locating a casting for machining

I'm having some trouble grasping the concept of locating a casting on the given X and Y datums. I understand fixturing concepts when walls are straight or using a precision hole to locate off of (in the sense of making parts from billet).


But how does one locate off of a drafted surface? I am aware that locators can be made into the casting but they would still have draft in order for the pattern to pull from the sand.

Any pictures would be helpful along with written response. Thanks.

I can't tell if you are familiar with the process of "layout" and asking for specific procedures to use with rough, angled surfaces, or if you are unfamiliar with layout. If "layout" is not a term you're comfortable with, hit a machine shop textbook of the appropriate vintage (Machine Shop Practice by Karl Moltrecht is a good one).

In a nutshell, you block/shim the casting on a floorplate or surface plate, and establish the datum planes by scribing on the exterior of the casting using height gages and squares. The finesse in layout is making sure that the planes you establish (and the features they locate) will, in fact, clean up to the specified locations and dimensions. You then use the scribed lines on the casting, together with rules and squares, to locate it on the milling or boring machine square to the appropriate machine axes.

Added in edit: Fixturing of rough castings is a bit different than carving out soft jaws. (Precision investment castings are another story.) Several general features are often required, including equalizing clamps, spring-loaded/lockable supports and stops, and adjustable locators. You either operate on the working assumption that the rough casting has sufficient material to clean up, regardless of inconsistent placement in the initial fixture(s), or you perform layout on the part and use adjustable locators to position and align the layout lines within the fixture. Once you get one or two faces cleaned up, you can, of course, start locating off those.

Sometimes on a casting you end up splitting the difference or choosing the most critical features to preserve.

I actually enjoyed machining iron castings.
Same part, but the layout and set up was the challenge.

The real fun starts, if you’ve got one with a shifted core...

Duramaxhp said:

(in the sense of making parts from billet).

Click to expand...

(Pained expression emoji) A rolling mill makes plate or bar from billet. (OK, some super-heavy raw forgings might be made from billet.) Machinists make parts from plate or bar (or sheet or tube, or forgings). Nobody makes parts from billet. This is a silly phrase introduced by marketing guys, mostly in the aftermarket auto-parts industry, because it sounded "cool" to customers in advertising copy. In the trade, it just sounds ignorant. You asked a serious question, and should up your game.

Duramaxhp Sir,
What sfriedberg said ! Really he's right.. Here's some things I've learn from machining castings over the years...
Your really like a sculpture.. Usually I try and make a first cut/hole and all reference work comes from that. If you have a fixture then that becomes the reference. There is thread somewhere on this site ( antique forum maybe ) about the Cleveland twist drill co. and it has alot of pictures of this type of fixture and some jigs too. You just have to think more about work holding than with standard stock ( like a round in a lathe or a rolled piece of steel. Think about critical dimensions and such and where and what they need to reference from.. Shims are your friend for the initial set up if you have no planes or other things to reference from. Paper between the part and table and or vise can be a great help in hold things down to.. Just some ramblings for you to mull over.
Stay safe
Calvin B

a pic of the casting would be helpful.

on the layout table, doing castings approx in the range of 3' x 3' x 3', with cast bosses and bores, it worked like this at least in the 80's


put cstg on table with blocks and 1 jack or more to allow leveling.


put wood scrap targets in the bores. put blue dykem on, let dry, cover with yellow chalk.


use a toe in caliper or scriber to mark the center of the cast bore, but center from the outside of that cast boss, goal is equal wall thickness of the boss when bored, not equal stk removal boring the part necessarily.


level with the jack(s). I was fortunate enough to be using a pdq machine, or portage machine. like a big (6') 3 axis surf gage that had visual scales on all 3 axis, and some rotational capabilities and scales to match too. it followed machined tracks in the table which were a foot apart if I remember, the table was covered in tracks, and you could pull up the roller guides, and re insert them in any track needed, thus maintaining squareness when moving the whole arm setup to extend range if needed.


the casting sat on a big hand rotary table with scale, so it could be spun as you wished.

after leveling the rotary was used to check relationships plane to plane to the center marks you made on the wood. you bumped the casting itself around gently with a soft hammer to establish the best balance between the center marks.

