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Measuring x-axis alignment on a manual lathe

Jacob3650

Plastic
Joined
Jan 30, 2023
Hi
I am wondering if there is any way to measure how well my cross-slide is aligned with the bed/guideways on my manual lathe. I've tried to illustrate this below.

Thanks in advance
 

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That doesn’t mean much on a lathe. Set your cross slide level and make very sure you don’t have a twist in your bed ways from end to end.
Hi
I am wondering if there is any way to measure how well my cross-slide is aligned with the bed/guideways on my manual lathe. I've tried to illustrate this below.

Thanks in advance
 
Looking at your sketch in the third picture. If you're wondering how the top of the crosslide aligns with the way travel you can just mount an indicator on the bed or spindle and see if the top of the crosslide is parallel. As Just Dave says, that has no effect on ordinary operations.

If you're thinking that there is some absolute bed plane and wonder if the crosslide is parallel to it - there isn't Per your sketch, the crosslide could be at practically any angle and you just set the tool so it intersects the lathe centerline.

If you think about CNC slant bed lathes, a crosslide could be on the back side at a 45 degree angle, and if the tools are set it will cut just fine. There isn't a lathe bed absolute flat angle, so long as it doesn't change over the length of the bed. That's the twist that Just Dave points out.
 
Not entirely clear to me from your 3 pictures and from the wording of your question what you are asking. From the sketch in your third picture, it seems that you are concerned not about alignment drift during carriage travel but about whether the top plane of your crossslide is parallel to the plane across the tops of the two ways of your lathe. Is that correct?

-Marty-
 
Not entirely clear to me from your 3 pictures and from the wording of your question what you are asking. From the sketch in your third picture, it seems that you are concerned not about alignment drift during carriage travel but about whether the top plane of your crossslide is parallel to the plane across the tops of the two ways of your lathe. Is that correct?

-Marty-
Yes - The turcite underneath is worn, and i would like to change it. However, one of the guideways is a prism and the other is flat. Therefore it would be nice to know if the carriage is level with the ways. If i know it is level i can take off the turcite and measure the thickness and buy the equivalent thickness.

But if it is crooked and how much, i think it would be possible to calculate the right thickness so the difference in angle will make up for it?


Because of the angle on the prismways, i guess i need to know the right ratio because different thicknesses (but equal on both sides) will make the carriage angeled in X-axis

I've tried to illustrate what i mean in the photo attached to my reply. hope it makes sense.

Thanks
 

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Looking at your sketch in the third picture. If you're wondering how the top of the crosslide aligns with the way travel you can just mount an indicator on the bed or spindle and see if the top of the crosslide is parallel. As Just Dave says, that has no effect on ordinary operations.

If you're thinking that there is some absolute bed plane and wonder if the crosslide is parallel to it - there isn't Per your sketch, the crosslide could be at practically any angle and you just set the tool so it intersects the lathe centerline.

If you think about CNC slant bed lathes, a crosslide could be on the back side at a 45 degree angle, and if the tools are set it will cut just fine. There isn't a lathe bed absolute flat angle, so long as it doesn't change over the length of the bed. That's the twist that Just Dave points out.
I see your point, that you just set the tools so they are in center. The reason im asking is because im changeing turcite, and because of the angle on the prismways i might get a major angle on the carriage in the x axis if the turcite is not the right thickness. So that it might not rest evenly on the ways?
Im new in this world, just my thoughts:)

Thanks

Like this:
 

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You have to scrape the Turcite to fit the bed after it's glued in. That's the thing that matters most. Where that winds up, by itself, doesn't matter much. However, if the apron is bolted to the saddle as it usually is, the angle across the bed will also affect the angle of the apron and the spacing to the leadscrew, feed rods, etc.

You might assume, and this is truly just an assumption, that the builder originally made the crosslide ways parallel to the two flats i see on the outside edge of the bed adjacent to the tailstock ways. If we take that as true, then you would make an initial fit of Turcite to the bed, then measure the distance from the crosslide ways down to the flats, then scrape the Turcite further as required to make the measurements equal front and back.
 
You have to scrape the Turcite to fit the bed after it's glued in. That's the thing that matters most. Where that winds up, by itself, doesn't matter much. However, if the apron is bolted to the saddle as it usually is, the angle across the bed will also affect the angle of the apron and the spacing to the leadscrew, feed rods, etc.

You might assume, and this is truly just an assumption, that the builder originally made the crosslide ways parallel to the two flats i see on the outside edge of the bed adjacent to the tailstock ways. If we take that as true, then you would make an initial fit of Turcite to the bed, then measure the distance from the crosslide ways down to the flats, then scrape the Turcite further as required to make the measurements equal front and back.
Good point about spacing to feed rods and leadscrew. I had not thought about using the flats on the tailstock ways to measure, but good idea, I'll try that. Thanks for the tip.

- Jacob
 
I think you determine wear first, by careful measurement of worn area to the un-worn ( or least worn) area(s) of the ways, front and back. then you know how much to add to the worn areas. wear on the bottom of the carriage is a separate issue, and I think you determine that by referencing the machined but non contact area to the worn contact surfaces, if those are intact. building up and truing those surfaces is obviously part of a proper rebuild.

determining the compromises that are part of a "practical" rebuild is part of this too. for instance, if your front ways are worn say, .006 near the headstock, but your rear is 2-3, what happens is the carriage tips down in front, pulling the tool away from the work. if you build up the front to match the rear, the carriage stays "level" , even though it still drops down the 2-3 thou and the error is minimized, to the point it is likely insignificant for all but the most demanding applications. if you add that as extra to the front bottom of the carriage, it will be "level" near the headstock and high down to the right of the bed, this might be a way to only add to the carriage, and depending on the type of work done on the lathe, get an acceptable result.

emphasis on "I think", as im not a millwright or rebuilder!
 








 
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