Losing concentricity

intothefiregw · Apr 22, 2020

Im trying to maintain a high amount of concentricity but for some reason I often realize half way into making a part that the center drill hole is all of the sudden a few thou out. I have no idea what I'm doing to cause this. I feel like Im being easy enough on the work but its like I bump it off center all the time... Am I the only one??

DDoug · Apr 22, 2020

intothefiregw said:
Im trying to maintain a high amount of concentricity but for some reason I often realize half way into making a part that the center drill hole is all of the sudden a few thou out. I have no idea what I'm doing to cause this. I feel like Im being easy enough on the work but its like I bump it off center all the time... Am I the only one??

Face off for no nub, and use the facing tool to make a starting cone for the center drill.

Lightly...LIGHTLY bring the center drill in to play.

mach2 · Apr 22, 2020

digger doug said:
Face off for no nub, and use the facing tool to make a starting cone for the center drill.

Lightly...LIGHTLY bring the center drill in to play.

Center drill as Digger Doug says then drill undersize and bore to size with a single point boring bar.

If that doesn’t work then you have unwanted movement in the work, work holding or machine.

redlee · Apr 22, 2020

Was it concentric when you started ?
Many materials can move whilst being roughed out.
Or is it actually moving in the chuck.
I found some materials required remachining the center before finishing.

But if its not straight to begin with then follow advise above.

awander · Apr 22, 2020

You guys are all assuming he is using a lathe...

crossthread · Apr 22, 2020

I'm certainly assuming he is using a lathe.

intothefiregw · Apr 22, 2020

Definetly using a lathe

intothefiregw · Apr 22, 2020

i turn down the diameter, center drill, drill and bore... everything is spot on at that point as far as I can tell. Then i use a live center to support it and start to shape the rest of the work. Im not sure when it starts to go south from there. I recently upgraded from a south bend Model A to a Feeler 618e so I guess theres a bit of a learning curve... I brought the spindle speed down from 1100 to 800 today and it seemed to help a lot... :confused:

theperfessor · Apr 22, 2020

Are you centering up on a sharp edge? Or do you put an accurate bevel at the hole's mouth to line up the center. Using a sharp edge doesn't work real well for me.

jims · Apr 22, 2020

After centering use an indicator on center to see if it is true. A center drill can walk off center.

eKretz · Apr 22, 2020

If your center is true to begin with and you are taking more than a skin cut after you center you probably aren't doing anything wrong. Steel moves as internal stresses are relieved when you cut a bunch of the original material away. This is why whenever you have much stock to remove you need to spring and reset the part. If you need very tight concentricity and it moves much during roughing, it's a good idea to recut the centers also.

intothefiregw · Apr 23, 2020

yes I am, I bore the end and throw the live center in... so there isnt full contact...

eKretz · Apr 23, 2020

Definitely add at least a small 60° bevel if you are currently running on a sharp corner - that is a no-no. Both for maintaining centerline and for maintaining your center! You can always re-tool it later if the part moves a bit due to stresses being relieved when material is removed.

On big machines with longer work that is fussy, this is standard practice. Rough complete, then re-tool both centers and finish.

dian · Apr 23, 2020

you can also use and offset countersink or even faster (21:00):

YouTube

mountie · Apr 23, 2020

dian said:
you can also use and offset countersink or even faster (21:00):

YouTube

Here's a direct link to the start of that part of the video: Making an accurate center

ETA: Basic gist is using the cross slide to nudge a drill chuck in the tailstock holding a combined drill and countersink to make sure it cuts on-center

intothefiregw · Apr 23, 2020

eKretz said:
Definitely add at least a small 60° bevel if you are currently running on a sharp corner - that is a no-no. Both for maintaining centerline and for maintaining your center! You can always re-tool it later if the part moves a bit due to stresses being relieved when material is removed.

On big machines with longer work that is fussy, this is standard practice. Rough complete, then re-tool both centers and finish.

I’m fitting several different materials together, I make spinning tops. So if I lose concentricity and re cut the center hole I’ll stuff be off I think...

donie · Apr 23, 2020

intothefiregw said:
Im trying to maintain a high amount of concentricity but for some reason I often realize half way into making a part that the center drill hole is all of the sudden a few thou out. I have no idea what I'm doing to cause this. I feel like Im being easy enough on the work but its like I bump it off center all the time... Am I the only one??

This can be caused by a 3 jaw chuck, the clearances needed for the scroll chuck to function can shift under high speed or cutting forces. Often encountered on performance machines like the Monarch ee.

Rudd · Apr 23, 2020

Something that strikes me - he's using a chuck, drills, bores, then uses the live center. And it is a new-to-him lathe.
Is the tailstock aligned with the headstock?
Try chucking up a piece of scrap, turn a kinda 60 degree point on it, then bring your live center up and try to "pinch" a 6" scale between the points. What happens?

(And yes, you need a 60 deg bevel in that hole for your center to seat in)

intothefiregw · Apr 23, 2020

Rudd said:
Something that strikes me - he's using a chuck, drills, bores, then uses the live center. And it is a new-to-him lathe.
Is the tailstock aligned with the headstock?
Try chucking up a piece of scrap, turn a kinda 60 degree point on it, then bring your live center up and try to "pinch" a 6" scale between the points. What happens?

(And yes, you need a 60 deg bevel in that hole for your center to seat in)

I haven not checked alignment yet, there is VERY little taper when turning OD but I'll go ahead and check it this way next time I run it.

intothefiregw · Apr 23, 2020

Thank you all SO MUCH for the advice!!!

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