Where did I go wrong???

[jgarrett]

I attempted to make a part for Elmer's horz. mill engine. It is the head and guide. I did not have a piece of 5/8" large enough to make it in one piece so I was going to try and make it two piece. Here is what I did. I took a piece of 7/8" round stock and cut it 5/8" sq. on the mill. Located the edges and drilled the 4 corner holes. Came out right on the money. I then centered it both X and Y with a DRO and used a 1/4" center cutting bit to make a hole in the center. I measured the same on all sides. I then took it over to the lathe, parted it in the 3-jaw and eyeball centered it in a 4-jaw chuck. (this is my fist attempt at centering anything in a 4-jaw). I put a short piece of 1/4" drill rod in the tail stock and adjusted the jaws until the rod would slide into the 1/4" hole without interference. I proceeded to turn the shoulder to the dia. I wanted. I then took a boring bar (small insert type) and bored the center hole out to 7/16".
Everything looked fine untill I measured and the hole is off center by .015".
Is my method wrong or did I just goof up somewhere??? What is the best/correct to center a square piece in a 4-jaw???
Thanks,
Julian

 

[mklotz]

Since the part originally had the 1/4" hole centrally located (assuming no errors in doing that), I would have used a pump center to center the hole in the 4jaw.

A pump center is a long (say 10") rod with a male center and a female center on the other end (often spring-loaded, hence the name 'pump'). The male center goes in your 1/4" hole and the female center is supported by a dead center mounted in the tailstock.

A DI applied to the rod near the part will read the eccentricity of the part as the chuck is rotated. Adjust the 4jaw until it reads the same all the way around.

 

[jgarrett]

Marv, Where can one of these tools be purchased or is is best to make one??
Thanks,
Julian

 

[Cedge]

Julian
I'm curious. Why not chuck up in the lathe first? Drill the center hole and cut the shoulder in one chucking so you'd have a clean concentric alignment. Then you could move to the mill where it's just a matter of cutting the square faces and locating the bolt holes. Then you could rechuck the work piece for cut off or face it off on the mill.

I guess I've just always used the lathe as a primary mode of attack, so my mind naturally works from a lathe oriented point of view. There is no right or wrong way and please don't think I'm being critical, just inquisitive. It's fun to see how others think through a problem like this one and I never fail to learn something by observing how other do things. Believe me.... I'm definitely still learning...(grin)

Marv...
By chance do you have a photo of the set up you are describing? I'm pretty sure I have a fair mental picture of the tool and using the DI or DTI has just about become second nature since my scrap box overflowed... LOL.

Steve

 

[jgarrett]

Steve, Being a novice I guess I did not even think of doing it that way. Since my round stock was 7/8" I have NO room for error to get it to 5/8" sq. Just something I need to learn to do. I will start another one on the lathe first. Like I said this is first attempt at centering something in a 4-jaw so I probably was not paying close enough attention. Thanks for the tip.
Julian

PS: What part of SC are you located???

 

[Cedge]

Julian
I'm located in Greenville, in the north western part of the state. Not many hobby machinists in this area that I'm aware of, so welcome, neighbor.

Steve

 

[1Kenny]

A very simple way to center a 4 jaw chuck is to mark the jaws 1,2,3,4. Jaws 1 & 3 work together as does 2 & 4. I used numbered stamps on the chuck to mark the jaw locations. Once you are close to center by eye ball, adjust 1 & 3 till that axis is in the center, then adjust 2 & 4 till that axis is centered. You may have to repeat the process a couple of times but the part will be centered.

Kenny

 

[mklotz]

jgarrett:

I made my own. Take a piece of, say 1/4", drill rod of suitable length (maybe 10") and cut a 60 degree cone on one end. On the other end, drill a 60 degree female center hole.

The male end goes into the workpiece hole to be centered. The female center hole is supported by a dead center in the tailstock.

Mount a DI on the compound (mine slaps in place with a magnetic holder) such that the DI axis is at the same height as the lathe axis. Use a flat, plate-like tip in the DI.

Bring the DI tip up against the drill rod close to the workpiece. Now, it should be obvious that, as the workpiece is rotated, if the hole center is not aligned to the lathe axis, the drill rod will move in a circle and the amplitude of that circle will register on the DI.

Adjust the 4jaw as you would normally so the DI shows no displacement as the chuck is rotated. When this happens, the workpiece hole will be centered on the lathe axis.

If you're new to centering work in a 4jaw, you may want to read my treatise on the subject written for the HSM club to which I belong. The url is:

http://www.schsm.com/html/centering_work_in_the_four_jaw.html

Cedge:

I, too, would have started in the lathe as you suggested.

This is all part of that most difficult aspect of machining - learning to sequence the operations in the proper order.

Nevertheless, learning how to center a hole in the 4jaw is a necessary skill. Inevitably, a situation will arise where one has to do that, e.g., centering a bushing to rebore it.

Sadly, I don't have a picture of the process. If my description above doesn't make things sufficiently clear, I can set one up and take a picture sometime.

 

[jgarrett]

Marv, That is great info. I will try it tonight. I guess it goes back to "cutting is easy...setup is the hard part!"

Steve, I am about 1 1/2 hr. away. I would like to come over one day and go to your scrap dealers with you. The ones here won't even talk to you unless you are selling.
Thanks,
Julian

 

[mklotz]

For other similar approaches to this subject, go to:

http://bbs.homeshopmachinist.net/showthread.php?t=24133&highlight=centre

and page down to posts #14 and #16 by John Stevenson. His rig is self-contained and fits into the tailstock.

The closest illustration I could find that parallels my approach is mentioned in Frank Ford's post in the same thread. It's illustrated at:

http://www.frets.com/FRETSPages/Machining/Tooling/WigglyCenter.jpg/wigglycenter.html#Hardening

 

[tattoomike68]

This is no joke...

Eyeball the part, you can see 1/16"+/- not too tough.

I have made bigger mistakes, im sure we all have a fancy scrap pile. :wink: