taper problem.

[wheeltapper]

Hi
here's a weird one.
I'm turning a No2 morse taper, so its a fair chunk of metal, not a flimsy thing.
I have the saddle locked, I'm turning the topslide feed with a socket on the centre nut driven by a cranked handle so no side to side wobble.
I get near size and take three cuts without changing the cut so no spring, lovely finish,

and the damn thing is fatter in the middle than the ends!!

have I got a special topslide that goes round corners or am I just going round the bend?

any ideas.
Roy

[lordedmond]

your tool is not at precisely centre hight hence the barrel shape

[DMIOM]

or maybe the top-slide gib(s) aren't evenly worn - certainly on a mill you can find they're slacker in the middle...

Dave

[Mike Cole]

Geo H Thomas gives the figures in his book 'The model engineers workshop manual' . But the old story of tool height not being on center gives a barrel shape is a bit of red herring. The error with a tool 30 thou out is something like 0.000012 in or one 12 millionths of an inch. Look to your tailstock fitting and the gibs.

Mike

[wheeltapper]

the results so far, tools dead on centre height checked with a facing cut, no pip.
tailstock not used.
no discernable sideways movement of the topslide throughout its travel.

Roy

[DaveH]

Roy,

I supose it has been take out of the chuck by now.

If not you could put a dti on the topslide and see if it "tracks" zero along the taper.

 
DaveH

[BillTodd]

How is the top-slide fixed to the cross-slide?

I'm wondering if the top-slide could be warped by the fixing bolts if overtightened or tightened down on a small piece of debris.

Bill

[wheeltapper]

thanks for the suggestions lads.

I just stripped the topslide down to its component parts, polished the dovetails and the gib strip with a fine diamond stone, washed it all in white spirit and reassembled it.

I've adjusted the gib so there's an even drag all the way along.

tomorrow I'll try another taper and see what happens.

Roy.

[jim]

is the top slide parallel to the spindle?? that's about the only thing i could see that would cause this

[DaveH]

Roy,

Let us know what happends -
 
DaveH

[wheeltapper]

Oh you are going to love this.

to recap, cutting a taper, toolpost in usual position, large end of taper at chuck end, cutting right to left.

as you know, I stripped and refitted the top slide.

tried just now with a new piece of steel.
same thing, cutting barrel shaped.

then I tried something silly.

I moved the topslide assembly to the rear of the cross slide, put a different tool in, swung the topslide round a few degrees , ran the lathe in reverse and cut on the back of the taper.

and it cut a WASP WAISTED taper 

so I've got a topslide that likes to turn right. (sounds like a Dr Who story).

at least it'll fit without a wobble 

if the tool was being pushed away from the cut as the topslide extended you would think the cut would be the same no matter what side it was cutting.

I don't think I want to turn anymore tapers.

cheers

A thoroughly confused Roy

[DaveH]

Roy,

Thanks for letting us all know.

Quite a clever way you sorted it out

I would tend to agree with you, nothing is pushing, I think it must be your top slide, or at least the dovetails.

I’m glad you have at least a morse taper you can use.
 
DaveH

[John Hill]

So Roy,  does this indicates the top slid dovetails are not straight?

[wheeltapper]

Haha, as far as I can see they describe a very large circle.

what I might do is strip the thing down (again) clamp the slide part onto the mill table and run a D.I. along the dovetails to see if they are straight.

I haven't got as far as working out what to do if they aren't.

if I do this I'll let you know the result.

Roy

[DaveH]

Roy,

Haha, as far as I can see they describe a very large circle.
Roy

Sorry to say I think you are correct
 
DaveH

[John Hill]

Haha, as far as I can see they describe a very large circle.

what I might do is strip the thing down (again) clamp the slide part onto the mill table and run a D.I. along the dovetails to see if they are straight.

I haven't got as far as working out what to do if they aren't.

if I do this I'll let you know the result.

Roy

It does seem a bit odd though, if the dovetails are a bit squirely one would expect the slide would bind at some part of its travel.

[hopefuldave]

Quote:
It does seem a bit odd though, if the dovetails are a bit squirely one would expect the slide would bind at some part of its travel.
/Quote

Depends whether the wear's at the ends or in the middle, and whether the gibs are taper-with-adjusting-bolts (which would bind) or parallel and adjust from the side with a bunch of setscrew and nuts (which could be adjusted to fit the worn curve)?
Assuming that the wear would be in the middle of the pressure face where it has done the most work and the gib is adjusted to match, it'd give the deviation the OP described... How much deviation, if not extreme it could probably be scraped out? A good test would be to put a known bar (uniform diameter. ground silver steel or HSS blank?) in the chuck and run a tenth-thou" DTI along it, would at least indicate how much needs to be taken off to straighten it. Luckily it sounds like the error's in the topslide, so there's not the same criticality - unlike the topslide, it doesn't need a fixed 90-degree angle to the ways - you'd be swivelling it anyway

Dave H. (The other one)

[wheeltapper]

Hi
the lathe isn't that old so I wouldn't expext much wear in the topslide dovetails.
most of the time the topslide is in a cupboard and I have a chunky toolpost that goes directly on the cross slide.

I only fit the topslide for tapers.

Roy

[hopefuldave]

Ah, so it was made that way... Damn.

If it were mine, I couldn't leave it as-is, I'd be in with the Prussian blue and a scraper (assuming it isn't grossly curved) - how much of a bulge did you get when cutting the 2 Morse taper? It doesn't take a lot to make them rock in the socket, I know!

Dave H. (The other one)

[John Hill]

Of course a taper can be cut without using the top slide but by using centres and offsetting the tail stock (which I understand is quite easy if one of those boring bar holder thingies are used instead of actually disturbing the tail stock position).  Bit hard to do an inside taper that way though..

[wheeltapper]

Hopefuldave;
I didn't measure the bulge but when I cut at the back and got an iwards curve it was about 3 thou in 2.5 ins.

the outer bulge did seem to be more than that tho.

Roy

[flutedchamber]

Wheeltapper,

Is it possible that there isn't enough relief under the cutting edge of your toolbit?  That would allow the bit to bite in the beginning of the cut until the material being cut pushes the toolbit back just a bit.  The same basic thing happens at the end of the cut, causing a barrel shape.

Are the spindle bearings good, with no detectable movement?

[philf]

My guess is that the topside isn't running parallel with the headstock axis in the vertical plane.

If this is the case the tool will only be exactly at centre height at one point. This could account for a wasp waisted part.

Firstly, I would clock the top of the cross slide to make sure it is parallel to the bed.

If that's OK then clock the bed from the topslide (by moving the topslide and not the saddle).

Hope this helps.

Phil.