Stuart H10 Crankshaft Bearings

[James Hall 3Participant @jameshall3]

I'm well into my first model from castings, a Stuart H10, and (fingers crossed) all seems to be going well.

However, and it may be because I'm a novice and not practiced at reading engineering drawing, I'm puzzled by one thing.

The brass crankshaft bearings are asymmetrical – the turned portion on one side where the flat part is machined away shown as projecting1/8" and on the other side of the bearing as projecting 5/32". I can't see from the drawing whether the longer projections face inwards toward the crank or outwards.

Perhaps they are intended to limit end-float on the crankshaft but it would be pretty large either way round. I think that these bearings would be the same for a 10V so the same question would arise.

While I'm here, any suggestions for a tool to machine the crank pin – it needs to be fairly narrow bur has to project from its holder to the depth of the crank webs (i.e. the crank offset plus a bit).

Thanks for any advice that you can offer.

[James Hall 3Participant @jameshall3]

Which way round for brass bearings

[Paul KempParticipant @paulkemp46892]

Never built a Stuart so can’t comment on the bearings. On the crank pin it would be good practice to have a small fillet radius where the pin meets the webs. Really depends on the lathe and the tool size it can accommodate and how much you need to machine off. You could use a narrow parting tool and rough out by plunging in and then take the last few thou by traversing the parting tool. Or you could use a vee tool to rough out the centre and then a left hand and right hand tool to rough the sides, finishing with a parting tool as previous. If you have a long overhang to get in and a small section tool limited by the tool post you can give the end of the tool a little more support by making a “pin” that will rest under the tool onto the cross slide so it has support closer to the tip. Plenty of options what you do ultimately depends on your kit and available tooling.

Paul.

[JasonBModerator @jasonb]

They are to retain the crankshaft movement axially.

Not sure how you get the "pretty large"

If you take half the 5/16" width of the bearing body and the 5/32" projection you get 5/16" from centre line of bearing to it's edge.

Crankshaft is 5/8" over the webs

Ctr to Ctr distance of bearing stud hole sin the base is 1 1/4"

Now add two times 5/16" for each bearing and the 5/8" for the crank width and that also totals 1 1/4" Just enough for some "Fitting" as required

I use an insert parting tool with the middle of the insert ground away to finish the pin, moving the tool side to side with very small depth of cut once most material has been removed with a conventional parting insert and action.

[James Hall 3Participant @jameshall3]

Thanks Jason B:

I'd missed the relationship of the bearings to the centres of the bearing stud holes and, as you demonstrate, the wider projection will give a zero end float and just a little fitting. I'd based my misconception of a large end float on mounting the crankshaft but with the bearings not properly positioned axially. Beginner's mistake.

More question on tooling for machining the crank pin.- more newcomer's ignorance I'm afraid – so please bear with me.

I have an indexable parting tool, but only c.1/16" wide do you think it feasible to grind away the centre? More generally one would have to use something like a diamond slip or very small wheel to do this. My inserts are gold coloured so presumably coated (TIN?) I suppose the underlying carbide is fine without the coating, but are there unexpected negatives to removing it – I have wondered about his in respect of other inserts and giving a little sharpen with a diamond slip as I would with an HSS tool.

I do have a more (unused) substantial HSS parting tool which would be wide enough to bifurcate but this, squaring the end, and small roundings to provide the fillets would obviously require a regrind to return it to its original function.

Sorry to extend my original question, but now seems the time to ask.

[James Hall 3Participant @jameshall3]

Paul Kemp:

Thanks to you too for your response which gives some very helpful hints on machining a crankpin. I'll let you know how I get on!

[John OlsenParticipant @johnolsen79199]

If you have the equipment, milling away most of the material with a slot drill is a less stressful way of machining crankshafts. It does require having either a vertical mill with a way of rotating the crankshaft between centres on the table, or something like a Potts milling attachment for the lathe. The final finish needs to be done with a lathe but by that stage only tiny cuts are needed.

John

[JasonBModerator @jasonb]

The main reason to relieve the middle of an insert or HSS parting tool is to recuve the width of the cut by having less cutting edge in contact with the work as you move from side to side which will help to reduce the chance of ant chatter.

With your 1/16" insert you are on the border line of it not really being worth it as the cut is quite narrow anyway. If you can get inserts made for non ferrous metal rather than the general purpose ones then these are "sharper " and will both reduce the risk of chatter and be better for taking light cuts

The only thing to watch is that as your parting tool becomes narrower it will be more prone to flexing sideways particularly with the amount of stick out needed to clear the crank webs

Don't be worried if the final finish is by a strip of emery backed by a strip of wood of needle file (with handle)

What is the vintage of your casting kit as some had cast crankshafts, some flat bar and others were intended to be built up in which cast you don't need to machine the pin.

[James Hall 3Participant @jameshall3]

Thanks again JasonB.
Not sure of the exact vintage of my 10H, but I got it on Ebay and had plainly been living in someone's attic for a while. It's old enough to have the cast crankshaft.
If needs be I'll finish the crankpin with emery, though pride and the challenge say that I should try to machine it to a finish. I'm wondering whether, as an alternative to emery I might lap it: a simple lap should be easy enough to make, or using the big end (which can be progressively tightened up as it is split) hence lapping the two together. I think that I can take the bulk off the inner faces of the crank webs and the pin with my heftier HSS parting tool and the try various strategems for finishing it while still over-size to see what woks best (this worked well with the rest of the crankshaft).

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