Thread: Off centre drilling--lathe

I had that problem before as the cutting edge is longer on one side, I replaced with brand new drill bit and that sorted. It was out but not 3mm at the other end like yours!

Originally Posted by BaronJ

Hi Guys,
When drilling small deep holes, it is vital that both edges of the drill cut exactly the same, otherwise you will get the effect that you have experienced.

This is what I thought as well.
So I put it to the test using offcuts of 25 mm diameter bar..

I took a 3/16" bit and reground the tip it so that the sloped cutting edge was about 2x longer than the other (point still centred).
Used a centre bit, and did noticed more swarf coming out of one side but despite several trials it still drilled true.
Tried it without a centre bit - still worked pretty well - remember the point is still centred.

I didn't bother testing it with the same edge angle but with the point off centre (as per Shedhappens post) as I have done that many times to get a slightly wider hole and had no problems. However I have always used a centring bit - maybe without a centring bit this would not be the case

Then I tried the following with an correctly formed bit tip.

I faced off the end of a 30 mm long bar but left an ~1/2mm diam and <1/2 mm tall dimple in the centre of the face.
I also held the drill bit by about 1/2 the shank so that it would flex more easily.
No centring bit, so it would be easily forced off centre by the dimple.
The final hole was ~1/2mm out at the face and ~1mm out at the other side - this was over 30 mm so it should reach 2mm out over 60mm.
It was obvious that the bit was wobbling while it was drilling.

Bear in mind it does not need to be a dimple - it could be a small dip or dent in the face that would catch the drill tip.

So for newbies its pretty obvious what to do.
- generate a smooth face
- use a centring bit
- if the drill is wobbling - its going to be problematic - so stop drilling.

Thanks everyone for your help.
I did face the end and ensured there was not a dimple before starting with a centering bit, so that's why I was shocked to see how far the drill had run out.
I keep a set of sutton drills aside for 'special' jobs and had selected the 5mm drill from this store.
Even so I did have the drill as fully extended as it could safely be held in the drill chuck in the toolpost, so from the good advice Ive been offered it seems that may have been the culprit.

I will have some time over the next few weeks to experiment with a dial gauge and be able to take some measurements and of course try with some stub drills to see if I can improve the job as per BobL's suggestion)
Thanks again (as always) for the helpful and good advice from everyone
warm regards
Brian

Hi Brian,

Try a spotting drill to create a centre ! Seems to work better than a centre drill.

Brian you've had some very good advice but I'll throw something completely different out there. Did you have the stock extended from the chuck and running true, or back in the bore of the spindle? To be 3 mm off centre I suspect it was the latter and the far end of your stock isn't running true.

Originally Posted by Briangoldcoast

Even so I did have the drill as fully extended as it could safely be held in the drill chuck in the toolpost, so from the good advice Ive been offered it seems that may have been the culprit.Brian

In your quote above, you state that you had it in the tool post, is this a typo instead of being in the tailstock?
If you did have it in the toolpost, I'd say that this is where the problem lies. Also having the drill fully extended will/can create the same problem.
Next time, insert the drill right into the chuck, drill to the end of the flutes, removing the swarf as you go. E xtend the drill out as deep as you need to go, then drill a bit, about 5-6mm, back the drill out to remove the swarf, keep repeating this as you go. Should you need to go deeper than the standard jobber drill get a long series bit. DON'T drill the full length of the long series bit before cleaning out the swarf, as this could also cause the same problem.
HTH
Kryn

Thanks Kyrn, yes indeed I did mean tailstock, and yes I started with th minimum length of drill extended and yes I retracted the drill every 5-6 mm travel and cleared the swarf.
i tried twice to do this job with similar results then drilled from both ends of the 60mm length to solve the problem for is job, but I'm aware the problem still exists and I need a quiet morning with no interruptions to explore the issue sensibly and systematically.
Thanks again to all you helpful friends for your good advice.
warm rgards
brian

Originally Posted by Briangoldcoast

Thanks Kyrn, yes indeed I did mean tailstock, and yes I started with th minimum length of drill extended and yes I retracted the drill every 5-6 mm travel and cleared the swarf.
i tried twice to do this job with similar results then drilled from both ends of the 60mm length to solve the problem for is job, but I'm aware the problem still exists and I need a quiet morning with no interruptions to explore the issue sensibly and systematically.
Thanks again to all you helpful friends for your good advice.
warm rgards
brian

I appreciate the issue has been solved for the moment Brian, but I'm curious for my own experience the answer to my above question as to whether the stock was fully inserted in the chuck or sticking out. It may indicate other issues with the chuck.

