Problem Parting Off On A 101.28980 Lathe

[AMDtrucking]

You measured the spindle nose thread major diameter, not the register diameter. The actual thread diameter isn't particularly critical. Your first photo appears to be made up from three photos. What you need to measure is the unthreaded area to the left of the threads as shown in the lower right corner of your first photo. And the inside diameter of the unthreaded area before the threads start as shown in the lower left corner if the first photo. As I wrote earlier, the register diameter should be 1.5000"/1.4995".

Got it. Here it is. My Soviet micrometer's jaws grabbed some of the threads, that's why I double-checked with thin jaw calipers.

 

[wa5cab]

I assume that in the 4th decimal place, your digital caliper only displays 0 or 5. But in any case, the spindle register is fine. And it is unlikely that three different chucks are over-size in the register area. I would re-set the spindle bearing preload, tighten the compound and cross slide gibs, and re-check that the parting off tool holder is perpendicular to the spindle axis.

I note that your micrometer is reading about 1.50027" but that could as you said be due to catching the thread.

I would suggest also switching to the ThinBit holder and cutters. At the very least, if it happens again, you won't damage the holder. I don't know what the ThinBit cutters cost today as it has probably been a decade at least since I last had to buy any. But not damaging the cutter holder will save considerable.

 

[Silverbullet]

What your going to find is the machine is flexing or moving as the force of the cutoff blades pushing down bending the cross slide and tool post . Many LATHES just aren't strong enough to use a cut off tool. Mines one too an 11" Logan. The only way I can do it is to mount a solid block with extra support across the ways and snug under the tool itself . I've watched the whole carriage twist under the pressure. I know others will say different so try and see. Make you own observation have your machine video taped from different angles. Do your cutoff routine and watch the machine bend and flex .

 

[bobl]

Looks to me like you're doing everything right, AM. If the work is climbing on top of the blade then there has to be clearance somewhere to allow that, which makes the spindle bearings suspect or excessive slop in the saddle/compound gibs. Another possibility is that the tool holder is moving in your tool post. Try eliminating these things first.

Sounds about right but also is your parting tool at the correct height re the centre just a small amount below to much will give this problem also as I know from experience also chip buildup


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[wa5cab]

Right. I would strongly recommend getting one of the relatively inexpensive bubble-level centering tools. Eyeball on center often isn't good enough.