View Full Version : Castings, Castings.....Well uh...
08-23-2006, 10:52 PM
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.......
08-24-2006, 06:10 AM
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.
08-24-2006, 07:39 AM
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.
08-24-2006, 10:47 AM
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.
08-24-2006, 11:21 AM
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.
08-24-2006, 11:00 PM
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..........
08-25-2006, 07:45 PM
What are you Buildin'?
08-25-2006, 10:18 PM
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.
08-26-2006, 04:15 AM
Hey I need one of those. Would Lindsay's have that?
08-26-2006, 09:12 AM
I'm building a 1/2 scale Root and Vandervoort horizontal hit n miss.
I need that book as well. Reprints available?
08-26-2006, 01:00 PM
As an example of "fun with castings" here's one on the mill right now. It's a crankcase for a 3 cylinder vertical, the upper surface in the image is the bottom, as yet uncut for the bearing mounts. It's about 9.5" end to end, that's a 1" EM hanging down.
The fun with this is in getting a reference surface - the front has details and a slanted surface, making it a surface you don't really want to clamp on. The parting line is lengthwise, so the top and bottom have the relief, and the back has mounting points that are not square with anything else on the casting. The only choice is to mount it upside down on packing and to rough mill the legs, then flip it over and rough mill the top while holding it in with toe clamps, then correct any angular error with packing and a second pass (the top matters, it's got some bosses you want square and nice), then flip it back over on a doubled piece of printer paper (otherwise cast iron to cast iron will slip) and mill to thickness. That whole process is fun as you have about 1/16" to play with.
Milling the bearing holders to thickness is fun - there are pockets on the front that will show when done, so I clean the ends up, measure the wall of the boss to the end of the casting and cut the fatter one to the narrower dimension, then take equal amounts from each end for the 9.5" length. This leaves the casting as you see it here - the pocket is a bit thinner on the outside than the inside, but it's more important to make it bilaterially symmetrical than to have one end fat and the other thin. The far end cleaned up about the same this morning. The middle pockets have to fall in where they fall, I can't affect their positioning. The bearing holders need about .160" taken from each side so they can take a while to mill.
This is about 1/3 through the milling operations, I still have to pocket out between the bearing mounts then mill the mounts - an .875 slot lengthwise and take about .062 from the top to leave a .125 rim on each side of the top. The pocketing takes the most time, it's lock axis, unlock other axis, mill to other dim, lock axis, unlock other axis, repeat. Those are the times I just wish I had a CNC.
I just thought I'd show an example of some of the considerations you have to take when ssetting up a casting.
08-26-2006, 10:42 PM
Wow, that looks the real deal to me. I must admit a small amount of intimidation lurks in me when I look at what you all are doing. I'm gonna get it but first I must make it clear in my head. As with all things, the first is the scariest. Thanks! Onward and upward!!
08-26-2006, 11:12 PM
The gist of the deal in the book is that.....
First, you find center points (or other appropriate dataums) as closely as possible at the farthest apart ends of the casting. That way your positioning is unlikely to call for metal where there is none.....
Having marked them, possibly bridging openings with a piece of wood jammed in, you set the piece on your surface plate and get it leveled.
"leveled" means that the center poits at the ends are at the same height, and the part is also set so the two sides have their approximate centers at equal heights. This would be most appropriate for a cylinder block, etc. Other parts might have other requirements.
Then, having the part packed up in that position, with marking medium on it, you scribe a line all around it.
Next, you turn it 90 deg, so that line is vertical, and the center marks at the ends are still at same height. Scribe another line around. You can also mark a vertical thru each center before turning it, in which case you set that line horizontal before scribing around.
Now you have centerlines in two axes, and can go on to develop other positions off of them, taking into account as-cast holes etc, and scribing lines for them. You can always set the thing in your original two positions as-needed, since you have lines to follow.
As you develop other locations, you may have distances to set relative to one or more of them, and not to your original marks.
Eventually you mark hole centers with punch marks, scribe witness lines and punch mark to preserve them, etc, etc, and use those to guide machining the part.
You are "finding the part" within a given irregular lump of cast metal.
Obviously, if you start at one end, and measure and mark from there, you may run out of metal due to measuring errors caused by part irregularities, etc, etc. By finding the overall centers, if the patternmaker did his job, you will never run out of metal.
08-27-2006, 08:11 AM
From your print you should find referance points for the rough casting ( roughly 3 points to referance X and Y then 2 points to referance Z ) . You will then machine fixturing holes or flats. ( preferrably holes drilled and reamed ) Now you are ready to machine your casting from the fixturing holes. All machining will be located off of these holes until the product is finished. This way everything will be true and square to the original casting.
