A good rule of thumb for mild-steel is:

RPM = 10000 / diameter of drill (mm)

This formula gives 400rpm for a 25mm drill in mild steel, compared with the 382rpm recommended in Jason’s table.  Close enough.

Jason’s table also gives speeds for other metals, and the ratio between them can be used pro-rata on the mild-steel rule of thumb.   For example, the table shows Aluminium cuts roughly twice as fast as mild-steel, so a 25mm drill should be spun in Ali at 400×2 = 800rpm.   Cast iron cuts at roughly half mild-steel speed, so that’s 400/2 = 200rpm.

Although the formula and table both get into the right ball-park, be prepared to experiment.  Best speed in practice depends on a bunch of other factors such as the machine’s power and rigidity, the exact alloy being drilled, state of the cutter, the feed-rate, and whether or not coolant is used.

Dave

Sparey gives a simple rule———80 over 1/4 of drill dia for MS.

This works out at about 320 revs for 1″ drills.

Have been using  this for years and found it to work well.

A lot depends on the machine, coolant, and the grade of steel [not always easy to tell !] , but still a good rule of thumb.

Now you have your drilling machine, it’s time to get some drill bits. Do Not buy a boxed set of cheap ones, infact don’t buy cheap ones at all, they will cause little but trouble. Seek the advice of a good tool store as to names that will be reasonable quality. Start with a few of the sizes you think you will need, then as you use them buy others to suit the work. Then there is the issue of sharpening, a bench grinder will do ! Do a bit of reading on the matter and study the end of a new drill. A drill will more oft than not drill oversize, how much may well be governed by the sharpening. Not something to worry about now.

Your question about speeds has been answered by others and there are numerous charts, some will cover the small ones and the eye watering speeds recommended.

With stainless steel more common now drilling it is a skill ! Start with a sharp drill bit, use good lube eg RTD(this is also good for tapping/dieing – it’s not cheap ) and IF the bit stops cutting STOP ! Try to resharpen, but if this doesnt work then the hole has work hardened and will need a special type of bit.

If drilling holes in sheet be VERY careful ! Clamp the work and allow the drill to break through slowly ! DO NOT hold by hand – if it grabs it becomes a rotary knife. There are special drills for this work, but not often seen !

Ah well, good luck. Noel.

On 11 July 2024 at 11:58 SillyOldDuffer Said

Small diameter drills used to making printed circuit boards have to driven very fast, as do engraving tools, and high-speed spindles can be bought for that kind of work – up to about 40000rpm.

My old school bench top PCB drill does 10,000rpm and the high speed spindle on my CNC mill goes to 24000rpm. For commercial PCB manufacturing SoD is an order of magnitude out; some drilling machines are now running at 500,000rpm. At one time the number of holes was a key parameter for getting a PCB quote. That doesn’t seem to be the case now. Some of the RF boards I’ve laid out have patterns of closely spaced vias to tie planes together. Board manfacturing quotes are not noticably different to a board with no RF circuitry, and far fewer holes.

For drills much below 10mm I don’t waste time sharpening them; just buy new. I can sharpen by hand and also have a Clarkson T&C grinder plus the drill/tap sharpening attachment which I have used for sharpening home made taps. But simply not worth wasting the time to setup and sharpen small drills. The cheap attachments for bench grinders never worked for me; if one must sharpen a jobbing drill do it freehand.

Andrew

Drilling holes is not really a precision operation and no hole made with a twist drill will be truly round, that’s why we either use a reamer to finish or bore where possible on jobs that require a truly round hole. As regards the quality of drills, I have full sets of Dormer/Presto/Osbourne etc. up to 1″ on plain shanks and Morse tapers and both number and letter sets but I doubt if any of them would now be made in the UK although they are still available at a price. When necessary I sharpen those above about 3/16″ with a Reliance drill grinding jig. Within the last year or so I bought a super cheap set of metric sizes from the local household discount store just to use in a hand held cordless drill on household maintenance jobs and was really surprised by the apparent quality, so you never can tell it seems. As regards drilling speeds, I have never done anything other guess it, faster for small sizes and slower as the size increases. Just like any other cutting tool, a drill bit will let you know when its not happy and you can then make the necessary adjustment. As always, it works for me.