View Full Version : Lathe questions
07-12-2004, 05:46 PM
I have an old Sebastion lathe that I picked up for free. I have it set up and it is working fairly well for its age. Two questions - how do you get a nice smooth cut? I am using a machinable steel but as I turn it or bore it the tool bit doesn't seem to leave a nice smooth finish. I am using slow speed. The second question is I cut round stock on my bandsaw. It is not perfectly round or the two sides parallel. When I put it in the lathe should I cut a face on it, flip it over , cut a second face on it and then cut it to make it round? Some parts I made I centered the piece in the chuck as best I could, bored it, then recentered off the bore and then cut the outside to the diameter I needed. But without the two ends being parallel I wasn't happy with the parts. So what is the proper procedure?
07-12-2004, 08:44 PM
Hi Andy, Congrats on your "new" lathe. I'm no machinist, but I have made a couple chips here and there.
I am assuming you are having trouble getting a good finish on plain old mild steel. If you are using HSS try honing it to a razor's edge. and take your last cuts on the light side - say .005" or .010" per pass. Depending on your stock size you might play with the speed as well. A blue or yellow colored chip for HSS is generally TOO fast. Step it a tad back if you're in THAT range. Also make sure you've got some TOOL CLEARANCE (relief angles - cutting tools NEED "room"). Tool nose geometry of course helps as well - using a tool with a radiused cutting edge will help --- as will a tool with a "flat" cutting edge - sorta burnishes it as you go. Lots of "tricks". By far mild steel is generally quite difficult to get a good finish on. It is far easier to obtain a better finish on a free machining steel - like the LEADED steels - 12L14 for instance (they generally don't weld for crap). The HARDER steels will also give better finishes.
If you're running carbide you might want to INCREASE your D.O.C. (depth of cut) carbide likes a load. It also likes SPEED (in general)
Keep in mind typically carbide is VERY brittle and HARD - as such they actually put a small radius on your cutting edge to prevent it from cracking- this is why it can be difficult to take light cuts with it - it sorta "plows" the metal instead of actually cutting it. It is the same reason why it works like crap on non ferrous metals UNLESS you can crank the RPM's. There are a zillion grades of carbide - all with different applications - so double check you've got the right stuff for the job.
If none of this helps - you'll need to get a little more specific on your set up.
In general - stock needs to be round first - then face the ends - then make your part.
Turning between centers is always a option. Learning how to "play" with your chuck jaws - is always valuable. And of course the 4 jaw chuck is always nice to have as well.
If possible - it is always desireable to do as many of your operations in one set up. This is an easy way to ensure concentricity. I have found that over time I am getting better at looking at a part and determining what the "best" method is - that will come with time - hopefully it will happen FASTER for you that it did me.
Jacin in Ohio - The Barefoot Machinist
07-13-2004, 05:32 AM
Don't believe what Jacin said!
I mean the part about him not being a machinist. :) Everything else he said was right on. With some steels it's difficult to get a good finish. The small radius on the tool really helps. Sometimes a little emory cloth and a file will polish it up real nice. Carbide does need to be run pretty fast and if your old lathe is like mine it's pretty scary trying to run it that fast. I usually stick with HSS. I'm not trying for production speeds.
07-13-2004, 07:18 AM
You often want a small radius on the tip of your HSS tool. It results in a better finish. When turning something down (a step) you want to leave that small radius in the corner anyway. This avoids a stress riser where the part would be likely to break.
Face both ends of your part to make a nice finish and square them up. The squarer the part is in the chuck, the better it will come out. Use a dial indicator to get it square or just use the end of the tool bit for reference and do it by eye. Even with a tightened 3-jaw chuck you can usually tap the workpiece around to square it up within a couple thousanths.
07-13-2004, 01:03 PM
I also would like to know if you are using High speed steel or carbide. One other thing to look at is tool center heighth. The tool should be close to your spindle centerline to get accurate cuts. If you are getting tearing or galling on your part you may need to use cutting oil or adjust speed to eliminate it. Honing the cutting edge of your tool will help some. Also like the others have stated, try a little more depth of cut. Also, a little positive rake on the cutting tool helps sometimes. When I finish with hss on mild steel I tend to use a sharper tool and finer feed as it tends to cut better and tear less.
One thing to try for round stock is to cut your part a little longer and chuck it up. Bump it around to get it runing as true as possible. Then cut your part, inside and out and face one end while still chucked. Then you can saw or use a parting tool to cut the length and turn around (if necessary) to get a true 2nd end. If you do it this way, you will only have to indicate the part on the OD 2 places (near the chuck and near the end)to get both ends parallel.
Hope this helps a bit. Let us know what tooling you are using and your setup, speeds, feeds etc.
07-13-2004, 07:43 PM
What is the ballpark minimum speed to be able to use carbide tools effectively? I suppose that it is probably not expressed in RPM, but in inches per some unit of time.
07-13-2004, 08:20 PM
I am trying to remember numbers to tell you, after doing this for 20 years you'd think I'd know, but I watch my chips and adjust as necessary. I am thinking that with HSS on mild steel it will be somewhere around 125- 150 sfpm (surface feet per minute) and carbide will be 250 - 400 fpm. as with everything there are tons of variables. If I am wrong about these numbers I apologize, but these seem to stick in my head. FWIW. Please correct me if I am wrong.
07-14-2004, 02:44 AM
Thanks for the info. I am still learning. I am using HSS tool bits. I have some carbide tools but haven't used them much
07-14-2004, 09:05 AM
Hi Peter, Like you suspect the speed is related to SFM (surface feet/ minute) - which will be given for various inserts used with various materials - actually it can all get quite daunting for a week end warrior. A somewhat easier method to to simply watch your chip color - unless they're coming off and immediately turning blue - you aren't taking full advantage. The thing with carbide is that it is also somewhat suseptible to thermal shock - meaning that you should either run it WITH coolant - non stop -(assuming you are pushing things to the limit) or try to run it at a lower load a and just let is stay dry. Erratic application of coolant can lead to chipping - so long as your not trying to make production time a home guy can usually not be too affected by this stuff. I just keep a "flux brush" nearby (previously dunked in cutting oil of course)and leave it drag on my part while cutting giving it a constant supply - albeit mighty small. Otherwise I will leave it cut dry. All depends on how acessible your cut is - boring holes it isn't always practical to "dab" onthe lube so I will go dry.
The BIGGEST thing I have learned is how to part off reliably - in a nutshell:
1. RIDGID TOOL (shortest overhang possible)
2. RIDGID PART (shortest overhang possible)
3. RIDGID TOOL HOLDER -a NARROWER cutter HELPS minimize this
4. TOOL ON CENTER
5. TOOL HONED TO A RAZOR EDGE (HSS)
6. TOOL POSITIVE RAKE
7. TOOL RELIEVED ON TOP (helps chips clear)
8. START SLOW - increase speed slowly only if things looks GOOD
9. LAST and one of the BIGGEST factors use a GOOD CUTTING OIL - SULFUR BASED OIL works great at the LOWER SPEEDS - if it starts to smoke - your probably pushing your cutt off luck.
10. (I almost forgot) - DON'T dilly dally - you GOT TO give it some FEED!!! Too much and you'll break a tool - too little and you risk chatter - chatter - more oil - less rpm's - sharper tool etc
Keep in mind this are my personal preferences - based on working with mild steel, stainless (304), aluminum 6061-t6 and 4130 mostly - all on a small home machine - 12" and under.
YOUR mileage may vary
Jacin in Ohio aka the BAREFOOT MACHINIST
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