Thread: Lathe basics: Spindle centre error

Dave J, the whole taper is only 40mm long. It doesn't really have a middle to remove! Pipeclay, don't take it personally, but I want to try again to verify what I actually have. I'm scheming to reverse-engineer the method outlined in Woodlee's link. Graziano, that would stick out a fair bit. Could still work though, if the small end is no bigger than about 35. Originally I made mine stick out enough so the tip was flush with the face of the chuck so I could drive off a jaw. Now it's so, um, minimal, I'll have to use the faceplate.

Im not upset at all,just thought you might like to know what the taper/inch was so you could set your compound without having to jam something in your spindle.

Yep, that's great info, thanks pipeclay. But I'll just try to confirm by measurement that's all. Been busy with other stuff today.

I've spent some time on this over the last few days and I feel I've made some progress, though not as much as I'd like. For those who came in late, I'm attempting to reproduce the taper in my lathe's spindle. It's a size that's hard to find (MT 4.5?). I want to be able to make adaptors to fit various standard tooling. It's become an exercise in error-chasing, and a great learning process.

So I made a plug gauge, with one end fixed (press fit) to a shaft, and the other, exactly one mm larger in diameter, a sliding fit. The idea being to fit this snugly into the taper, mount between centres and set the compound slide from it. Here's a picture of my first attempt to do this.

https://www.woodworkforums.com/attach...1&d=1280832135

And my purpose-built, centre-height indicator holder (not a machined surface on it )

https://www.woodworkforums.com/attach...1&d=1280832135

I used a toolbit as a parallel to run the indicator on. You can see it's only contacting the leading edges of the two diameters. Yes they are rubber bands, and yes, that is a knurl. So I got it dialled spot-on, and cut a taper.

https://www.woodworkforums.com/attach...1&d=1280832292

The result was not flash. Bluing showed all the contact at the small end, and there was a bit of runout too. Closer examination of my plug gauge revealed the small end had a lot of runout, which I still can't really explain. The part was turned at one setting and should have been concentric.

I fudged it by setting it at the average position. This time I ditched the toolbit and just got the indicator riding on the very corner (easy to do actually), pulled the stem back, wound the compound along, and found the same sweet spot on the larger diameter. Again, got it spot on, took another cut.

Here are 2 pics of the blue reading:

https://www.woodworkforums.com/attach...2&d=1280832129

https://www.woodworkforums.com/attach...3&d=1280832129

Not a bad contact area I thought. What's interesting is in the second pic you can see the contact reduces. I got this by rotating the workpiece a full turn, BTW. You may just see a texta dot at the lower left of the workpiece. That marks one of the high points. There are two, about 90 deg apart.

Total indicator readings were .01 at the small end, nearer to .05 at the big end. I took these readings by mounting the piece lightly between centres and turning it by hand, so spindle error was eliminated. I tried it with the part reversed to eliminate my cheap live centre. Same result. Negligible runout at the small end; two oddly spaced lumps on the big end, always at the same spots.

https://www.woodworkforums.com/attach...4&d=1280832129

https://www.woodworkforums.com/attach...5&d=1280832129

Now, I can think of a better way to set the compound: Use a lever type indicator mounted in the toolpost to read directly from the taper. I don't have such an indicator but I think I may very soon (Anyone have one to sell before I get an ebay cheapie? ) However. I don't think the angle is the problem. I think I'm cutting lumpy circles and I don't know why. That's as far as I've got.

Thanks for reading this far. Any thoughts?

Hi Bryan looks like you have been busy, I hope your wife has recovered and is ok

The cheaper dial test indicators work fine, as there not for measuring only comparing. I would recommend you get a micron indicator, especially for this job as you will see more movement in the needle for the same amount, compared a 0.01mm one.
It could be some of your problems at the moment.

Here is a reasonably priced one, he is also on ebay if that suits you but he is a bit dearer on there.
MICRON DIAL TEST INDICATOR #D11

I bought one of his 0.01 test indicators in the link below that come with the small mag base for general use, to save using my more precision and expensive ones. The mag base is useless but the test indicator works fine. Mine has Mitutoyo printed on it, but it is only a copy.
DIAL TEST INDICATOR WITH MAGNETIC BASE #C91

If you have a bit of time there are good quality ones that you can get on ebay for good prices. I have picked up a Mitutoyo 0.001mm indicator for around $35 and Girod test indicator for about $59 etc.

Dave