'Tee' bolts are traditionally used - these have a rectangular head that slides into the grooves machined into the drilling machine pedestal.
In practice you can get away with a traditional bolt with a washer under the head as normal.
John
Easy to get hold of, but at a price.....Wixroyd, Boneham, Buck and Hickman etc.
Even coach bolts with the squared head below the dome could be converted.
John
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