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gr8life
12-25-2009, 01:42 PM
I am having a problem facing this piece. It is either hot or cold rolled steel. I am using a new rcmt and getting a nice mirror finish on the outer portion but you can see the problem as I get to the center. I have tried faster/slower rpm, feed rate & depth of cut. I have used mist, oil and dry. I am cutting on center & have enough hp, can not understand it.
Could the metal be that much softer in the center?
any help?
thanks
& Merry Christmas
ed
http://i461.photobucket.com/albums/qq336/gr8lifeLV/FacingProblem001Small.jpg
http://i461.photobucket.com/albums/qq336/gr8lifeLV/FacingProblem002Small.jpg

beckley23
12-25-2009, 01:57 PM
Common problem. As you approach the center, the SFM decreases. If your lathe has variable speed, increase the speed as you approach center.
Harry

Oldbrock
12-25-2009, 03:05 PM
Get a round nose hss tool with LOTS of top rake. Get it set up then hone a small flat in line with the face maintaining clearance. Go slow rpm with feed and keep wet with coolant. Called slow finishing. Very light depth of cut. You should get a mirror finish. Works for me anyway. Merry Christmas, Peter

airsmith282
12-25-2009, 03:20 PM
if you want it flat and nice looking , you need about 900 to 975 rpm, use any HSS cutter you got that you can angle so the tip is whats touching, lock the carriage,then take a few very light cuts about 2 or 3 thou each cut use your compund to feed it in for the depth and use the cross clide of course for the faceing but keep the speed slow but stady no farting or stalling your pass ,you must be with in 1 thou below dead center, get it as close as you can and then touch up with 150 grit emery cloth then 1500 paper that you will use cutting fluid on when you use the 1500 make sure your speed on your lathe is maxed out on RPM...

if you think your going to get a mirror fininsh only using a cutter id like to see that happen .

also make a note here you will never turn accuratly if your tool is right on dead center,you will dull tools fast carbide will chip and brake faster then they should and so on and facing well thats another story, also parting off cuts just a slight hair below center .. things work better tools stay sharper longer and so on and so on ..

Carld
12-25-2009, 03:44 PM
It's like Harry said, the SFM decreases as you go from the OD to the center and since your feed stays the same that means the feed gets more coarse at it gets to the center. There is no way to get it as perfect as you may want it. For small work I feed by hand and on larger work I feed auto until it gets to about 1-2" dia and then feed by hand if I want it to look nice without any hand work.

One other thing that helps some is a very small radius on the tip of the cutter, about 1/64" R. will do.

Take what you have and try a 1/64"R cutter at about 500 rpm and about .002-.003" feed if you want to do it with the auto feed. Just take about .010" off the face and it should clean up. Sometimes to fine a feed will only make it look worse. Faster rpm helps too.

Machinist-Guide
12-25-2009, 03:54 PM
I agree with beckley your circumference is changing as you get closer to the center.
Something that works for me is try cutting from the inside out set your speed & feed for the smaller diameter.

hwingo
12-25-2009, 04:56 PM
I agree with beckley your circumference is changing as you get closer to the center.
Something that works for me is try cutting from the inside out set your speed & feed for the smaller diameter.

I'm insufficiently experienced to offer too much advice. I too have experienced this problem.

I agree with Beckley and Machinist-Guide that circumference is changing as you get closer to the center thus rotation becomes slower. Years ago I had an old machinist to tell me, "The closer you get to center, the slower the center turns. In theory, the *exact center* does not turn". Well that made sense to me and I've tried not to lose sight of his way of explaining things. His final bit of information explained speed ..... so that I could understand. When teeny-tiny screws are made using a lathe, e.g., screws for wrist watches, etc., a VERY HIGH RPM is used to make the center spin a little faster than "dead stop". Speed is needed because the center is turning soooo slowly.

I too have found that I get a better finish at high speed when coming from center out. Another thing is, angle of approach, i.e., angle of cutting surface to the face of piece being cut. "Radius at tip of cutting tool" has been pointed out. I have had good results using a 60 deg handmade threading cutter. Often I will turn the long side of the tool so that the side is but a few degrees from being parallel with the face of the piece being turned. Starting at the center, I will advance the compound until a small amount of metal is being removed by the point and side of the cutter and then slowly turn the cross feed until the cutter clears the outer circumference. That's worked for me.

Harold

gr8life
12-25-2009, 06:08 PM
With a little elbow grease & some 400-600-1200-1500 and polish it all worked out. Saved the other half and will experiment when I have time.
Thanks 4da help
ed

http://i461.photobucket.com/albums/qq336/gr8lifeLV/FacingProblemSmall.jpg

Carld
12-25-2009, 06:17 PM
There ya go. :D

spope14
12-26-2009, 04:56 PM
Though Surface footage is the issue, I have cheated it a bit by doing a .002 facing action center outwards to the edge.

Machinist-Guide
12-26-2009, 05:09 PM
Though Surface footage is the issue, I have cheated it a bit by doing a .002 facing action center outwards to the edge.

I agree it's the surface feet getting slower because the circumference is getting smaller. I also like you said have better luck going from the inside out.