Center Height Gauge for 820 -- What do you do?

tmenyc

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In case you see both boards, yes I've also posted this on the groups.io board.
I'm puzzled by what to use for an accurate center height gauge on my 820. The saddle support arms extend too far forward to mount anything across the ways close enough to the QCTP, and the base is too pitted and covered in oil and chip to stand something there that would reach up to above center with repeatable accuracy. Can't attach anything flat to the cross-slide, because it's sloped and also somewhat scarred with old paint. While generally I turn the QCTP and touch off the tailstock taper, I'd like to not have to do that every time. Finally, the work I'm going to be doing will be pretty small, so I'll need precision; the "close enough" of the 6" rule trick is not precise enough. Would appreciate learning your ideas or solutions.

thanks,
Tim
 

ErichKeane

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I've got one of these that does a great job:


You simply put the angled side up against your workpiece (it works best if you've taken a cut since putting it in the chuck), then put the other side on the top of the tool. Raise/lower the tool until it reads level and you're there!
 

RJSakowski

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The rule is actually fairly accurate if you have a piece that is concentric with the spindle axis and it is fairly small in diameter and the cutting edge is making point contact. Another device that works fairly well is this:
1565703993478.png It also relies on the workpiece being concentric with the spindle axis and the lathe being level front to back.
A third option is to cut a piece of stock that can rest of the lathe ways and is just long enough to touch the spindle axis. This allows you to set the tool height with no work in the lathe. I believe that Joe Pieczynski did a recent video on making one.
I set mine up with a dial indicator which allows me to precisely set offsets from the centerline. I made a custom stnd for a dial indicator and a calibration standard. Post # 3 in this thread. https://www.hobby-machinist.com/threads/making-tips-for-a-dial-indicator.67209/#post-561864
 

tmenyc

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thanks for the quick responses. I'm looking for a setup that does not depend on having work in the spindle.
RJ, the other issue is that I can't mount a magnetic base on the cross slide; it's sloped and with a rough finish from old paint. If it were just the paint I'd clear that off, but the slope makes it pretty impossible to mount anything. I'll look up Joe Pieczynski's video.
thanks again.

Tim
 

Alexander McGilton

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Another option is just to place a shim between the tangent of work and the cutting edge. A small change in height will drasticly change the angle of the shim, no need for a bubble level as shown. You can also scrip a line in on the tail stock quill, provided that the quill does not rotate much in the housing. Fast forward to (3min 25sec)
1565706218514.png

 

RJSakowski

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The success of using a rule to set the lathe tool height depends largely on the ability to detect small deviations in angle. For 1" stock, a .01" offset will result ind the rule being off the perpendicular by .37º. Without a reference, it would be difficult to detect that difference by eye. In addition, using zero relief tooling like N type carbide inserts present a flat vertical face and the work making contact with the rule anywhere opposite that face will result in the rule being vertical. This is especially bad for that type insert as the tool must be at or below the center in order to cut.
Adding the bullseye level vial to the rule as in the post above improves the situation but it would be well nigh impossible to set the height to within a thousandth or two.
 

RJSakowski

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A comment on the need to set the cutting edge of lathe tool exactly on the lathe centerline. In any turning situation, there is a line between the axis of the lathe and the edge of the cutting tool. That line can be above or below the horizontal and the tool will still cut as long as the proper relief and rake angles are maintained. Raising a tool has the effect of decreasing the relief angle and increasing the rake angle. The opposite for lowering the tool. The following illustration shows two different tools that present exactly the same rake and relief angles to the work simply by changing tool height.
Lathe Tool Geometry.JPG

There is an effect on the forces with regard to rigidity but any lathe is a complex system of forces that varies from lathe to lathe by virtue of its design. I suspect that is why a particular recipe work well on one lathe and totally fails on another. The other side effects are that when the cutting edge is offset from the center line, moving the toll a thousandth does not take two thousandths off the diameter when cutting threads you vcan only have correct thread geometry when the tool is horizontal with the spindle axis, and cutting below the centerline leave a nub while above the centerlin will shear the nub off with the non cutting edge or, if the difference is great enough, deflect without cutting.
 

mikey

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If you crank the cross slide forward, that exposes the dovetail of the saddle. Is it possible to perch a height gauge there and swivel the tool post to reach it with the tip of the tool?
 

tmenyc

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RJ,
This is exactly why I, with my first lathe and a vintage lathe to boot, want to know its accuracies and inaccuracies, so want to be precise before long habituation allows me to let down my guard a bit. I appreciate the directness of the writing.

