That sounds quite bizzare mike, i'd just go for HSS fully ground drill, normally with plastic and aluminium it will drill slightly undersize, in most metals it will drill slightly oversize.

You can drill bigger than 3.3 and it will still produce a thread on an M4x 0.7mm tap. 3.4 should be okay even if it doesnt do the same thing as your cursed 3.3's.

Michael W

If it persisted in that particular material, if it's not a bad drill then you could even ream to 3.3 before you tapped it if it came down to that?

MichaeL w

It might sound patronising but to drill a hole 3.3mm diameter you need to use a 3.3mm drill!

Its not that simple however, standard twist drills are not precision hole cutters (although you dont need to be that precise if its a hole for tapping) and they cut holes about the nominal drill diameter.

The size of hole a particular drill will produce will vary with the material, speed of penetration and other factors. In your case it would seem that the drills you purchased may be wrongly marked if they are only drilling 3.19 holes (how did you measure that by the way?)

When you start tapping a hole you should get an immediate indication of whether the hole is undersize as the tap will need a lot more than normal of torque to turn. At that point its best to stop and review the situation rather than breaking the tap.

What is the material you are tapping, what is the thread? what type of tap, CS, HSS, spiral, thread forming, etc?

If you are working in SS, Titanium or Inconel best to have the tapping hole oversize.

Ian P

If you measure the shank of a drill can be undersize. This presumably helps stop drills jamming in deep holes. You need to measure across the widest part of the flutes.

A basic jobber drill at 3.3mm diameter will have a tolerance of size to -0.018mm(h8). A new correctly ground drill should give a H12 hole, 3.3/3.42 diameter. These are the sizes for named manufactured drills e.g. Dormer, Gurhing etc. If the shank is less than the marked size then suspect the drilled hole will be undersize also.

The shank is the same diameter as the twisted portion. Drill length should be chosen so that the flute is always clear of the hole, otherwise swarf will jam the drill. Also tap thread form should never be run in beyond the top of the hole as they will tap in both directions causing oversize thread form.

Howard

It is a long time ago but I think I was taught to expect a drill to drill oversize. I have always worked with this in mind and piloted holes when possible. If you finish an existing hole with a slightly larger good sharp drill you should get a hole diameter corresponding to that drill (with the usual caveats – rigidity, material, material thickness, etc). Obviously use the best drills you can afford.

By the way tri-lobular holes generally have a constant diameter – measure the diameter of a 50p piece.

JA

A simple way to gauge if the drill is cutting to its nominal size or the shank size is to drill a hole and then stick the shank of the hdrill into the hole.

If its a firm fit then its likely to be cutting to the shank size if its an easy fit then probably cutting to nominal size.

Not always so. All my metric and imperial drills are within ±0.01mm of nominal size when measured at the tip, as near as I can tell. However, the shanks are 0.03mm to 0.1mm undersize, depending upon the drill size, and also taper slightly away from the tip.

Andrew

PS: All my metric and imperial drills are Dormer, bought new in sets

Howard

Do you mean that a tap can cut two sizes or that the cutting portion of the tap must not go below the face of the hole?

Ian P

A tap will cut on the way out if the threaded portion goes below the start of the hole. If you look at the Emuge catalogue, a good nights reading, they give maximum thread depths for their taps. As has been said befor in other threads full depth threads don't need to be more than one to two times the thread diameter. Softer metals require deeper threads ( depth refers to length of thread in hole not major diameter of thread).

Howard

Question the design! Usually you would counterbore to reduce the thread length to keep it within design limits of the cutting tools.

Howard