What did I do wrong?

[Kmot]

I used two new procedures for the first time today on my lathe. Taper turning, and boring. My goal was to make a funnel for a smokestack.

All went really well, for awhile. I was feeling pretty good about it all. Then all heck broke loose.

While trying to do a taper turn on the inside of the part where I had tapered the outside already the tool for some reason grabbed and ripped the part out of the chuck damaging the part. It crushed in one side of the tube. I was able to straighten it sort of, and re-mounted it and tried to finish cutting the inside taper. This time the tool bit grabbed and not only did it rip it out of the chuck jaws but it also partially yanked the cutter bit out of the tool holder.

I was able to salvage the part, by reverting back to the boring bit and boring out most of the inside of the taper. Then I was able to finish the job by doing a taper turn cut again.

I am guessing that trying to remove so much metal from the inside was the problem. It was just asking too much of the cutting tool? The outside taper cut went easy and I expected the inside to be exactly the same but it was anything but smooth and easy.

If I have to do something similar again, is the proper procedure to drill, bore, and make stepped cuts on the inside taper area until there is just a small amount of metal to remove using a taper turn cut?

Why does the outside taper cut so easy in contrast to the inside taper?

Thanks for your advice.

Outside taper being cut:

Part has now been bored:

Starting the inside taper:

Damaged part:

 

[joe d]

Kmot

I would cut from the bore outwards. Going at it your way you are plowing through a lot of material. If you start in the middle
and work outwards, you can crank in a comfortable depth of cut, and just do as many passes as it takes.

Hope this makes sense,

Joe

 

[Kmot]

Dang, that makes a whole lotta sense Joe! That would have saved me the trouble of setting up the boring bar again too. Thanks!

 

[rklopp]

You have to take off the material in a bunch of small cuts rather than one big chomp. Based on a guess at the scale you're working at, I'd stick with <.050" depth cuts. As you gain experience, you'll learn what is suitable for the equipment, tooling, and materials you have. It looks like you had plenty of chatter. That's not good and a sign of worse to come, as you apparently discovered. To stop the chatter, reduce the depth of cut, reduce the spindle speed, and INCREASE the feed rate.

 

[doc1955]

It would also help to turn up a plug to slip inside where you are clamping in the 3jaw a nice slip fit once chucked in the 3 jaw it should be tight. Plus what already has been mentioned take smaller cuts (sneak up on it that way it won't get away) :big:

 

[Kmot]

Thanks guys! Another great tip Doc, making that plug for the inside. Would have saved it from getting bent and crushed.

 

[Drei]

Mayde you hit the inside face by accident and the tool tried cutting to much at once and ripped the part off the cuck etc... sometimes it also happens to me even when i bore and thers a chance that tings go wrong. ..

Drei

 

[kvom]

I'd bore it out as the last op, so that when cutting the inside taper you can chuck on a more solid bar.

 

[Captain Jerry]

It may be my inexperience but I would go at it differently and try to avoid the re-chucking that might lead to loss of concentricity.

1. Face the end and drill a hole equal to the depth of the tapered section.
2. Turn the inside taper using the compound set to taper angle.
3. Turn the straight part of the part that is next to the outer taper to diameter.
4. Turn the outer taper using the compound (still at the angle).
5. Drill and bore the inner diameter to length and finial diameter.
6. Turn outer profile (two diameters)
7. Part off.

My thinking is that inside cuts are more likely to go wrong due to visibility and unsupported length of bit and the more material still left on the outer surface provides stiffness and dampens chatter so I try to do inside cuts first.
On steep tapers like this, I try to cut from inside to outside or from left to right. Going this way, if the bit grabs, it tends to pull out of the material rather than into it. Keep the bit sharp and the cuts light on thin wall parts.

I could be wrong.

Jerry

 

[Ned Ludd]

Are you using the top slide to make your tapers (correct way) or just turning the tool in the tool post and using the side of the tool (wrong way)?
From your first picture it looks like you are just using the tool rather like a form tool.. The trouble doing it that way, you have an enormous load due to the area of tool touching the work. Enormous loads tend to make things fly out of chucks! Using the top slide, set to angle required, you can take small cuts in the same manner you would when taking a facing or sliding cut.
So to answer your question "what did I do wrong" ,I think, you expected too much, but you can only know this from experience, which you now have more of. At least you tried something new to you, keep at it and good luck.

Ned

 

[Kmot]

Yes, I used the compound slide to move the tool inward. I realize now that I was trying to chunk off too big a bite at one time. I was lulled into it because the outside cut was also a large cut using almost the whole side of the insert and so I thought the inside taper cut would go the same. But I realize now that it was trying to cut two sides of a three sided insert when doing the inside taper.

 

[Ken I]

I see the same thing as Ned - you are "plunge" cutting not turning with the compound slide as you should have been (alternative taking an overoptimistically large cut).

For a long internal taper I normally do an autocad drawing of a number of drill stages to rough out the metal using drills and then like Captain Jerry suggested start at the bottom and work out - at least until you are happy that your boring bar works at the small end of the taper - charging in from the outside only to find it doesn't fit at the small end is a route to trouble.

Ken

 

[rake60]

In any turning application you want the tool pressure to be going toward the center of the chuck.
That isn't always possible but when the method can be rethought to do that, do it!

Over the past quarter century I've thrown a few parts out of machines while turning them.
Every one of those events involved face turning or outward boring situations.
In almost every occasion I knew what could happen but believed I could pull it off.

Damaged small parts stink.
When you send a 500 pound housing airborne people in dress clothes come running out of their
offices to see what happened. You just smile and wave, then get the crane to set it back up and
try again....
:shrug:

Rick

 

[Kmot]

Everyone, thanks for your comments.

I'm not certain I understand though, what some of you are saying I should have been doing. Here is a pic, and I placed an arrow on it to show the direction the tool was feeding.

I understand I was trying to hog out too much at one time. As far as I can tell though, I was cutting the taper by using the top slide or compound I think it is called, the correct way.

If there is a video of cutting an inside taper on the opposite side of an outside taper that someone can point me too, I would appreciate that.

 

[mu38&Bg#]

It's OK doing it that way, but you start at the small through hole you have in the middle and work your way out to the OD removing maybe .020-.050" at a time, until you end up at final size. Same with the outside taper. I'm sure you could get away without the tailstock if you used small depth of cut.

This is CNC, but the process is the same.

[ame]http://www.youtube.com/watch?v=zjS3m4IOcKg[/ame]

 

[mu38&Bg#]

Here's a sketch of how I'd make this part. I made something just like it but 2" on the big end and something over 1" on the small end. I set the compound at the angle for the taper and feed it out when I get to the taper on the OD. I machine toward the headstock on the OD. On the ID if you have a lip make the first, then machine from the inside to the lip at the angle. The hardest part is knowing how thick the angled wall is. I didn't have a tool appropriate to measure with.

 

[rake60]

Kmot

If I were cutting that taper, I'd start with a boring bar in the center of the stock.
I'd step it in roughly with the tool pressure going straight into the chuck.

After most of the meat was out of the way, I'd go to freehand smoothing up the
taper.

It's easier to do than it is to put into type.

I'll try to do a video of my method tomorrow.

Rick

 

[Kmot]

Thanks guys! I think I've got it now.

 

[compspecial]

Its just a thought, K mot but you could probably get a good result by flaring the tube after annealing, just as the plumbers do with copper fittings