21-05-2013, 11:55 AM
INFLUENCE OF HUMECTANT QUANTITY AND MIXING TIME ON THE QUALITY OF ABRASIVE STONES USED IN HONING PROCESSES
INFLUENCE OF HUMECTANT.pdf (Size: 337.65 KB / Downloads: 28)
ABSTRACT
In the manufacturing process of abrasive stones with cubic boron nitride (CBN) as abrasive and
metallic binder, some differences were observed regarding abrasive density at different heights of the
stones. If abrasive density is too low, honing stones do not cut the material properly. Honing stones
are obtained by means of sintering processes. Previously, a humectant material is added to abrasive
grains, and then they are mixed with metallic binder. Humectant is used to assure covering of each
abrasive grain with binder. In the present work, along the manufacturing process of honing stones,
quality of the mixing process of components was assessed by means of two different parameters:
covering degree of grains by binder (CD) and homogeneity degree (HD) of the mix before sintering
process. Both parameters are calculated by means of weight measurement in the mixing step. Different
quantities of humectant and different mixing times were taken into account. A higher quantity of
humectant implies a higher covering degree, leading to more homogeneous honing stones. For this
reason, highest quantity of humectant is recommended within the range studied. However, a certain
mixing time is required to assure homogeneity of the mix prior to sintering.
INTRODUCTION
Stones used for interior honing are usually made of an abrasive material and a binder. In addition,
during their manufacturing process a humectant material is added in order to assure that each abrasive
grain is surrounded by the binder. Abrasive materials can be divided into two categories: conventional
ones made of ceramics such as Al2O3, SiC or ZrO2, and superabrasives, like cubic boron nitride and
diamond [1]. Common binders are resins or polymers, ceramics or metals. In order to obtain stones
with superabrasives, the most employed metals are bronze, iron or nickel [2]. Owing to the differences
in the electronic and chemical structure between metals and superabrasives, good adhesion between
them is difficult [3]. In abrasive stones, it is usual to find defects such as sedimentation of abrasive
grains on the lower part of the stone (next to the base) or the formation of groups of abrasive grains or
lumps. Different abrasive concentrations will lead to different material removal rates as the stone
wears out. In order to assure that the stone cuts the material properly, cutting conditions of the honing
machine need to be varied as the operation proceeds.
MATERIALS AND METHODS
Materials
CBN abrasive was used with metallic binder. Components were mixed in a Turbula machine. Density
of CBN is 3.5 g cm-3, while apparent densities of metallic powder range between 0.7 g cm-3 and 3 g
cm-3. Three different quantities of humectant were used: QH, which corresponds to one drop of
humectant per g of abrasive, 1.25QH and 1.5QH.
Methods
For preparing the mix, first the humectant was added to the abrasive; afterwards, binder was added and
the mix was placed in the Turbula machine for mechanical mixing. Important variables in the mixing
operation are quantity of abrasive, quantity of humectant, quantity of binder, and time of mechanical
mixing in the Turbula machine. In all tests a fixed quantity of abrasive and a fixed quantity of binder
was used. Quantity of humectant and mechanical mixing time were varied.
In order to assess quality of the mechanical mixing operation, two new parameters were defined,
homogeneity degree (HD) and covering degree (CD).
CONCLUSIONS
Within the range studied, the higher the quantity of humectant, the higher covering degree is at a
certain mechanical mixing time. A higher covering degree will lead to less lumps of abrasive in the
mechanical mix. It will also minimize the fact that the abrasive grains tend to descend towards the
base of the mould before sintering. Thus, homogeneity of the stone will improve. However, a certain
mechanical mixing time is necessary to assure high homogeneity degree of the mix prior to sintering.