Mike,
Don't know! But will find out tonight!
Tim
 

tmenyc

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Mike,
Reaching the point of a tool from over the cross-slide is not possible with the indicator stand I have, not by some inches. I have not yet invested in Noga, and my cheapo stand just doesn't go there.
Others,
Many thanks for the responses.
I think, for now, I'll face a 1.5" x 4" piece of aluminum rod I have, mark its center as exactly as I can, scribe a line across that center, and chuck that in place when I need it. Much as I like Joe Pieczynski's idea, I just don't have the flat space to place anything. And, as others have noted, the more I dedicate AXA tool holders to particular tools the less often it's actually needed, so I'll do this until it gets annoying, then figure out Plan A.
Tim
 

wa5cab

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I will point out that although a good AXA tool post is not cheap, 101 tool holders are. I have more than a dozen of them, most loaded with pre-centered cutters that I seldom use, but they are ready when needed.
 

tmenyc

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Robert,
yes, I've been steadily adding to the crowd when I see them on sale, which definitely reduces the need for height measurement.
thanks.
Tim
 

mikey

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Mike,
Reaching the point of a tool from over the cross-slide is not possible with the indicator stand I have, not by some inches. I have not yet invested in Noga, and my cheapo stand just doesn't go there.
What I meant was to see if you could perch a tool height gauge on the dovetail and then rotate the QCTP around so the tip of the tool reached the height gauge.

logan.png
 

wa5cab

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Note that setting the cutter height does not have to be done with the piece that you are getting ready to whittle on. Having a short piece of 3/4" to 1" round that has been marked for the location of the #1 jaw and then had a cleanup pass done on it can then be used any time that you need to set the height of any cutter. You would use one of the bubble type setting indicators.
 

Jimsehr

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I have a piece of flat stock that I use to check center. I keep it with the lathe. I check a piece of round with a height gage. Divide half of stock and then have center.https://share.icloud.com/photos/0vk8cJuXvVs9R0aBHuIZqMNyA#Soulsbyville,_CA
 

pontiac428

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I keep one of these at the lathe for setting tool height. Unlike a pillar gauge (that you can make with any piece of material that happens to be on hand), the square allows you to hang the gauge over the void between the ways. You can reach tools in a variety of positions. Lots of ways to skin this cat!

 

tmenyc

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What I meant was to see if you could perch a tool height gauge on the dovetail and then rotate the QCTP around so the tip of the tool reached the height gauge.
Mike, yes, I can do that but it's pushing the cross slide or compound as far as it goes to get enough purchase for a piece of round stock.
I'm super busy with my pen work right now (and have this day job thta somehow keeps me out of the shop until evening...), so built a pro tem device:
I touched my CCGT tool exactly on the tailstock taper, faced and skim-cut a piece of 1.5" aluminum round in the chuck with it, and touched the tailstock taper to the center point on the face to confirm. There was a match (and there had been no nub left before), so I aligned the tool on the center point, dialed it in a thou, just enough to leave a good horizontal scratch along the face, wound it back, and I now have a center height. It's much much better than the rod I was using before, which I had newbie-like not made sure was absolutely level. Its only drawback is that it only works in the chuck, which is progressively less necessary as I get tools aligned.

When I have time this fall I'll build a "Joe Pie" model, and maybe perch it on a square like Pontiac's above. Or I'll build one of the rounds with the embedded magnet and just stick it on the flat way.

Thanks!
Tim
 
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