Thread: How did I bore a taper?

Hi all, newb question here - I used a boring head for the first time the other day. I've used the lathe up until now. I used the auto-downfeed on the mill to widen a 1" ID 3" deep bore out about 0.020". So a 0.010" cut really.

The base of the bore is narrower than the top by about 0.0015" - so the bore is a bit of an inverted cone. The taper is not a disaster - in fact it's probably pretty handy for keeping the shaft snug in there - but not what what I was trying to achieve.

I can't get my head around how I could have bored a taper. Off-center maybe, or at an angle if mill wan't trammed and so on - but a taper? Maybe some ever so slight movement in the cheap rubbish boring head? Not sure so asking for opinions.

What I did realise afterwards is that I forgot for take a spring pass or two - could that be it?

Here is the set up:

IMG_20190504_155327.jpg

All comments welcome,

Greg.

I think the problem is that the machine is way too clean

10 thou might have been a bit eager and I am guessing that you might have had a bit of vibration and that
might have caused your boring head adjustment to move.

cheers, shed

Tool flex?

But that doesnt make sense either...does it

Spring passes maybe.
Too much bar overhang out of the boring head?
Is the boring bar cutting edge the only edge touching the bore being cut?

Andrew

Small amount of slop at the bottom of the quill housing? The further out the quill is, the more it would be able to move around.

Tool getting blunt the further into the cut, leading to springing.

Tool getting blunt the further into the cut, leading to springing.

My thought too. Although the tool may have been blunt at the start. Sharpen and take finer cut.

Cheers
Roger

After setting the bore size did you tighten the boring bar slide to stop movement?

That's a lot of replies for two hours! Don't you guys have jobs!!

Thank you all, very helpful and valid points there.

Some more info. The quill is has no detectable slop - it is in very good nick. The cutting tool was a fresh out of the box chinesium carbide boring tool - quite sharp. I chose the second-thickest in the set to cut down on flex - if there was going to be any. I took two cuts, the first was maybe 0.005" and the next was about 0.007" for a total ID expansion of 0.022". The boring bar head was the only thing touching.

I do recall now that I had not locked the table and saddle.

Some other possibilities I guess could be:

- a different in the material hardness from one side to the other.
- a small rotation of the cutting face. Infamously, these cheapie chinese boring tools cut with a negative rake so maybe some small rotation away may have occured?

But ... the important thing is nobody said "tramming" or such, so in theory, the auto-feed *should* have bored straight as I thought (yes?). I may have to do an experiment or two. I'll have more boring to do so figuring it out is important.

(and no, it is not that it is too clean )

EDIT: snapey: yes, I did.

I've experienced the same thing, I picked it up while while checking the size during the job and so transferred over to the lathe for the final few cuts to size with no issues - be interested to see what sort of suggestions on possible causes you get. My explanation to, myself was flex in the quill assembly the further out it comes the more it can whip around and flex.

The only thought I can contribute was that the tool worn as per RC's suggestion. Was your tool speed correct for the hole, and what is the finish inside the bore like (both top and bottom)

Michael

Thanks Michael,

I won't say the finish was amazing, but acceptable. Likely I could have run it faster - I was on about 500rpm or so for the 1" bore. The 'wear' thing is maybe realistic. The points on these carbide tools come very sharp new - it only has to take a off a metric smidgeon I guess.

I might see if I can repeat it on a scrap piece this weekend. I have some cast iron about .. but quite different material .. like from old elevator weights so they are likely to be full of junk, hard spots, dentures, and landfill. Might be a good test for wear!!

Hi Greg,

“The cutting tool was a fresh out of the box chinesium carbide boring tool - quite sharp”

I seem to remember seeing something on YouTube about those chinesium boring bars not work working “straight out of the box”. It might have been Stefan G, reworked a set to make them cut.

Cheers

Piers

If the head was out of tram your hole should still follow the axis of the spindle and your hole should still be parallel just not square.
Is the hole tapered or is it stepped? If it is stepped then a chipped tool is likely and i reckon you would of heard it change tune if that happened.
At this point i would check that the cutter is sharp, is at the centre of the tool, has some back rake and then be getting a good look at the tool in the bore and ensure that you have clearance behind the cutting edge.
Is that pulley cast iron? Looks like it has a keyway also, i dont know actual speeds and feeds as i just seem to know, if you get my drift. So cast iron and a keyway to bounce over and about 30mm i reckon i would start at about 200 rpm and see how it goes.
cheers shed

Agree with Shed, although I'd probably run the tool slower - 100 to 150rpm to start with and maybe increase from there. Depends on how rigid the boring set up is.

Michael