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Eccentric turning

Trunch Trojan sheave

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Arthur Goodwin22/08/2020 22:38:00
41 forum posts
18 photos

Hi all

It is 54 years since I did my one and only eccentric turning as a fifteen year old in the training school.

I am now faced with my second attemp in doing the eccentric sheave for my engine.

My Q is would it be accurate enough to use a 3 jaw with a spacer on one jaw or do I need to use the 4 jaw with all its incumbent setting up.

I have seen vids on you tube and the 3 jaw is certainly much easier.

What do you guys advice.

Regards Art

Hopper22/08/2020 23:21:12
5027 forum posts
111 photos

Yes you can use three jaw and packing. But the thickness of packing is not exactly equal to the offset you end up with . It's easier to use the 4 jaw if you have it. But one of the old books by either GH Thomas or Tubal Cain - I dont remember which - had a chart in it of packing vs offset. But you can play around with different packing Ang measure the offset with a dial indicator until you get it right.


Edited By Hopper on 22/08/2020 23:23:01

Jeff Dayman22/08/2020 23:58:35
1969 forum posts
45 photos

Hi Arthur, Just FYI I don't do my eccentric boring on the lathe any more. I switched years ago to turning the OD and flange in the lathe, leaving the eccentric on a stub of the parent bar. I then load the bar eccentric end up in the mill, locate centre, step over by the needed offset, then drill and bore or ream on the mill. Less quesswork, in my mind anyway, no need to find packing exactly right, no risk of packing slipping out, etc. Just mentioning it as an alternate method. If you or someone you know has a mill, give it a go.

Paul Lousick23/08/2020 05:04:53
1636 forum posts
610 photos

My eccentrics are made the reverse of the conventional arrangement and have the female slot in the eccentric instead of the strap. Turning an external slot in the eccentric is easier than boring an internal slot in the strap.

eccentric 1.jpg

The faces of the eccentric straps machined and the casting bore cleaned up (left undersize). Position of cut marked out using digital readout on the mill. Holes for bolts drilled. Straps cut in half and the mating faces machined flat.

eccentric 2.jpg

The 2 halves of the strap bolted together with a piece of shim material in between. (replace with thinner shim later to adjust the gap). The strap then clamped between 2 plates and centred on the lathe.

Then the eccentric strap can be bored straight thru to the finished diameter. No difficult internal slots to bore. The outer, larger bore then turned.

The strap then turned over to machine the opposite side. A dial indicator used to re-centre the strap but is not that critical as the outside recess is only a clearance fit with the mating eccentric.

eccentric 3.jpg

Material for the eccentric being turned and and the strap used as a gauge for a running fit. Then parted off and face cleaned up.

eccentric 4.jpg

Eccentric centred on the mill and an undersize hole drilled thru, using the DRO to measure the offset from the centre.

Then returned to the lathe and used a centre in the tailstock to position the eccentric on the chuck. Chuck jaws carefully tightened to hold the eccentric in position for the filal boring of the hole. (could have used a boring head on the mill but did not have one at that time)

JasonB23/08/2020 07:11:50
19504 forum posts
2131 photos
1 articles

Another vote for using the mill to put the hole in after turning the OD and cutting the groove. If you then drill for the grub screw you can use that to hold the eccentric onto the end of a piece of bar back in the lathe so that the boss can be turned concentric to the hole.


Edited By JasonB on 23/08/2020 07:13:38

Roderick Jenkins23/08/2020 09:13:58
2002 forum posts
524 photos

Tubal Cain's formula for eccentric turning in a 3 jaw SC chuck:

tc eccentricity formula.jpg

The shim mentioned is wrapped around the job to protect the finish from marking from the chuck jaws. I have used this method successfully.

Stay well,


Andrew Johnston23/08/2020 16:37:41
5824 forum posts
662 photos

I did all the boring and grooving on my eccentric straps and sheaves on the lathe using fixtures in the 4-jaw chuck. The assembled eccentric straps were located with four buttons for boring and grooving:


Similarly the sheave was mounted on a plate for boring the offset hole:


The plate is set true by indicating on the central hole. Not visible are two buttons at positions calculated to give the correct offset for the hole in the sheave.


Arthur Goodwin23/08/2020 16:52:06
41 forum posts
18 photos


Thanks guys. Thats a lot of useful info to get my head round.


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