bob ward
14th Mar 2018, 09:26 AM
I needed to remove/drill out 20 sheared off bolts and studs from tapped holes in an old engine (1935 Hudson 8), they were a mixture of 5/16" UNC and 3/8" UNC.
The eternal problems with this sort of work are:-
a)precisely finding the bolt centre
b)getting the drill to start on centre - bolts and studs never break off nice and smooth
c)keeping the drill on centre
Drill guide plates solved all of the above problems, I was fortunate on this job that the bolt layouts were measurable and I did have some clean holes that I could bolt the guides to. Using the mill and DRO I made few drilling plates, all from 12mm flat steel, to suit the different configurations of holes that were available for fixing and holes I needed to tackle.
Each drill plate had 2 sets of fixing and drilling holes. The first set of holes positioned a 6mm hole through which I firstly put a 6mm spotting drill 2 or 3mm into the offending bolt or stud, then I swapped over to a 6mm jobbing drill and drilled right through. After that the second set of holes positioned either a 6.35mm drill for the 5/16" threads or a 7.8 mm drill for the 3/8" threads and drilled right through. Those drill sizes are larger than the minor bolt/stud diameter and smaller than the hole tapping size. It goes without saying that you only use fresh sharp drills.
372457
After that its a matter of teasing the remnant of thread out of the block. Final score Bob 20, bolts and studs 0.
372458
The eternal problems with this sort of work are:-
a)precisely finding the bolt centre
b)getting the drill to start on centre - bolts and studs never break off nice and smooth
c)keeping the drill on centre
Drill guide plates solved all of the above problems, I was fortunate on this job that the bolt layouts were measurable and I did have some clean holes that I could bolt the guides to. Using the mill and DRO I made few drilling plates, all from 12mm flat steel, to suit the different configurations of holes that were available for fixing and holes I needed to tackle.
Each drill plate had 2 sets of fixing and drilling holes. The first set of holes positioned a 6mm hole through which I firstly put a 6mm spotting drill 2 or 3mm into the offending bolt or stud, then I swapped over to a 6mm jobbing drill and drilled right through. After that the second set of holes positioned either a 6.35mm drill for the 5/16" threads or a 7.8 mm drill for the 3/8" threads and drilled right through. Those drill sizes are larger than the minor bolt/stud diameter and smaller than the hole tapping size. It goes without saying that you only use fresh sharp drills.
372457
After that its a matter of teasing the remnant of thread out of the block. Final score Bob 20, bolts and studs 0.
372458