Taper Tantrum

[G-ManBart]

Okay, not exactly a tantrum, but it got your attention, right?

Edit to add: To save folks from typing out how to set a taper using indicators, I know how to do that, but appreciate people covering the concept. I'm simply trying to figure out why the taper scale seems to be significantly inaccurate...like 30% error.

I finally got to a project where I needed to use the taper attachment and I'm clearly missing something, or doing something wrong. The lathe is a Sheldon M series 13x36 with a telescopic taper attachment. The taper has a degree scale on the tailstock end, and an inches per foot scale on the headstock end...pretty typical. I will say the manual doesn't even talk about the taper attachment, but it is mentioned in catalog pages and describes it as I have. I can't guarantee the catalog is from the same era as my lathe...just in case something might be different, like not in inches per foot.

The lathe has been leveled and I haven't noticed it turning taper previously...certainly not on anything just over an inch long. I've got a couple of pieces about 12" long I turned recently and I'm going to double check those for taper.

I used my normal setup with the compound at 29.5*, the tool holder perpendicular to the chuck as confirmed with a 123 block and a fresh CCMT insert in the holder.

The part is 1.15" long, .622 on the small end and .682" on the big end. That should be 1.494* or .626" per foot using online calculators. I was using a piece of .75" diameter 1144 stress proof with maybe 1.5" of stick out from the chuck, turning from small to big, towards the chuck. I will say I was happy to exactly nail the diameters and the finish was really nice....glass half full!

I set the taper at 1.5* on the degree scale, confirmed that multiple times, checked my tool height carefully so it was on center, put the carriage close to the part with plenty of travel for the taper, locked the taper clamp screws down, turned the part using power feed and wound up with 1.1* rather than 1.5*.

The part looks nice, it's just not even close to the right taper. The compound was definitely moving smoothly in the direction intended and I thought I went more than far enough towards the tailstock to eliminate backlash. I wasn't taking heavy cuts and the surface finish was very good the whole time.

I've got to be missing something here. Hopefully you're already laughing because it's something stupid like my Trig is terrible, but could it be something mechanical?

I haven't moved anything on the taper, so I went back confirmed the setting on the angle scale at 1.5* and looked at the inches per foot scale and it's on the 5th line where the graduations are .125" so it's set for .625" per foot. The online calculator I was using (I tried two) gave me .6285" per foot when I used 1.5* so that's pretty close.

I can get the indicators out and set the taper angle using inches per foot, but I'd hate to think the taper scale is that drastically far off (it doesn't appear to have ever come off the lathe and the serial number is a T prefix which suggest a factory taper attachment.

On the Sheldon group page someone suggested setting the taper to 0 and turning a part with it engaged...seems like a good idea. It would seem if it's moving less than it should I would get a negative movement when set at 0....we'll see.

I can make the part, I'd just love to hear what I'm doing wrong, or what might need to get looked at. I know taper scales are just ballpark, but this seems a bit much. Thanks in advance!

 

[twhite]

We need pictures of the taper attachment set up. To help us out.


Cutting oil is my blood.

 

[twhite]

Is this what yours looks like? Not a Sheldon mind you. If so there needs to be a threaded handle on the top to lock the crossslide to allow the taper attachment to function. See circled picture.

Cutting oil is my blood.

 

[G-ManBart]

 

[G-ManBart]

Is this what yours looks like? Not a Sheldon mind you. If so there needs to be a threaded handle on the top to lock the crossslide to allow the taper attachment to function. See circled picture.

No, this style only requires you to tighten the tail clamp for it to engage and vise-versa.

Pictures posted above. This machine is pretty tight overall and the gibs on the taper seem to be adjusted properly. Everything is oiled, cleaned and moving smoothly.

Barring something obvious I'll have to go back to basics and check to make sure something hasn't shifted lately to cause it to turn taper under normal circumstances.

The only thing that comes to mind is I recently powered up the radiant heat in my shop, so the floor would have changed temperatures...that was a couple of weeks ago, so it should have stabilized, but that doesn't mean something hasn't moved.

I'm sick as a dog, day two of testing positive for Covid so I won't be able to try much for at least a few days....just looking for things to check at this point. I walk the hundred yards to my shop and almost need to take a nap

 

[twhite]

Perfect. Now for the obligatory obvious questions.
1. Is the taper attachment clamped down to the ways tightly? Picture 1 of yours. The bottom 2 bolts in picture.
2. Is the screw tightened fully on the cross slide and the taper attachment? This would be my guess as to the issue.

Now as a check. May I suggest setting the taper attachment to max angle. Then you can easily see it move when hand dialing the carriage. If you do not have a telescopic lead screw. You will not be able to move carriage.

Start here and let us know how that works.

Good luck.


Cutting oil is my blood.

 

[twhite]

Deleted post. Bad info.

 

[G-ManBart]

Perfect. Now for the obligatory obvious questions.
1. Is the taper attachment clamped down to the ways tightly? Picture 1 of yours. The bottom 2 bolts in picture.
2. Is the screw tightened fully on the cross slide and the taper attachment? This would be my guess as to the issue.

Now as a check. May I suggest setting the taper attachment to max angle. Then you can easily see it move when hand dialing the carriage. If you do not have a telescopic lead screw. You will not be able to move carriage.

Start here and let us know how that works.

Good luck.


Cutting oil is my blood.

Thanks!

The clamp is definitely tight on the ways...no movement there. I think I already checked them, but will recheck the nuts that lock the bar to the tail clamp to make sure they're tight.

I will have to confirm that the screw is tight. I turned the cross slide handle with the cover removed and didn't see any movement that shouldn't be there, but I will check it for sure.

It does have a telescopic lead screw so checking at max angle will work. I'm also going to set it to zero, keep it engaged, put a dial indicator on the cross slide and see what happens.

 

[tq60]

To test place anythig handy in like a pipe.

Mark 2 places that indicate an easy to hut mark on your taper, some place what the difference should be say 0.25

Keep lathe OFF for this.

Place a Dial indicator in your tool holder and place it on one of the marks.

Engage everything to operate the taper.

With hand wheel move carriage to other mark.

Did it change 0.25?

Repeat until you get it right.

Next test on pvc or scrap.

Adjust until perfect, then do real part.

Sent from my SM-G781V using Tapatalk

 

[f350ca]

The scale on the taper attachment is an indicator not a measurement.
Just finished cutting a Jacobs taper on the Hardinge, the indicator was close.
I use a test bar between centres, with a dial indicator on the tool post and a dial indicator on the bed or the DRO
In your case zero both indicators, move the carriage 1.15 inches and the tool post should move .030, half the taper.
Adjust the taper attachment until you get that.
Make certain the indicators are parallel to the movement or you'll get sine errors large enough to affect the measurements.

Greg