PDA

View Full Version : Why do I get a taper when turning?



irving2008
15-09-2009, 12:20 PM
I've been practicing machining the ends of my leadscrews using a sacrificial bit of 12mm threaded rod. I am having problems with surface finish and a small amount of taper.

I don't have a steady so have been taking very fine cuts using a brazed tip tool (like the 12mm one in the photo but a 16mm to fit the lathe).

I thought that any movement of the workpiece away from the tool, due to lack of a steady or movement in the spindle bearings, would result in an oversize at the end, and on/under-size nearer the chuck. In fact the reverse is true, the taper being .05 - .07mm in about 30mm.

I've checked the topslide movement relative to a test bar and the topslide runs parallel to the bar within .02mm over a 100mm length. Also the run-out of a test bar in my newly ground chuck jaws is less than 0.02mm over the same length.

If the workpiece was canted in the chuck am I right in my belief that that should not result in a taper, although the machined end would be off centre relative to the rest of the workpiece?

Should I be worried about this level of error, given the age of the machine (90-ish years)?

I am aware there is some movement in the slides, with end float and backlash, and I am assuming this is the main culprit for the somewhat rough surface finish, however slowly I take the feed, although the spindle speed is only 160rpm (I think, I've not measured it accurately yet).

This is clearly too low for such a small diameter workpiece... I need to change the drive pulley arrangement to give me a higher top speed (700rpm?) so I can bring it down if need be with the VFD (and backgear for threading)

BillTodd
15-09-2009, 02:15 PM
Can you support the end with a centre from the tail-stock?

Or, knock-up a tool-post mounted steady; just something to stop the work piece lifting (which can cause the tool to cut deeper as the work rolls towards the tool) and pushing away from the tool.

[edit] have a look at these: http://www.practicalmachinist.com/vb/showthread.php?t=185659&highlight=steady


I've checked the topslide movement relative to a test bar and the topslide runs parallel to the bar within .02mm over a 100mm length. Also the run-out of a test bar in my newly ground chuck jaws is less than 0.02mm over the same length.

Have you checked it on the top/bottom as well as the side? Height changes will cause your work to taper.

irving2008
15-09-2009, 04:54 PM
I didnt think I could use the tailstock as the part I am cutting is only 50mm long and the saddle is quite large and wont let the tailstock near enough really even with the barrel extended. tbh I've not tried it. Also not sure how centred the tailpost is, I've not tried to adjust it yet.

I'll have a look at making a steady, I have some 20mm aluminium plate that might do the job.

bikepete
15-09-2009, 05:23 PM
Might be worth experimenting with faster speeds for better finish... 160 sounds slow for 12mm stock (what material?).

EDIT re speeds I should read your post fully before opening my gob!

Incidentally if you are machining a fairly long ballscrew or whatever you might want to make sure the back end of it is supported to stop it flailing around - a collar type arrangement to plug into the back of the spindle hole would do it.

Robin Hewitt
15-09-2009, 08:59 PM
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, 11:36 PM
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
16-09-2009, 12:08 AM
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 :beer:

irving2008
16-09-2009, 12:13 AM
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 :beer:OK, I'll have a look at that tomorrow..

btw, its a brand new tool :confused:

graffian
16-09-2009, 12:29 AM
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, 03:25 PM
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, 11:39 AM
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
17-09-2009, 12:03 PM
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
17-09-2009, 12:59 PM
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
16-12-2012, 09:59 PM
nnnnnnnnnn

mocha
16-12-2012, 10:05 PM
onecut, check your PM, you have mail!

birchy
16-12-2012, 10:48 PM
Tickling the swarfe off a bar usually ends up with some weird results, including tapers and poor finishes. Most people seem to be scared to use the correct speeds and feeds, particularly when using carbide tips because you have to give it some welly and take deep cuts, often 2 or 3 full turns of the handle at a time!