Problem- center drilling: Out of center

[skipper]

Hey Guys,

I have a 9x20 HF lathe, what is the correct type of drill to use to drill a hole to be able to use the live center, is it a "center drill"? The live center I have has a real sharp point, I a bit confused about center drills, or spot drills or counter sink drills as to there uses. Also, the reason I want to start using the live center is that it seems that if I am working with a round stock that is out 3 or more inches (not supported on the other end of the chuck) the run out is attrocious. The lathe will not cut a concentric cut on the bar. Does this make sense, the cut material circle will be out of center with the center of the rod, like a cam. Not sure how to correct this other that trying to support the other end with the live center. The chuck I am using is the stock 3-jaw chuck. Will using the live center fix this problem?

Thanks,
Skipper

[stevehuckss396]

Originally Posted by skipper Hey Guys, I have a 9x20 HF lathe, what is the correct type of drill to use to drill a hole to be able to use the live center, is it a "center drill"? The live center I have has a real sharp point, I a bit confused about center drills, or spot drills or counter sink drills as to there uses. Also, the reason I want to start using the live center is that it seems that if I am working with a round stock that is out 3 or more inches (not supported on the other end of the chuck) the run out is attrocious. The lathe will not cut a concentric cut on the bar. Does this make sense, the cut material circle will be out of center with the center of the rod, like a cam. Not sure how to correct this other that trying to support the other end with the live center. The chuck I am using is the stock 3-jaw chuck. Will using the live center fix this problem? Thanks, Skipper

The live center will fix the problem providing you drill the hole for it dead center.

By "runout" I am guessing that the unsupported end wobbles more than the end in the chuck. If that is true, what i do is grab the stock snug with the chuck and using a small hammer and a dial indicator mounted on a magnetic base, tap the end until the stock is running as true as the chucked end. Sometimes it will even run better.

If you want to drill the end for the live center, use a center drill.

[cam1]

Hi:

Yes, use a center drill to prepare the stock for a live or dead center ( 60 degree included angle).Center drills by their design were invented for this purpose.
Spot drills are short, so that they can be used on many jobs without having to firsr centerdrill the hole location.Counter sinks are exactly what they are described as. Jobber drills are a compromise, in that they can drill depper holes.For a socket head capscrew you would first drill the hole for the screw body clearance, and the counterbore for the bolt head.

Rule of thumb for turning: any more than 3 X diameter of workpiece sticking out of the chuck requires the use of a center. eg 3 inch diameter round stock requires a center after 9 inches of stickout from the chuck. Obviously the depth of cut and numerous other parameters will dictate how much stickout you can tolerate for a given setup.

Depending on what you have to work with, a three jaw chuck is not reliable ( I'm not talking about set true or soft jaw setups) to expect to turn a part with no runout. This is why you would select a suitable rough stock size to compensate for the camming effect.
PS how much runout are you measuring just past the chuck jaws.

regards

[diecutter]

My HF 9x20 came with a "follower" which bolts to the carridge and supports thin/long workpieces near where the tool is cutting by moving with the cutter. It looks like a large circle with three adjustable metal bearings which ride against the workpiece.

[RICHARD ZASTROW]

Skipper, There are many things that influence where the "center" is. Think of the center of rotation about the headstock bearings.This is point "A". The tailstock center is point "B". A line from point "A" to point "B" is the center of rotation. If the point of where the live or dead center is not on that line, there will be a taper cut on the workpiece.

The tailstock must be in alignment with the headstock bearings along the length of the tailstocks travel wherever it is positioned along the bed of the lathe. (that would be perfection which seldom exists)

Generally, a dead center is more accurate than a live center but has lubrication problems. A live center brings with it run-out problems relative to the accuracy (and price) of the live center's bearings and total assembly as well as force limitations on the bearings.

Often, eccentricity (runout) is caused by a tailstock assembly not rigid enough to handle the forces exerted on it.

What is commonly called a "center drill" is actually a " combination center drill". This is a cutting tool with a drill point and behind that a centering taper matching the angle of the dead or live center's tip. Usually 60 deg.

If the drill part of the "center drill" is short (typically because it was ground back during re-sharpening) the tip of the center will contact the bottom of the hole. This prevents proper contact with tapered areas on the center or the workpiece, causing the workpiece to "wobble" around the live or dead center.

