Castings, creating a datum before milling

[Garry Smith 7Participant @garrysmith7]

The Above are castings of the bearing brackets for the Tina Mill Engine.

Having practiced milling on my new mill by making basic clamps I am now ready to mill castings. Can experienced members advise me the best way to get a datum face on the castings before I set them in the milling vice. I think I need to make one face true before I start milling to give me a datum ( is this a true statement) easy simple methods to achieve this will be most welcome.

Thanks for reading.

[Garry Smith 7Participant @garrysmith7]

Milling Castings

[JasonBModerator @jasonb]

What you really need is MEW 289 Milling for beginners that is about dealing with odd shapes and castings!

Without the drawings and castings infront of me it's hard to be exact but I usually go over the castings with files first to remove any unwanted mould part lines etc.

Next compare the castings to the drawing to see where metal needs to come off and how much machining allowance has been given, this will save you taking too much off the first surface and then finding you are short on the opposite face.

At a glance I would make the bottom surface my datum and first surface to be machined flat (does not have to be to finish yet) I would hold in the milling vice upside down with the length of the casting along the length of the jaws, slip two equal height packets (drill bits, lathe tools, parallels) across the two jaws to support the work and set whet will be the upper bolting surface horizontal. Then mill the base flat, you should also be able to take it to width and length at the same setting.

[Garry Smith 7Participant @garrysmith7]

Thank you Jason. Could you just confirm the third line from the bottom “set whet will be the upper “.Not sure how I know the casting would be vertical with your set up in the vice before milling. If it’s not vertical then the base would not be horizontal when milled ?

[Martin Johnson 1Participant @martinjohnson1]

As Jason says, it all rather depends. First step is always to knock the lumps off with a file. A good next step is to mark the castings out, with (say) the base underside surface and shaft centreline. Also mark out the base in relation to it's axial position with the bearings and spot where the holding down holes will come. You will soon see where there is not enough metal (happens on commercial castings too often) and too much metal, and can adjust your marking out accordingly to get the best shot at the casting.

I would then probably go for a preliminary cut on the under foot and the long side of the foot – not to size at first, just to give you square edges.

On complex stuff it can be like knitting with fog to get a start. A dodge then is to cut card templates to actual size and sight them against the casting to get the best shot.

Hope that helps,

Martin

[Garry Smith 7Participant @garrysmith7]

Thanks Martin, Card templates great idea I’ve used them before so must have had a senior moment not to remember them.

[IanTParticipant @iant]

Castings are definitely a mixed blessing Garry.

For your castings I'd do exactly as Jason advises and clean it up and then very lightly machine your reference surface. It's easy to assume that a casting has enough 'meat' all around and to try to get down to a clean surface on the first pass – and then you find that there is not enough left elsewhere to end up with the stated dimensions.

I've recently machined several pairs of cylinder castings for use in a published locomotive design. There wasn't enough metal to get smooth surfaces all around in either pair- so you really have to decide where to compromise or simply change the design. For these castings of yours (and any other castings involved) it will pay to look at the item "in the round" and make sure you don't have to adjust something before you start.

In the cylinders I had to balance the machining at each end-face because the friend I was doing them for had already ordered laser cut frames (to the drawing) and the cylinder cut-out is oversize to the available metal in the castings supplied. Both ends could have been cut back to clean surfaces if the frame cut-out could have been reduced. The design also specified a 1/2" bore but on both cylinder sets the core was already 12mm and the cores weren't central either (one cylinder was offset by nearly 3mm) so to bore & ream them to the given size was impossible.

The flanges need to be symmetrical to the body, so I always set-up based on those (rather than the bores). The final bore size (pre-reaming) on this pair was about 16mm & I obviously had to do both cylinders the same. I'm sure it will all pan out but with castings you really have to have a good look at them and think ahead a bit.

In many ways small fabrications, whilst they need more work up-front, are a bit easier when you come to actually machine them.

