Slot drill hole size?

I was trying to drill a 10mm deep 6mm diameter flat bottomed blind hole in a piece of aluminium.

I thought that I would use a 6mm 2 flute milling cutter (slot drill) held in an ER collet in the mill. However I produced a 6.3mm dia hole! I tried a different 4 flute 6mm slot drill, but got the same result.

A 5.9mm twist drill held in the same collet produced a 6mm hole.

Was I over optimistic in assuming that a 6mm slot drill will produce a 6mm hole? Or am I doing something wrong?

Wonky bearings

A cheap chuck

loose spindle housing

an error in the collet

To name a few

There are so many reasons, it depends on the drill

A decent DTI goes a long way towards getting an answer

see from 1min 41secs

Edited By Ady1 on 26/11/2020 00:37:48

If you need an accurate size hole, you must always drill undersize & then ream to the size you need.

Steve.

It’s not recommended to cut a slot with an end mill, using a mill at 100% of slot width - it will invariably be slightly too wide, so likely the same if simply boring with one.

Unless I need a completely flat bottomed hole, I would drill a pilot slightly deeper, drill to, say, 5mm and then follow that with a 6mm end mill. I don’t generally need to worry about the odd fraction of a mm with my work and have never checked the hole tolerance at that size.

Reaming will be the best solution, but only for through holes, as all reamers have a lead-in to the flutes. A ‘D’ bit, made for the job, is the usual suggested solution to your problem, I think.

I'd have put a 5mm drill in as a pilot then finished with a 6mm 3-flute cutter. A machine reamer will give a better sized hole almost to the bottom but you have the issue of finding something to give a reaming size flat bottom hole first unless you can do without the flat bottom.

No, it should be perfectly possible. However, slot drills are ground slightly concave on the end, so if you want a truly flat bottom to the hole you'll need to make your own D-bit. Here's a secondhand 1" slot drill making a hole 1.5" deep:

The hole was predrilled 9/16" as that size goes deeper than the 1" diameter anyway. The hole measures 1.001". So it is possible. To the notes from Ady1 I'd add cheap cutters. If a 5.9mm drill produces a 6mm hole then something is wrong. Out of interest I just measured a hole I drilled 6.4mm a while back. It measures 6.42mm. I was using a quality 4-facet drill from Dormer. My guess would be that the problem is caused by poor quality cutting tools and/or tooling.

Andrew

You'll get a much more accurate finished size if you do what Andrew did with his 1" hole above and drill a smaller pilot hole first. Just plunging a 6mm slot drill straight down into metal is always asking for it to wander about a bit. Better to drill something like 5.5mm first then finish with your 6mm slot drill. Most drills will go 0.02mm or so oversize anyway. Sometimes you can get away with finishing with say a 5.9mm drill bit if you have one and end up with a 6mm hole. Have a play in some scrap metal with different combinations of pilot hole sizes and finish drills and slot drills and get a feel for how it goes.

He wants a flat bottomed blind hole.

Although I said I prefer a 3-flute cutter I did a test with a 2-flute "slot drill" which is what you used in some 6082

Hole on the left was first drilled with a 5.0mm Dormer A022 stub drill with no punching or spotting and then followed up with a standard length 2-flute HSS cutter from ARC's premium range and that is a good fit on the shank of the cutter that I measure at 5.98mm.

Hole on the right just done with the 2-flute cutter, that is a 6.1mm drill in the hole who's shank actually measures 6.07mm

So without a pilot the hole came out approx 0.1mm oversize or 0.004" in old money. In both cases I had the quill locked and lowered the head to feed the cutter.

Peter, I’m useless at sharpening drill bits but what I have done on a number of occasions is to put a flat grind on a twist drill to produce flat bottom holes.

In the video I put up the cheep chuck on the cheep drill is about 70% of the error

A decent replacement chuck from the same brand is about 70 quid, 1/3 of the price of the drill

They must be turfing branded copies out for a tenner a pop from china for mass market purposes while still doing better quality gear from other sources

Thanks, I will go and experiment with the things using the DTI.

The milling cutter (and the twist drills) were held in ER collets in an MT2 ER collet holder mounted in the mill spindle. I will go and investigate the runout.

I tried several ways including drilling a pilot hole at 5.8mm and then running the end mill in without touching the x-y position. This produced the same sort of hole size as without the pilot - so it's the milling cutter cutting oversize I think.

As it's only a pivot for a motor, the 5.9 drill produces an adequate solution at this stage. I wanted the blind hole more for aesthetics (so that the pivot doesn't poke out of the carrier block) than necessity.

It's all about learning what works and what doesn't!! The advice here is always helpful on the climb up the learning curve.

I think it would be remarkable if a slot drill could produce a hole of its nominal size, they are pretty rigid and so any spindle or chuck runout will make the hole bigger, zero runout would be nice but in the real world there will be some. As Andrew said above most slot drills will not produce a truly flat bottom and a reamer will not hold full diameter right to the tip, a D bit is probably the simplest way to achieve a good diameter with flat bottom and best of all they are straightforward to make from silver steel.

Mike

Mike is quite correct with what he said also in reality it is rare that you need a truly flat bottomed hole.

Tony

I also notice some drills have a nice screw-in chuck but no register

Even cheap lathes have a register because of the accuracy and stiffness advantages this approach gives

If I want a decent hole I put the workpiece in the 3/4 jaw and use 3x tailstock drills, a spotter,a pilot driller and the finishing drill

You can feel and hear whats going on with a lathe

Edited By Ady1 on 26/11/2020 14:26:52

That happened to me once. I used a 6mm which cut oversize and another which was much better. The difference was the sharpness, one used and the other new. Solid carbide will cut a better size hole because it is stiffer than steel. For critical size holes, a smaller size first and a reamer to finish as long as the taper at the end of the reamer is taken into account.

Edited By old mart on 26/11/2020 14:07:02

Edited By old mart on 26/11/2020 14:09:25

I can't readily imagine another cause than the 6mm slotdrill running out. If another 6mm cutter produces the same result, the most obvious reason would be swarf or grit in the collet cone, tipping its axis.

There may well be enough parallel shank exposed to get a clock finger onto it to verify concentricity, though 0,3mm TIR should be pretty clearly visible when running.

When I make salt/pepper grinders, I regularly mill a slot in the delrin cap with a 1/4" two-lip slotdrill held in a 3-jaw chuck in my Warco, with the the cap held in the vice on my vertical slide. This is always a tight hand-push fit for a piece of 1/4" square-section brass bar (usually measuring .249" ) .

So no, an oversized cut is not necessarily to be expected.

Hopper's point about starting with an undersized hole is good. There might be risk of deflection when an outer corner of the slotdrill contacts the work first, especially if the feed's severe. If the slotdrill runs true, best keep down the work it has to do.

Edited By Mick B1 on 26/11/2020 14:27:47

I seem to recall reading some famous ME chap saying "it should not be possible to push a drill into the hole you just drilled with it" kinda thing

The way to do that is grind the drill perfectly & then stone off the 2 corners, it does work.

Tony

Thanks for that

I've done it a few times but had no idea how I managed it, the flanks must have been worn down from previous drilling