Start with the least costly possible problem (short or wrong tool) and work your way to the most costly until you find out what causes the problem.

A countersink or spotting drill is not the proper tool. A combination center drill should suffice as long as the tip hasn't been reground.

Hope that wasn't too confusing. LOL

Dick Z

Add: Many good postings while I was typing with the Columbus method, find a key and land on it.

[ronsii]

Hey Skipper, I had a similar problem with an enco 13x44 lathe I bought 15 years back, as it turned out the 3-jaw chuck I had was not in plane with the z-axis (travel axis). This took a while to figure out as I would setup a dial indicator on a short .25" dowel in the chuck and it showed about 2/10,000ths out and my center lined up perfectly(within 2/10,000ths) but when I finally chucked a piece of 2" x 36" long drillrod and put the indicator out at 32" from the chuck I would see the dial showing 70 -80 thousanths out of round so I took a sharpie and marked where it was out then loosened the chuck rotated the drill rod 180 degrees in the chuck and the dial still showed~70 thousanths this tiem opposite the mark which showed me that the drillrod was straight (more or less) and that my chuck wasn't pointing straight inline with the ways on the lathe. Your lathe may have the same problem as it sounds like how mine was acting at the time, Hope this helps, RonSII

[deji]

Pretty good info in this thread. Especially if more than 3 times the diameter is sticking out to use a center.

Made me realize that I need to get a better understanding of live and dead centers. When we learned to cut a taper, we did it between centers. But not sure if the center in the chuck was live or dead. But we used something called a lathe dog that attached to the part so the chuck could turn it. Guessing live since technically the center was turning.

[cam1]

Hi:
Center in chuck is dead, as it does not rotate relative to workpiece. A live center rotates relative to the workpiece.Dead centers at the tailstock require lubrication @ the point, as they don't rotate.
Liver centers at tailstock do not require lubrication (other than internal bearing lube periodically).
Live centers are not used at the headstock for the operations you are descricbing.

regards

[BobWarfield]

Originally Posted by ronsii Hey Skipper, I had a similar problem with an enco 13x44 lathe I bought 15 years back, as it turned out the 3-jaw chuck I had was not in plane with the z-axis (travel axis). This took a while to figure out as I would setup a dial indicator on a short .25" dowel in the chuck and it showed about 2/10,000ths out and my center lined up perfectly(within 2/10,000ths) but when I finally chucked a piece of 2" x 36" long drillrod and put the indicator out at 32" from the chuck I would see the dial showing 70 -80 thousanths out of round so I took a sharpie and marked where it was out then loosened the chuck rotated the drill rod 180 degrees in the chuck and the dial still showed~70 thousanths this tiem opposite the mark which showed me that the drillrod was straight (more or less) and that my chuck wasn't pointing straight inline with the ways on the lathe. Your lathe may have the same problem as it sounds like how mine was acting at the time, Hope this helps, RonSII

This is good info. Lots of potential sources of inaccuracy. Here are some thoughts:

- 3-jaw chucks are just not very trustworthy. Even if they start out good, they can wear in the scroll and wind up with runout almost invariably. Take your 4-jaw out and measure your result after dialing in the workpiece.

- Get a small mallet and see if you can LIGHTLY tap your workpiece into better alignment.

- The jaws are a source of error. You may find it helpful to use a boring bar to lightly clean them up. Not trying to remove much material here! Just make them true.

- The backplate your chuck is mounted to may not be true. You can clean that up in your lathe as well. Remove the chuck and get your dial indicator and see whether the backplate mounted by itself is true. Next would be to check whether the back of chuck is true. Don't just wing it. Track down one of the many posts or articles on how to properly fit a new chuck to a lathe. Go through those operations to make sure your chuck is mounted true.

- Start projects with trued up and square workpieces. When you insert stock in the lathe, true it up. Give it a light turning and facing to make sure it's good and square before you get on with your actual machining. I find this is even more important in the mill.

- Use the tailstock, steady rest, and follower. Most amateurs use these too infrequently.

- As for live vs dead center, get a really good live center. Skoda and Bison make good ones. If you can find a Royal at a good price on eBay they're excellent. You are very unlikely to exceed the accuracy capabilities of one of these live centers if they're in good shape.

That's a few things that came to top of mind. Should give you some areas to think about.

Cheers,

BW