THERE are many shapes and sizes that can be made as broaches. However, in the home workshop the usual shapes are to make square holes and key-ways in gears or pulleys. The sizes often needed for these shapes can be reduced to a few standard sizes e.g. 1/8", 5/32", 3/16" & 1/4" for square shapes and 0.125" , 0.093" width size for key-ways.
There are a few things to consider before making the broach and these generally relate to the type of material to be cut and the restrictions of the broach cutting length, which is determined by the method used to pull or push the broach. In my case I made a simple broaching jig which pulls the broach which is much less likely to cause breakage. However, by design, this limits the overall length of the broach.
Broaches can, of course, be bought from commercial suppliers but these are very expensive, precision made and primarily designed for production use. The broaches I use are aimed at my needs which is basically occasional use and thus, by necessity, low cost and designed specifically to fit my broaching jig.
The other major consideration is the material the broach is made from. There are several obvious materials such as silver steel (primarily for square shapes), oil hardening gauge plate and ordinary mild steel case hardened. In the case of square shaped broaches it is best to use silver steel as it is inexpensive and hardens well.
Key-way broaches are less demanding as they cut on a single face only and using case hardened mild steel does provide a solution that will last for some time. Indeed, many commercial suppliers use this approach although they will have better facilities to harden the broach.
The problem for the amateur engineer in making long broaches is that of hardening and keeping the broach from deforming badly. Long and thin will deform more, so it pays to make the smaller square broaches shorter in length.
In my experience square broaches made from silver steel harden well by plunging vertically into agitated cold water and generally the deformation is relatively small and thus acceptable for my needs. Tempering should be done, but with care, as the broach is under considerable tension when cutting but, equally, if over tempered the cutting edges are quickly dulled.
Key-way broaches present different problems when it comes to choosing materials and hardening methods. Initially, I tried using silver steel which was machined from round bar down to a rectangular shape but, on heating, the machining stresses caused excessive twisting and bending which ruled out this approach.
Commercially available oil hardening gauge plate seems ideal as the exact thickness can be easily obtained but once again during the hardening process heating caused bending problems however these were able to be 'adjusted' after tempering but I found it difficult to ensure the teeth cutting edges remained hard.
The other material for making key-ways seems the most unlikely candidate and that is mild steel! This is very inexpensive, easily machined or hand filed and then can be case hardened which actually provides a few unexpected benefits in that the cutting edges are very hard but under the surface the material is soft and elastic thus any bending deformation can, with care be more easily corrected with gentle persuasion!
Making square broaches
Next comes the critical task and that is the manufacture of the broach. Unlike commercially made broaches, which are precision machined, we model engineers can get by with more basic methods and still achieve reasonable accuracy.
The purpose of a (square) broach is to change the shape of a round hole into a square hole which has equal length sides, square corners and to size. As long as the broach has these qualities then it will produce results which are accurate and certainly better than those done by other methods. It may seem strange that the actual teeth that perform the cutting task need not necessarily be machined with absolute pitch precision since the only thing that really matters is the amount of metal each tooth has to remove rather than the spacing of the teeth.
With this in mind I designed my broaches to provide size accuracy, equal length sides and square corners, such that they can be easily made using a lathe and hand filing. The task of ensuring each tooth removes the designed amount relies largely on the taper used when making the broach.
The drawing shows the design of a standard set of square broaches.
The making process is as follows:
-
1.Obtain a length of silver steel either square or round. If round, then this needs to be machined by milling (and surface grinding if available) to the exact size of the square required. This sizing is crucially important as this determines the finished hole size in the work-piece.
-
2.Machine the RH end of the square bar to form the 'pilot' end. It is essential the centre of the bar is exactly at the centre of the square section so care must be taken when setting this up in the 4-jaw chuck.
-
3.Machine a taper starting from the pilot diameter. The use of a lathe long-taper attachment helps but it can be done in stages using the top-slide off-set.
-
4.Grind a lathe cutting tool as shown which is used to shape the teeth.
-
5.Determine the optimum pitch of the teeth required so that each tooth has strength (form) and will remove the correct amount of material (aim for 0.002" for brass, 0.0015" for steel).
-
6.Using the lathe cutting tool cut each tooth recess by the same in-feed amount and then feed the cutter towards the headstock end a very small amount to form the relief angle (approx. 3 degrees). Repeat this process until all teeth have been done. Note, it is not essential that the pitch of each tooth be exactly the same and,in some cases, it is
