I promised some time ago to do a post on my method of making knurled thumb screws. I used these screws when making cutting guages such as the one pictured below (see a previous post).
A cutting gauge featuring my decorative knurled thumb screw
These screws are very useful around the shop for jig making and are often needed if you are a hobby tool-maker. You will need access to an engineering lathe for this project.
There are two principle ways in which a knurled thumb screw can be made. One is to turn the entire thing on the lathe from the chosen metal blank (I call this the “one-part method”) but if you are like me and mainly use brass thumb screws, this method can be relatively expensive in terms of brass used. Between three and five times as much brass will be used than if the second method is employed, depending on the length of the thumb screw.
The second method, which I call the “two-part method”, is the method I intend dealing with here simply because it is my preferred method (because it’s cheaper). Essentially, it involves making the thumb screw in two parts; the knurled head being one and the threaded shaft (cut from either a suitable steel bolt or a length of threaded bar) being the other (you can of course, use plain bar and thread it yourself if you wish). The shaft is then threaded into the head and the two parts bonded together with Loctite superglue (I have never had a problem with one coming apart).
Anyone interested in learning how to turn the entire thumb screw on the lathe should have a look at MACHINE SHOP TIPS ♯ 106 Making Knurled Thumb Screws for South Bend Lathe, Tubalcain, on YouTube. He actually goes through both the one-part and the two-part method of making knurled thumb screws. Clockmaker, Clickspring also does an interesting video on turning small screws although these are polished rather than knurled. It’s a well-made, crystal clear video though and worth a look: How To Make A Clock In The Home Machine Shop – Part Three – Making The Washers And Screws.
So, to begin, we cut a length of bar (brass in my case) of the required diameter, long enough to make the number of knurled heads we require, allowing extra length for chucking and parting. If our bar stock is too large in diameter, we simply turn a piece down to the required radius on the lathe. Everyone’s exact requirements will be different but for the exercise at hand let us assume that we are making thumb screws of the size and type that I used making my marking/cutting gauges. These screws were 1and1/8“ long overall, with a 15/16“ dia. head, with a button insert. The head was 7/16“ thick. The thread was M6 with 11/16” of length.
Knurled Thumbscrew Designs
The thickness of the heads varied ever so slightly, one from the other, at the time I made the gauges as each was measured and marked by hand before parting off on the lathe. I have since made a carriage stop for my lathe which will pretty much guarantee that all heads from now on will be exactly the same thickness. Not that the difference in thickness was particularly noticeable to the eye but I was aware of it, so I made the stop.
The picture above shows three different types of knurled thumb screw that I made for the gauges. On the left is what I call my basic thumb screw; just a threaded shaft and a simple, flat knurled head. The middle screw has an imitation ivory button insert in a flat knurled head. The right-hand screw is a bit more elaborate and has a mother of pearl diamond inlay in a black button insert and chamfered edges to the knurled head. These two latter designs make a handsome adornment to any tool or jig you might be making. There are any number of designs that can be made of course; just use your imagination.
The Carriage Stop I made (from brass off-cuts) for my Lathe. It sits under the chuck, bolted to the bed, and has two adjustment phases; one by loosening the beam and a micro adjustment by turning the screw on the right. A lock nut holds the setting.
A crash course.
Before we knurl a bar we must ensure that the bar is turned to the correct “knurling” diameter. If it’s not, the knurl will, more than likely, over-track and ruin the appearance of the finish. That being said, there are many variables involved in the knurling process and even having the correct knurl diameter does not necessarily guarantee a perfect finish. However, we can but try.
The first thing to establish is the circular pitch (CP) of the knurl you are using. Knurl pitch is simply the number of knurls (or teeth, if you like) per unit length of measure. If you did not find out the pitch of your knurl when you purchased it (don’t worry, I didn’t either!) you can work it out as follows:
Ink the knurl with layout dye (or even oil if you’re stuck) and roll it across a sheet of paper. You will end up with something like this:
Now count out 100 of these lines and measure their distance with a calliper (you can buy a handy digital calliper on Amazon). Divide that distance by 100. The resulting number is the circular pitch (CP) of the knurl.
