Set up in 1933, the Thor Hammer Co is the UK’s only manufacturer of soft hammers. It produces some 6,000 a week at its base in Shirley, Birmingham, and can claim to have one or more of its tools carried by most skilled fitters.
They are used by the MoD and any number of engineering companies and workshops around the world, as well as by classic-car owners, who still order them in ‘original’ specification to tighten and loosen ‘spinners’ on the wire wheels of 1950s and 1960s sports cars. Moreover, it was a Thor copper/rawhide combination hammer that was seen in the press photographs taken when HRH Prince Charles visited the Whitechapel Bell Foundry and struck the first Royal Jubilee Bell — appropriately named ‘Charles’ — which heralded the Diamond Jubilee Pageant.
Securing the shaft to the head of a Thor hammer (heads can be made from a variety of materials including cast iron, a die casting, aluminium or aluminium bronze, lead, rubber, laminated compressed wood with flush fibre hoops, polythene, pure extruded copper, beryllium-copper, rawhide or brass) is far more involved than just knocking in a handle and pressing or tapping in a wedge. Safety is critical, especially when the heads of the largest solid-copper mallets are some 85mm in diameter, 173mm long and weigh 9kg.
For machining its components, Thor uses a pair of XYZ machines, both controlled by Siemens 828D systems and supplied by Burlescombe-based XYZ Machine Tools Ltd
(www.xyzmachinetools.com). The 710 VMC vertical machining centre features a mechanical fourth-axis index unit and is used to produce the shaft hole in the copper and brass hammer heads. Meanwhile, a Compact Turn 52 CNC lathe turns the rolled, pinned and pressed blanks of water buffalo hide used as facing for the hammer head.
Managing director Derek Mathers says: “When we replaced our existing VMC at the end of last year, we checked out the market for suitable machines. The XYZ 710 VMC gave the best ratio of price against productivity gain we felt we could achieve. Moreover, the level of support we received from the XYZ applications team was such that it was a ‘no contest’ when we came to increase our turning capacity to meet increased demand for rawhide facings. We installed an XYZ CT 52 lathe.”
Elliptical bores
Thor processes some 10 tonnes of brass and copper a month. Machining hammer heads from these materials involves producing diametrically opposed elliptical shaped bores up to 38 x 48mm with 3deg ‘locking’ tapers that meet in the centre of the head. When the appropriately shaped handle (in most cases produced from hickory, a hard, dense and shock-resistant wood that in Thor’s case comes from 100-year-old trees in Tennessee) has been fitted to the head, a wood and steel wedge is inserted into the head end of the handle to expand the wood so that it locks securely into both tapers.
Works manager Chris Last says that machining brass is straightforward, but machining hammer heads from 99.9% pure electrolyte copper (7/8-3.25in-diameter bar or 0.75-3in square-section bar) is a totally different matter. The highly ductile, fine-grain structure of copper makes it difficult to cut; and because the material has a high level of plasticity, it does not ‘chip’ effectively, resulting in continuous coils of ragged-edged swarf. Furthermore, any heat generated by the cutting process can cause the swarf to rapidly expand and “bind up the drill in the hole”.
Once the material has been cut to length, it is held in the chuck of the fourth-axis unit on the 710 VMC. A ‘slave cup’ is applied to the other end of the billet material, to avoid any damage to the end face when it is supported by a tailstock. A pilot hole is drilled through the billet at 1,500rev/min and 0.05mm/rev feed, using a Seco Crownlock exchangeable-head drill (this features a reduced shank diameter to prevent any binding of the copper swarf). The ellipse is then generated, using three-axis interpolation and a Seco Minimaster milling cutter running at 8,000rev/min to ‘waterline’ machine with 1.4mm step-down stages to create the required 3deg taper. Once one side of the tapered bore is completed, the index unit is rotated 180deg and the second taper is machined.
Mr Mathers says: “Our development of the machining program with the XYZ application team means we can now produce our largest copper hammer head in just 8min — half the time taken on our previous VMC.”
Material inconsistency
Turning the blanks of water buffalo material to create the face for the hammer head introduces a totally different set of challenges. Thor uses a particular grade of rawhide that is sourced from the Far East; it has a tough structure that resists splitting and absorbs shock from the hammer blow. To prepare the cylindrical blanks, the material is soaked, cut into strips, coiled, pinned and pressed up to three times over a period up to six months. There are six sizes of cylindrical blanks; these vary from 25mm in diameter ¥ 25mm long to 70mm in diameter x 40mm long.
Mr Last says: “The problem for a machinist is that rawhide is the skin of an animal, so the blank’s characteristics and dimensions can vary from part to part. Another potential process problem is caused by the fact that the chuck jaws must not mark the material — and neither can the blank be allowed to slip in the chuck, despite the clamping pressure having to be low. The machining data has to be tolerant of this fact.
With the need to undertake some 6,000 turning cycles each week to machine the front and back faces of the rawhide blanks, up-time and machine utilisation were paramount for Thor. Working with XYZ, the company changed from using high-speed steel tooling, which needed regular re-grinding and re-setting, to carbide button tooling. “Tool life increased dramatically and — most importantly — the young lady who operates the machine can easily rotate or change the insert; this means that we lose just 10min of production at most. Change-over to a different material size rarely exceeds 15min.”
With the XYZ CT 52, cycle times are between 18 and 38sec (subject to the blank size), with the machine run at 3,000rev/min with 0.4mm/rev feed. The machine has a maximum turning diameter of 220mm and 280mm turned length; it is powered by a 15kW 5,000rev/min drive.
The family-owned Thor Hammer Co can trace its roots back to 1910 as the Stephens Belting Co, producing drive belts for cotton mills; the production of rawhide hammers evolved to make use of material off-cuts. In the war years, the company became a major supplier to MoD engineers, so they could work ‘silently’ on anything metallic; it even supplied 250,000 hammers for the D-Day landings.
Today, it is one of only two international soft-hammer makers in the world working with rawhide, and almost 50% of its product range is exported — to 81 countries. Mr Mathers says: “Authors have even written books on the use of Thor hammers; and in certain countries our nylon hammer version is used to test wooden telegraph poles. If the pole does not ‘ring’ when hit, it is deemed rotten and in need of replacement.”