Slant Pro Taper on Cuts - Causes?

Troubleshooting a bit of an odd issue here - originally I thought it was just me pressing my luck, but I am realizing it’s maybe a bit more systemic with my machine. I was curious if I am missing something in terms of calibration or set up on the machine.


  • I have been noticing that I will see a taper on the cuts I am making in 6061 aluminum.
  • Originally, it was larger/longer parts, which I was sort of chalking up to being one of those “it is what it is” things in terms of maybe not enough rigidity or my chuck not being able to apply enough pressure.
  • Recently, I had a couple jobs come in where this became a bit more important, and I ended up having to program in a reverse taper on my parts in order to get them straight, so I figure it’s time to finally dig into this and see what i am missing.


  • I am seeing a fairly substantial taper over 2" long parts. Currently, I am doing a 2" long x 1.125" diameter part, and I am seeing 0.004" of taper along the part.
  • Initially, I was noticing this with my 3 jaw chuck, so today I decided to try the experiment in a 5C collet, and I got the same results, so it’s not just a chuck thing.
  • Other than this, the machine seems to be working well, and doesn’t actually have a ton of hours on it or anything.


  • Is there some adjustment that can cause this sort of taper? I was wondering about the spindle alignment to the machine bed. Is that something that could be out of alignment?
  • What else could I check to troubleshoot this?



If its consistent from part to part and from Chuck to collet with same material then it sounds like it would have to be alignment issue. Can you setup a test bar and sweep it with an indicator to see if there is alignment issue?
I do a very light spring pass cut on my 8l to overcome any collet or material flex. But I dont see a taper unless the material is flexing or moving due to cutting force.

You did not mention how old your lathe was.
The slant pro uses timing belts between the axis steppers and the lead screws, so the belts can become loose over time, it’s just something to check.


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Wow, didn’t know it had axis drive belts. Just looked at the exploded parts view. Hmm.

It’s simple to check, the belts are under cover. I found mine were loose and a simple adjustment returned the lathe to it prior performance.

Yes get a ground linear guide from McMaster Carr, 0.750 is what I have. ±.0005? .0002? Put a .0005 indicator on a mag base then the carriage. Wiggle the bar and close collet a few times. Seated best you can. With 6 inches sticking out run indicator on the side and top. Draw a diagram. Figure out which way its pointing. I shimmed my spindle to horizontal, and pivot to straight. Pull tapered pins before you start. Ran a tapered reamer and bought slightly larger pins.
Could take a full day or two to get it right. Great fun.

Thanks Bruce. Where are the tapered pins? This part seems to elude me. I was hoping there would be a guide somewhere for how this is done. I loosened the 4 bolts but couldn’t really move the spindle, so I assumed I was missing something and it has to be the pins.

Look for two nuts, on spindle flanges. Little stud like protruding. Tighten to remove pins.
I probably should have emphasized that you should measure carefully before changing critical alignment of a major component. Check all possible influences on your measurements. This includes carriage and the gibs. You can dig yourself a hole that might take a few days to get out of. But keep at it. You’ll be smarter after the pain stops:)

How’s it going Big T?
Couldn’t remember what my numbers were so i checked them today.
On a 0.0001 indicator, I’m getting about 5 or 6 tenths over 6 inches on a 3/4" ground bar.
Runout at the end, at 6.5 in was about 0.0024. That could be the collet. Carved on the face of that on for a project.

Trying to get the tailstock lined up for a new project.
Remember you are looking for repeatability.

Thanks Bruce. That helped immensely. I got those pins out, and backed off the four bolts leaving the top left one fairly tight as my pivot. After that, I sort of followed a similar process to tramming in a vise on a mill and got things square. I would say my number match up pretty close to yours there now - the needle on my indicator doesn’t move sweeping over 6” or so, which is pretty good. I skimmed a one inch bar after as well and my mic says it’s within 2 or 3 tenths over the distance.

Tormach tech support confirmed that approach as well, so I think it works. One thing they said is that the tapered pins may or may not need to go back in. They are there from the factory to hold it in spec, but are not necessary if it’s been adjusted. Mine happened to basically go back in, so I tapped them in gently and rechecked and everything was still square so they’re fine.

I did a cursory centre line check with the turret as well and things seem fine, but I’ll have to go do a more thorough check. Maybe more luck than brains this time around, but it all seems to be back in spec. I’m still not exactly sure what happened to knock things out of whack, but I did make some errors early on when I got the lathe, so that was probably it.

Thanks for the help! I’m back on the square!

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Good, glad to hear it. Tracking down alignment issues can be a real headache.
I don’t remember what the taper was but you should buy a tapered reamer and oversized pins and
re-ream and put new pins in. Everything flexs and everything moves. This will keep your alignment in place.
BE CAREFUL it is real easy to go too deep with the reamer. Tapered holes are deceiving. A little goes a long way. But this will lock everything in place. it’s those little bumps that don’t seem to do anything that will get you.