Now the diametrical pitch (DP) of a knurl is π/CP and what we want is the inverse of that; CP/π because the dia. of the bar we want to knurl must be some multiple of that number. [The number π, or pi, is a mathematical constant; the ratio of a circle’s circumference to its diameter, and approximately equals 22/7 or 3.14159].
If the 100 lines you rolled out on the paper measured say, 4.00 inches, then 4.00/100 = 0.040. Now, 0.040 tells us that we have a 25tpi knurl because 1/ 0.040 = 25. This we should note for future reference as 1/25 = 0.040 is the CP of the knurl or the distance between the teeth. Other common CP of knurls in tpi are 10, 12, 14, 16, 20, 30, 35, 40, 50 & 80. Now, dividing our CP by π gives 0.040/3.14159 = 0.0127”. This figure becomes our multiplier.
If we were trying to knurl a 0.5” bar, then 0.5/0.0127 = 39.370 which is an uneven multiple. The .370 “spare change” is our problem. We need to get rid of the “spare change”. So, in order to get the best chance of a good knurl pattern we should reduce the dia. of the stock to 0.4953”. Now where did I get that number? Well, from 0.5/0.0127, we know that the base multiplier integer is 39, so multiplying our multiplier, 0.0127 by the base multiplier, 39 gives 0.4953” turn diameter (which, you can see is a smidgen less than 0.5”). This reduction in dia. may seem insignificantly small but it should help to ensure that the knurl tracks correctly, ceteris paribus, or all other things being normal (whatever normal is!).
Unfortunately the above calculations do not hold precisely under all conditions. I have found that the same knurl can track differently on different occasions. This could be down to variations in the sharpness of the teeth, how clean the knurl is, wear on the knurl etc. and of course, variations in diameter of the stock or even just my own level of concentration at the time being (in particular, how firmly I engaged the knurl tool to the work in the first place).
Moreover, by the nature of the operation, the contact between the knurl and the work will tend to have some ‘slop’ in it, thus the knurl will be inclined to climb or slip enough to land in existing grooves. As knurling involves pushing or squeezing material out of the way rather than cutting material, the knurl will tend to follow the path of least resistance. The secret then is to get those grooves well established at the start; this means the knurl (following the path of least resistance) should then follow the path you want it to follow and not find some other new path.
To get the grooves firmly established, we need to be brave and jam the knurling tool tight into the work from the beginning and this implies that the work is firmly held in the chuck and supported at the tail stock end by a live centre. In this regard, only the length of stock needed should protrude from the chuck. There is no point in having the bar reaching the entire length of the lathe if we are only knurling an inch or two. Short and stubby is easier to hold firmly in place than long and lanky.
In this regard also, a straddle type knurl holder is preferable to a ‘bump’ type knurl holder. As considerable sideways force is involved with the ‘bump’ knurl holder, this may cause flex in small diameter stock which varies and disengages the contact between the tool and the work, leading to uneven tracking and disappointment. Furthermore, frequent use of ‘bump’ type knurl holders can lead to wear and damage to the lathe bearings and the cross-slide. Not good! The straddle type holder pinches the work with an up & down clamping force that is independent of, and has no effect on, the bearings. There is also a better chance of a more even contact between the knurling tool and the work with a straddle type holder. Furthermore, the contact can be made firmer by tightening the screw on the straddle holder, a facility that is not available with a ‘bump’ holder. With the ‘bump’ holder you have to force the knurl further into the work with the cross slide causing stresses all around. Straddle holders are a bit more expensive than “bumpers” but if you intend doing knurling on a regular basis, a straddle type holder is well worth the money. Accu Trak Tool Corporation (www.accu-trak.com) manufacture good quality knurling tools & holders.
