Getting a better finish on my mini lathe

J

Jon K

Active User
Registered
Joined
Apr 23, 2012
Messages
55
Hey guys - I know a lot of people are going to say "You need a bigger more rigid machine" and thats ok, but I actually enjoy having a small guy because it doesn't take up much room and I can do 99% of the work I want on it. Anyhow, I made some bushings and stuff that I really want a better surface finish on.

8208298344_1cb0586af0_c.jpg
DSSR assembly by Jon Kensy, on Flickr

Here you see a stainless rod I turned down - it was to be welded into the receiver there. Fit was great, and in this case finish doesn't matter, but how can I get a finer finish? I tried HSS and carbide and both seem to give me similar results. On aluminum its more obviously got an almost extremely fine thread look to it. It's not bad, but I see some guys turn brass and bronze and aluminum and get like... gloss finish parts.

Its a 7x12 Grizzly mini lathe with the stock original gearing in it - no changes. Am I right in thinking that if I slow the screw travel down I will get a better finish?
 
Two things that made a big difference in the finish i got with the stock ratio on my 7 12 Cummins was the leadscrew was binding and "jerking" although I don't see evidence of that in your photo. The second was how the HSS bit I was using was ground. More radius on the "lead in" side of the bit and a bit more top angle made a world of difference on my machine. There are other who have posted different way to slow the feed and i am sure that would help as well. Spend some time with the grinder and work on the bit profile, it will make a difference. Tubal Cain and other have videos on the bit sharpening as well.
 
I never turned SS on my 7x14, but in general, make sure you don't have too much stickout, either on the tool, or the part. Try some cutting fluid and a low depth of cut with a sharp tool for the final pass.

Some people have also added a DC motor to the leadscrew to allow for finer feed rates. The lowest feed rate on these lathes is actually pretty coarse. You may also try a "vertical shear tool" http://www.gadgetbuilder.com/VerticalShearBit.html. They give great surface finish on steel and alum.
 
It would appear that the cutting tool is everything use a round nose tool or a carbide. I make different cutters for different metals and different type's of cutting such as rough in or finish work. Ray
 
What kind of tool are you using? This looks like a brazed carbide cut done at too low a cutting speed. HSS can do this, too, if your nose radius and side cutting edge angle is too small.

Is this 303 stainless? If not, what alloy is it?
 
mikey said:
What kind of tool are you using? This looks like a brazed carbide cut done at too low a cutting speed. HSS can do this, too, if your nose radius and side cutting edge angle is too small.

Is this 303 stainless? If not, what alloy is it?
Click to expand...

Its 304L but I wish I had more pictures of my brass/aluminum.

Actually, here's a video of me turning some bronze:

http://vimeo.com/54826152
 
In addition to the suggestions above, you can put the "finishing" touch on with a file. That is, leave it in the lathe and with the part spinning, carefully touch it up with a sharp mill file.
 
Hi...

We'd love to help and it makes no difference what your machine is. Can you give the specifics of how you obtained that finish? Type of bit, style of bit, RPMs, DOC (Depth of Cut), Feed Rate, Material etc are all factors. It may be as simple as changing 1-2 parameters rather than guessing all over the place. Without knowing at least the material, we can't even suggest the basic parameters to try. Don't worry, we'll help but need the basic data.


Ray
 
I'm new myself, but if I was getting that finish, I'd:
  • Try a piece of aluminum. If you get the same result, you do not have rigidity issues.
  • Coolant/cutting oil
  • Check your speeds and feeds. Assuming about 40fpm for stainless, and a 1/2 dia part, I think you want about 300-350 rpm.
  • Rough with a 'sharply pointed' cutter if you like, but do your finish pass (final 2-3 thou) with a round-nose cutter.
It looks to me like you need the last suggestion the most.
 
I may be a wimp but I hate 304, L or otherwise. That stuff work hardens if you dwell in the cut for even a heartbeat so I only use it if I have to weld it. Sorry, I know you have to cut it so I'll stop whining.

If using carbide inserts, try a nose radius of 0.007-0.008" with a chip breaker. Depth of cut should be no less than the nose radius so the smaller radius will help. If your lathe is manual, get the cutter moving and do not enter the cut hesitantly or stop the cut at any time. If you do work harden, take a deeper cut on the next pass to get under that hardened area. Use a lot of cutting fluid (I use some sulfur based stuff) and keep your speeds on the lower side (550-600 SFM for carbide).

If you can use HSS then I suggest a side rake of 25 degrees and 10 degrees of back rake with a nose radius of about 1/64". Again, this gives you a decent depth of cut and gets those chips out of there in a hurry. Use plenty of cutting fluid and don't dwell in the cut. Cut speed down to about 100 SFM.

For the aluminum, I highly recommend using a sharp HSS tool with a 1/32" nose radius and side rake about 35-40 degrees. Use as much lead angle on the tool as you can manage. Turn at higher speeds and you should get a mirror finish, especially if you use cutting fluid. I use WD-40 for most stuff and Relton A-9 for when the finish matters.

Hope that helps.
 
You must log in or register to reply here.
Back
Top