Measure squareness of lathe saddle.

This is a continuation of https://www.woodworkforums.com/185969...e+lathe+saddle. I finally got some spare time - don't you hate it when work gets in the way of your hobbies!


I re-leveled my bed - it's 0 on both ends, and about a 1/4 of a graduation off in the middle (1 graduation = 0.42mm per meter). I'm not sure if there is anything I can do about this twist - the lathe is fixed at 3 points.

Using a test bar, I aligned the head so the error was less than 1/4 graduation on the DTI (1 graduation = 0.0005")

I mounted a straight edge in the chuck and ensured that was square to the spindle rotation.

I am planning on using the straight edge to check the saddle squareness to the way.


So, my question, given all these little accumulative errors in setting up, is there a better way to check the squareness? I was thinking of mounting a square to run parallel to the way and parallel to the cross slide, but I don't have a precision square like that, and I'm not sure how I'd mount it.

Frank

PS For the record, may saddle is showing approx 0.1mm convex over about 100mm.

Hi there,

I could be wrong but I'll have a crack anyway!

I think you need to be careful what you choose to be the datum when checking for squareness, parallel etc. etc. The saddle should be perpendicular to the spindle axis which in turn should be parallel to the bed ways.

If you relate all measurements back to one datum then its less likely to create a accumulation of errors. As such, I would be measuring the saddle squareness (or the cross slide) to the bed ways, not the spindle. How you go about measuring that I would have to think about that...

Simon

If i understand what you are saying it should be fine. Put a large parallel/straight edge in the 4 jaw. Using a dial on the saddle adjust it so that you get a 0-0 reading at the ends, by turning the spindle over 180 deg, not moving the saddle. Then run a dial across the parallel with the cross slide, this will give you the numbers. There must be a Utube vid on this surely? Ew

Quote:

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Originally Posted by Ueee

If i understand what you are saying it should be fine. Put a large parallel/straight edge in the 4 jaw. Using a dial on the saddle adjust it so that you get a 0-0 reading at the ends, by turning the spindle over 180 deg, not moving the saddle. Then run a dial across the parallel with the cross slide, this will give you the numbers. There must be a Utube vid on this surely? Ew

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That's what I'm planning on doing, but if my spindle isn't set correctly (through accumulated errors), I'm going to stuff up the saddle squareness. I was just wondering if there was a better way of doing it.

Frank

Yes, but it means having the saddle off the machine, and you need a precision square.

As long as you bed is level and your happy with the test bar results doing it the way you plan is fine IMO

Ew

My understanding is that the squareness of the saddle to the machine axis is measured with a facing test - either face a face plate or a piece of material attached to one. If the saddle is square then the faced surface should be flat. Limits from Schlesinger are 0 to 0.02mm per 300mm of diameter (hollow or concave only)

Michael

Quote:

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Originally Posted by Michael G

My understanding is that the squareness of the saddle to the machine axis is measured with a facing test - either face a face plate or a piece of material attached to one. If the saddle is square then the faced surface should be flat. Limits from Schlesinger are 0 to 0.02mm per 300mm of diameter (hollow or concave only)

Michael

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It is not desirable for the saddle to be square to the ways or the spindle center line. All new lathes have the saddle slightly offset, such as to ensure that the lathe faces concave. This offset is quite large for cheap lathes, and pretty small on an expensive precision lathe, but there always is this offset in concave direction. A lathe that faces convex is a worthless reject that cannot be sold.

You do not check this measuring the angle between the cross slide dovetails on the saddle to the lathe bed. You check this very simply with a facing cut, because a real cut takes into consideration every contributing error. And this is not something that you can adjust. The only way to change this is by taking off metal from the saddle, either from where it meets the bed, or from where it meets the cross slide.