Machining eccentrics

[Nick WelburnParticipant @nickwelburn]

Working on the eccentrics for Minnie. I’ve been end milling the faces and then boring the centre. It’s working OK. But the metal is a bit smeary and I am sometimes getting so variation in levels with passes over the surface.

Wondering if I perhaps should be flycutting the big faces? Would that give a better finish?
move got a boring head. Could I effectively fly cut with that?

[Nick WelburnParticipant @nickwelburn]

[EmgeeParticipant @emgee]

Nick

If you are getting variations in level of machining on the face perhaps your mill needs tramming, using the boring head will, if the sweep of the tool is greater than the width of the part, not show a different level but may make one side thinner because of the out of alignment of the head to the table.

Emgee

[RedsetterParticipant @redsetter]

Assuming that these are relatively small eccentrics –

Chuck a piece of bar stock long enough to make the required number of eccentrics, plus parting allowances. Use 4 jaw for preference, but 3 jaw is possible.

Turn full length to o/d, and add any grooves required using a parting tool or similar.

Offset stock in chuck to produce the required throw. Bore and ream all through.

Part off as required – cautiously, due to interrupted cut. Just saw them off if you cannot part deeply enough. Then, if necessary, re-chuck them individually, holding lightly by the rim, to finish the faces.

All the eccentrics will then have the same diameter and the same throw.

.

[noel shelleyParticipant @noelshelley55608]

This is a job for the lathe, as Redsetter had said. Noel

[JasonBModerator @jasonb]

If it is the eccentrics and not the straps then as said face and groove in the lathe 3-jaw, offset in the 4-jaw to drill and ream/bore. If you don't have the book then look at the pdfs of the original article, Mason describes how to make each part, just change the number in the pdfs url or other issues

Also look at how sharp your milling cutter is as it may be riding up on the work if getting blunt. This will be worse on the Gun Metal straps if that is what you are describing as the softer metal needs a sharp cutter

Edited By JasonB on 04/12/2022 07:03:41

[RedsetterParticipant @redsetter]

If the OP is referring to the eccentric straps, then that is also a job for the lathe, as indeed the original article describes.

[Nick WelburnParticipant @nickwelburn]

Interesting, yes I’m talking about the eccentric straps. I’ve been milling every face. Of which there are many, when it seems I should have faced each side in the lathe and filed to shape. That in retrospect might well be easier!

[JasonBModerator @jasonb]

They can be done in the mill, just the fact that Mason only had a lathe when he described their making. Though if you don't have a boring head then you will need to set up in the 4 Jaw to do the bore so may as well face off the casting at the same setting.

Do be careful about how much pressure you put on the chuck jaws the GM is quite soft and can easily distort.

Same applies to turning the soft GM needs a sharp tool either a HSS one or if you use inserts then use a **GT type that is meant for aluminium and other non ferrous metals.

[Anonymous]

For my eccentric straps I did the following:

• Face each side in the 4-jaw chuck on the lathe
• Mill bolt lugs to width
• Slit the castings
• Clean up lug mating faces on the mill
• Mill the eccentric rod face
• Drill bolt holes and drill and tap holes for eccentric rods
• CNC mill the fancy curves on the periphery
• Make square head bolts and hexagon nuts

Basic eccentric strap:

• Bore and recess centre for eccentric using a fixture for repeatability; fixture has a central hole for centring same in the chuck

• Drill and tap oil hole

The brass oiler as shown has been binned and replaced by proper wick feed oilers

Andrew

[Ramon WilsonParticipant @ramonwilson3]

The problem with machining gunmetal eccentric straps in a four jaw is the ease in which the strap can be distorted by jaw pressure or insufficient grip of the strap if done up to prevent such. Gun metal is very soft and will move with ease if not careful.

IMHO its much better to bore the hole first either in the mill or on a faceplate to the required size including any groove holding the strap by clamps rather that any outside edge.

The strap can then be mounted on an expanding mandrel for the outer faces to be machined without any fear of distortion. The outer profile then shaped as desired by methods available.

If it's of help then there are several images covering this process here for four identical eccentrics and straps for a Double Ten which should explain it visually. Scroll to the bottom of the page and then onto the next. Personally I would never consider using a 4 jaw to machine these small parts – a faceplate and a scrap of ali for a fixture is all that;s required for repeatability.

If the gun metal is 'smearing' then the cutter is, without doubt, worn. Doesn't need to be worn much either to have that effect. – You can improve that by grinding a small 45 degree (approx) angle on each tooth by hand on the off hand grinder – yes it changes the end of the cutter but if its worn it doesn't matter – you've just got some more life out of it – and no it doesn't matter if they don't match but the closer they are the better.

Best – Tug

[Howard LewisParticipant @howardlewis46836]

If optimum finish is to be achieved, the tool must be sharp (May be worth "finishing" a freshly ground tool with a light touch from a diamond hone ), and set spot on centre height.

HTH

Howard

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