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How to turn/grind between centers with deliberate small eccentricity?

sfriedberg

Diamond
Joined
Oct 14, 2010
Location
Oregon, USA
There's another thread active about turning eccentric (1/8") long (8') shafts on a manual lathe. I've got a smaller scale problem where I'd like to ask for advice. I plan to teach some highschoolers some rudiments of precision measurement, and part of that is testing for taper and TIR, with bench centers, micrometer and dial (test) indicator. I plan to make several test bars about 6" long, turned and ground between centers. Material will probably be 4140PH. One cylindrical, one with about a 0.001" taper over the length, and ideally one with a cylindrical profile and about a 0.001" eccentricity relative to the center axis. I'm assuming 1/2-3/4" at each end will be turned down below the finished profile for dog driving or chucking. Drilled centers will remain on both ends of each bar.
I am scratching my head on how to do the eccentric bar. There's no way the "drill two centers" approach will work with that little eccentricity. I am considering several options and would appreciate additional suggestions. Whatever I do needs to carry over from the lathe to the tool&cutter grinder (Cincinnati #2). The shop does not have another cylindrical grinder.
Option 1: Turn/grind a cylinder, face off the ends. Indicate position in a V-block in the horizontal mill and drill new centers. This seems like my best option, but there's a whole lot of indicating going on to get the part correctly centered on the spindle before making the desired offset. So highly skill- and patience-dependent.
Option 2: Turn a cylinder between centers. Using 4-jaw chucks both ends, indicate the desired eccentricity, grind to finish. I don't currently have a 4-jaw that fits the work head on my T&C grinder, much less one for a height matched tail stock. But this is possible.
Option 3: Drill center holes on the blank. Build tiny offset center adapters with female center one side, male center offset on the other side, and some way to clamp them in offset alignment at each end of the blank. Turn/grind the blank on the adapters. With my skills and equipment, this option has the best chance of consistently reproducing a specific small offset (if I want to make multiple pieces), but I don't think this will stand up to cutting forces. Might work for the grind step, though.
Option 4: (variation on option 3). Make two disks with an eccentric bore, put them over the turned-down-between-dead-centers drive ends of the blank, clamp them in offset alignment, and chuck/center on the disk ODs. Could use chamfer the disk edges and use matching female centers at both ends. If the disks aren't as wide as the turned-down area, I can arrange a separate drive dog, so the offset disks don't take the cutting force.
 

implmex

Titanium
Joined
Jun 23, 2002
Location
Vancouver BC Canada
Hi sfriedberg:
Overall the first option is by far the easiest.
It's truly not that hard to clock in the bar and then offset it and put in your new centers after you've ground the bar..
Also, you don't NEED to care that it be exactly 0.001" eccentric...you're just making a point with these demo bars if I understand you correctly.
If you wanted to be real brisk about it, you could just bash the female centers at a bit of an angle with a throwaway dead center and a hammer...0.001"ish runout is not hard to make.
Kinda crude though.

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com
 

Peter Colman

Stainless
Joined
Sep 22, 2004
Location
Rugeley UK
Why not turn the blank with centres both ends then transfer to the grinder and grind cylindrical between centres then slip packing under the tailstock and grind again, you can calculate the packing needed by simple trig
 

eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
You can use a live center at the tailstock end and shove a bit of shim stock or even paper in there on one side only to create runout. As Marcus said, it doesn't need to be an exact amount of runout if it's for demonstration purposes. It will be pretty difficult to get much runout relative to the centers if using dead centers, that's pretty much the whole point of using them.
 

mathamattox

Aluminum
Joined
Nov 1, 2009
Location
Charlotte, NC
I was going to suggest the same thing as eKretz. Thats how we used to machine journal bearing surfaces eccentric to existing centers.
 

michiganbuck

Diamond
Joined
Jun 28, 2012
Location
Mt Clemens, Michigan 48035
Guess it would be good to have 1. a dead straight, 2. a tapered, 3. an eccentric, and 4. a wobble.
The shim idea is good, and likely two shims would go to the live center(tail) and to the dead center (head spindle). for the eccentric shims straight across from each other..and for the wobble 180*opposite from each other..
Definitely teach high schoolers how to inspect squareness

and how to make a lathe stub chuck-held on one end mandrel
and how to turn a chuck-held headstock center.
 
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jccaclimber

Hot Rolled
Joined
Nov 22, 2015
Location
San Francisco
Shimming between centers was my first thought. Chucking a tad eccentrically and dressing the center chamfers with a boring bar in the compound after was the second.
A few more complicated ways to do it involving attaching things at the ends and proper eccentric grinding setups, but as easy as the first two are I wouldn't pursue them.
We ordered some parts with journals just a smidge (0.005" to 0.020") eccentric several years ago for testing the impact of shaft eccentricity on seals in our product. The shop put the headstock end in a 4 jaw and had a slip over or screw on cover for the tailstock end that moved the center hole by the same amount.
 

Peter Colman

Stainless
Joined
Sep 22, 2004
Location
Rugeley UK
If you make a cylindrical recess on each end of the bar and grind the bar true between centres. you then make a pair of plugs to fit the recesses with centres on each eccentric and glue them into the ends of the bar.
The plugs could be turned in a collet with a slip of packing on one side or in a 4jaw, after all we are after an approximate runout
 

michiganbuck

Diamond
Joined
Jun 28, 2012
Location
Mt Clemens, Michigan 48035
On a lathe, the head end could have a turned in the 3 jaw chuck center for all 4 inspection shafts with the shim taped to the center and live tail center and the parts dogged to one jaw. I think I would finish them on the lathe so as to be a one-operation for each.

