Unwanted taper

[Meta Key]

Does your cross slide/ compound have a lock?

Yup. Both of them. They're set-screws and are adjusted with a hex wrench. I cinch them down to avoid the 'variable taper' problem. On my lathe, the cross slide locking screw is on the right side of the slide body, about at the mid point. On the compound it's on the left side. They're different size hex screws..

MK

 

[JimDawson]

I had tried using a HSS cutter, but I was getting a lot of chatter and couldn't figure out how to get rid of it.

If you are getting chatter with a HSS cutter on that cut, something is seriously loose. Tailstock, center in the tailstock bore, spindle bearings, tool post, or something. I can't tell from the picture, but is that a dead center you are using there? Should be using a live center. Without proper lubrication, a dead center will wear or wear the center hole out quickly. White lead is the best, but you can't get it any longer, since the 70's or so. They do make some kind of center lube, but moly-grease or STP might work

 

[mikey]

Hi all....Attached is a photo of piece I am turning down with the purpose of putting an MT2 on it. My problem is this: I am getting a .005" taper over 2 1/2 inches and I'm having trouble figuring out why.

One thing that I see as I cut from right to left, using the power feed, is that as the tool makes contact with the workpiece the tool appears to push back very slightly. Before I engage the half-nut I pull back on the tool post to remove any back lash in the cross slide.
Oh! The other thing is this....the taper I'm getting doesn't always stay the same....sometimes when I make a pass I'll get no taper at all.
So I'm a little miffed as to what can be my problem...I make sure everything is tightened up and still end up with the problem.

I'd be more than happy to hear any ideas you all might have. Thanks!

Wino, I have an opinion. You appear to be using an SCLCR tool with a CCMT insert. I cannot tell what the nose radius is on your insert but it looks big so maybe a CCMT 32.52? Can you tell us what insert you're using? You also do not mention the depths of cut you're taking and this makes a difference. The other thing that matters is the material you're turning and the speed you're using so can you fill in the blanks for us?

Let me clarify why I'm asking. A CCMT insert is a positive rake insert, which is good, but this insert has a radiused cutting edge; that is, it is not sharp. It also tends to have a generous nose radius that has a minimum depth of cut it must take in order to cut accurately; if you don't take a big enough cut it will not cut consistently and what's more, it will deflect a lot more than you would like. Your packaging will usually give you the recommended depth of cut for your insert and this is for a typical roughing cut. The minimum depth of cut your insert will take depends on a number of factors but, in general, you can get away with 1/2 the nose radius and get it to cut somewhat accurately. You have to try it and see what the tool wants from you in terms of how light a cut you can take and still get what you dial in. For example, if you have a CCMT 32.52 insert, the nose radius will be 0.03125" so the minimum depth of cut you might get away with is 0.016". Any less and the tool will deflect. Your most accurate cuts will be greater than 0.0400" deep.

This minimum depth of cut can vary with the material you're cutting, the speed and feed you use, whether or not you use cutting oil and the condition of your lathe. If the material is medium to high carbon steel or stainless then work hardening will really compound the deflection your insert produces.

So, give us some more info and include the cutting conditions (speed, feed, depth of cut) you're using and we can maybe give you better advice. At this point, I am guessing your biggest issue is deflection and all that implies.

 

[petertha]

There is a big long post I contributed to somewhere on the forum with lots of discussion on taper turning issues. Aside from the lathe leveling (which really should be called lathe twisting) another factor to be aware of is - many lathes have their head stocks bolted down to the ways. What can happen is the head stock can get out of alignment meaning it is yawed rotationaly looking down from top. A tiny amount can cause taper cutting havock that has nothing to do with the tailstock or lathe twist. There are typically adjustment grub screws which are uber-sensitive.

So what I did was inserted an MT3 ended test bar into the headstock spindle (actually into an MT3/MT5 socket on my lathe) and then run a DTI down the bar. If its pointing anywhere but axial to the ways, then this is a source of taper cutting. If you have an accurate test bar cylinder, you can clamp in a chuck, but technically the chuck could be adding a bit of error. Still, it might be useful for quick evaluation.

If you are using the Edge test bar I'm thinking of, it has centers on either end & ground rings (dumbell equivalent). This is useful but unfortunately masks the rotated headstock problem. You have to do the aforementioned step first, then you can do the center to center test.

 

[petertha]

Here's some pics of my lathe, hopefully makes sense.

Forgot to say if you have an integral headstock meaning cast with the bed ways vs bolted on, disregard the above. There is no 'rotational' adjustment you can do on those lathes.