Setting up a casting can take a very long time, so be patient. Others have summed it up pretty well. You need to find a point you can call "0,0,0" such that all the other key points (hole locations, surfaces, etc.) will be within the casting body (as J Tiers puts it, there will be metal where you need it).
08-28-2006, 11:42 AM
I learn more with each and every post. I again want to express my Thanks to all who took the time to respond. I know you are all busy and your efforts are indeed appreciated.
09-02-2006, 01:06 PM
I have found you need a reference point to always work from . . For each engine I build, I start with making a base plate . . usually a piece of 1/2 inch alum plate which I machine perfectly square and parallel . .then I mount the major casting [ usually the block] and use the plate for mounting on the mill .don't rely on just bolting the casting to the plate . . you need to use percision locating pins to assure exact placement when you unbolt and rebolt . alos add holes and slots for use to attach it on either the mill table, the vise or on an angle plate . . .
Plan your work to make all the axis machining operations possible without disturbing a set-up . . then go to the next set-up . . . The biggest help in getting the engine to run well is to keep the crank axis and the bore axis perfecty square . . "almost isn't good enough" http://bbs.homeshopmachinist.net/images/icons/icon10.gif
09-05-2006, 10:15 PM
I think that one way or another all the points of locating a casting (or forging) have been covered by prior posts, I wanted to try to clarify some of the information.
Consider what happens when you use a basic milling vise, when you place any part in the vise, it rests on the bottom of the vise or parallels. (The -A- plane, usually three points) Then the second place the part is located on is when it is pushed against the fixed jaw, (the -B- plane, usually two points) and to be able to repeat the position there is a work stop (the -C- plane, usually one point)
Most commercial drawing/designs for a casting incorporate a Datum structure. (a locating scheme) I think that they need one. If your does not have one you can still make one on your own after you study the casting and determine where the extra material (machining stock or allowance) is.
There needs to be a primary plane, It may be called the –A- plane and have target points described by size and location from an easy to find place like a center line. It is usually three points. Like a tripod there will always be contact of three points, the fixture may need to incorporate targets and matching raised pads to insure that the casting is repeat-ably located on the primary surface.
The Secondary plane is the same as a fixed jaw in the mill vise; it may be called the –B- plane, it can be described from cast features and from the -A- plane. It is usually two points they may also be raised and called out for the size of the contact point.
Finally the third plane may be called the -C- plane, it can be one point called from cast features, the other datums, or a combination of the two.
Adding more points than the three, two, one, scheme or using surfaces instead of target points can cause problems with repeat-ably loading machining and inspecting the casting/machined part. You do not see many four legged tripods, (would they be quadpods?) The basic machine fixture nest that uses two dowels on one side of the casting and one dowel on the end can be thought of the secondary and third locating planes.
Once the cast datum structure is established everything can be referenced, checked and machined from the datums. If you are on the datums, and you machine the part correctly and still run out of casting, the problem is with the casting.
I understand that this may be overboard for the guy who just wants to make a green sand engine casting, but it is the language of castings. Machine stock, draft, flash and shrink allowance can be planned around or compensated for with a good datum structure. The datum structure is how castings are described and locations of features are called from them.
09-06-2006, 07:46 AM
many good suggestions here, couple of points to add.
I agree with Reality that the first order of business is getting a reference surface, sometimes this is a bore, but usually a plane.
Many simply bolt the casting to the milling table or clamp in a vice without first cleaning up the surface to be in contact with the table. You must first create a flat plane to go against the table/vice else the act of holding it twists it out of shape. After machining it will spring back and you will forever be frustrated as to why nothing came out flat/square etc.
Using files, scrapers (power is so nice for this op) blue and a surface plate get enough of the first surface in the same plane so that when you clamp it you won't be distorting it!
another trick on very irregular shapes, to avoid distortion, is to us auto-body filler. put some filler on some sheet steel, set the casting on it (remember to use a release like petro jelly), the you should be able to clamp without distortion. i never bother with this mess though, just get the file and blue out and create a flat surface
09-06-2006, 12:14 PM
If the casting designer was good, you will find some bosses cast-in from which you can sufficiently accurately locate the casting to get started right. Common with volume production.
I think one can guarantee that production operations have NO layout table at the head end.......
if they were NOT good to you, THEN you may have to find the part in the metal the hard way.
09-06-2006, 11:59 PM
Again solid sound advice from those who have walked the walk. Thank you one and all. Having been in my job for 39+ years I was always glad to give the less experienced guys as much to work with as I could. I found it to be gratifying for myself in that tricks and methods gleaned over many years were passed on to the next gang who would have a need for it. I can see that all of you as well share that quality. Thank You one and all for sharing the wealth you possess
09-08-2006, 07:14 AM
Id love a book like that also. Wher can one be scrounged or purchased.