Cutting internal threads on Sherline lathe

[BiggerNoise]

I'm trying to cut an internal 3/4-16 in 12L14 stock.

The threads start, but before I am more than a few thousands deep, the cutter digs in hard enough to shift the part in the three jaw chuck.

I'm advancing .003 on every pass. I have done a handful of passes at the same depth (not sure if these are called spring passes when threading) just to make sure that I'm properly clearing the threads. I think the grab is somewhere around .030". I was trying to do .005 a pass and had the grab issue and dropped to .003. Not sure if I need to go further down from there.

I have a solid carbide internal threading tool (meant to get a shorter boring bar and bought this instead), so I don't think it's the tool.

I am backing out the cutter when I return to the starting position. The thread depth that is cut before the grab look pretty clean to me.

I'm going to step away from the tool for a bit and try making the part in Aluminum tomorrow. I'd sure appreciate any tips on how I might improve this setup.

Thanks,
Andy

 

[woodchucker]

well, for an underpowered lathe, cutting a big thread I would use HSS.
threading occurs at slow feed rates, slow rotation. Carbide likes High speed turning, and a higher feed rate.

so I think your carbide may be hurting you. Unless its a micro100 threading tool. But if its an insert, I don't think you have the HP, or rigidity.

 

[benmychree]

You don't say if you are plunging in straight with the cross slide or using the compound rest at 30 degrees.

 

[BiggerNoise]

well, for an underpowered lathe, cutting a big thread I would use HSS.
threading occurs at slow feed rates, slow rotation. Carbide likes High speed turning, and a higher feed rate.

so I think your carbide may be hurting you. Unless its a micro100 threading tool. But if its an insert, I don't think you have the HP, or rigidity.

It is a micro 100 (IT-320750 if it matters)

 

[BiggerNoise]

You don't say if you are plunging in straight with the cross slide or using the compound rest at 30 degrees.

I am not using the compound. AKAIK, the Sherline compound can only run 1/4" tools.

 

[BiggerNoise]

I watched the three part set of videos on YouTube from Sherline. She was talking about .001 and .0005 depth of cut, so the answer may be that simple.

The third video shows using the compound to cut threads. That looks interesting but will require a couple doodads that I don't have yet. I'll try this again with the very shallow depth of cut.

 

[mmcmdl]

I am not using the compound.

That most likely is your issue .

 

[mikey]

The Sherline lathe threads just fine using the cross slide feed alone. You DO NOT NEED to use the compound with this lathe for threading.

Micro 100 solid carbide internal threading tools are excellent for use on the Sherline, even at hand cranking speeds.

The problem you are having has to do with how you're staging your cuts. Your chuck is grabbing only about 1/2" of material so you cannot take big cuts. Here is what I suggest:

• Bore the work accurately and don't forget to chamfer the entrance to the bore. You must know your desired ID and you must know the estimated total depth of cut.
• Get the tool tip about 0.002" above center height. Trust me; this makes a difference. Also be sure the flat on top of the tool is dead horizontal.
• Do a scratch pass to make sure your gearing is correct. A scratch pass is 0.001" deep. If the pattern is correct, back out and dial in a 0.004" cut and make a pass, then make another pass at 0.005" deep. From this point on you need to reduce the depth of cut because your tool is cutting deeper and has more edge contact per pass. I suggest you take a 0.003" pass next, then 0.002" next, then 0.001" thereafter.
• Every 0.005" - 0.010" of thread you cut, take a spring pass (a pass made with no additional depth of cut dialed in) or four until the tool stops cutting. This really helps to condition the thread so it is consistent. You are hand cranking the cut so variation in the thread is unavoidable; spring passes take care of this.
• Estimate your total depth of cut. One formula that will get you close is: (Major dia. - Minor dia)/2. There are others. Begin checking for fits well before you hit this estimated number. You can take the chuck off the lathe and fit it to the mill spindle threads to check for fits, then replace it on the lathe to continue cutting the thread until you get the fit your need.

This work flow works well on the Sherline lathe, at least for me. I've cut hundreds of Class 2A/B and 3A/B threads on this lathe using exactly this process. Let us know how it works out for you, Andy.

 

[BiggerNoise]

Thank you. I think I had most of that worked out, but I wasn't doing enough on the spring passes. Didn't have tool above centerline either. I'll definitely give that a whirl.

BTW - I bored the ID with a micro 100 boring bar (that you recommended a while back), finally getting to use it in a rigid setup. The cutting was just glorious.

 

[BiggerNoise]

Good tips, particularly the multiple spring passes. Three or four every five thousands or so really made a difference.

I'd add one more: be sure that you're clearing the chips between passes (duh, I know. Or, now I know).



Probably need more chamfer on the bore, and definitely on the outside. I was really trying to avoid activities that might possibly shift the part in the chuck.

At any rate, here's the part on the lathe with the entire thing ready to do the Criterion side (7/8-20 male threads).