Amount Of Material Removal On Spring Cut

[Deerslayer]

Cutting a pin out of 1144 stress proof today, the round was 2" to start with. Target diameter was 1.595" , after cleaning up the round I set up my dial indicator to accurately measure cross feed travel because I can't hardly read the old dials on the machine. After several cuts of .025" removing .050" of material at a time and measuring between cuts and consistent to the .001, I found myself at 1.610. Dialed .005" movement on dial indicator to allow me to sneak up on final dimension, after a nice cut I check OD and to my surprise I am at 1.587", now the .008" undersize will hardly matter on this but it's aggravating me. Could this be spring in the part?

 

[f350ca]

Spring in the part and probably mostly the lathe. I usually do a spring pass before measuring as I'm getting close to size. Making a .025 cut will spring a light lathe quite a bit, your .005 thou cut relieved the spring and amount you dialled in.

Greg

 

[4GSR]

I thought that was normal, to take a .005" cut and it winds up being undersized by .008". Used to happen to me all the time.

What size lathe do you have? How old is it?

 

[FOMOGO]

Check out this Tubal Cain video. /www.youtube.com/embed/n_dZe3K0a5M It may clear things up for you. Mike

 

[Wreck™Wreck]

Try this: Assuming that you are not making thousands of parts.

Make several rough and finish passes at the start well before finish diameter, note which DOC and feedrate gives the desired finish, write this down.
The difference will be the spring with that tool, material, speed and feed.

Rough the part close to finish leaving enough room to do the above again near final size, Use the same parameters at near finished size. The spring will change as the diameter and other conditions change.

Good Luck

 

[JimDawson]

I used to have the same problem, then I first figured out planning the cuts to take the last cut at about 0.025 DOC (0.050 off of the diameter) pretty much eliminated that problem. Then the second trick I learned is to do my turning with the tool set below center, no spring at all. Take a look at this thread http://www.hobby-machinist.com/threads/backlash-or-just-poor-technique.41725/

 

[Wreck™Wreck]

I used to have the same problem, then I first figured out planning the cuts to take the last cut at about 0.025 DOC (0.050 off of the diameter) pretty much eliminated that problem. Then the second trick I learned is to do my turning with the tool set below center, no spring at all. Take a look at this thread http://www.hobby-machinist.com/threads/backlash-or-just-poor-technique.41725/

I am interested, where did the spring go?
Geometrically speaking of course.

 

[Deerslayer]

I reckon in the future I will make a spring cut without any feed before

I thought that was normal, to take a .005" cut and it winds up being undersized by .008". Used to happen to me all the time.

What size lathe do you have? How old is it?

I have a leblonde model 21 (24" diameter X 8') prolly older than both of us together if your under 80.

 

[Deerslayer]

Check out this Tubal Cain video. /www.youtube.com/embed/n_dZe3K0a5M It may clear things up for you. Mike

Watched, thank you, however I was using an indicator movement of tool was pretty accurate.

 

[JimDawson]

I am interested, where did the spring go?
Geometrically speaking of course.

Quote from pstemari, post: 358238, member: 38046 "Setting the tool below center reduces the rake and increases the clearance. It's also going to increase the inward force on the tool."

There is a lot of discussion on this in the above referenced thread. It seems to have to do with the cutting forces on the tool. I personally could never really explain why it works.