Regards,

IanT

 

 

Edited By IanT on 20/12/2019 17:25:53

[Garry Smith 7Participant @garrysmith7]

Thanks Ian Advice is gratefully received.

[JasonBModerator @jasonb]

Something like this

[Garry Smith 7Participant @garrysmith7]

Jason thanks for the CAD drawing that makes sense to me and I will adopt it and hopefully make changes as and when other castings need it. As for MEW 289. The one month I don’t buy it there an article that would be useful to me. I can’t get one anywhere at the moment so I’m hoping I can get a back issue next month. Thanks for your time and help it’s very much appreciated as a newbi.

[JasonBModerator @jasonb]

It's not out in the shops until next week, should be available in digital any day.

Just don't take too big cuts as the GM castings can bend, if in doubt add a support/stop behind as you will see in the mag.

[Henry BrownParticipant @henrybrown95529]

For what its worth, and probably a bit OTT for this, we used set the parts on adjustable jacks and paint on hard blue where there was to be metal to be removed and then scribe lines where the removal was to be from. If the casting wouldn't clean up it was adjusted slightly until best match could be achieved. You might be ok using Plasicene to set the parts on but, of course you will need a surface table of a good flat surface and a scribing block.

[IanTParticipant @iant]

Purists look away now….

I recall having great trouble with some bronze frame castings for a 3.5 inch NG 'Lew' many years ago (I was attending evening classes). I machined the first one and took too much metal off, so purchased a second one and did exactly the same thing again (and No, I've never met Einstein)

I was really 'miffed' shall we say – but Pete (one of the Instructors) said no problem and we walked over to the Welding shop and he soldered a lump of brass on it and then I was able to machine everything to size!

Not best practice but sometimes much better than buying another casting…

Regards,

IanT

[Garry Smith 7Participant @garrysmith7]

Posted by IanT on 21/12/2019 14:06:47:

Purists look away now….

I recall having great trouble with some bronze frame castings for a 3.5 inch NG 'Lew' many years ago (I was attending evening classes). I machined the first one and took too much metal off, so purchased a second one and did exactly the same thing again (and No, I've never met Einstein)

I was really 'miffed' shall we say – but Pete (one of the Instructors) said no problem and we walked over to the Welding shop and he soldered a lump of brass on it and then I was able to machine everything to size!

Not best practice but sometimes much better than buying another casting…

Regards,

IanT

[Garry Smith 7Participant @garrysmith7]

Great 👍👍👍 😊😊😊 More than one way to skin a cat.

[JasonBModerator @jasonb]

I've done the same thing Ian when a casting has been undersize, just mill a flat surface and solder on a new machining allowance

[Dennis RParticipant @dennisr]

Garry

There was a series by Stan Bray in Model Engineer under the heading of Bray's Bench starting in Vol 176 No 4009 in 1996 about building "Tina" from the castings from GLR Kennions together with a follow on of building the boiler.

Harold Hall also did part of the engine in one of his series in MEW, I think on using a milling attachment he had made.
You may find some useful information in the articles.

Dennis

[Nick Clarke 3Participant @nickclarke3]

I have the opposite issue with a pair of 71/4" locomotive gunmetal cylinder castings. No names, no pack drill!

Not got there yet, but they have no cores so I am facing drilling them out to allow a boring tool to enter, where I am worried about drills grabbing and then seeing a not inexpensive amount of gunmetal swept into the swarf bin!

As I say, not got there yet, but as I write this I am wondering what is the chance that they will be very close to size on the outside dimensions to add to my cup of joy?

[John OlsenParticipant @johnolsen79199]

Nick, To drill gunmetal with less risk of grabbing, it would be worthwhile to stone the rake off the cutting edges of the drill. This can be done by hand with a small stone. To be clear, this is not done on the end facets of the drill, it is done on the flute so that where the drill is cutting it has zero or negative rake, rather than the positive rake that the flute gives as it reaches the cutting edge.

A drill treated this way is not as good for cutting steel as it used to be, but if you can manage it, it can be good to have a few treated this way for brass and gunmetal.

John

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