info needed for turning excentric

[lordedmond]

I need to turn up a cam with lets say for the question with a 2mm offset

can I just put a 2mm shim under one jaw of the three jaw or do I need to far around with the 4 jaw and use a dial gauge to clock the offset

BTW on the job it will need to be pretty close to dims.


Stuart

 

[Jasonb]

2mm shim won't give 2mm offset in a 3 jaw, there are formulas about for working out what size shim is needed, I expect Marv has one.

4 jaw is what I would use and clock it in with a dti.

J

 

[lordedmond]

thanks for that I have been trying to get my brain cell in gear to work it out

so much for the easy way out


thanks

Stuart

 

[Jasonb]

Just looked on marv's site and as I thought there is a program that will work out the thickness of packing, its called eccent.zip

http://www.myvirtualnetwork.com/mklotz/#shop

J

 

[Ken I]

If you don't have a long throw DTI - bring up the butt end of a tool as a reference and measure the min. & max. gaps to measure the eccentricity.

2c

Ken

 

[lordedmond]

J
thanks for the info but marv's programs are of no use to me , I use a mac and exe file will not run

Ken I do have a long throw dial Gauge but will bear your idea as a back up cross check


trouble is the 4 jaw is 165 mm and the shaft is only 6 mm with the cam bit 16 mm


Stuart

as I am a dev for apple I will try xcode to see if I can recompile Marv's c code

 

[Jasonb]

Just google "eccentrics in a 3 jaw" and you will get the actual formulas and can put in your sizes, Marv's just makes the maths easier.

Though if you are holding by a 6mm shaft that may throw the formula out as most hold the main OD.

J

 

[lordedmond]

thanks

looks like i would need some of Marv's .h files to compile the code

Stuart

 

[Ken I]

I doubt you will be able to push a 6mm shaft over 2mm in a 4 jaw - the jaws will probably collide with each other (been there done that).

If you must hold by the 6mm shaft make a split eccentric bush - yeah more tooling.

Ken

 

[rake60]

I read somewhere that a throw can be machined in a 3 jaw chuck by shimming one jaw.

The formula is: Offset = 2/3 Shim Thickness

To get a .050" I needed a shim of .075"
I happened to have some 1mm thick brass that is .039" thick.
Two pieces stacked would be .078", close enough for me.

I chucked the piece up with the shims and checked it with an indicator.
Perfect .100" total run out. .050" throw with no indicating!

It isn't perfect, it just works.

Rick

 

[Maryak]

Yep,

2/3 it is. I first came across this from an old, (1920's), workshop manual when I needed to make a piston ring with tapered horns. I sometimes think our forebears were pretty clever. They did all this longhand, no calculators, no computers, just some log and trig tables.

Best Regards
Bob

 

[lordedmond]

Thanks guys

 

[tel]

Hasn't our Marv got a programme for that?

 

[Jasonb]

Keep up Tel, I mentioned Marv in my first reply, linked to the program in my second an its beed discused in about half the posts ;D as it won't work on a mac.

J

 

[mklotz]

Keep in mind that Rick's formula is only an approximation since it doesn't take into account the width of the tip of the jaws.

The complete formula is:

P = 1.5*E - R +0.5*sqrt(4*R² - 3*E² - 2*E*W*sqrt(3) - W²)

where:

P = required packing
E = required eccentric offset
R = workpiece radius
W = width of jaw tip

If you must turn eccentrics in a 3jaw - a bad practice in my estimation - a better and safer procedure is to use a tube cut to the appropriate size. Here's an extract from the text file that accompanies my program...

Imagine a tube bored to be a sliding fit on the parent stock. A slot
of sufficient width to pass one of the three-jaw chuck jaws is milled in this
tube. The parent stock is inserted in the bore and the whole thing clamped in
the three-jaw chuck such that one jaw passes through the slot to seat on the
parent stock while the other two jaws seat on the circumference of the tube.
Obviously the centerline of the parent stock is offset from the lathe spindle
axis and turning will produce an eccentric. If we know the OD of the parent
stock and the amount of the offset required, it's possible to calculate the OD
of the tube needed to produce just that offset when clamped as described
above. That's exactly what ECCENTUB does.

 

[MachineTom]

Recently I had a similar need, .300" offset on a 1" bar. To hold a small 6mm bar, turn a disk 15mm or so, then on the mill, center the disk, drill, ream a hole 6.0+mm with an offset of 1/2 the size of total travel needed. ie: 2mm offset yields 4mm travel. put in a couple of set screws and done. Chuck it in the 3 jaw and turn away.

 

[lordedmond]

would the simple equation hold up if I disregarded the jaw tip width , let me explain I have a set of er32 collets and one of arceuro hex to er32 holders has this has six flat sides when I put in the shim from the simple equation on one of the flats then would not the jaw tip width be negated

http://www.arceurotrade.co.uk/Catalogue/New-Products/Stevensons-ER32-Collet-Blocks

boy the more you delve in to a problem the more complicated it becomes

I have a tolerance on 0.05 mm

Stuart

 

[mklotz]

As soon as you put a shim on one jaw in a 3jaw the other two jaws are no longer clamping across their full face width. Make an exaggerated drawing and you'll see what I mean.

Given your desired accuracy, why aren't you using a 4jaw?

Yet another approach...

Center the work in the mill, move off center by the desired amount and make a tiny dimple with a drill. Mount in lathe and use a pump center to center the dimple. Turn eccentric.

 

[Blogwitch]

The mill trick you said Marv is the way I do most of my eccentrics, but I drill all the way through on the mill, then mount onto a mandrel and turn the boss around around the mandrel.

That is the way I showed how to do it in my Paddleducks engine build.


John

 

[lordedmond]

thanks guys

John the cam in question is in the centre of a shaft much like a IC engine cam shaft , BTW your method is the one I also use for steam engine eccentrics

Marv yes I agree with your statement but with the hexagonal bar it would be on the flats not two in the vee and one one the lips


I view of the smallness of the shaft I think it will have to be the 4 jaw

Thanks again for all your input

Stuart