Why do I get a taper when turning?

**Robin Hewitt** · 15-09-2009

Face the end, that will tell you if you have the tool height spot on.

Only possible culprit I can add to the above is angle the tool edge 5 past 12 on the clock. That way it pushes against the work rather than pulling on it. If it shies away from the tool you can fetch it square on the next pass. If angled 5 to 12 you expect a crummy finish and awkward taper.

**irving2008** · 15-09-2009

Hmmm, checked tool height by expedient of trapping a thin steel rule between work and tool - it remains essentially vertical.

Not sure what you mean by "angle the tool edge 5 past 12 on the clock", or rather I understand the words, not the intent... When the tool is square in the toolholder the leading edge is already angled to the work...

BTW, here is a pic of the test piece (its not intended to be a finished item), was just practicing turning to a diameter 12mm->10mm-> 8mm and using a die to thread the end. The 8mm ID thrust bearings and ballraces are a nice fit and a nut screws on nicely.

**Robin Hewitt** · 15-09-2009

Originally Posted by irving2008

Hmmm, checked tool height by expedient of trapping a thin steel rule between work and tool - it remains essentially vertical.

Yes, but carbide will cut with 90 degrees at the tip so you could be above centre and pass that test. Facing the end so it doesn't leave a pip, or snap a pip off, is more of a test. I'd say the tool is either too high or it's lost it's tip. Something rubbing is the usual way to that blistered appearance. :nope:

If the tool leading left hand edge is set perpendicular to the axis of rotation (12 o'clock) you could do well to rotate it clockwise a tadge (12:05). Any rotation counterclockwise (11:55) can be seriously bad news because it pulls on the work rather than pushing against it.

If I got a cut like that I'd start by sharpening the tool

**irving2008** · 15-09-2009

Originally Posted by Robin Hewitt

Yes, but carbide will cut with 90 degrees at the tip so you could be above centre and pass that test. Facing the end so it doesn't leave a pip, or snap a pip off, is more of a test. I'd say the tool is either too high or it's lost it's tip. Something rubbing is the usual way to that blistered appearance. :nope:

If the tool leading left hand edge is set perpendicular to the axis of rotation (12 o'clock) you could do well to rotate it clockwise a tadge (12:05). Any rotation counterclockwise (11:55) can be seriously bad news because it pulls on the work rather than pushing against it.

If I got a cut like that I'd start by sharpening the tool

OK, I'll have a look at that tomorrow..

btw, its a brand new tool :confused:

**graffian** · 15-09-2009

You want to try with a sharp, small nose radius bit of HSS and more rpms.
Carbide I would be turning that at max rpm, which is 2250 and isn't really enough for that diam. HSS probably around a thousand rpm.

**irving2008** · 16-09-2009

OK, swapped a few pulleys around and now got a 2:1 reduction from motor to spindle, and I think I can improve that with a further change to get 1:1 if I change the motor pulley a bigger one.

At 2:1 and '50Hz' on the VFD I reckon I am getting 700rpm (4 pole motor) at the spindle, but I need to take it to 52Hz to avoid a nasty resonance in the drive train, bench mounting or whatever...(think its the flat belt joint 'pinging' the countershaft mount into vibration, may need to put another piece of 19mm ply under the bench top in that area to increase the rigidity of the top, or maybe some lengths of 2" angle-iron)

Anyway, running at higher revs makes for a much better surface finish.

I tightened up the top and cross-slide gibs so they are fairly stiff but I'm still getting some movement somewhere which I think is contributing to the 'taper'. If I wind the cutting edge towards the chuck on the top-slide, taking off a moderately fine cut, if I reverse direction the tool digs in and cuts deeper, yet I can't detect any significant movement in the slide itself. Any ideas how I can validate this? One thing I was going to try was to put the DTI onto the bed and see if there's any movement in the saddle when the topslide is wound in for cutting, but I can't tighten the saddle gibs any more else I can't move it!

**Robin Hewitt** · 17-09-2009

Originally Posted by irving2008

taking off a moderately fine cut, if I reverse direction the tool digs in and cuts deeper, yet I can't detect any significant movement in the slide itself.

I like guessing games :dance:

It's a Vee bed, n'est ce pas? If so I'd look at the saddle bearing face opposite the Gibb strip. Has it worn into a bow? If so no amount of tightening the Gibb will stop the saddle twisting and you have some scraping to do.