Why do I get a taper when turning?

[graffian]

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]

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]

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.

[irving2008]

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.

It is a vee-bed, the outer edges are angled at 60deg approx. The tailstock runs on the centre of the bed.

I haven't taken the saddle off yet. The gib strip is at the back so I assume that is where the wear will be, no? Is this a case of replacing the gib strip?

[Robin Hewitt]

I haven't taken the saddle off yet. The gib strip is at the back so I assume that is where the wear will be, no? Is this a case of replacing the gib strip?

The Gibb strip can bend, it's the other side that may be the problem.

Suggest you loosen the Gibb, seize the saddle with both hands, shove it hard up against the Vee on the non-Gibb side of the bed and then twist it.

Does it rock or does it roll? You should be able to feel if the contact face has bowed.

[onecut]

nnnnnnnnnn

[mocha]

onecut, check your PM, you have mail!