions will be measured off the centerlines of the bore and crankshaft. It helps to chalk up the casting, paint it with white out or even spray paint it flat white and, using the surface gage, layout these lines on the casting. Using this method, any discrepancies will be apparent before metal is removed.

Once these two centerlines are established, I invert the castings, and clamp them down. I use the surface gage or other means to assure the centerlines are as close as possible to being right with the world, and then mill the base of the engine. Once a solid, flat base is established, the major machining operations become easier.

Another thing to lookout for is that many drawings are made after the original model has been completed, and dimensions are taken from the actual model. This can result in some off the wall numbers. Do not get too involved in trying to follow the drawings to the nth degree, rather, establish and get the relationships of the major parts correct and fit to them.

[caddy]

My Thanks to those of you who replied. It takes time to do and I appreciate it. I have printed all the responses and find that for me reading multiple times always helps to visualize the information provided. THANKS so much guys, I do appreciate your efforts. Now, where's my sledge..........
Caddy

[Sprocket]

What are you Buildin'?

[J Tiers]

Sounds like you need a copy of this:



This used to be an important skill for one-off and proto manufacturing. In volume production of course there would probably be fixtures or jigs.

[lynnl]

Hey I need one of those. Would Lindsay's have that?

[caddy]

I'm building a 1/2 scale Root and Vandervoort horizontal hit n miss.
I need that book as well. Reprints available?