The Marking Knife and why you need one…

This article was originally published in the Narrow Gauge & Industrial Railway Modelling Review in April 1996, Issue 26 as part of a series called “At the Workbench” which examined basic tools and techniques for fine craftsmanship. I have updated the article and prepared new illustrations for republication here.

A Marking Knife is quite simply a specifically shaped and ground knife with the sole purpose of scribing accurate layout lines on materials to be cut or otherwise machined. It is used in conjunction with precision squares, straight edges and other layout tools. It is far superior to scribers, hobby knives, pencils, pens and markers that are often used for the same purpose. There are Marking Knives available commercially. However, any that I have seen are usually a wood-handled tool combining a heavy metal blade at one end and a scriber/awl at the other. In all the commercial examples I have seen, the knife blade has bevels ground on both surfaces of the blade which effectively renders the tool useless for reasons that I will explain later. It is a simple matter to make your own precision Marking Knife from readily available materials. I made the knife pictured above over twenty years ago and continue to use it to this day.

The most important component of the Knife is the blade. The blade is made from a high-speed steel cabinet scraper, available at most woodworking suppliers. Cabinet scrapers come in different shapes, a variety of thicknesses and sizes. Buy a standard rectangular scraper, usually around 2.5″ W. x 6″ L. Try to find one that is between .020″ and .030″ thick. It must be made of high-speed steel. Kunz and Sandvik are popular brands. Do not get a carbide or carbide-edged scraper. High-speed steel is the material of choice because the Knife blade must be readily ground and honed to maintain a sharp edge.

Material for the handle is a matter of personal choice. I used some scraps of hardwood I had laying about the workshop, walnut and ebony. Hardwood is probably preferable. You will be able to get a smoother finish for greater comfort and the handle will last indefinitely if the Knife is properly cared for. The use of contrasting woods is not for aesthetic purposes, it serves an important function that simplifies the use of the tool which I’ll explain later.

The first step is to cut your blade from the cabinet scraper. Refer to the exploded view above for general layout and assembly. The blade length and width dimensions are not critical, something in the range of .75″ W. x 5.75″ L. should be fine. Once you have determined the size blade you want, cut the rectangular blank from the scraper using a hack saw. File the edges flat and smooth by drawing the blank along a flat file which you can rest on your workbench. File one end of the knife blank round. This is the handle end. Now mark out and cut a point on the opposite end. Note that the angle of the point must be less than 90°. If your point is 90° or greater, you will not be able to use the knife in a corner or scribe a line up to a vertical surface. Two cutting edges need to be cut so the knife can be used right or left-landed. Set the blade aside now and turn your attention to the handle.

Cut and carve the wood pieces to form a handle to your liking and comfort. Be certain to select contrasting materials. Laminate the wood pieces to either side of the blade using a contact or epoxy cement. The side with the darker piece will be the back of the blade. Determine where you wish to locate your assembly pins and drill holes through the entire knife assembly. Select a drill dia. that will give a pressure fit to whatever material you are using for your pins. For my knife I used brass rod which I then tapped into place with a hammer. To finish the handle I filed off the excess pin material and sanded all edges and surfaces of the handle smooth. All edges were rounded slightly to avoid any sharp uncomfortable edges in the hand.

You are now ready to grind and sharpen your knife. You will need a bench grinder and sharpening stones. A word or two about sharpening stones is probably in order at this point. It is a worthwhile investment to equip your workshop with a pair of quality oil or water stones. There are several varieties of each available. You only need two stones, a medium and a fine grit, either oil or water. Oil stones can be either India, Carborundum, Washita and hard black Arkansas. India and Carborundum stones are artificial. Washita and Arkansas are natural stones. Use a medium stone for general sharpening and a hard black Arkansas for the final honing and polishing. Be sure your stones are flat. Check them with a precision straight edge. An out-of-flat stone is useless for sharpening. As their name implies, oil stones need to be lubricated with honing oil (a 50/50 mix of Kerosene and lightweight machine oil will work just fine) during sharpening. Water stones are less expensive and just as effective, though they do tend to wear faster and need regular flattening. Two stones are needed, a 1200 grit and a 1600 grit. These stones are stored and used in water. Flattening or lapping can be done on a coarse carborundum stone (as long as it is flat) or on coarse sandpaper over a known hard flat surface.

