Dressing a vitrified bond CBN wheel?

[michiganbuck]

A porous dressing sticks of vitrified abrasive grits softer than diamond are great for abrading away the bonding material that holds the diamonds in the wheel, so letting the diamonds stick out to contact the work/part, and so also make space for chips and coolant

One very good if not best material for this is a scrap green wheel broken into pieces.

 

[michiganbuck]

Diamonds would be a good abrasive grit for steel, perhaps better than aluminum oxide and CBN.
It is not used because it is far overkill and very expensive.
Perhaps a diamond wheel made for steel might last 10% longer and cost 10 times the price. ( I'm just making up the comparison numbers)

Aluminum Oxide and CBN grit does have sharper facets than most natural and lab-created diamonds, so that would be a factor..

Aluminum Oxide is a 9 on the hardness scale so plenty had for steel.
Steel might be 4 to 7 hard. (I'm not sure about 7 that might be CPM, a file is about 6.5)
Some CPM materials may be so hard/tough that diamond wheels may be considered.
Diamond wheel use on steel is uncommon but is being done.

I once ground a test CPM (compressed powder metal) that was on the verge of ungrinsable with AO and CBN wheels. I think the grinder was 7 HP and that material would try to stall the spindle.

Going to a softer wheel it would just eat the whole wheel.

 

[CarbideBob]

What's the use case for these kinds of "rotating wheel" dressers? Why are they called "brake dressers"? Are they only for diamond and CBN wheels or also for SiC or AO ones? Are there some standard rule of thumb about the size/speed/materials for the dressing wheels? Motor driven versus free wheeling? What rpms or surface speeds are appropriate?

Actually named brake controlled truing devices.
On the back end are brass shoes loaded by a coil spring and a bolt inside a cup. One can adjust the screw to limit rpm where the brake "turns on".
Since the dressing wheel or stone will not go up to the speed of wheel a grinding action occurs that removes the bond and the abrasive falls out.
Motorized the same as the motor now limits the speed. Note that here both "wheels" spinning the same direction just at different surface speeds.

There is a whole different motorized dresser for AO wheels that use a plated diamond roll for form work. In this they usually rotate against each other.

Cup removed and shoes shown below.
Bob

 

[michiganbuck]

The brake dresser actually puts on the brakes so the dresser wheel is turning slower than freewheeling.
Freewheeling would only crush dress and crush dressing is not suitable for super abrasive wheels. The slower dresser speed makes a grinding action to the wheel being dressed. Additionally, the brake action restricts the dresser RPM to be less than the blow-up speed which is about 8200 to about 12.000 depending on the dresser wheel brand..

We would take a pencil eraser end to give the dresser wheel a little spin so it would start on the wheel with no radial bearing vibration or start-up shock to the grinding wheel.
Angle brake dressers were mad that had a dresser wheel in a shaft to set on the Mag at 45*, the angle would restrict the RPM, and with being an angle would cause the needed grinding/dressing action.

I am not sure about using a brake dresser on an AO or another non supper abrasive wheel. It seems that most break dresser wheels were about 100 to 15O grit and very, very hard, the hardness might tend to dull an AO or green wheel's grit.

A green wheel will dress a diamond wheel(likely a CBN also?), but it takes a lot of green wheel so only good when machine time is a factor. A diamond wheel can be put on the likes of a motorized TC grinder workhead and dressed. Yes you may use $15.00+ worth of green wheel to take .005-.010 off a 6" diamond wheel. Suppose a CNC grinder is making $100 and hour+, and the apprentice can put a fresh sharp corner on 3 or 4 wheels in an hour (perhaps a 60* 6" x 1/4 x 1 1/4 with a max .o10 corner radius wheel)
Where shutting down the CNC and setting up the brake dresser on the CNC might take a half-hour to dress one wheel. To do this one likely has to have a TC grinder pre-set-up so it runs dead-true with no fiddling to make true. *YES, a break dresser mounted on a TC grinder would be better.

Penn tool and National Labs often have a good new price on new brake dressers.
The old Nortons were very good and often to be bought used on ebay or the like.