3.3 mm drills are reasonably flexible and will take the path of least resistance in a deep hole, i.e. relative to the diameter of the drill. The fact that the bend appears to occur at a specific location suggests that there may be a hard spot in the aluminium ( not an unusual occurrence ) at that point and the drill has deviated either enough to go around it or to the point where it will not bend further and forces its way through.

I suspect that someone will be along shortly to provide a formula for drill diameter, hole depth and flex in a uniform material but I don't think that is what is happening here.

Pero

When I had to work for a living I was making some workshop instructions. One of the points that can remember was the depth of the holes which can be reliably performed without too much care…..this was three times the diameter. In the case of a 3.3mm diameter hole, a reliable hole depth could be 9.9mm. Obviously this is a worst case, as mentioned, pecking is a cure as is cleaning the drill, using a sharp good quality drill etc etc

cheers

Bill

I would pilot drill and complete the hole, to size, with a suitable end mill – they are much stiffer than a common twist drill.

Certainly need much more difficult, once more than 5 times the diameter, but attainable with care.

Not done it with holes as small and deep as the OP wants, but it works OK with intermediate holes that are a bit tight to bore easily.

Far too long ago to think about I worked in a jobbing machine shop where we did a fair amount of work for two local blow moulding factories. One of the items we did an awful lot of were 'blow pins'. Turned from stainless in sets of three they had a uniform shaped threaded top end and a long varying much smaller diameter lower end to suit the particular moulding (bottles) and varying lengths about 150-170mm long

They had a hole drilled right through which was done from both ends. Agreed, it only had to pass air which meant accuracy and perfect alignment wasn't an issue and yes the drill did wander at times but more often or not the holes would allow the drill to be pushed right through after drilling. If I recall correctly the largest drill was 5mm diameter and with some 4 and occasionally 3mm the latter a particularly fraught time. I well remember doing one when the drill wandered enough it broke through the side of the pin which was about 7-8mm diameter.

Done by hand on a Colchester 2000 lathe only basic HSS jobbers drills were used – standard and long series so nothing special and the drilling began with a short series drill to get a good concentric start.

As has been said pecking and frequent withdrawal with plenty of coolant was essential to aid swarf removal – a tiring exercise when the tailstock was constantly being moved and several batches were sitting there to be done.

My, that was along time ago

Tug

Edited By Ramon Wilson on 14/07/2022 09:02:01

Another vote for Joe Pie. When you listen to what he says, it makes sense.

Back in the 70s, standard instructions for drilling in automatic lathes was first peck after 3 diameters depth, then 1 peck for each diameter thereafter.

But I suspect that was more for surface finish in the hole than straightness – although of course they're not entirely separate issues.

I've come to the view that – given the grind is symmetrical on angle and lip length – pressure is the most significant variable under user control. If you feel vibration through the tailstock handwheel, there's likely catch-and-slip going on at the cutting edge. If you can eliminate it by backing off a little and then applying light forward pressure you might well keep the hole straight enough.

Endmills or slot drills are much more rigid than jobber drills, but they also cut on the helical flute land, which a twist drill doesn't, so you can be exchanging the risk of a bent hole for an oversize one unless tailstock alignment is very good.

Like a lot in engineering, it's a matter of particular circumstances, priorities of different features of the hole, time, cost and patience pressures on the operator, etc.

Edited By Mick B1 on 14/07/2022 10:42:05

Tug, I just finished a set of 3 pins this week with 5mm holes through. The secret to drilling the long straight holes is the very start. I used to start with a Centre drill, and they always had some wonder. These ones I started the holes with a 6mm ball end mill to about 5.5mm diameter. Then went in with a stub 5mm drill, 15 mm deep. Then went in with a jobber drill, then with a long series drill. Sharpening the drills , so that they are even cutting from both sides and carefully either web thinned or 4 facet sharpened. I find the chisel point is good as it has very low pressure required to make it cut, compared to the conventional drill end grind. These pins are from 420 stainless, so not easy to drill. We also drill about 250mm deep with 8mm holes for injection cores, and the end of the hole is required to be quite concentric to the outer diameter by 0.1mm TIR. Quite often the drilled holes are well within the runout and roundness. The last batch the holes were round to about 0.02mm-.03mm tir at the front of the hole, and about the same at the end. We have a way of setting them up and trueing the hole to the stock, then finish turn the outers to suite. When I drill those holes, I put a very small radius on the outside corners of the drill. Gives a very nicely finished hole, and often very round too. Pushing the drill or forcing the drill , seems to lead with moved holes, same with the material getting hot. It seems to me that you just can not have too much coolant and regular pecks. On the long holes hand drill with .5mm pecks, and retract every 1.5mm-2mm. I used to gun drill the 5mm and 8mm holes, until the drills got damaged and were never replaced. I prefer for long hole drilling the shallower or longer point grind of the drill, so is about 120 deg included angle, compare to the standard of 135 deg. I have never had much luck with the flatter grind angle I see on alot of the long series parabolic fluted drills for making very straight holes.

Neil

You would need a new drill from a top manufacturer to minimise the wander. My old firm used to make aluminium hydraulic manifolds for Airbus. They were about 600mm long with a 6mm hole drilled from both ends done on CNC mills. The mismatch in the centre was about 0.75mm, good enough as long as any burr was removed.

A deep hole boring machine is like a lathe, but rotates the drill as well as the workpiece, they produce straight holes.

Edited By old mart on 14/07/2022 17:02:43