With the grinding jobs at hand, we often need to achieve very close tolerance of internal dimensions, say, resonant cavities or some rings, spacers, etc. that require internal grinding.
By looking to various classifieds, I found someone offering a large set of internal grinding arbors, accessories and internal grinding media for just a bit over 100 EUR, a steal.

The set of tooling and parts used to belong to an internal grinder, likely, a Kellenberger brand, with a HJN-828 Fischer spindle, operated in the southern part of Germany.

Many high quality grinding media, 16 mm, 20 mm and larger diameter, all in quite suitable grain size 46~60.

These are simply mounted to the arbors by epoxy glue. So you can re-use the arbor many times. With my light use, the grinding media will last a long time, unless I need to do some dressing for geometry, taking off a large amount of grit.

The adapters HJND size were all a bit rusted, but they are made from case-hardened steel, so they don’t loose precision too quickly.

Gave them all a nice polish, and some light stoning.

These are normally expensive, and I would like to use them, but the Saacke UW II has a Blombach spindle with a 1:6 ratio taper nose, there is no adapter from such cone to match HJND size internal grinding holders, unless you custom-make it – considerable effort managing balance and precision to micron levels. May consider it later.

Another problem is the necessary cutting speed, normally 20-40 m/s at the perimeter of the grinding body, which is beyond the capability of the Saacke (6000 RPM).

The HJN-828 is certainly a great spindle, 9 kg of mass! But the price… 20 kEUR, well, that’s above the budget!

Looking around, there are many options, like, used spindles, spindle motors, water cooled spindles, air cooled spindles. After all, settled for this VEVOR brand 1.5 kW air-cooled spindle, 3.5 kg mass, ER11 collet (can fit up to 8 mm shank), and 24000 RPM at 400 Hz. I will be running it in the 12000-24000 range surely, so there is less concern with the air cooling, which does require high RPM to function – if you want to run high frequency spindles at low RPM, better get a water cooled version!

It is specified to 2 µm runout – in fact, I could barely measure any runout, maybe 1 µm, on my surface plate. It is at the limit of what can be reliably clamped in a ER11 collet, and what can be reliably measured. Fair enough.

For the internal grinding, we need to use grinding media of small diameter, say, 20 mm diameter for a 30 mm inner bore – this requires rather high RPM.

The spindle comes with a VFD (variable frequency drive), at 400 Hz, the spindle runs at 24000 RPM, and I have added some configuration about the start and stop ramps to operate the spindle gently. An analog output is used to drive a display of the current RPM (voltage output 0-5 V scaled to show RPM as volts).

For the speed selection, start-stop, display, I repurposed some of the existing controls (and added some new labels and cables internally). All quite easy to do. Documentation is a little sloppy, because it will be a little different for each VFD, and easy to figure out. Power is taken from the contactor for the axis drives, so if the axes encounter an error, the spindle will power down.

The spindle is a 65 mm diameter, so I decided to mount it underneath the primary spindle, using existing M8/10 mm T-slots of the Saacke UW II.

After all that, some test run, without any holder – the Vevor spindle is working great and quietly.

The Vevor spindle’s primary purpose is engraving, wood cutting, etc., but not sure how long it will last if grinding dust will get into the bearings. So, we better protect the bearings. Sliding seals are no good option at these high RPM. So I decided to use an air seal, pushing compressed air through a narrow gap, at very high linear velocities.

Contemplating about the material, aluminum alloy EN AW-2007 was in stock here in suitable diameters, and in case of some incorrect adjustment, clogging, etc., it will be soft enough to avoid damage to the spindle nose (made of steel).

Inside, a groove at the perimeter to take a silicon O-ring, and air supplied from the side, with a screw-adjustable air flow regulator.

The air is supplied by a 6 mm PU pneumatic tube.

All mounted such that it doesn’t get into the way of grinding.

Now, we can do some test. The high frequency spindle fits well to the Saacke, the T-slots are integral to the grinding head and solid, there is no vibration or anything.

Surely, we won’t use that for very fast cutting, high efficiency grinding, or anything like that. These are all good items for productivity increase of industrial grinding processes, but here in my workshop it is enough to cut a few micron at the time, minimal load to the bearing and machine.

For some first test, ground a 45° chamfer on a steel part, just roughly machined, mounted in the A axis, and then ground the chamfer in steps of 0.02 mm, easy!

Another addition – an air blasting gun, optionally can also add cutting oil or water – minimal amount lubrication is not quite effective for grinding, but may help. For the time being, I just use it to blow away the dust, and then use a vacuum dust collector to avoid a too big mess – but eventually, just cutting a few 100 milligrams of metal, so it is not too large a mess anyway.