I have a strange dip in the middle of my turns

Dave K · Jan 2, 2018

Haas, SL10. Strange situation. When turning something kinda long, and there's a bit of a taper from front to back, we program out that taper. I'm talking about .001 - .0015 ish taper along about a 6-7 inch length. Yes, this is supported by tail stock. When working out the taper, we almost always have to go with the X axis needing to go in the plus direction as the Z is turning, or, make the side close to the chuck, larger, so it matches the front.

None of that is a big deal, but what's weird is we get this dip in the middle of the shaft that's always undersize. About .001 under, yet both ends are perfectly matching each other. However, I noticed one day when running another job, the taper had to go the other way. In otherwords, it was larger toward the chuck, so I had to taper it with a - U move while turning. When I got both ends to match each other, the dip was non-existent. So it appears when I need to trim the X in the - direction, that dip is gone, and I get a perfect straight turn.

What the hell is this? Is this a back lash issue in the X axis?

HuFlungDung · Jan 2, 2018

Is this on a fairly stout piece or is it flimsy? By flimsy, I would describe the part as barrel shaped (if the path is straight and parallel to the centerline). Straight tapers indicate an alignment issue of the spindle with the bed, and/or possibly a tailstock centering issue.

If you check all those out, then there is still the issue of turning between chuck and center. The tailstock cannot pull an eccentric centerhole perfectly onto the centerline. There will be some residual precession of the part, as well as some bending of the part (check for precession of the tailstock quill as the part is turning). In short, the center hole must run within a couple of thou of dead true before you put the rotating center in it, or you'll encounter random appearing tapering and have trouble controlling size at the end near the tailstock. I think the usual method is to allow the part to flex in the chuck, so that the tailstock can better straighten the shaft position. This might mean chucking on only a short piece at the end of the stock, not a full diameter or two worth of stock width, as that gives the chuck jaw too much restoring power compared to the tailstock's ability to pull the part onto the centerline.

yardbird · Jan 2, 2018

It could be? Long skinny work will have some deflection witch will make it shaped like a football.

Larger diameter work will tend to show the amount the tailstock is away from center and repeat better after you've sorted the taper out.

To minimize backlash I try to maintain one direction. If I need to taper down I come off the part and reposition the tool with a downwards feed before heading across. Doing this sometimes will screw up my relationship with other diameters on the part and have to lie in the program to get them to all jive.

Brent

EndlessWaltz · Jan 2, 2018

If doing a featherlight pass to clean up "on size"..I have noticed that sometimes if taking a .001-.003 cut that on a smaller diameter shaft that is longer, that if your center is off just enough, it may start to runout after the roughing and that is why you will run skinny in the middle. So interrupted cut, bad center, or gripping on untrue o.d. to the center drill you will have this.

I have done alot of shaft work 1"-1.5"dia.

Bobw · Jan 2, 2018

yardbird said:
It could be? Long skinny work will have some deflection witch will make it shaped like a football.

Like Sabrina, the teenage? Glad I'm not the only one that does that.

Or conversely, depending on DOC, feed, and tool geometry, it can suck the material in and give you an
hour glass.

yardbird · Jan 2, 2018

Bobw said:
Like Sabrina, the teenage? Glad I'm not the only one that does that.

Or conversely, depending on DOC, feed, and tool geometry, it can suck the material in and give you an
hour glass.

Funny it looked good when I posted which now that you mention it was kinda stupid.

Lol...

I can also see the tool being below center possibly causing like you say also.

Brent

Dave K · Jan 2, 2018

HuFlungDung said:
Is this on a fairly stout piece or is it flimsy? By flimsy, I would describe the part as barrel shaped (if the path is straight and parallel to the centerline). Straight tapers indicate an alignment issue of the spindle with the bed, and/or possibly a tailstock centering issue.

If you check all those out, then there is still the issue of turning between chuck and center. The tailstock cannot pull an eccentric centerhole perfectly onto the centerline. There will be some residual precession of the part, as well as some bending of the part (check for precession of the tailstock quill as the part is turning). In short, the center hole must run within a couple of thou of dead true before you put the rotating center in it, or you'll encounter random appearing tapering and have trouble controlling size at the end near the tailstock. I think the usual method is to allow the part to flex in the chuck, so that the tailstock can better straighten the shaft position. This might mean chucking on only a short piece at the end of the stock, not a full diameter or two worth of stock width, as that gives the chuck jaw too much restoring power compared to the tailstock's ability to pull the part onto the centerline.

Wow, lot to think about here. I don't have this issue when turning shorter parts with no tailstock, only on the parts that require tailstock. Even if it's larger diameter, it's still there. I don't think it's a chucking issue at all, as I mentioned, as soon as you reverse the taper move, the dip is gone, even when all other things are equal.

Dave K · Jan 2, 2018

Bobw said:
Like Sabrina, the teenage? Glad I'm not the only one that does that.

Or conversely, depending on DOC, feed, and tool geometry, it can suck the material in and give you an
hour glass.

That's what I'm having. Hour glass shape.

Vancbiker · Jan 2, 2018

If the headstock and tailstock are not at the same height off the bed you will get taper or an hourglass shape.

Dave K · Jan 2, 2018

Vancbiker said:
If the headstock and tailstock are not at the same height off the bed you will get taper or an hourglass shape.

I understand the taper. Can you explain why the hourglass effect though? And what I don't understand is , why when the taper goes the other way, the hourglass effect is gone.

