An Accurate Radius Cutter

Posted By Richard Barker on 10 May 2014

Posted in The Vintage Machinery Almanac

This article first appeared in Practical Engineering 1940 Vol1 No22. The article pertains to information that is valid as of 1940. This article highlights developments within the Manufacturing sector at the time.

Form and radii grinding on surface grinders is now so much in demand that many small firms find themselves without a reliable fixture which is both compact and accurate.

The cost of manufacturing the fixture is very small, and the fixture can be used for cutting both concave and convex radii up to an accurate radii of lin. either way. Dimensions must be governed by individual requirements and the different types of surface grinding machines in use. When the lin. setting block is in position on the setting bar, the diamond must be at dead centre. The first principle for setting the fixture is: for cutting a concave radius on the grinding wheel, subtract the size of the radii from the lin. setting block, put in the new size and move the diamond up to the face of the setting bar.

Concave Radii

As an example, to cut a 1/4in. concave radius, put in a 3/4in. setting block, move the setting bar until the block is held, then move the diamond up to the face of the setting bar and tighten the set screw on the diamond.

Convex Radii

To cut convex radii the size of the radius must be added to the lin. setting block. This means that for a 1/4in. convex radii, the setting block would be 1 1/4in. It will be seen that if care is taken with the setting of the diamond, and if workshop slip blocks are used for setting blocks, a very accurate setting can be made.

The fixture itself must be accurately made, because no lasting degree of accuracy could otherwise be expected. The base may be made of cast iron and, has V-slides and a liner held by three adjusting screws, as is the usual custom for slides of this type. It is important that these screws should always be carefully adjusted.

The base has an apron screwed to it, this being tapped to accommodate the knurled feed screw  which moves  the pillar block along the V-slides. This vernier feeding arrangement is a very important part of the fixture, because although all surface-grinding machines are equipped with a vernier cross slide there is rarely any fine feed on the traverse. Consequently, any fixture without a fine feed has to be used in conjunction with the traverse handwheel; this method is precarious, because with such a coarse feed fine limits are obtained by a series of small jumps.

Pillar Details

The pillar block may also be made of cast iron machined all over, bored out and recessed for the pillar bearing and distance pad, drilled to take the feed screw at the end and drilled and reamed for either one or two stationing pins to fix the lead screw. A hole should be drilled to take the stop pin and the V's should be machined.

The stop pin must be a drive fit in its hole, and when it is in position the protruding end should have two flats ground on, parallel to and central with the sides. The width between the flats must be exactly twice the distance which the side stops project beyond the centre line, to permit the pillar being rotated in a semi-circle.

The pillar itself must be built up; a solid one is, of course, more acceptable and should be adopted if possible, but the large size of material required to produce the crank may prove prohibitive. The pillar should be of sturdy design and great pains must be taken to determine the exact dimensions from the centre of the pillar bearing to the inside of the pillar face.

The diamond must be a good fit in the pillar hole, and have a shank long enough to permit of its being moved at least lin. beyond the centre line of the fixture without losing any of its bearing. The diamond itself should be small and sharp, since it is impossible to cut small radii with a large rounded diamond.

The setting bar should also be a good fit in its pillar hole, and a well-fitting bush of suitable proportions should be fitted on the end and held by a taper pin. A flat face is filed on the underside of the setting bar. This is so that when the diamond has been set, the slip blocks may be removed and the setting bar rotated until the setting face is upright.

The flat will then be on top, and the setting bar should be pulled right back to the pillar face and held there by tightening down the set screw on to the flat. The setting bar is then used as a handle to rotate the diamond while cutting radii on the grinding wheel. The complete setting bar should be ground between centres on the inside of the bush and on the setting face.

The Procedure

With the fixture made, the next step is to set the grinding wheel in order to get true radii. First, raise the grinding head until the collet centre corresponds exactly with the height of the diamond point. (It would be a good plan to mark the position of the grinding head when once it is set, so that any future radii cutting will be facilitated.) Next place the fixture against the back fence of the grinding table, with the feed wheel facing the operator—it may be necessary to swing the grinding-wheel guard out of the way.

Move the cross slide until the diamond is in the required positions-centre if a central radius is to be cut. Using the traverse, bring the diamond almost up to the grinding wheel. Start up the machine and commence cutting, gradually feeding the diamond to the grinding wheel with the knurled hand-wheel while swinging the fixture from side to side right up to the stop pin. The only possibilities of error are due to not remembering that the work radius is the opposite of the grinding wheel radius, in not centring the diamond with the grinding-wheel collet, or in using a worn diamond.