Unwanted Taper ?

[Danuzzo]

I am at my wit's end trying to figure out why I consistently get a taper on the three 7x mini lathes that I have tried. I am now on a third 7x mini. I was getting a taper in all 3. The taper always the same; that is, the chuck end always thicker than the tailstock end. All pieces were turned without a tailstock.

This is my thinking at this point. The common denominator on all 3 lathes is Me. I am now reluctant to think that all 3 headstocks were out of alignment, and all in the same direction. The 1st one had a significant taper of about .001 per inch. The second one was a little better, especially after I bolted it to a bench with a shim on the tailstock end away from me.However, the next day, I went back to cutting a taper with the same lathe that did so well the day before.

Now on my 3rd one. At first, it was cutting a taper; but, I think it has improved because I shimmed the bench's foot at the tailstock end way from me. The lathe itself sits on the bench on the rubber feet. The lighter the cut (e.g. .001 for a total removal of .002), the greater the taper. The current piece I am testing is aluminum 1" stock sticking out about 3" from the chuck. The taper has been reduced to about less than .0015" over the 3"; except the last piece where I said "one last try for the night", and I took a cut of about .010 (actually .020 taken off) and I got much less taper. In fact, the first 2" from the tailstock end measured about .0005 off only at the first 1/4" or so. After that, it was probably less than .0001 off. But; then I get to the last inch and it tapers about .00075.

This was all yesterday, I haven't gone out to see what happens today. Help would be greatly appreciated.

 

[Danuzzo]

Okay, went and tried again. Taking a cut depth of about .013" (total of .026"), the 2" measured from the tailstock end are pretty good varying about .00025". Still, when I get to around the last 1" closest to the chuck, it is about a .001" thicker than the other 2 inches.

Please help me figure this out .

 

[Smokey Joe]

I had the same problem with mine. I ended up shimming the head of the machine .003 to straighten it out.

 

[Danuzzo]

Smokey, where did you place the shim? Was it for the height or from side to side? Thank's.

 

[Smokey Joe]

The raised part of the bed that locates everything. On the far left side on the front of the raised part I had to put a .003 shim about 1/2 long to shift the head CCW when your looking at the top. Mine had three bolts holding the head on. Two in the front and one in the back. I hope this is understandable. I'm not real good at describing things. I took the covers off and took a picture so you can see what I'm talking about.

 

[fcheslop]

When bolting the machine down the bed could be twisting
In the ML7 Manuel it shows a simple way to level /wind the machine in
I would try this before shimming the head
A link to the book
http://vintagemachinery.org/pubs/4654/9521.pdf

 

[Danuzzo]

Thank's for the replies. Still at it; once, I got lucky and minimal taper. I think I will bolt it to the bench and try the shims on the far side of the tail stock end. Not quite ready yet to take it all apart to try to shim the headstock.

 

[Danuzzo]

Okay. Got the taper down to a repeatable .00025" with lathe bolted to bench and a .007" shim on the tailstock end away from me. Several different shims sizes were used. If I recall correctly, the 1st shim was .016", and the taper was about .002", but with the narrow end at the chuck (opposite of what had been going on). Tried several different shim sizes until I went with the .007".

Done for the day. That 1" aluminum stock is down to around .70" diameter now. Will try again tomorrow, hoping for the same results.

 

[goldstar31]

Okay. Got the taper down to a repeatable .00025" with lathe bolted to bench

This is 'a. quarter of a tenth of a thous' but it does raise the questions of
1. Over what distance the measurement was taken
2. on what instrument(s) was it taken



and a .007" shim on the tailstock end away from me.
Again, I wonder what test was used to necessitate such an addition.

Several different shims sizes were used. If I recall correctly, the 1st shim was .016", and

the taper was about .002", but with the narrow end at the chuck (opposite of what had been going on). Tried several different shim sizes until I went with the .007".

With a chuck in the test????? Unless one has a 'Griptru' or something similar, the accuracy of a chuck on on a 7 x 14 is rarely better than being better than 3 whole thous holding a fairly accurate ground 'test bar' which is suggested when my good friend Frazer described how the old Myford firm suggested in its various manuals- of which I have copies

Being a Doubting Thomas of almost 90 now, I hae me doots

Norman

 

[bluejets]

Worth watching.........

 

[teeleevs]

Okay. Got the taper down to a repeatable .00025" with lathe bolted to bench and a .007" shim on the tailstock end away from me. Several different shims sizes were used. If I recall correctly, the 1st shim was .016", and the taper was about .002", but with the narrow end at the chuck (opposite of what had been going on). Tried several different shim sizes until I went with the .007".