I should have mentioned, before even putting in the wood targets, the general areas of important machined surfaces were painted with a whitewash made of chalk powder and water.

the leveled and balanced casting now could have lines put on the whitewash, to indicate there was stock to mill and bore all around. if not, some retapping, adjusting, and remarking was done to insure 100% stk to finish everywhere.

once good, sent to machining which in those days was manual vtls, hor bore mills, and the casting was set up to your marks and machined. some good drawings would indicate an x, y and z centerline all around the casting, if available, those were put on too.


was a good job, if you failed, you could just repaint and restart.

My apologies for referring to bar stock/solid stock as billet.

I guess I should clarify a little bit more here. I am making the initial patterns (on match plates) with machine stock on critical features. The end goal is to have this product end up in production.

Ideally I will have 3 rest buttons on the bottom of the part (Z) and then locate with stops in my X and Y axis and then clamp down on the rest buttons in the Z axis.

My main problem is establishing a reliable and repeatable method (X and Y on a drafted surface) to secure these things down for the initial machining process. Which like you guys stated as soon as I can get this one surface machined I will located off of the machined features for the down stream operations.



Edit: I'm positive I have enough clean up stock to locate this part within +/- .030" on location of my first initial feature and still have clean up for everything else. Referring back to your last lines of your first post-sfriedberg. This will be a rough sand casting.

Colvin and Stanley, and Albert Dowd, wrote a lot (multiple books' worth) about tooling, jigs and fixtures circa World War 1. There was quite a bit about fixturing of rough castings for turret lathes, vertical boring mills, horizontal boring mills, etc. And frankly, not much of what's considered good practice has changed in 100 years! You can buy (expensive) modular fixturing kits that didn't exist then, but clamping and locating principles have not changed, and most of the standardized tooling components you can buy today were quite recognizable back then.

Much of their written work was published through American Machinist magazine or McGraw-Hill (some of it before the two publishers merged). This stuff is out of copyright. Most was available from Lindsay Publications, and now is available from various too-slack-to-list-contents print-on-demand vendors.

Was for a fairly small part (fit in a 6" cube with multiple cored holes), but the last pattern maker I worked with insisted on at least 1/8" allowance for machined surfaces. I imagine that scales up fairly proportionally until you get to rather large parts.

Machine Shop Practice Books

Duramaxhp said:

I'm having some trouble grasping the concept of locating a casting on the given X and Y datums. I understand fixturing concepts when walls are straight or using a precision hole to locate off of (in the sense of making parts from billet).


But how does one locate off of a drafted surface? I am aware that locators can be made into the casting but they would still have draft in order for the pattern to pull from the sand.

Any pictures would be helpful along with written response. Thanks.

Click to expand...

Machine Shop Practice Books 1 & 2 have very good explanation and many pictures of layout I don't remember which one of the books has the layout section

I have read the "Jig and Fixture design" (Hoffman) one of the most recent books I could find (rather dated). But principles are the same, I was disappointed to find it didn't go into advance detail on fixturing a rough sand casting.

As for machine stock I am running between .100" and .125" stock. This part is would literally fit in a 6 inch cube. Cast from 356A.

Duramaxhp,
Now your getting down to the meat of the matter..! If you are designing an item to be sand cast you really should allow a minimum of +1/8th or more on ALL features that will need machining. Remember you need at a minimum .030 just to get under the scale on a casting. Flip the casting over and there .060+ gone already.. Just some things to consider.
Just as an aside.. when ever I have the privilege to visit a fellow tradesman's shop I always look at the setups rather than the machines.. One never has enough "fixture/jig" knowledge..
As for reference materials the ones previously mentioned are all good (+1 for Moltrecht's books ) and I'd add "A Treatise on Milling and Milling machines" by the Cincinnati milling machine co.. Books from back in the day when cast iron was a mainstay of industry.
Stay safe
Calvin B
PS volume 1 machine shop practice ( K.H.Moltrecht)chapter 3 is the layout chapter

