Lathe cutting a taper

[Chris MateParticipant @chrismate31303]

I bought a new CQ6133 lathe few years back. I worked around the taper cuttng problem, but are back at it again.

Note-1:Shimming & measuring=OK
-I Fitted a 16mm HSS grounded bar with 110mm stickout from 3 Jaw chuck. MY 3 jaw chuck had been modified to true adjust. So I adjust the chuck to run this bar without runout=OK.
-I managed to ajust/shimmed rear of bed inside bolt to .014″ shim. With this the .01mm Dial Indicator run true zero deflextion/taper along the HSS bar=OK.

Note-2:Cutting another Test Bar, setup as above=OK.
-As soon as I take a lightish cut with new insert(10mm shank type), I get a taper not good up to .1mm taper.
-Now I can duplicate this by pulling on the bar with one finger, I can pull it easily up to .1mm, with force up to .18mm.

Note-3:Checking headstock versus Spindle backplate versus MT5 versus face of backplate.
a)The spindle run 100% true(I was surprised), the spindle face run 100% true(I was surprised again).
b)Fit chuck & HSS Bar. Fit Dial indicator to headstock & measure from headstock to spindle, to chuck, to HSS bar. No movement with finger, zero. If I grab the bar with hand and pull as hard as I can, the deflection is minimal at less than.01mm, which certainly is not the problem I am looking for.

Note-4: Check headstock fit to Bed .
-The headstock is tight, I cannot tighten the bolt anymore.
-At this stage the bed is tight to the plate on cabinet, and run without taper from crossslide with shim, and without cutting or pulling.

Question: Where does this cause of the taper comes from-?

If you fit a bar to your chuck and pull on it, how much deflection do you get-?
-The pulling wih one finger to seems like similar taking a light cut.

If it was the spindle bearings, I would thought I saw that in the headstock/chuck/Bar test-?

[JasonBModerator @jasonb]

Note-1:
-I Fitted a 16mm HSS grounded bar with 110mm stickout from 3 Jaw chuck.

Note-2:Cutting another Test Bar, setup as above=OK.
-As soon as I take a lightish cut with new insert(10mm shank type), I get a taper not good up to .1mm taper.

 

To me 110mm of 16mm bar sticking out is not a good idea, Ctr drill it and use tailstock support

[bernard towersParticipant @bernardtowers37738]

your turning test should be done with a dumbbell shaped bar and with compound set to 5.75 deg. and then test cuts of tenths can be taken (3 or 4) then measurements taken.

[Chris MateParticipant @chrismate31303]

What puzzels me the most, is not cutting a test bar, but the fact that measured from the Headstock nothing moves(Hard pull), but measuring from crosslide it moves a lot(Soft pull), and that make sense to the taper it cuts under very light load…….I just cannot see where this is coming from yet. The bed cannot be that flimsy…..

From the headstock to the tip of the bar(Chuck & chuck jaws included) nothing moves, my 1st thought it was the spindle bearings, but it does not look that way.

[bernard towersParticipant @bernardtowers37738]

If you are going by soft pull and hard pull thats not very scientific you should let the lathe do the work then measure and yes a lot of latje beds are flimsy to a small degree. the twist can be adjusted with adjustable feet.

[Michael GilliganParticipant @michaelgilligan61133]

Slight digression, but I think it’s worth looking at Schlesinger’s design for a test mandrel … It puts the flexibility of all such items into the proper perspective !

Details are on p5 in the widely downloadable book.

MichaelG.

[Author]

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