Turning compressed buffalo hide, drilling and taper cutting opposed elliptical forms in pure copper and brass, are a far cry from normal machine tool operations. But two recent installations by XYZ Machine Tools is not only enabling the UK’s only manufacturer of soft hammers to maintain its high orders of quality and safety in use, but the XYZ machines are also helping produce some 6,000 hammers a week.
Set up in 1933, Thor Hammer Company, based in Shirley, Birmingham can claim to have one or more of its tools carried by most skilled fitters. The tools are used by the MOD and any number of engineering companies and workshops around the world as well as classic car owners who still order them in ‘original’ specification to tighten and loosen ‘spinners’ on the wire wheels of 1950s and ’60s sports cars. Indeed, it was a Thor copper/rawhide combination hammer that was portrayed in press photographs when HRH Prince Charles visited the Whitechapel Bell Foundry. During the visit he struck the first Royal Jubilee Bell appropriately named ‘Charles’ that heralded the Queen’s Procession down the Thames for the Diamond Jubilee Pageant.
Securing the shaft to the head of a Thor hammer which can be 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 some 85 mm diameter by 173 mm long weigh up to 9 kgs.
The two XYZ machines, both controlled by Siemens 828D systems, a 710 VMC vertical machining centre with a mechanical fourth-axis index unit, produces the hole for the shaft in the copper and brass hammer heads, while a Compact Turn 52 CNC lathe turns the rolled, pinned and pressed blanks of water buffalo hide used as facing for the hammer head.
Said Managing Director, Derek Mathers: “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”. Such was the impression gained prior to installation, and from the level of support from the application team at XYZ, he added: “It was no contest when we then decided to increase our turning capacity by investing in the Compact Turn to meet increased demand for rawhide facings”.
According to Works Manager, Chris Last: “The order for the CT 52 lathe was the first time Thor Hammer had ever acquired another machine from the same supplier, but such has been the on-going support from XYZ that when we replace our older lathe, it will surely become the same again!”.
Thor processes some 10 tonnes of brass and copper a month. Machining both these versions of the hammer head involves producing diametrically opposed elliptical shaped bores up to (38 mm by 48 mm) with 3 deg ‘locking’ tapers that meet in the centre of the head. When the shaped handle (in most cases produced from Hickory, a hard, stiff, dense and shock resistant wood – sourced from 100 year old trees in Tennessee) is fitted to the head, a wood and steel wedge is inserted into the end of the handle to expand the wood to positively lock into both tapers.
As Chris Last explained, machining brass is straightforward but 99.9 per cent pure electrolyte copper, which is produced from 7/8 inch up to 3 1/4 inch section round or square material with 3/4 inch to 3 inch section is a totally different matter. The highly ductile, fine grain structure of copper is difficult to cut, not helped by high orders of plasticity and it is especially awkward to control the continuous coils of ragged-edged swarf which will not effectively chip. In addition, any heat generated during the cutting process can cause the swarf to rapidly expand and bind up the drill in the hole.
The material is cut to length and held in the chuck of the mechanical index fourth-axis unit set on the 760 mm by 460 mm table of the 710 VMC. A slave cup is used for the other end of the billet material, which is supported by a tailstock to avoid any damage to the end face. A pilot hole is drilled through the billet at 1,500 revs/min and 0.05 mm/rev feed using Seco Crownlock exchangeable head drills with a reduced shank to prevent any binding of the copper swarf. The ellipse is then generated using 3-axis interpolation and Seco’s Minimaster, a shank and insert milling system at 8,000 revs/min, to waterline machine with 1.4 mm stepdown stages to create the 3 deg taper down the bore.
Once one side of the tapered bore is completed, the toolchange switches a blank into the spindle to initiate a 180 deg rotation of the index unit. The second 3 deg ellipse is generated using the same stepdown technique to break into the previous bore. The 710 VMC has a 15kW, 8,000 revs/min spindle with travels of 710 mm in X, by 450 mm in Y by 500 mm in Z, and when installed at Thor, was fitted with the optional Renishaw OMI-2 tool setting probe.
Said Mr Mathers: “Our development of the machining program with the XYZ application team means we can now cycle our largest copper hammer head within 8 mins-half the time taken on our previous VMC machine”.
Turning the blanks of water buffalo material to create the face for the hammer head introduces a totally different set of operational characteristics. Thor uses a particular grade of rawhide sourced from the Far East for its tough, cross grade structure that resists any splitting and absorbs shock from the hammer blow. Prior to turning both faces, the material is soaked, cut into strips, coiled, pinned and pressed up to three times over a period up to six months into six sizes of cylindrical blanks. These vary between 25 mm diameter by 25 mm long to 70 mm diameter by 40 mm.
Said Mr Last: “The problem for a machinist is that rawhide is a natural material and as a skin of an animal, its characteristics vary from part to part let alone dimensionally, which can easily be up to 0.5 mm. Another potential process problem is the delicate location in the 150 mm chuck of the lathe. The clamping pressure is so low which means the cutting cycle has to be tolerant of the material and not induce skidding or marking from the jaws of the chuck”.
With some 6,000 turning cycles performed each week to machine the front and back faces of the rawhide blank, up time and machine utilisation was paramount for Thor. Working with XYZ, the change to carbide button tooling from high speed steel, eliminated any previous tool regrinding and resetting. Said Mr Last: “Tool life increased dramatically and most important, the young lady who operates the machine is able to easily rotate or even change the insert and we lose at most 10 mins production. Changeover to a different material size rarely exceeds 15 mins” he said.
Each part is also able to be approached at an angle through the use of the button tool so as not to mark the outer diameter. The part is then faced and programmed to create a slightly convex surface to the centre to allow for later shrinkage of the material once secured to the hammer head. Around 3 mm is left for the second operational cycle and a smooth blend radius created on the OD. Once a batch is produced, the machine is reset to face to length and radius the OD.
With the installation of the XYZ CT 52 machine, cycle times are between 18 secs and 38 secs according to the blank size, with the machine run at 3,000 revs/min with a 0.4 mm/rev feed. The machine has a maximum turning diameter of 220 mm by 280 mm turned length, and is powered by a 15 kW 5,000 revs/min drive.
The family-owned Thor Hammer Company was named after the Norse God of Thunder and the son of Odin who owned the most miraculous throwing hammer. It can trace its roots back to 1910 as the Stephens Belting Company producing drive belts for cotton mills, which led to the production of rawhide hammers from material offcuts. In the war years, it became a major supplier to MOD engineers to allow ‘silent working’ on anything metallic such as assembly of Bailey Bridges, vehicles and weapon repairs and even supplied 250,000 hammers for the D-Day landing campaign.
Today, as one of only two international soft hammer makers in the world working with rawhide, almost 50 per cent of its product range is exported by the 34-people business to 81 countries. Customers even include the motor sport industry for tricky gearbox assembly, special malls created for stone masons and a special heritage selection for classic cars, trains and aircraft.
Said Mr Mathers: “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’s deemed rotten and needs replacing”.
(Above) Chris Last hammers home the benefits of XYZ machines.
(Above) XYZ’s 710 VMC is ahead of other machines producing copper hammerheads at Thor Hammer.
(Above) Some 6,000 turning cycles are made each week on blanks of water buffalo hide at Thor Hammer using XYZ’s CT52 Compact Turn CNC lathe.