help new machinist - cannot get a good finish

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mioduz
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help new machinist - cannot get a good finish

Post by mioduz » Sun Nov 20, 2011 5:13 pm

I am new to machining, have a 10X22 Grizzly lathe. I have turned a few projects down with great success. I have run into a problem with this project. I cannot get this 4" piece of tubing to cut properly. I need to turn down the OD as well as the ID. I have the machine running at 150 RPM, and the Feed set to as slow as the machine will go. The depth of cut doesnt seam to make a difference, but ive tried from .01 to .001 with no success. See pics below

https://lh4.googleusercontent.com/-dULo ... 164030.jpg
https://lh3.googleusercontent.com/-cEAU ... 164009.jpg

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GlennW
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Re: help new machinist - cannot get a good finish

Post by GlennW » Sun Nov 20, 2011 5:38 pm

You need to support the far end of the tubing with the tailstock center.

Make a shouldered disc to fit in the end with a center in it.

I'm surprised that it has not come out of the chuck as it is a VERY unsafe setup!
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!

mioduz
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Re: help new machinist - cannot get a good finish

Post by mioduz » Sun Nov 20, 2011 5:49 pm

Glenn Wegman wrote:You need to support the far end of the tubing with the tailstock center.

Make a shouldered disc to fit in the end with a center in it.

I'm surprised that it has not come out of the chuck as it is a VERY unsafe setup!

I thought that would help. And i can make something to throw in there easy enough. My next question would be how about boring? wont I have the same issue when I bore the ID? how do I hold it more securely then?

Torch
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Re: help new machinist - cannot get a good finish

Post by Torch » Sun Nov 20, 2011 7:56 pm

You would use a steady-rest to support the OD whilst boring the ID. That said, it looks like a pretty long tube. Could be a real challenge boring it accurately with a long boring bar flexing away inside there.

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Orrin
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Re: help new machinist - cannot get a good finish

Post by Orrin » Sun Nov 20, 2011 9:40 pm

After you get your workpiece properly supported, here are some tips that will help you get a good finish.

1) If using HSS, make sure your cutting edge has the proper geometry; and, make sure the cutting edge is sharp.

2) Brush a little cutting oil onto the workpiece now and then. It makes a huge difference in the finish.

3) On the finishing pass, take a light cut with a tool that has a bit of radius at the nose. Beware, however, of having too much radius or else you might develop tool chatter.

I'm currently building a low-tech project using crap-pipe about the same diameter you have. It's crummy stuff to machine, but I'm getting a beautiful finish. Some folks complain about the finish they get on hot-rolled stock; but, if they'd follow the three suggestions, above, they'd be more than satisfied.

I think the rule of thumb is to have no more than 3X workpiece diameter "stick-out" from the chuck. Your item exceeds that by a big factor.

Good luck!

Orrin
So many projects, so little time.

dly31
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Re: help new machinist - cannot get a good finish

Post by dly31 » Sun Nov 20, 2011 10:40 pm

If possible, the best way to bore that would be to mount the cylinder on the carriage and use a rotating boring bar between the chuck and the tailstock center. That takes a lot of careful setup and operation but I don't think you will be able to do it on the lathe otherwise. A boring head on a suitable milling machine might do it.
Don Young

stevec
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Re: help new machinist - cannot get a good finish

Post by stevec » Mon Nov 21, 2011 7:37 am

How long is the piece? Do you actually have enough bed length to bore it's full length?
How much precision do you need for the bore? I ask because, after you get success, by following Glenn and Orrin's advice, you might bore halfway then turn the piece around and bore the other half. That way you'd get by with a b. bar half the length.
Don's(dly31) suggestion is good but you'd need even more bed length, if I'm not mistaken.

mioduz
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Re: help new machinist - cannot get a good finish

Post by mioduz » Mon Nov 21, 2011 8:29 am

stevec wrote:How long is the piece? Do you actually have enough bed length to bore it's full length?
How much precision do you need for the bore? I ask because, after you get success, by following Glenn and Orrin's advice, you might bore halfway then turn the piece around and bore the other half. That way you'd get by with a b. bar half the length.
Don's(dly31) suggestion is good but you'd need even more bed length, if I'm not mistaken.
Accuracy is not critical for either the inside or outside. I just need something RELATIVLY true. Boring from one side and turning the piece around was my plan

mioduz
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Re: help new machinist - cannot get a good finish

Post by mioduz » Mon Nov 21, 2011 8:30 am

to support the end with a live center, can i tack weld on a piece of bar stock across the diameter of the peice and place the live center on that?

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GlennW
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Re: help new machinist - cannot get a good finish

Post by GlennW » Mon Nov 21, 2011 8:38 am

You sure can!
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!

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fourjc
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Re: help new machinist - cannot get a good finish

Post by fourjc » Mon Nov 21, 2011 9:17 am

If you need any degree of accuracy in bore size and concentricity I suggest you Bore first then turn OD on mandrel between centers or chuck in finished bore on one end and center with disk on other end as suggested. To bore a tube this long accurately you need to investigate www.microbore.de/info/boring_tools/default.aspx.
It’s already been suggested to load the boring bar between the chuck and live center and the tube on the carriage, you can make the Microbore boring bar yourself and purchase the brazed cartridges from eBay. The boring bar needs to be twice the length of the tube plus chucking with the cartridge in the center. This method will give you the most rigidity and accuracy. I know this method involves a bit of extra work but this boring bar will come in handy many times over. I hope this brief is helpful.
Good luck.

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