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Phil McCrackin
01-10-2009, 09:38 AM
Hi all,

I have more questions about taking finishing cuts on my lathe.

If you don't remember, you guys helped me out when I had questions about my 9X20 lathe chattering when I was trying to turn a piece of Home Depot steel.

I took all of your suggestions and did the following.
1. Got a piece of Non- Home Depot mystery steel.
2. Ground my bit according to your recommendations and sharpened with an oil stone.
3. Set my bit height to center of the work.
4. Made a solid/rigid tool post. (had to use Al, but it still works better than the original)
5. Slowed my speeds way down.

Now I can take HUGE cuts(.070 or more), with no chatter. Just a nice long piece of steel string. Although the 'birds nest' it makes is annoying, still no chatter.

My question is simply this, can someone show me a picture or a drawing of a properly shaped finishing bit, and what doc should I use for steel.

I am actually getting pretty nice finishes with my current set up, I just want to get a super nice; almost mirror finish. Can this actually be accomplished?

HSS
01-10-2009, 10:01 AM
Try this link, it helped me a lot. It's about chip breakers and it will help you rid your turnings of birdnests, somewhat.;)


http://www.chaski.org/homemachinist/viewtopic.php?t=77187

Pat

Yankee1
01-10-2009, 01:20 PM
Hello Phil
I have a 7x12 lathe and I installed a animation motor on the right end of the lead screw. It turns at about 3 revolutions per minute.
I remove the gear from the left end of the lead screw. I run the lathe at about 600 rpm. The results are a finish that looks like it has been chromed.
I learned this technique from a website called " Varmint Al's"
He informs as to where these motors can be purchased. I made a coupling that stays on the right end of my lead screw in order to install the motor quickly. The motors are inexpensive.
Chuck

Phil McCrackin
01-10-2009, 04:03 PM
Thanks guys, I'll check those sites.

Is there a difference in tool bit shapes for roughing and finishing?

dan s
01-10-2009, 04:09 PM
a lot of parameters affect finish, what's your feed rate and tip radius?

miker
01-10-2009, 04:19 PM
Phil, have a read of this thread.

http://bbs.homeshopmachinist.net/showthread.php?t=29333

On page 2 you will find pictures and story of "The Contrary ground Finishing Tool" .

I have tried it with very good results.

Rgds

Phil McCrackin
01-10-2009, 04:31 PM
a lot of parameters affect finish, what's your feed rate and tip radius?

Slow and Small (lol). Seriously though, I do not have rpm meter on my lathe and the tip was ground sharp and then just hit lightly with my stone.

The book I have shows 4 basic shapes. They are roughing, finishing, facing, and round nose. The finishing bits look like they have a larger radius tip. Is that what I need, basically the same grind, just with a larger radius?

However this book does not seem to be much of a help. The advice I got here was what really set me straight.

dp
01-10-2009, 05:41 PM
I found that video I was thinking of in that thread. The tool is fascinating:

http://www.mmsonline.com/articles/hard-turn-without-leaving-feed-marks.aspx

dan s
01-10-2009, 05:45 PM
Phil,

Don't take this the wrong way, but you need to know these things.

1. RPM - You don't need to know it exactly. You have a KBC 9x20 right? Thus you lathe has 6 RPM settings. Take a look at your manual and let us know what you are currently using.

2. Material - We need to know more than its just steel. We need to know if it's 1018, 1095, 4140 etc. different steels must be handled differently.

3. Diameter of part - By knowing the diameter of the piece, and the rpm your turning, we can calculate the SFPM at the cutting bit. Each material has an "optimum" SFPM. Take 1018 for example, if your SFPM isn't high enough your never going to get a good finish, no matter how your tool bit is shaped.

4. DOC - from previous posts, I know you understand.

5. feed - Your lathe has a quick change gearbox if I remember correctly, thus your manual should tell you what feed rate you are using. feed is very closely correlated to surface finish, if it's to course you can "normally" forget about getting a nice finish.

6. tip radius - just like feed is closely correlated to surface finish. If it's too small, your part will feel like a cylindrical file, to large and you won't be able to take a very deep doc without chattering.

You already know about rake and relief angles and that is good, because they to play a part in surface finish.

OK its time for a disclaimer so I don't get jumped on by the "seat of your pants" guys. Everything is a range. There are no exact values, by this I mean what works on one machine might not work on another, for any number of reasons. Additionally even on the same machine, you might be able to get a good finish with several completely different setups. As you gain experience you will start to "feel" when your set-up is correct. When your new to it all, it's usually faster, and less frustrating to look stuff up.


If you can provide some of the information above, one of us can probably get you squared away in short order.

Malchira
01-10-2009, 06:22 PM
On page 2 you will find pictures and story of "The Contrary ground Finishing Tool" .

Just an FYI for those looking for it: since the linked thread doesn't have the original, I found another thread (http://bbs.homeshopmachinist.net/showpost.php?p=312223&postcount=14) here where John Moran lists the original file's location, the 7x12 Yahoo group's files (http://groups.yahoo.com/group/7x12minilathe/files/) section.

