I have the Wahl cordless clipper and the blade head keeps on coming off.It is not damages any ideas/

blades dull --misaligned --damaged

Go here and see if you can find instructions:
Wahl Home Products Home Pet

http://home.wahl.com/Products/HomePet

Bend the 2 spring clips at the front of the clipper down a bit and try again.

Clipper won't turn off - Fixya

www.fixya.com/support/t23019355-clipper_turn_off

Jump to Open Question - Wahl clippers: I have the cordless model. Can not get blade to stay on. pops off the minute I turn clippers on. Bend the 2 spring ... More results from www.fixya.com

Click the blade off, inside the front end there should be 2 screws, undo them and force the case halves apart from the front. Battery pack will need to come from Wahl, not normally available as a non genuine part.

Model 8550 is a Pro Series clipper. Blades for this model can be found at shaveroutlet.com. See link below

http://shaveroutlet.com/animal/wahl-size-10-replacement-clipper-blade.html

IS there a screw in the middle of the blade near the bottom? or a hole where the screw should be?

Should have a flat head screwdriver handy... on the side of the clipper there is a dial where you can insert the tip of the flat head.. Turn on the clippers and turn that knob with the screwdriver slowly till it hums nice and even..

from the picture you have here, That I see you have a Wahl 8554 Wahl Alpha cordless clipper. but you stated the model 9918-6171 Groomsman Beard and Mustache Trimmer model, You need to be sure of what model you have. He are the two picture of each model. if this is correct, Mannys Sharpening has the charger for this cordless clipper, I had the same problem and he sent me one it took about 2 weeks for delivery.
The clipper on your Left is Wahl 9918

This clipper is the Wahl Alpa 8554 cordless
Call Mannys Sharpening he has all the replacement charger for Wahl.
323 263-4697

Have you been oiling the blades? If you ran out of oil you can use any light machine oil available at any hardware store. If you feel comfortable opening the unit you will be able to see the electromagnet inside and watch what is going on. If all else fails Buy a new set (the exact kind) and keep the receipt. Swap them out and return it from the store where you purchased it. You paid good money for them and they should not have died so quickly. The store will get full credit for it when they return it to the manufacturer so it is not that morally bad. The manufacturer should bare the burden because of the poor quality control that allowed your clipper to die in such a short period of time. Hope you find this solution helpful and the last option is a surefire fix...
George