Originally Posted by jack620

When drilling deep holes I drill from both ends where possible. Even if the holes don't line up in the middle, the subsequent drill bit tends to align the two holes. I always start with a centre drill as mentioned above.

I'm with Chris on this one. If you start where you want the drill to exit, and drill about half way, then turn your workpiece around and drill till you hit your existing hole, you should be on the money. You can step up to final size, and if you still have issues while using good engineering practices, you could do the same with your final drilling. If all else fails, then the suggestion already made of using oversized stock, drilling first followed by turning between centres will definitely work.

The only scenario where the turning between centres trick won't save you is when the wall thickness isn't there or the long flexible drill has made a bannana shape through the work. In that case you get a nice of but a shaft won't run without binding in the hole.

When required gun drills are not massively expensive, although you need to jury rig through coolant.

Peter Stanton on YouTube covers gun drills really well.

Hi Guys,

After reading this thread and knowing the problem, and that I needed to do this, I took pains to note the conditions I used.

I wanted to make some aluminium rounds for a job that I'm doing, to use as grips on a three jaw chuck.
Anyway I used 1" inch diameter bar and cut a 70 mm long length. I put it in the three jaw to drill it 6 mm down the centre.
I drilled all the way through and then parted off 15 mm long lengths.

By the time I parted off the last slice the hole was almost exactly 0.5 mm off centre. For this job it didn't matter.
Two observations, bearing in mind that I can sharpen my drills four facet. One: the alloy bar is not round ! I did try a soft wrap around the bar to get it to run as true as I could. Two: the alloy is not homogeneousness, it seems to vary in density. The character of the swarf
changes with depth whilst drilling.

27-10-2018-007.JPG 27-10-2018-010.JPG
In the first picture I had completed drilling and had started to part the first piece off. The second picture is the last piece. Its very difficult to see, but the hole is 0.48 mm closer to one edge, its also a fraction off in diameter.

The aluminium would have to be awfully crap to not be essentially homogenous (for our purposes) through the entire length.

As I said above, I think a lot of people are chasing a furphy here. Take the same piece of stick, wrap some tape around it and hang it out the front of the chuck. It would be nothing short of amazing if it were to run true within 0.5 mm. We can see that runout and often correct it. What makes you think it will run any more true when it's facing the other direction?

Just because a piece of stock is sitting inside a spindle and running true at the small section poking out, doesn't mean the opposite end of it will be running true. with good chucks and decent stock it's normally good enough for our purposes, but sometimes the tolerances stack. If you need to drill deep holes like this, either set up a steady and ensure it's running true at both ends, or use a spider at the spindle end to keep it running true inside the spindle.

Hi Pete F,
I held the 60mm length of stock in the chuck with about 20mm sticking out and faced the end before spotting the end with a centre drill. The tailstock was retracted so the 5mm mark was my starting point.
Baron J--you pretty much repeated my experience.
Pete F--Sage words, thanks.

However, I have to say that I have always assumed (note assumed) that in a reasonable three jaw chuck, a short piece of bright stock would run true and a hole drilled from a centre on one end would run true to the centre of other face.
I still have to take the time to run some checks but I recently took some time to adjust the tailstock so a 250mm cut only varies by .02mm on the length of a 25mm diameter bar. I also realise this could be a totally different matter
I'm also still very appreciative of the energy and effort so many knowlegable and thoughtful guys have put into this enquiry--thanks again
Brian

Originally Posted by Briangoldcoast

However, I have to say that I have always assumed (note assumed) that in a reasonable three jaw chuck, a short piece of bright stock would run true and a hole drilled from a centre on one end would run true to the centre of other face.

That's a reasonable assumption Brian. You don't need elaborate setups to achieve acceptable results for non-critical work. We're only talking about a hole depth of 12D here. There will, of course, be some error at the other end as the drill wanders in the stock. As I said earlier, if that's important you can fix it by drilling from both ends with a smaller drill and then drill or ream to size from one end to finish.

In your case (3mm off centre) something was clearly amiss (dodgy chuck, out of round stock, swarf on one of the chuck jaws?).

Let us know if you ever work it out!

Originally Posted by Briangoldcoast

However, I have to say that I have always assumed (note assumed) that in a reasonable three jaw chuck, a short piece of bright stock would run true and a hole drilled from a centre on one end would run true to the centre of other face.

Yes but you said it was a piece of aluminium you were drilling right? If this is consistent and not just a one off, take a piece of the same stock and hang it out the front the same amount. How true does it run?

If a hole isn't concentric both ends yet not oversize, then two things may be happening. Either the drill is bending (due to uneven sharpening etc) or the stock isn't true. I strongly suggest a 5 mm drill won't bend that amount in that depth, therefore it is the stock that is not true.