Lathe cross-slide tolerance goals?
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  1. #1
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    Default Lathe cross-slide tolerance goals?

    Hi,

    I have a Kent TRL-1340 lathe which is quite new but had premature wear
    in the cross-slide and saddle casting. Largely due to the cross-slide being
    surface ground, and the saddle casting being poorly scraped and not deburred around
    the oil grooves.

    I have scraped the underside of the cross-slide flat. Now I intend to use it
    as a template to scrape the saddle. However I wanted to check the tolerances for
    alignment and parallelism.

    To check alignment as suggested in Connelly, I put a parallel in the chuck, and
    rotated the chuck 180degrees with an indicator showing zero-zero (Figure 26.63 in
    Connelly). When I test the sliding compound against this, I get a few tenths, and
    in the right direction (would face a concave disc) as per Connelly Figure 26.64.

    So far so good.

    When testing the existing saddle mating surface for the cross-slide however, it is .0035"
    high in the rear vs the front, as measured from the flat ways of the bed. On this lathe
    bed the flat ways are parallel, the v-ways are different heights.

    Since the lathe bed is hardened and unworn, that is an easy reference surface to use, to ensure both lateral and longitudinal parallelism of the cross-slide / saddle surfaces versus the lathe bed flat ways. What I'm not sure of is whether I should leave the rear high slightly to compensate for tool pressure?

    Some pictures (apologies, I'm having problems getting decent resolution pictures posted these days, the uploader seems to downsample them badly)

    cross-slide-inspection-2.jpgcross-slide-inspection-1.jpg

    Thanks,
    Phil

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    I looked in Schlesinger's Testing Machine Tools and do not see a tolerance for the elevation of the cross slide relative to the ways.
    I did not expect one because in a lathe the work turns- and it is the relationship to that axis that matters. They make slant bed lathes...

    You do want to make sure the cross slide is parallel to the lathe ways and spindle axis as a way of ensuring the compound travel is parallel to the spindle axis. There is a standard for that. .0012 in 6 inches in the vertical plane and .0008 in 12 inches in the horizontal plane

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    Quote Originally Posted by J_R_Thiele View Post
    I looked in Schlesinger's Testing Machine Tools and do not see a tolerance for the elevation of the cross slide relative to the ways.
    I did not expect one because in a lathe the work turns- and it is the relationship to that axis that matters. They make slant bed lathes...

    You do want to make sure the cross slide is parallel to the lathe ways and spindle axis as a way of ensuring the compound travel is parallel to the spindle axis. There is a standard for that. .0012 in 6 inches in the vertical plane and .0008 in 12 inches in the horizontal plane
    Hi,
    Yes this makes sense, I also looked in Testing Machine Tools and didn't find a specific value either for this specific area.

    I was assuming however that keeping tool height consistent during a facing operation would imply wanting some precision for the cross-feed motion, but a few thou is probably ok?

    From the compound requirements you mention, I gather than means if the compound is turned to 90degrees, in the vertical plane that implies the cross-slide parallelism to itself (left and right sides) should be .0008"/12".

    Thanks,
    Phil

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    After I replied I had some further thoughts, with one related to facing. If you adjust the height of the bit to be on center at the centerline, you will have no nub. If you use a tool to adjust the tool height using a level, you may need to adjust it for you lathe- and then reset it if you re-level the lathe, or the foundation shifts.

    If you do the math, a low tool bit does not make much difference in diameter unless the bit is very low, or the diameter very small.
    Being .005 low in tool height means what should have been 1.00000 inch in diameter is 1.00005. Being .010 low makes the anticipated 1.00000 come out 1.00019. Being .020 low you would cut 1.000798. Make the anticipated diameter .25000 and .005 low cuts .250198. Being .010 low cuts .250798, and being .020 low cuts .25318.

    In using the lathe cutting off and facing are the two operations where we cut very small diameters. The shape of the nose and backangle of the bit can mean it cuts at one height- except for facing cuts.

    For the compound- yes, it is turned 90*, so its motion is parallel to the axis of rotation. The vertical plane refers to measuring the TOP of the test bar. This is a way of seeing if the tool bit edge raises or lowers as the compound is extended.

    I was in error in my comment about the horizontal plane standard, as that standard is if the compound has an automatic feed. There is also a different vertical plane standard of .0012 in 12 inches if the feed is automatic.
    Last edited by J_R_Thiele; 11-18-2020 at 05:16 PM. Reason: spelling

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    Thanks. In practice I need to scrape the surface anyway, so getting it to .001" or so laterally and longitudinally won't be very hard. Thanks for the advice!

    -Phil

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    Being it is an Asian lathe and on most Asian lathes the head-stock is setting on a flat surface and can rotate on a pin under it so it can be aligned with the bed. If I were scraping it I would square the cross slide / saddle to the bed first and either with a test bar in the spindle taper or a 2 collar test after the cross slide is finished turn the head so it is .0004" in 6" concave or TS end pointing toward the operator to compensate for push away.

    Look at page 18 https://www.kentusa.com/wp-content/u...ion-Manual.pdf

    also: scroll to min 17 --- Monarch 16" Lathe Resoration - Part 13: Measuring the Saddle for Scraping - YouTube

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  8. #7
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    in the video keith sais that the ways can be tilted down in the back (so something would roll away from operator). what is the idea?


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