Hard turning vs grinding

[gfphoto]

After reading Evan's post I looked up "hard turning" and found this article.

The article says: "For some shops, though, the process of repeatedly turning parts that are harder than 45 Rc to grinding-level accuracies is still a bit cloudy."

What kind of grinding are they talking about that is so accurate?

Is there some way that what Evan showed could be done by grinding?

Any explanations appreciated.

Thanks,

Gary

[Evan]

It's very difficult to beat proper grinding by any sort of turning operation except in the few circumstances where grinding cannot be easily applied. Hard turning is a very commonly used operation but when you need a really accurate ID or OD on something grinding is the method of choice.

As for what sort of grinding, a good tool post grinder can do both ID and OD grinding on a lathe. It will have accuracy to whatever level of positioning the lathe is capable of as well as the quality of the tool post grinder and the type of stone or wheel used.

[The Fixer]

The bearing race he was turning would have been factory ground for both precision and for surface finish.

[gfphoto]

Thanks for the info. I guess I'm just surprised that grinding would give a better finish and be more accurate. Will research more...

Gary

[digger_doug]

Thanks for the info. I guess I'm just surprised that grinding would give a better finish and be more accurate. Will research more...

Gary

Gary,
I believe you are basing that assumption on what you do on
a bench grinder. In the business this is known as "offhand" grinding
or "snagging" and Norton or other grinding wheel literature
describes wheels for just such usage.

Precision grinding is considered more precise than machining,
as well as producing a far better surface finish.

For even more, google "superfinishing" using abrasives
in a different way, to improve a ground finish even more.
It's used on hydraulic engine valve lifters to give them a
mirror finish, and correct up the roundness just a little more
from what the grinders left.

[METALARCHER]

To carry this one step further, how would surface grinding as opposed to using a face mill or fly cutter work? using a flaired cup wheel in a mini mill?

[pcarpenter]

Cylindrical grinding operations are generally a better way to reach high precision and better surface finish than any turning operation would hope to. It's the reason that they make "toolpost grinders" for lathes (for shops that don't have and can't cost justify the high expense of a cylindrical grinder of some sort). They make a mess and require huge amounts of care in shielding the lathe ways etc. to keep the grit off, but they will do for the rest of us....on the rare occasions that we need that sort of precision.

Don't think of some dude standing in front of a bench grinder.....rather, think of the part spinning in precision bearings with a diamond trued wheel counter rotating and taking off a few ten-thousandths of an inch. In effect, then, rather than a single point of contact with the work, you have thousands of tiny points of contact that are constantly rotating past the spinning part, creating an aggregate cut that is very smooth and very precise. Depts of cut may typically be in the tenths range.

For even more precision, some applications call for "centerless" grinders in which the part is turned between two wheels....and idler that effectively rides on the back side of the part, behind the grinding wheel and the grinding wheel itself. This way, the only thing that establishes the part dimension is the part itself. It takes any issues of the precision of spindle bearings (or lack thereof) or heating of the spindle etc. out of the equation.

If you think about a normal turning operation, say you had a precision specified down to "tenths" (ten thousandths of an inch or a "tenth" of a thousandth). If your next to last pass yields a part that is only .0005" over tolerance and your tolerances are (-.0002,+.0000), how are you going to make a turning cut that takes off .0005"? Cutting tools tend not to work very well when not loaded properly. Steel is being sheared in normal turning. You could probably finish by hand with an abraisive strip....but that's not so precise. In the cases where this sort of tolerance is called out, the part might be finished on the lathe to within a few thousandths and then finish ground to size.

If you still tend to think of grinding as an inherently crude operation, remember that eye glass lenses (and lenses far more precise than that) are ground using purpose-built machines. There you are grinding both to the point of optical clarity, as well as to a precise curvature to make your prescription different than another.
Paul

[digger_doug]

To carry this one step further, how would surface grinding as opposed to using a face mill or fly cutter work? using a flaired cup wheel in a mini mill?

Well not exactly, you need a higher traverse speed for starters.

Look at surface grinders, these are what you would use.
There have been surface grinders built as you descibe
(the wheel orientation, not the actual contruction)
Pratt & witney made one.

Also please notice how dedicated grinding machines do not
have exposed ways. Everything is covered/mounted behind
so as to stop the ingress of the grinding dust/grit.
Some also incoporate flood cooling as well, with the added
gutters on the table, the settling out apparatus, and
a holding tank with pump.

[DR]

One of the punch and die companies maintains an inventory of heat treated blanks.

When you want a specify size punch it's a simple matter for them to hard turn one of the blanks to the required size.

Prior to their use of hard turning the process involved rough turning a soft blank, heat treating the rough turned punch, then grinding to size. Relatively long lead time as opposed to the hard turning method.

[Glenn Wegman]

O.D grinding a hardened steel pin. That's a 14" wheel.


I've ID ground sprag clutch housings (similar to what I believe Evan is making) but can't find the pics or I'd have posted one of them.