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How to mark out features which use the centre as a datum without a DRO?

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William Ayerst27/05/2021 19:49:47
259 forum posts

Flush with success from facing it, I'm looking at the slightly pyramidal baseplate for my Stuart 10V.

There are a number of features which take their datum from a centre line.

I don't have a mill or a DRO, just plain old manual hand equipment - and I've realised I have NO IDEA how I would mark out (for example) bolt holes which locate the bearing caps - shown in yellow below relative to a theoretical centreline of the crank shaft (yellow line), or the bolts for the standard (green) vs the theoretical perpendicular centre line:

I gather I could probably measure the length of the top surface of the part in a given axis to get an external datum, and use a height gauge to scribe it - but surely there's a better way?

Any thoughts or help would be gladly taken!

JasonB27/05/2021 19:58:08
21652 forum posts
2495 photos
1 articles

Traditional way would likely be to stand it on end against an angle plate and pack up until the crankshaft ctr line is at a nice whole figure on your steel rule say 2". Then take a scribing block and set that to half the ctr to ctr spacing of the bearing cap holes and scribe their position above the 2" line so 2 5/16". repeat for the same distance below. 2-5/16 = 1 11/16"

Now lay on it's side and do similar for the positions across the sole plate.

These days if I do mark out something likely that I use the height gauge rather than a scribing block but that was all I used when I did my 10V and it was a home made scribing block at that.

The other alternative is to just locate the ctr line and machine/file the "U" shaped cutout. Then make the bearings and sit them in place spotting through the hole positions in the base from the bearing holes.


Edited By JasonB on 27/05/2021 20:16:02

Neil A27/05/2021 21:45:32
103 forum posts

Have you looked at Harold Hall's web site for his article on machining the 10V. It is well worth reading even if you decide not to follow his examples.

He does not drill the holes in the sole plate for mounting the bearings until he has made the crankshaft so that he can get the end float right.

For the standard mounting he decided to make a fixture that he used as a jig for positioning the holes in both the standard and the sole plate.


William Ayerst27/05/2021 22:15:38
259 forum posts

Jason B - excellent advice as always and much appreciated. I have  a nice height gauge so I will do just that. I gather I'm essentially guessing the centre line, right? Or I guess a rod turned to a cone at one end set in the crank bearing holes might work also...

Neil, thank you - I will check out Harold Hall's website also.

While awaiting advice I've been boring out the standard (my first ever boring operation!) and like a complete idiot I've just bored it out about 30 thou over size on the cylinder end - so that's an expensive mistake

Edited By William Ayerst on 27/05/2021 22:21:47

Andrew Johnston27/05/2021 22:41:50
6325 forum posts
681 photos

If I was marking out without a DRO I'd use the aforementioned height gauge along with an angle plate and surface plate. I'd also machine reference surfaces on at least two orthogonal sides of the base so that the base sits accurately on machined surfaces when marking out.


Thor 🇳🇴28/05/2021 05:58:03
1438 forum posts
41 photos

Hi William,

If the bore in the Standard is parallel but slightly oversize, you should be able to turn the Crosshead and Cylinder Cover to fit. If not parallel, is there enough material to turn it parallel? Here is a write up of how mush I have managed to do on my vertical steam engine.


Edited By Thor on 28/05/2021 06:03:20

Dave S28/05/2021 07:19:35
266 forum posts
56 photos

Or it could maybe be sleeved to reduce the size.
How did it end up over? Could be a useful learning opportunity there to avoid doing what ever it was in the future (such as the cylinder)


JasonB28/05/2021 07:31:55
21652 forum posts
2495 photos
1 articles

Yes you will need to determine the ctr of the casting by measurement, eye, or adding a ref surface like a bar in the cut outs.

As Thor says make the lower covers spigot larger so it's a snug fit in the trunk guide. You may be lucky and have enough material on the cross head casting to use that at the larger diameter if not make on from a bit of brass or bronze bar

Howard Lewis28/05/2021 08:47:02
5562 forum posts
13 photos

It might help to machine a small datum surface on one end and one side, rather than working off an as cast face, (which will be at draft angle of the casting )

If you do not have a mill, the datums can be machined, using the 4 jaw. they don't need to be deep just enough the clean up the casting, so that you measure off a consistent face..

If you have access to a Height Gauge, the scribed lines can be positioned much more accurately.

You could probably use the bed of the lathe as a surface table, if one is not available.

If you want to make one, a piece of marble about 50 mm thick will suffice, as long as it on a fairly level surface, and unstressed. (It WILL bend! ) A local stonemason / headstone maker may let you have an off cut cheaply.

I made one. It sat on an angle iron base, carrying a wooden base to which the marble was bedded into polyfilla while it was still semi liquid



William Ayerst28/05/2021 14:13:50
259 forum posts

For marking out, thank you - I have both a surface plate, angle plate, scriber and height gauge so I think I'm covered in that regard.

Also tThank you for the advice on the over-large standard bore. It makes perfect sense. Re-clocking the standard it looks like it is parallel and central, but about 0.668" instead of 0.625". Unfortunately the cast crosshead is about that across the widest point in unmachineed form.

I have some 3/4" brass rod handy, so my thoughts are to open this out to 11/16" / 0.6875", machining the base of the cylinder cover to fit. I will need to fabricate a new crosshead but I don't think that should be too hard with my vertical slide?

The reason it was bored over size is that I was impatient to get started so was using some jury rigged clamping which shifted during boring, ending up with hole eccentric to the (now turned) outer rim - I thought I had miles of room so re-clocked and bored concentric - and then found myself in this spot.

Re: Harold Hall's site - I've read the build and seen some of the clamping and fixtures which look promising, I have ordered some plain aluminium to fabricate some of those pieces, too.

Edited By William Ayerst on 28/05/2021 14:14:31

Edited By William Ayerst on 28/05/2021 14:33:52

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