A modelling knife starts where a scalpel gives up. It has a thicker blade, a sturdier body and takes real pressure, so it copes with styrene sheet, heavier profiles, balsa and the tougher materials of a diorama. The category covers hobby knives with replaceable blades in wide and narrow patterns, fixed-blade carving knives for draw work, aluminium and ergonomic grips, and complete knife sets in a case. It is bought for rigidity, which is exactly the property a fine blade cannot offer.
When the material resists. Styrene sheet over a millimetre, timber strip, foam, hard rubber, a heavy profile — all of them need a stiff blade that will not flex or snap. The second reason is shaped work: a fixed-blade carving knife is built to be drawn and to remove material progressively, which a thin scalpel blade cannot do at all.
The most used group. A wide blade resists pressure and tracks a straight cut, a narrow one reaches detail. The holding system decides which edges will fit, and the stock is in spare blades.
The blade is set permanently in the body and built for repeated draw work. They are stiffer, hold direction better and are used for shaped removal, for carving timber and compounds, and for work where the blade is steadied against the thumb of the other hand.
With a knife the grip matters more than it does with a scalpel, because you are pushing. Aluminium and profiled handles spread the load into the palm and a non-slip surface stops it travelling. On a long session that is safety rather than comfort.
Several handles and blade shapes together. They make sense for anyone mixing materials — on a diorama with timber, foam and plastic you will swap tools ten times in an evening.
Always cut away from yourself and keep the other hand out of the blade's path. Guide straight cuts along a steel edge from rules and squares and put a cutting mat under the material. Use sensible force — if you genuinely have to push, either the blade is blunt or you are on material that belongs to a saw.
For regular circles and repeated shapes, a tool from circle cutters is more accurate.