You need to expect a few screw ups until you get it figured out You will be nervous as hell at first and after you figure it out you will start to relax. Be sure to bring a couple of pairs of underwear to work that day. LOL
As I read, I was thinking 3 jacks or 3 shimmed points as the use of 3 points is a lost art too. I agree on the machining under the crust to establish a starting point. Look for the simple set up to establish a flat area that can be set on and not distorted when it is clamped down on a 2nd, 3rd, 4th,etc. operations. One very important thing when machining cast iron is to wear a dust mask or some sort of respirator, if not you will be coughing up black dust. Been there done that when I was foolish. You are in a learning curve here and you need to be sure to over clamp everything on your practice pieces. Also ask and look into the cutters you use, speeds and feeds, etc. Have fun and be careful as you figure it out. If you have a shop go find a few experienced operators to help you in person. :-) Rich

Duramaxhp said:

I'm having some trouble grasping the concept of locating a casting on the given X and Y datums. I understand fixturing concepts when walls are straight or using a precision hole to locate off of (in the sense of making parts from billet).


But how does one locate off of a drafted surface? I am aware that locators can be made into the casting but they would still have draft in order for the pattern to pull from the sand.

Any pictures would be helpful along with written response. Thanks.

Click to expand...

.
casting is stock divided using extreme ends to get it parallel to a axis to .020" and centered using work offset. often just use a spot drill and a big ruler and small block of metal. of course a probe can be used with a program to avoid using a ruler and a spot drill. parts does not need to be at center of table it needs to be centered on expected program position. that can be a big difference of many inches or feet
.
draft on sides not a issue as amount to machine is normally far greater. cored holes usually small enough to be not a issue. laying out lines not needed with a cnc. all you do is center enough so all sides got enough stock to be machined and not still see raw casting surface. basically you dont want to machine over 1.0" on one side and not have enough on opposite side to machine off the raw casting surface
.
hole depth on raw casting surfaces often use a spot drill with length comp active going 1.0000 away from expected position and verify in handle mode and position screen then set a P offset for the top of hole. can even be programmed that way drill stops 1.0000 or 2.0000 from cast surface and at M0 it asks you to check and set the P offset for that hole. obviously if probed and P offset set automatically its like 10x faster. sometimes it even more crude at M0 is just says verify spot drill hole 0.300 to 0.320" wide(might be programmed to make 0.1 wide spot hole and always need a bigger adjustment to avoid too big a spot) . if no spot hole than a long drill can wander off position and break obviously

garychipmaker said:

Machine Shop Practice Books 1 & 2 have very good explanation and many pictures of layout I don't remember which one of the books has the layout section

Click to expand...

Volume 1, Chapter 3 (2nd edition, but I imagine that's the same in all editions)

From my experience, the drawing will call out datums that are machined features. The actual casting will have a profile tolerance to the datum.

Die castings are very accurate and repeatable. Sand castings can be all over the place. Often the pattern makes take all kinds of liberties with the draft, gate locations, stock amounts, etc. Something like a lost wax or permanent mold would be in between. Plus you have variables like core shift and warp.

Don't get too carried away with the fixture before you see the casting. BTDT. 100% guarantee they will put a gate or riser right where you have a locator or clamp.

I like the advice, just some little things I’d add;

3 points to establish a plane, then plan to restrict part movement for 6 degrees of freedom. Some machining may need 3 very solid points in 2 planes if you have to go hard at things.

The plan for smart machining should involve putting heavy thrust forces against the fixed points (not clamps) if possible.

If planning to re-use said fixture often (especially) with different machines, have a plan to locate it. Being very old school myself, I still use the table slots (with custom funiture). Drop it, pickup datums & run like a thief.

Good luck,
Matt

ewlsey said:

Don't get too carried away with the fixture before you see the casting. BTDT. 100% guarantee they will put a gate or riser right where you have a locator or clamp.

Click to expand...

+1 on this!