Cut my very first chips, inevitably, I now have questions...

[grhm]

So, after working out the issues with my lathe, I've managed to acquire a bit of 12L14, sharpen up one of the tools that came with the lathe, and cut a few chips in something that isn't PVC pipe. I think it actually went decent for someone who's never touched a machine tool before!

I do have a question, though. I was just making facing cuts at first, and as I wound the tool into the work, as I got close to the center, say 1/16" nub left, the cross slide got "tight" and didn't want to wind in as eagerly as it did when I started. I would have guessed that this is a symptom of either having the tool too high above the centerline or having the tool pointed away from the work, but I don't think that either of these is the case. I triple-checked the tool height, and it seems to be correct, and I pointed the tool toward the work by about 15 degrees, and that didn't improve the situation either.

Is there something else I should check?

Watching Quinn Dunki on YouTube, she has her tool pointed toward the work whenever she's turning or facing. Watching Adam Booth and This Old Tony, though, they don't seem to. I'm not entirely sure how I should set up my compound and tool post angles, maybe that's got something to do with it?

 

[KevinM]

Make sure that your tool height is on center. When facing toward the center the radius decreases therefore the cutting speed decreases dramatically. Try it with a higher speed (rpm). What material and RPM are you turning?

 

[Bi11Hudson]

It may sound a little silly, tool position is a major factor in getting a "clean" cut. You can check center (not the most accurate) with a sheet of shim stock, which I use. A ruler will work almost as well. Simply press a ruler between the tool and a finished surface of the work. The tool will either lean toward you (low) or away from you. (high) When it is vertical, you're on center.

How the tool "points" is a factor of the shape of the tool. It can vary (widely) on this alone. Almost everyone has a slightly different approach. By the way, welcome to the "rabbit hole". You will be learning until the day you look at grass from the brown side.

There is another factor here. The machine you speak of is used, is it not. It may have endured many hours of use with the cross slide gibs a little too tight. Or perhaps too little lubrication. What you are describing is fairly common. My larger machine, an older Atlas/Craftsman is sorely worn. I refer to it as being "loose". But I have some experience in dealing with it.

The cross slide dovetails may well be worn a little. It can easily be checked by loosening the gib setscrews a fuzz. By "fuzz" I am refering to a measurement of slightly more than a snake hair but less than a "royal red #### hair". Loosening the cross slide will cause some problems. But will indicate if wear is where your problem is. If your gib setscrews have a "hex" head, about half of one flat will be enough.

.

 

[mmcmdl]

Make sure that your tool height is on center. When facing toward the center the radius decreases therefore the cutting speed decreases dramatically. Try it with a higher speed (rpm). What material and RPM are you turning?

SS ( surface speed ) is the issue as mentioned above . You're pushing material vs cutting it getting closer to center . Take your rpms up or just turn that dial a little harder . CNC lathes have the CSS option that eliminates this issue . As your X axis decreases , your rpms automaticly increases . Yo DO need to put a max rpm value in these cnc lathes or nasty things happen . Ask me how I know .

 

[markba633csi]

If you are getting a "nub" when facing then either your tool height is wrong or the compound/cross slide is moving down as you cut (too loose)
-Mark

 

[grhm]

Make sure that your tool height is on center. When facing toward the center the radius decreases therefore the cutting speed decreases dramatically. Try it with a higher speed (rpm). What material and RPM are you turning?

I'm turning 12L14, 3/4" or so (I'm not near the machine at the moment), and speed is... not super high. Maybe 320 RPM?

There is another factor here. The machine you speak of is used, is it not. It may have endured many hours of use with the cross slide gibs a little too tight. Or perhaps too little lubrication. What you are describing is fairly common. My larger machine, an older Atlas/Craftsman is sorely worn. I refer to it as being "loose". But I have some experience in dealing with it.

Yes, the machine is used. The cross-slide moves freely when I'm not cutting, however, so I'm not sure that's the issue here. I don't get the impression that this machine saw a *lot* of heavy work, but I'm not really qualified to say, honestly. I would like to go through it and completely disassemble it and clean it up, adjust it, and make it "new" (or as close as I could), but It'll be a while before I have the skills for that.

If you are getting a "nub" when facing then either your tool height is wrong or the compound/cross slide is moving down as you cut (too loose)
-Mark

I have a nub because I haven't gotten all the way to the center yet. I don't know that the nub would persist if I did. I do know that a nub generally indicates that the tool is below center, but I've also been told that increasing cutting pressure as one approaches the center of the work indicates that the tool is above center. I'm relatively confident that's not the case, but I don't know just how precise it needs to be.

 

[KevinM]

When turning 3/4" 12L14 you should be turning about 1000 rpm. When you get close to the center (< 1/2" diameter) you can increase speed to 2000+rpm if available.

Here is a link for calculating cutting speeds:

Cutting Speeds

This page gives cutting speeds for turning, milling, and drilling of several different materials. It includes a calculator to figure the spindle speed required.

littlemachineshop.com

 

[grhm]

When turning 3/4" 12L14 you should be turning about 1000 rpm. When you get close to the center (< 1/2" diameter) you can increase speed to 2000+rpm if available.

Here is a link for calculating cutting speeds:

Cutting Speeds

This page gives cutting speeds for turning, milling, and drilling of several different materials. It includes a calculator to figure the spindle speed required.

littlemachineshop.com

Ah, that's super useful, thanks! I should probably grab a copy of the Engineering Black Book, but I haven't yet...

 

[homebrewed]

A magnifying glass can be helpful in this situation. When the cutter becomes hard to move, stop the lathe and examine the relationship between the cutter and center nub. That will tell you exactly what's going on.

I find this to be a better indication of tool height than the old method of using a metal rule captured between the work and tool. It wouldn't take much of a bump or bit of swarf to cause the ruler to lie to you.

 

[BladesIIB]

All great advice above. The only think I can add as you are trouble shooting is take a very light pass. May not get the best finish but you will be able to see the tool path and see if it cuts all the way to the center of your piece of material. This should help you rule out height, cross slide movement etc. If that is good then RPM and tool sharpness, and angle could be the cutting issue. Sometimes trouble shooting is about quickly eliminating what the issue is not to help you focus on what it is.