Curiosity question on surface grinders.

[astro_al]

Hi guys, just popped into my head...

I've never used a surface grinder and I was wondering how they produce a flat surface by running work under a spinning grind wheel - doesn't the wheel wear away during the process so that the last point to be ground is not at the same height as the first? I guess this question applies to axial grinding - like crankshafts etc, too?

Anyone got / used one and know the answer???

Ta, Al.

 

[rtbcomp]

It has to be that the wheel wears within the tolerance of the work. Doing successive passes in opposite directions might help.

 

[Welderpaul]

The wheels are very hard, and don't wear that quickly. The wheel should be regularly dressed so the outer edge remains flat, otherwise it becomes tapered, with the most worn part on the side that is on the progressing face of the cut (If that makes sense).
You would usually have the narrowest width of the object parallel with the wheel so the effect of any wheel wear during the passes is reduced. Usually the degree of wheel wear is well inside the tolerances for surface grinding.

 

[astro_al]

Weird, when you think how easy it is to wear away a wheel on a pedestal / bench grinder.

Ok, well, thanks for the replies!

Al.

 

[scott0031]

Your thinking is along the right lines. To fully understand what happens during a grinding process you really need to understand fully how a grinding wheel works.

A grinding wheel like all forms of cutting it uses single point cutting, as in a lathe tool or milling cutter. A cut is made by a sheering process on the material to be cut. The geometry of the tool is what creates to surface finish attained.

The same principle applies to a grinding wheel the major difference is a grinding wheel has ‘multiple' single point cutting, each grain of grit acts as one cutting tool and sheers away the material.

The type, size of the grit and the bonding on the wheel all play a factor in the end result.

When a grinding wheel cuts the grains that make up the wheel dull off, as they dull off this creates more force on the bonding agent of the wheel. As that load is increased it gets beyond a critical point where the bonding cannot resist the load any longer. When that happens the grain departs from the wheel. In turn a new grain is exposed underneath and hence a fresh single point cutting tool is exposed. When you see a wheel glazing up this is due to the bonding being to strong. This does not allow the grit to depart from the wheel. So there fore you get lots of very dull cutting edges still attached to the wheel. These factors with right wheel choice are what gives you the correct end result as in surface grinding.


Depending on the style of grinder, its normal to do a finishing cut this is very low load as such with the correct wheel it does not dull off the grains to any degree. They stay sharp and compared to heavy cuts they do not depart from the wheel, hence you will end up with a flat surface.


That’s the basic theory behind grinding wheels and how you achieve the end result.


Hope that’s some help to you.


Kind regards,

 

[astro_al]

Hi, thanks for the excellent reply. Makes sense - I figured on a minimal final cut to reduce the diameter change to a minimum.

Why do people use them - is the finish so much better than just milling the surface flat?

Ta, Al.

 

[Rubber Duck]

Hi guys, just popped into my head...

I've never used a surface grinder

Please, please, please make sure all the saftey guards are in place. If in doubt get someone knowledgeable to show you how to work it.

Grinding wheels can be increadibly dangerous if not used properly!!!!!!

 

[astro_al]

Good advice - I don't have one, I was just wondering about them, but good advice nonetheless.

Al.

 

[scott0031]

There is a multitude of reasons. That question does open up many other questions, so to keep it simple.



1)The most common being the ability to work with hardened materials.
2) Pure accuracy and squarness/flatness not achievable by other means.
3) Surface finish not achievable by other means
4) The ability to produce accurate forms and shapes not achievable by other means
5) The ability to achieve complex forms and shapes not achievable by any other means. (Normally press tool orientated and jig/ fixture orientated) that require high tolerance generally 10% of the part tolerance. So super high accuracy is achievable. IE: if a part has a tolerance of + and – 0.1mm the tooling will be made to within + and - 0.01mm. So the bigger the part and the tooling, the harder it is to achieve the tolerance for the tooling. The example stated is only an example. The tighter the part tolerance the tighter the tooling tolerance. Tolerances go much tighter than + and - .01mm




Kind regards,


Scott (ward eng)