Yankee1
01-10-2009, 06:24 PM
Hi Again
This is the information I was refering to.
http://www.varmintal.com/alath.htm#Ultra_Fine

Check it out.
Chuck

tattoomike68
01-10-2009, 06:50 PM
If you want a nice finnish keep a few strips or worn out emory cloth stashed in a drawer and use it polish the last .0005" - .002"

Works real well on seal surfaces and things that have to have a good finnish.

you should be able to hold you finger up to the shaft and see your finger prints in the refection.

Emory cloth and a file are your friends just be super carefull on the use of them, they can bite you bad.

Phil McCrackin
01-10-2009, 08:05 PM
Dan, first off I am not offended easily, I am kind of a smarta$$ by nature :) , and I really appreciate everyone's feedback; and I even enjoy good natured 'ribbing'.

I converted my lathe to a variable speed treadmill motor about a year ago, because, not only am I brilliant physicist :) , I am lazy too :( . Therefore I can not tell my rpms very accurately. I am guessing at 80-120 rpms


All of my metal is purchased from the scrapyard and is not labled. It is very shiny and has not one speck of rust on it after sitting on my shelf for well over a year. I am guessing SS. (strike 2 for me)

tattoomike68, I just tried your idea and it works very well, but it feels like I am cheating. I don't suppose the "lathe police" will knocking on my door anytime soon, but I would like to be able to get a great finish with a tool bit for my own personal satisfaction. But in a pinch your idea works well.

All in all, I am very satisfied with where I have gotten in the past few weeks with everyone's help.

My original 'tool shape' question in this thread was more of a general question I suppose. Because my first tool bit in the other thread was sooooo far of the mark, I was just wondering if there was a general rule for tool bit finishing shapes.

tattoomike68
01-10-2009, 08:10 PM
dont expect to get a great finnish off a tool unless you are running a cnc at 4,000+ rpm and oil blasting at 100 psi right on the tool.


Use the emory cloth, it cost less than 10 cents to get a 6 microfinnish. to get a tool to cut mirror is unrealistic.

dan s
01-10-2009, 09:03 PM
My original 'tool shape' question in this thread was more of a general question I suppose. Because my first tool bit in the other thread was sooooo far of the mark, I was just wondering if there was a general rule for tool bit finishing shapes.
Generally you want a larger tip radius, the lowest feed you can get, a doc large enough to avoid work hardening issues (~.005 -.01), and a material appropriate cutting fluid. Rake and relief angles can stay the same.

Paul Alciatore
01-10-2009, 09:12 PM
dont expect to get a great finnish off a tool unless you are running a cnc at 4,000+ rpm and oil blasting at 100 psi right on the tool.


Use the emory cloth, it cost less than 10 cents to get a 6 microfinnish. to get a tool to cut mirror is unrealistic.

Hummm. That's what I said in the other thread Miker mentioned above. Another plus of emory cloth is you can control the final diameter down to tenths fairly easily (but with a bit of work if a lot needs to be removed). I keep abrasive cloth/paper on hand with grits down to 2000.

I am interested in some of the other tool geometries mentioned here and in that other thread and will probably try some of them on some cheap steel.

dp
01-10-2009, 10:31 PM
Dan, first off I am not offended easily, I am kind of a smarta$$ by nature :) , and I really appreciate everyone's feedback; and I even enjoy good natured 'ribbing'.

I converted my lathe to a variable speed treadmill motor about a year ago, because, not only am I brilliant physicist :) , I am lazy too :( . Therefore I can not tell my rpms very accurately. I am guessing at 80-120 rpms
Phil - you need a cheap digital tachometer like this: http://cgi.ebay.com/New-Digital-Photo-Tachometer-Tool-for-Wood-Metal-Lathe_W0QQitemZ310114314639QQcmdZViewItemQQptZLH_D efaultDomain_0?hash=item310114314639&_trksid=p3286.c0.m14&_trkparms=72%3A1205|66%3A2|65%3A12|39%3A1|240%3A13 18|301%3A0|293%3A1|294%3A50

Other sellers have them at under $15.00.

I bought one because I didn't have any way to know the speed of my Grizzly mini mill. This thing works great, and I quit ruining tools and work.

macona
01-11-2009, 05:31 AM
Dont slow down. Thats part of your problem. And try not to sneak up on your final OD. Its a bad habit to get into. Leave some metal for the finish pass.

Avoid scrap steel. You never know what you will get. Steel assortments from places like online metals are pretty reasonable. Go for something like 1144 stressproof for general turning. 12L14 is supposed to be good as well but I have never used it. It rusts fast and cant be welded.

Keep your feed up. Slowest feed does not mean the best finish. If the feed is too slow the trailing edge of the tool will rub on the next turn and mess up the finish.

If you are turning SS make sure you do not dwell in one spot. Stainless work hardens.

You might be able to do a spark test with the grinder to get a rough idea what you are turning. Search the web for article with pictures of different spark trails.