Holding Problems


Inspection, grinding, turning, locating, positive chucking without part distortion, Machine tools and spindles have constantly improved accuracy year after year and spindle accuracy now can be held as close as .0001 to .0002 with no trouble. With these accuracies maintained, why is it difficult to hold concentricity of .002 to .003 on normal machining and grinding jobs? Obviously, the loss in accuracy occurs with the method of holding the work piece on the machine spindle.

In recent years many types of expanding tools have been developed in an attempt to assist the tool engineer in solving the problem. The diaphragm chuck, the split collet arbor, the stresses arbor or chuck and the tapered arbor are some of these products. Great time and effort are spent in getting tools to expand or contract while they remain extremely true and parallel. However, the surface or the work piece to be held is practically never true or parallel. Quality control studies show that only 10% of parts produced fall within the perfect classification.

This means about 90% of parts have chucking surfaces spread over the tolerance range and we have therefore applied tooling to "perfectly" hold 10% of the parts while "imperfectly" holding 90% of all parts produced.

Let us examine some of the methods used to chuck close tolerance jobs and show why they fail to produce quality parts. Assume the part to be held is a gear blank 2" OD x 2" long. The print calls for .005 tolerance on the OD but it is decided to hold .002 for processing purposes. This means the OD can be tapered in either direction depending on how the part is chucked; also it can have a .002 bulge or dish.

Chuck Jaws Ground In Place On Machine

This method of holding is good, however, it is limited. It is true that the chuck jaws will compensate for some of the taper, however, in the case of a bulging, only line contact can be obtained, thereby allowing possible distortion of the part within the chuck jaws. Also the chuck jaws may not compensate for all of the taper, thereby again giving line contact.

Diaphragm type Chucks Ground On Machine

This method again produces a check with a surface that will indicate as true as the machine spindle will run. However, since the grinding must be done at some specific diameter, usually the mean dimension, perfect chucking will only take place on parts whose diameter just happens to be that to which the chuck was ground. Due to the arc of travel of this type chuck, all other parts will be chucked with line contact and not total surface contact. If chucking pressures are increased so as to distort the diaphragm to get total surface, then they may also distort the part due to the excessive pressure.

Split Collet And Draw Bar Type Arbors And Chucks

This type of equipment is also designed to expand or contract parallel, and therefore introduce the same problems as diaphragm chucks and ground chuck jaws as well as the possible error from dirt and chips. When the collet is actuated it moves away from the basic mandrel, and grit, dirt and chips may get between the collet and the mandrel, thus losing accuracy. Wear on the mandrel taper also takes place and introduces still further error.

Mechanically Expanded Arbors and Chucks, Closed Sleeve Type

Tools of this type made a great step forward in eliminating the dirt problem and are much more accurate than the open or split type. However, the main problem, inability to achieve total surface contact on approximately 90% of production parts, is still to be found here. The mechanical movement stops when the expansion picks up the smallest diameter of the hole or the largest diameter of the contacting surface, thereby giving only line contact. Another problem with this type of arbor or chuck is that expansion is accomplished by stressing metal against metal. This causes work hardening of the sleeve and produces very short tool life.

This analysis of the available types of tools attempts to explain why it is so difficult to produce close tolerances even with the best and newest of machine tools. Is it possible then to apply tooling to these close tolerance jobs which will transfer spindle accuracy through the part tolerance, absorbing that inaccuracy every time and give spindle accuracy at the work surface?

  • With Jerry Tools' arbors and chucks you can overcome most of these problems. Jerry Tools arbors and chucks have no openings or external moving parts and thus eliminate the dirt and chip problem.
  • Jerry Tools arbors and chucks expand within the elastic limits of the steel of which they are made and therefore will not harden or distort.
  • Jerry Tools products have their own built-in hydraulic system and require no outside source of oil pressure.
  • Jerry Tools products expand or contract to conform to the shape of the locating surface. They pick up taper, out of roundness, bell mouth holes, barrel shaped holes, or conform to any shape within their expanding limits. While conforming to these shapes they maintain spindle accuracy to the working surface; eliminating the manufacturing tolerances of conventional holding devices. In addition, tool chatter is reduced to a minimum because the work is, in effect, held within a thin layer of fluid under the steel expansion sleeve. This fluid acts as a dampening component, absorbing most tool chatter.
  • Jerry Tools products are practically maintenance free. If repairs are required, tools can be reworked, sleeves replaced, or other repairs made here at the factory fast and easy. Many components are standard shelf items, with quick turn around on minor repairs.