Drilling Holes

Drilled some holes in the cylinder support of an Elmers Open Column engine - 7BA clearance last weekend. When deburring the holes, I noticed that they looked quite large on one side of the component ( the top side whilst drilling ), and a lot smaller on the other side, Closer inspection of the holes showed that they were considerably oversize except for the last fraction of a millimetre where the drill broke through.

Looking at the drill point through an eye loupe revealed that the geometry was far from normal ( couldn't see enough detail without the loupe ). Although the point was in the centre of the drill and the point angle was similar on both sides, the Lip Clearance Angle ( according to the Presto cutting tool pocket book ) was wildly different one side to the other, in fact on one side there was more than sufficient clearance but on the other side, the cutting edge was below the flank causing that side to rub and, according to the book, causes exactly the symptoms I've seen in my holes.

Looking at all the drills in the set ( a budget set of number drills - long since forgotten the origin ) gave me a tally of roughly a third with incorrect lip clearance. Whilst I can probably just about re-grind the points on drills down to about No. 31 or 32, I find it too difficult to go any smaller, so a new set of Dormer drills were procured ( and inspected through the eye loupe - perfect ).

The other issue I've been having is with the actual drilling machine. Whilst I use a spotting drill to locate the holes from the readout on the Chester 626 mill, I find the spindle control has too little 'feel' with smallish drills, so I invested in a Sieg X0. The first real model engineering task for the X0 was drilling the hornstays / pedestal ties of my O gauge Josie clearance for 10BA. I found the speed of the drill very difficult to adjust - even on the low speed pulleys, which caused the tiny parts to become far too hot to handle. In addition, the drill seemed to struggle - only about a 1.8mm drill. I checked the speed with a tacho - should have been 0-3600 rpm was actually 0-7500 - no wonder the components got hot and there was very little power to make swarf.

I stripped down the drive motor and pulleys to investigate and the motor legend plate says 230V ac 150W and 18,000Rrpm - I wonder if the variable speed drive is only a half wave unit which may explain the top speed and the poor speed / power control. I spent some time looking to see if I could track down a lower speed motor as a swap-out, or maybe add a second stage pulley reduction but it doesn't seem that practical a solution.

Has anyone else on the forum got one of these machines and experienced similar issues and hopefully got some ideas ?

If you are having problems with Sieg drill speed you should contact the supplier with the problem ask them to rectify the problem or return for a full refund.

David