Thread: Best RPM for grinding with a Diamond-Cup disk?

Does it give you an rpm guide (for different processes/materials/surface finishes) in the Bosch grinder user manual?

Dear Vernon,

I've actually just got your basic Domestic-Garden-Variety 11,000rpm 4.5" angle grinder - rather than the pictured Bosch - at the moment...

By way of explanation, I'm just trying to work out whether I'm going to massacre the concrete if I hit it with a Diamond Cup at 11,000rpm. So far I've just been using a Zec-disk for grinding out some cracks, but I've now got a bit of adhesive to get off some surfaces that I want to remain flat. The Zec presents two problems in this regard: firstly, they clog up when you hit adhesive with them (whereas a Diamond-Cup can be soaked in turps overnight to get it clean again), and secondly, you can't grind quite flat with a Zec.

If full-bore 11,000rpm is too uncontrollable for the concrete with a Diamond-Cup, I'll probably need to look around for a variable-speed Grinder - and if that was the case, I wanted to line one up before I shelled out on the disk.

Thanks Vernon,
Batpig.

Batpig,
From what I understand (and this comes from reading info on concrete polishing, not from actual use), using a heavy hand will more likely damage the abrasive.

I would suggest you use your garden variety grinder and just use a light touch. BTW what grit is the diamond disk? If you use a fine grit disk, it should minimise the impact on the concrete.

Or try it out first on the footpath outside (or some other inconspicuous concrete), if you are worried.

Thanks Vernon,

I hadn't bought the disk yet, so I'm unsure about the effective grit rating, but I'd say you're on the money when you suggest high speed/light touch, because it sounds like a good recipe to avoid "chatter". There is certainly plenty of areas of the same pours and screeds as what I'll be grinding, that will be out of sight once the Kitchen cupboards go in, that I can therefore practice on.

Best Wishes,
Batpig.

I just used one similar to that on a 9" grinder to take the paint off the inside of my concrete swimming pool, it did the job very well.

I think you will find (as already said) high speed and a light touch gives best results, but they can be very aggressive and take material off quickly if you need to hog it off like I did.

I used a standard Makita 5 inch grinder with a diamond cup grinder.... very safe and easy to use. Used it to level before floating floors. There was np chatter and very easy to control. The no load speed drops really quickly when you contact the concrete.

Cheers
Pulse

Hi Batpig,

Pulse is right, a diamond cup disc on a full speed angle grinder is no problem. The large contact surface of a cup disc on the workpiece material requires much more torque, but luckily the working pressure needed is only very slight, so fairly quick results can be achieved at high rpm with medium power (from 1000 Watts or so upwards). There is a vast choice of diamond discs that all seem to have been optimized for high speed grinder units. And within this high rpm category there is already enough diversity that takes into account several grades of metal bonding hardness and abrasiveness, as well as element profiles optimized to reduce heat (disc holes and element vane shapes to enhance cooling airflow) or to resist clogging. There is also a vast range of grain sizes and element shapes. So there is already plenty of choice to adapt any ordinary grinder to many different diamond grinding and cutting jobs on many materials.

It would seem that reduced rpm offers benefits on soft or heat sensitive materials or with wet use (spattering, which can be more permamently damaging than soiling by dust). And with electronics, the basic tool is of course much more versatile for other jobs. But other than that, variable speed grinder type machines are more often applied in the region of wet grinding of stone and polishing jobs (furniture, automotive) and matt brushing stainless steel (railings, etc.). The assortment of the Flex tool brand (from Ackermann + Schmitt, the inventor of the series motor driven angle grinder and also an expert in grinding machines for stone and steel) shows good examples of that.

That the basic concept of an ordinary high speed angle grinder works well enough with cup discs, is shown by a Makita example. The tried and tested medium power 5 inch grinder unit 9005B was later developed into the PC1100 concrete planer, with alterations made to bearings and handling, but almost none to the drive unit itself. There are no electronics, it's the same high speed and the PC1100 had been around for a long time and still sells well. I worked with both models, they are very efficient. In theory, a 9005B with a cup disc from a PC1100 should behave the same and -leaving aside the handy guiding accessories and adapted lower ball bearing on the PC1100- it indeed does.

Do bear in mind though, that ordinary angle grinders will be worn down very quickly by the vast amounts of gritty dust thrown around by cup dics, since these tend to take off lots of material already at the slightest touch and pressure, The commutator copper and coil windings will be sort of sandblasted by sucked-in debris and the arbour's lower bearing will fail much more quickly because of the standard dust sealing that's not up to this sort of use. In concrete and stone grinders the motor parts will be fortified with extra tough coatings, an adapted airflow through the intake louvres and exhaust vents and an effective dust labyrinth on the bearings. The Bosch machine shown by you is a very good example. At max rpm it will achieve quick results on hard concretes and tiles, at reduced rpm it may be used for gentle results on brick and stucco, perhaps even without the need for changing the disc type. The unregulated PC1100 may bite into soft stone a bit too fast, so because of its single high rpm the right sort of discs must be chosen with more care.

regards

gerhard

Thanks to everyone for replying,

It certainly helps to straighten things out in one's mind before one does any Shelling-Out of Hard-Earned...

Best Wishes,
Batpig.