I have a proyect going that involves drilling and tapping a lot. I have 8 pieces of 15mm thick steel (flat bar 150mm wide 750mm long) and each needs 8 holes drilled 8,5mm and tapped M10 in them.Thats a total of 64 holes.
Of course, the more square and precise, the better.
So far I have done 2 pieces (16 holes) and the results are far from what I would like.
I am doing it as follows.
- Measure, scribe and center punch.
- Pilot drill (not all the way through) with a centering 3mm bit. I am only drilling untill I reach the "shoulder" of the bit (where the countersunking starts), thats like 4mm or so.
- Then I drill all the way through with an 8,5mm bit.
- Countersink a bit on both sides.
- Tap by hand with tapping fluid.
I have been drilling with a new (to me) substantial drill press, so everything is rigid. Before drilling all the way through I make sure the bit is well aligned with the pilot hole. With the drill running I touch the piece and if the bit wanders (meaning the alignment is not precise) I adjust via dead blow hammer and try again. I may have some holes of the first piece a tad off, but not much.
Then I start tapping. I had a few sets of 3 taps but I asked and I was told about machine taps. The ones capable of tapping in a single pass. And I bought one.
I am tapping with a hand held tap turner (?) so the alignment of the tap and the hole goes strictly by eye.
That said after tapping the 8 holes in the first piece I ended up with a few holes seriously crocked.
For the second piece, thinking that the single pass tap could be too aggresive, I used my old three pass taps. And the holes ended somewhat better.
Now my big question is... Is it possible to screw up tapping if the hole is perfectly square to the piece or I just had the holes crocked and the tap just followed it?
From now on I am going to chuck the tap into the drill press and, somehow, turn it by hand (probably loosening the belt so I don't have to fight the motor). That way I will make sure that not only the bit, but also the following tap is also straight.
What do you guys think? How do you do it? Any tips I could put to good use?
Thanks in advance,