Shrinking profits? Look out for tapping and reaming of cast iron parts!

We have had a number of cases where we had to increase the size of the tool while tapping and reaming of cylinder block and cylinder heads and some other parts made from GG variety of cast iron. Fortunately for standard size taps we could go for 6H X variety which is slightly oversize tap than the standard 6H tap used for getting 6H class quality of tapped hole. Same is not the case with reamers! We had to forgo the standard reamer manufacturing tolerance of 50-85% of indicated hole tolerance and make reamers of size same as the maximum tolerance value. To give an example, for getting tappet bores of size 28H8(0 to0.033 mm) in one of our cylinder blocks we have been using a reamer with a manufacturing tolerance 28.033-0.010 !

This was done to overcome the shrinkage of part size after reaming is done. Earlier when the reamer was new the holes were within tolerance but on mid size and lower. We were not getting sufficient tool life because number of grindings (resharpenings) were getting limited. Now with the basic size higher we get sufficient number of parts with more resharpenings to make the operation economical.In a nutshell this is as goos as saying use H8 size reamer for H7 size hole!

As an interim measure it is also possible to TiN coat reamers (and build size), confirm the result and then change the tool drawing value. To meet urgency slightly undersize reamers can be coated and used .By coating the size of the tool will be marginally higher.

Same technique can be used for taps. We have a few varieties of special sizes in which the basic size of the tap is higher than standard tolerance to overcome the tendency of shrinkage (and hence lower overall tool life because we do grind taps on lead as well as flutes). In the example we are using 6H class tap for 4H quality tapping.

Since reamers are best reground between centers ( CNC machines are costly) on T&C grinder, we have many reamers with a counter bore (recessed center) to retain the center during subsequent resharpening of reamers being used for reaming of blind holes. This feature will give many more grindings on a reamer. See the attached drawing