In the 3 jaw the center could be used as turned (true running) for the straight and the tapered ones.. for the eccentric and the wobble ones the chuck held center could be shimmed off set .002( or what is desired) for making the eccentric and the wobble.

This would be a very good lesson so the students understand the 4 common errors a shaft can have, plus learn how valuable a turned-in-the-chuck center can be.

but yes, it would be a good practice on the #2 doing it there
 
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michiganbuck

Diamond
Joined
Jun 28, 2012
Location
Mt Clemens, Michigan 48035
I think the exercise would be good/best to pick teams of 3 students to inspect the 4 parts with the limit of +- .005 and three would have .015 errors in different ways: Within spec, Taper, Eccentric and Wobble.....very cool for high schoolers.
 

implmex

Titanium
Joined
Jun 23, 2002
Location
Vancouver BC Canada
Hi All:
When I think about it, the fact that we all have to rack our brains over how to do it, makes me think this is hard to achieve in real life too.
I can see having an eccentric bashed up center in the workhead spindle and getting runout at one end.
I can see grinding a taper.
I can see grinding a wonky inchworm from overheating the work and bending it between the centers.
I can see grinding a barrel.
I can see grinding non round parts.

But what the OP wants to do...grinding a part between centers and having it eccentric to both is just about never going to come up in real life.
One end (the tailstock end) will be as round as the female center in the job permits but if the tailstock has a dead center, it's going to run concentric.
Now if the tailstock center is a live center in crappy condition you could get there, but you really have to work at it as our discussion shows.

So I gotta ask...what is the point of killing oneself to make a demo piece like this if it's also gonna be ridiculously difficult to make by accident in real life.
Maybe if the project is to demonstrate metrology principles I could kinda see it (even though it's a contrived example), but if it's any kind of cylindrical grinding seminar, I don't.

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com
 
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juergenwt

Stainless
Joined
Feb 2, 2008
Location
Wheaton, IL.
Grind the bar between centers. Make two small plugs (smaller dia as your bar} and solder them onto both ends of your bar. Clamp your part in a V-block. V-block in vise in a Jig bore or Vertical mill. Pick up end of bar (indicator}. Move desired eccentricity on the table and drill a new center into the soldered on plug.
Rotate V-Block for other end of bar to be on top. DO NOT UNCLAMP. Again indicate the bar and than move your table in the same direction as on first end. Put a new center in the soldered on plug.
Now take your bar and regrind it between the new centers to required dimensions. Remove the soldered on plugs on each end and your bar will now have the required eccentricity if you check it between the OLD centers.
Make sure you start with enough oversize to allow for eccentricity on the bar.
For an eccentric bar with gage quality you should use a jig bore.
For small eccentrics you can grind the bar right after jig boring the new centers into the soldered on end plugs.
For larger eccentrics you may want to take a cut on a lathe first. Also you may put a short shoulder on one side of your bar for a dog. By setting a slight angle on your grinder you can now grind a eccentric mandrel.
MAKE SURE YOUR PLUGS ARE SOLDERED ON GOOD.
We used to "sweat" them on. (Tin end of the bar with a soldering iron, tin one side of the plug with a soldering iron. Put two tinned ends together, hold using a pin and heat until they melt together.
Sounds like a long story but actually it is very easy and fast. Your accuracy is a good as your indicating and your movement on the table.

Ps.: You can harden your bar after putting in the fist set of centers if you dont need a later cut on a lathe.
From an old Tool and Die Maker.
 

cyanidekid

Titanium
Joined
Jun 4, 2016
Location
Brooklyn NYC
JESUS!!! parable of the blindfolded "wisemen" and the elephant comes to mind.. or the one about "every problem looks like a nail to a hammer."...

I could do maybe 100 an hour to + - .0005, co-axial or offset excentrics end to end.WITHOUT ANY MACHINE TOOLS. no grinder, no lathe, no shimming, no 4 jaw, no custom setup, no wanky adapters...

needed;
1) quantity of the cheapest import "test bars" you can get
2) decent bench center and two good indicators
3) "sharpie" marker
(that might be all you need, depending on how crappy they are, lol!)
4) 60deg. plated fine grit diamond points.
5) flex-shaft
6) cup of water with a few drops of dish detergent
 
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eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
JESUS!!! parable of the blindfolded "wisemen" and the elephant comes to mind.. or the one about "every problem looks like a nail to a hammer."...

I could do maybe 100 an hour to + - .0005, co-axial or offset excentrics end to end.WITHOUT ANY MACHINE TOOLS. no grinder, no lathe, no shimming, no 4 jaw, no custom setup, no wanky adapters...

needed;
1) quantity of the cheapest import "test bars" you can get
2) decent bench center and two good indicators
3) "sharpie" marker
(that might be all you need, depending on how crappy they are, lol!)
4) 60deg. plated fine grit diamond points.
5) flex-shaft
6) cup of water with a few drops of dish detergent

Yeah, some are getting a little carried away. A live center and a small piece of paper or shim is really all that is needed. Or even just a rotating center in the headstock and a piece of shim.
 








 
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