Vancbiker · Jan 2, 2018

Dave K said:
I understand the taper. Can you explain why the hourglass effect though? And what I don't understand is , why when the taper goes the other way, the hourglass effect is gone.

Visualize the tool is above center at the tailstock and below center at the headstock. At either end the diameter will be slightly larger than the middle where the tool is on center.

Dave K · Jan 2, 2018

Vancbiker said:
Visualize the tool is above center at the tailstock and below center at the headstock. At either end the diameter will be slightly larger than the middle where the tool is on center.

Ok, that makes sense. Now can you explain why it goes away when the taper is the other way?

Vancbiker · Jan 2, 2018

Dave K said:
Ok, that makes sense. Now can you explain why it goes away when the taper is the other way?

That's making me draw a blank. It's usually not tough to check and correct the machines geometry. I'd do that first.

I'm not experienced with the Haas control so not sure what their behavior around backlash comp is. It may be that some backlash comp or "reversal" comp is going on. Someone that knows those controls can blow that idea out of the water for me.

Spruewell · Jan 3, 2018

It could very well be a backlash issue on your X axis. As you cut a taper with a decreasing diameter, the lead screw is pushing the cross slide and tool against the part with cosistent pressure. Cutting an increasing taper, the lead screw must pull the tool out of the part as the part is also pushing against the tool making it difficult to maintain a consistent cutting pressure. It's not hard to check backlash, put an indicator against the tool holder, apply some pressure back and forth on the cross slide and watch the needle. I would also double check the alignment of your tail stock.
If you find that you have excessive backlash you may need to rebuild/replace your lead screw and nut. But be aware that zero backlash can be an expensive thing to achieve. You can also change the way you cut the part and turn your tapers from large dia. to small

machtool · Jan 3, 2018

Spruewell said:
It could very well be a backlash issue on your X axis. As you cut a taper with a decreasing diameter, the lead screw is pushing the cross slide and tool against the part with cosistent pressure.

I think he is onto something there. Back lash compensation doesn't come into play whilst your in-feeding. Soon as you change direction, out of the cut. It applies compensation, and yet you still have some free play / back lash. True position of the X axis is free to wander within that. You'd only need 12 microns of backlash to see a difference of a thou in work piece diameter.

Regards Phil.

Cycle1000 · Jan 3, 2018

Dave K said:
That's what I'm having. Hour glass shape.

I have spent a fair amount of time and money trying to keep a hour glass shape happy.. :nutter:

But on to the lathe parts. I bought a st10 just to make a .875 dia 11.75 long 6061 part. Before I moved I dialed in the taper and just ran parts. Since I landed in Fl I chase the Dia and taper all the time. I blame it on the humidity..or me..
Gary

Dave K · Jan 3, 2018

Spruewell said:
It could very well be a backlash issue on your X axis. As you cut a taper with a decreasing diameter, the lead screw is pushing the cross slide and tool against the part with cosistent pressure. Cutting an increasing taper, the lead screw must pull the tool out of the part as the part is also pushing against the tool making it difficult to maintain a consistent cutting pressure. It's not hard to check backlash, put an indicator against the tool holder, apply some pressure back and forth on the cross slide and watch the needle. I would also double check the alignment of your tail stock.
If you find that you have excessive backlash you may need to rebuild/replace your lead screw and nut. But be aware that zero backlash can be an expensive thing to achieve. You can also change the way you cut the part and turn your tapers from large dia. to small

Ok, that makes sense.

Dave K · Jan 3, 2018

machtool said:
I think he is onto something there. Back lash compensation doesn't come into play whilst your in-feeding. Soon as you change direction, out of the cut. It applies compensation, and yet you still have some free play / back lash. True position of the X axis is free to wander within that. You'd only need 12 microns of backlash to see a difference of a thou in work piece diameter.

Regards Phil.

I'm thinking he nailed it also. It's the only thing that would make sense as to why it would go away when running the opposite taper direction.

HuFlungDung · Jan 3, 2018

On some longish lathe parts, I've used 3 different vectors along the path, visualized as a raised plateau in order to get a 'straight' cut off the machine. This would be for instances of part flexure. On a slant bed lathe with a decently heavy carriage and turret, I'd think on a not too heavy finish cut, there would be enough weight there to help compensate for the dead zone in real mechanical backlash. That is to say there is a light preload at all times on one side of the ballscrew nut, something you wouldn't get with a flat bed lathe. It is also possible to have over-compensated for backlash in settings for that axis, which could affect the path.

DMF_TomB · Jan 3, 2018

Dave K said:
Haas, SL10. Strange situation. When turning something kinda long, and there's a bit of a taper from front to back, we program out that taper. I'm talking about .001 - .0015 ish taper along about a 6-7 inch length. Yes, this is supported by tail stock. When working out the taper, we almost always have to go with the X axis needing to go in the plus direction as the Z is turning, or, make the side close to the chuck, larger, so it matches the front.

None of that is a big deal, but what's weird is we get this dip in the middle of the shaft that's always undersize. About .001 under, yet both ends are perfectly matching each other. However, I noticed one day when running another job, the taper had to go the other way. In otherwords, it was larger toward the chuck, so I had to taper it with a - U move while turning. When I got both ends to match each other, the dip was non-existent. So it appears when I need to trim the X in the - direction, that dip is gone, and I get a perfect straight turn.

What the hell is this? Is this a back lash issue in the X axis?

.
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you using coolant ? temperature changes will cause problems

I have a strange dip in the middle of my turns

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