Non-perpendicular Cross Slide?

[Johnwright]

While reading a thread on parting, regarding a concave surface left on the parted piece, a member questioned: "Do you get a perfectly flat surface when facing the end of a bar? If not could be the cross slide is not travelling 100% perpendicular to the lathe axis." My question is now, if the cross slide is not traveling perpendicular, then what can I do About it on my 9 X 20 lathe? I'm just a newbie about all this. jw

 

[chips&more]

Well…is it not flat when you face? IMHO, it’s more like the parting operation left you with a not flat surface. And that observation is a common problem when parting. A sharp tool, properly square up and the cutting edge on center is a good start for set-up…Good Luck, Dave.

 

[randyc]

Hi,

I'm not sure what standards are used in manufacturing your import lathe but American lathes are set up so that the cross-slide intentionally cuts slightly concave. Not much, of course, and I cannot recall the amount at present. You could likely Google the topic and find a quantitative value.

There is good reason for this, by the way, consider mating two faced parts that might be slightly convex - a good joint could not be achieved. In the same situation, concave surfaces would mate properly.

Insofar as what you can do about it, my recommendation would be: nothing, leave it alone. Making the cross slide absolutely perpendicular to the spindle axis requires skills that you probably don't possess at this stage of your development, no offense.

 

[Wreck™Wreck]

Hi,

American lathes are set up so that the cross-slide intentionally cuts slightly concave. Not much, of course, and I cannot recall the amount at present.

I would be interested in knowing where you have seen this published.

 

[randyc]

I would be interested in knowing where you have seen this published.

Well, here for one:

"I do notice that on the Prentice Brothers inspection chart they say the the lathe should face .001 “ low or concave in the center."

The above quote is from this thread in "Practical Machinist": http://www.practicalmachinist.com/v...ory/accuracy-alignments-machine-tools-247127/

So far as I know, this is common knowledge among old-timers (at seventy, I guess I qualify and has been discussed a few times in the above forum.

 

[Wreck™Wreck]

Thanks

 

[randyc]

Also, FYI, I e-mailed a man who rebuilds machine tools for a living and has been doing it for over forty years (I e-mailed him before I found the above thread". He also teaches, conducting classes all over Europe and the U.S. I asked him to quantify the amount of concavity that he builds into his refurbished lathes. I just received his reply:

"That is correct plus the head is pointed toward the operator so when you get push away the shaft will turn straight. I believe it is .0002" in 12" in both cases.

I was always taught to do that to the cross-feed so when you set a shaft on a plate it doesn't rock if it is high in the middle (convex). I can email you a page out of the Testing Machine Tools Book by G. Sleshinsger if you want to see lathe tests."

Edited to add: He refers to the work of Doctor G. Schlesinger, which is the "bible" for machine tool testing. I don't have a copy, unhappily or I would have referenced it previously.

Second edit: Forgot to add his website: http://www.handscraping.com/

 

[John Hasler]

Hi,

I'm not sure what standards are used in manufacturing your import lathe but American lathes are set up so that the cross-slide intentionally cuts slightly concave. Not much, of course, and I cannot recall the amount at present. You could likely Google the topic and find a quantitative value.

There is good reason for this, by the way, consider mating two faced parts that might be slightly convex - a good joint could not be achieved. In the same situation, concave surfaces would mate properly.

That will depend on the parts, won't it? Consider pipe flanges bolted around the edges. If they are very slightly convex the bolts will draw them in tight. If they are concave you could get a gap.

 

[randyc]

Gaskets. Also, when I worked at Westinghouse Marine Division, all of the pipe flanges had a smaller diameter just inside the bolt circle that was turned just slightly "proud" of the flange surface. That insured a good steam or oil joint. Good question, by the way !

 

[randyc]

P.S. If the bolt tension of the pipe flange fasteners is sufficient to draw the flange together with its mating part when both are convex .... then similarly, the bolt tension should also be able to draw both flanges together when they are concave, in my opinion.

In any event, it's a non-issue since the amount of taper in the cross slide is so tiny - I looked up the Schlesinger reference a few minutes ago. It specifies 0.06 mm in 1000 mm which translates to a taper of 60 MILLIONTHS of an inch per inch.