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Sheep shearing blades are flat ground, not hollow ground. Human blades are the same as dog blades with the exception of a few different sizes and they are numbered differently. They should also be hollow ground.
The blades used on horses that fit the A5 or A2 type Oster or Snap-on blades used on Andis and Laube clippers are the same as dog blades. The horse blades that fit on the big Lister/Wolsley clippers are flat ground.
'Hollow Grinding' is a term used in the cutting instruments manufacturing industry. Hollow grinding is a method of achieving a very narrow, thin edge on a cutting blade, and still reserve the strength of a thick blade. After the initial grinding at the manufacturing stage, it is important that subsequent sharpenings do not grind off this hollow ground edge. An automatic hollow grinding machine is used, which holds both the comb and cutter on the turntable at the same time, matching them together. The two arms use magnets to hold the comb and cutter, and the two arms are adjusted to allow the blades to come in contact with the turntable very lightly and at the proper angle to achieve the hollow ground cutting surface on the blades. To achieve the hollow ground cutting surface, the arms have to be adjusted for each size of blades, 40s, 30s, 10s, 7s, etc. Before the process of honing starts, the turntable is sprinkled with a very fine honing compound that delicately polishes the cutting surfaces of the blades by removing less than .0015" of steel from the blade. The advantage to this is that it is not necessary to hold either of these pieces on this fast moving (400 rpm) table with fingers, which is required on the flat grinding tables.
On the flat tables, instead of magnets, the blade is held on the turntable with the grinder's hand and fingers. This same method is used on all sizes of blades. Finger pressure is needed to hold the blade on the fast moving table. If the grinder should place too much pressure on just one corner of the comb or cutter, then the blades will start wearing in that direction, just like an out-of-line tyre on your car. The cutter and comb have to be ground separately and hope they mate after the grinding. Flat tables use a very coarse grinding powder that rapidly cuts metal from the blades, reducing the life of the blade.
Ask your sharpener if they use the automatic machines that grind both the cutter and comb together. If they do, then I would think that if the sharpening service invested that much money in a 'single use machine' (a machine that has only one use, sharpening blades), then in all probability the machine will hollow grind your blades.
Also, if the grinder comes to your shop, you can take a look at his equipment and watch him sharpen some of your blades. If he is using a flat grinding machine, he will be holding the blades on the turntable with his fingers.
Flat tables are 'multiple use' machines and are used in machine shops for flat grinding any type of material that may require a flat surface, whereas a clipper blade machine is used for only one purpose… to hollow grind clipper blades. Many shops are not willing to invest in a machine that will do only one job. Also, you can buy three flat grinders for the price of one clipper blade machine.
A blade sharpener that uses the proper hollow grinding machine will save you a lot of money. We have found that just by removing .0015" of metal will sharpen most blades, unless they have been scored, perhaps by a grain of sand or such like. By removing only .0015" of metal with each sharpening, you extend the life of your blades and get many more sharpenings before you have to discard them. The result is you don’t have to buy new blades so often. So, I would think this is what you should look for in a blade service.
Factory specifications for hollow grinding
Charts and gauges are used to adjust the machines to the correct sharpening angle for every different blade size. This angle is the angle at which the blade makes contact with the polishing wheel and is specified by the factory for each size blade. The angles are set using steel blocks that are supplied by the manufacturer with the machine. In other words, there is a steel block of the correct thickness for each blade size. e.g. The machine is set up for size number 1A blades with a steel block number 093. This sets the arms that hold the blade on the polishing wheel at the correct angle for the 1A blade. Then the machine automatically sharpens the blade with a hollow ground edge. This procedure should be followed for every size blade that you send for sharpening. This machine is a single purpose machine and this purpose is sharpening clipper blades only. It is a little more elaborate, but in the long run, you can get many more sharpenings from a blade-set.
No matter what the animal is that is being sheared or clipped, all UNIVERSAL Snap-on blades used on the Andis/Oster/Laube groomers clippers are hollow ground. The blades are numbered for the length of hair that is being left on the animal after it has been clipped. So, a number 10 blade, no matter if it is an Oster, Andis, or Wahl blade will fit on any PROFESSIONAL clipper and will leave hair 1/16" long after it is clipped. It matters not what type animal is being clipped or sheared. The numbers range from a number '3' that leaves a 1/2" cut to a number '50' that is used by vets to shave the skin and leaves 1/150" cut. So a 'dog' blade can clip a cat, horse, rabbit, pig, sheep or bull. The guards or combs that are put on the blades will add to the length of cut.
I hope this clears up any confusion you may have regarding the blades and whether they should be hollow ground or not.