Using a bench grinder, grind a bevel (approximately 25° will be fine) on the top surface only of both edges of your knife point. Remember the back of your blade has the darker handle. Go slowly, quenching often to avoid overheating the edge. The metal is relatively thin so it shouldn’t take long to produce the bevelled edges. Then using your sharpening stones, lapp the back surface of the blade until it is perfectly flat and smooth, providing the finishing polish with your fine grit stone. The back of the blade must be flat in order to produce a sharp cutting edge and achieve accuracy with the knife. Do not skimp on this step. Then hone the bevelled edges, first with the medium stone, then finishing with the fine stone. You want to avoid rounding the edges and it will take some practice to get it right. The trick is to get a firm grip on the knife with both hands, lock your wrists in position and work from the elbows and upper arms, maintaining a constant angle between the blade and the stone. It is nearly impossible to describe in words, you just have to try it. Once you have finished sharpening the bevelled edges, polish the back of the blade once more on your fine stone to remove the wire edge. This is the same technique used to sharpen plane irons and chisels. A properly sharpened edge will shave hair off the back of your hand — this is in fact the standard test, but be careful. Your marking knife is now ready for use. Periodic sharpening will be needed to maintain an accurate edge.

The knife that you have just sharpened can be used equally effectively on soft and hard woods, metal, cardstock and styrene. In wood, a layout line is cut in the surface of the wood. On metal, the knife will produce a very clean, light line in layout die that has been applied to the surface of the metal. With cardstock, depending on the thickness, the knife will cut the material cleanly. For styrene, especially thin styrene, the cut produced by the knife can be deep enough to allow the styrene to be “snapped” along the line.

Why is this knife better than a sharp hobby knife, scalpel, any other sharp double-bevel knife, scriber, pencil, marker or pen? The answer lies in the blade’s flat back . As the drawings above illustrate, the flat back allows the knife to line up precisely with the straight edge and mark a vertical line precisely at the indicated dimension. All the other implements, being bevelled on both sides or round, will scribe a line at some distance from the straight edge causing inaccuracy. This distance will vary depending on how the tool is held while the line is being marked. Pencils and markers have the added disadvantage that the point wears down as the lines are being marked, introducing even more inaccuracy. In addition, the double bevel edge and conical points of scribers and pencils will tend, especially when marking on wood, to follow the grain pattern of the wood introducing more inaccuracy. The single-bevel blade uses this tendency to its advantage. The pressure of the grain on the front bevel serves to push the flat back of the blade against the straight edge, making it much easier to mark accurate layout lines. If you are working with wood and plan to remove stock with a chisel, say cutting a joint, you can insert the edge of the chisel in the line produced by the marking knife for absolute accuracy (assuming your chisel is sharp that is). You now have a layout tool that will fully exploit the accuracy inherent in precision squares and straight edges. You will no longer need to scribe a reasonably accurate line, cut your stock extra long, then take it to the sander to sand up to almost the right dimension, then continue the process through trial and error to get a correctly dimensioned piece. Measure twice, cut once is the rule.

The reason for the contrasting woods on the handle? I suggested you use the dark wood on the back side of the blade. This provides a very simple method of discerning the back from the front of the blade while working, without having to examine the knife edge itself. This is more important than you might think. If you are holding a straight edge in position on a piece of stock, your eyes are focused on maintaining the correct position of the straight edge. You don’t want to look away to determine which is the front or back of your marking knife before marking your layout line. By looking away, your straight edge is likely to move. The contrasting woods make it very easy to distinguish the back from the front of the blade without having to avert your eyes.

Tags: Craftsmanship, Marking Knife, sharpening, Sharpening Stones