Some factors which affect the tracking of the knurl are:
- The feed rate too fast
- Sharpness of the knurl teeth
- Hardness of the material
- Included tooth angle of the knurl (a sharper angle penetrates
- Width of knurl face (a narrow face penetrates easier)
- Method of the operator (while knurling may seem a straightforward operation, everyone has their own way of going at it).
- Cleanliness of the knurl (always brush the chips from the knurl with a small wire brush before starting and flush the work continuously while knurling).
- Whether you are using a bump knurl holder or a scissors or straddle type.
- The knurl may not be centred properly on the work.
- If you make the mistake of disengaging the knurl from the work during the operation (if, for any reason you do this, it may prove difficult to re-engage it exactly on-track).
- Contact between knurl and work too light at start so that the groove pattern is not firmly established.
- Tool post not perpendicular to the work.
Most machinists will tell you that knurling is a fickle business so don’t despair if your first attempts are a bit hit and miss.
A change in any of the above variables may correct (or cause) a tracking problem.
In summary then, “fingerprint” your knurl and measure the CP. Use stock with a diameter that is an integer multiple of the CP/π. Hold the work firmly at both ends, then establish the knurl groove-pattern firmly on the work from the start. Finally, turn the lathe speed down to about 120rpm while knurling and feed slowly and at a steady pace. It may be helpful to feed under power rather than by hand as this guarantees a steady rate of feed. I am not sure however, if this makes much difference, as from my own experience I have seen no real difference in the results between power v. hand feeding. Don’t disengage the knurling tool from the work during the process or it may lose the track. Keep the tool post exactly perpendicular to the work. Use plenty of lubricant during the knurling process.
Two straddle type knurl holders
A simple bump-type knurl holder
Completing the thumbscrews involves drilling the knurled head (while still in the lathe and before the head is parted off) and threading with a thread that matches the threaded bar you are using (M6 in my case). The head itself can be chamfered to tidy up the edges. Also, if you intend inlaying a decorative button in the top of the head, a shallow depression must be drilled in the head, again before parting off.
This depression must be drilled to a diameter that matches the material you are going to use for the inlay button. It could be, artificial ivory or even a decorative hardwood that has been turned to form a rod from which thin slices can be cut off for the inlay. I used two-part epoxy to glue in the inlays.
Once the inlay was glued in and dried, the thumbscrew was put back into the lathe for tidying up using a fixture I made for the purpose. The fixture is a rod, drilled and threaded (to M6 in my case) into which a thumbscrew can be tightened facilitating the trimming of the inlays etc. The fixture prevents the threads of the screw from being damaged by the jaws of the chuck. See below.
Fixture for re-chucking the thumbscrews to tidy up after inlay work. The 2nd photo shows (for demonstration purposes) a plain headed thumbscrew inserted into the fixture and tightened to “hand-tight”. Hand-tight usually works well enough for tidying up the inlay. If heavier work was being done an actual collet would have to be made. See later.
I knurled the fixture just to give something to grip. I have two such fixtures, one for M6 and the other for M8 threaded thumbscrews.
As a matter of interest, I also made an actual collet for holding the larger threaded thumbscrews that I use in the lever caps of the hand planes that I make. This means that if I want to do some inlay work in the heads of those, I have a way of re-chucking them.
I made this larger model as an actual collet, which tightens on the thumbscrew threads under chuck-jaw pressure for a solid grip but without the chuck jaws damaging the threads. See picture below.
Shop-made brass collet for holding lever-cap thumbscrews. The slots in the collet were cut on the milling machine.
If you want to go the whole way and make thumb screws with the diamond MOP inlay as I did, you are in for some hand work. Once the initial inlay is glued into the head of the thumbscrew and flushed off on the lathe, the shape of the diamond is carefully marked onto the inlay. The recess for the diamond is then carefully cut with a very small chisel. This is painstaking work that requires some patience as it is easy to make a mistake and ruin the whole piece. I used my threaded fixture clamped in a vice to hold everything steady while doing this work.
The MOP diamond is then epoxied into place and allowed to cure. It is then a matter of sanding the diamond flush with the inlay and finishing by polishing.