Done for the day. That 1" aluminum stock is down to around .70" diameter now. Will try again tomorrow, hoping for the same results.

Just a couple of observations, a lathe, especially a small one should never be bolted down to rubber as you can distort the bed even bolted to timber can be a problem as timber will move with changes of the weather. I have used shems to rectify problems with a very old lathe but before resorting to this you need to check the between centres accuracy. An over hanging cut can bend the job away from the toolpost, a very light lathe can even bend the mandrel but between centres it should be straight.

 

[David Shealey]

Okay. Got the taper down to a repeatable .00025" with lathe bolted to bench and a .007" shim on the tailstock end away from me. Several different shims sizes were used. If I recall correctly, the 1st shim was .016", and the taper was about .002", but with the narrow end at the chuck (opposite of what had been going on). Tried several different shim sizes until I went with the .007".

Done for the day. That 1" aluminum stock is down to around .70" diameter now. Will try again tomorrow, hoping for the same results.

You should also put a dial indicator on the carriage and run it up and down the TOP of a straight piece of rod in the chuck. If the head is pointed up or down relative to the bed that will also give you a taper. Not as pronounced as side to side, but still needs to be right.

 

[goldstar31]

Sorry but the chuck is not sufficiently accurate to be of use.

A 3 Jaw chuck on a 7x14 is not exactly my idea of something conforming to Schlesinger's `Limits

 

[Danuzzo]

Being a Doubting Thomas of almost 90 now, I hae me doots

Norman

Piece of aluminum sticking out about 3" from chuck, was used. Micrometer was used. The test used was "trial and error". I don't have fancy measuring equipment or a known true rod. Will see if it is repeatable later today, as soon as I can get back to it. I am hopeful that it will repeat. I will also try a piece of 1" mild steel.

 

[Danuzzo]

Bluejets, thank's for the video link.

Teeleevs, no rubber used. The bench is a formica covered one. The movement should be limited. Was not turned between centers.

David, I do have a 3/8 round drill rod; will that work, or is it to small in diameter, resulting in droop?

Thank's for all the responses.

 

[goldstar31]

Piece of aluminum sticking out about 3" from chuck, was used. Micrometer was used. The test used was "trial and error". I don't have fancy measuring equipment or a known true rod. Will see if it is repeatable later today, as soon as I can get back to it. I am hopeful that it will repeat. I will also try a piece of 1" mild steel.

I think that 'Boxford' lathes amongst other reputable lathe firms used to supply a test report using the Georg Schlesinger book on testing machine tools. It gave the abbreviated 'précis' of Connolly's Machine Tool Reconditioning.

This was ALL published in this fora years ago - and included how to get the standard book by Connolly from the 'internet.
Again, I wrote up how I restored a 'basket case' of a friend's Myford ML7 in Model Engineer.

I'll continue settle for what was tried and tested- and accepted.

 

[Danuzzo]

Goldstar, I wish I could get a hold of a Boxford or Myford.

 

[goldstar31]

Goldstar, I wish I could get a hold of a Boxford or Myford.

I think that I got £300 for a ML10- it was basic as I kept the accessories which went onto a Super7 with gear box and power cross feed at £3000- which was basic. Reconditioned from Myford and still very basic would have been £7500 to which accessories will- go sky high.
So my Sieg4 secondhand cost £350 and I added £350 for a new mill attachment but then I had to add- a lot- for accessories.

OK, I'm nudging 90 and a widower and have donated enough monies to see my grandchildren(4) through university when I'm dead and gone. With job problems with the coronavirus, both my kids are sort of unemployed and need a handout and , along with equally kind mates, we are donating to help people working for less fortunate people.

Model engineering simply limps along as a game that I've had- for 81 years or more!

I wish you the same good fortune

Norman

 

[peterl95124]

I would definitely not use bolting the lathe to the workbench with shims,
distorting the bed isn't a good way to counter a misaligned head.

I definitely feel for you though, when I tighten the bolts that hold the head to the bed
on my mini-lathe the tightening changes the alignment, so it's really hard to get it to
the correct offset such that it will come out straight after tightening. I've never gotten
better than a couple thou per 6" of length. If I need more accuracy I use the tail-stock.

Another problem, once you do get things all aligned, since you say the piece comes out
thicker at the chuck end and thinner at the far end is this, chucks (especially 3-jaw) are
not nearly as rigid as you'd think, the farther away from the chuck the cutter is the more
leverage it has to deflect the work, and the closer to the chuck the cutter is the less leverage
it has. on small diam soft material we all know it is the material that bends, but on thick diam
hard material other things, including the chuck, can distort. Something to remember for
the future even if it doesn't apply here.

and if that weren't enough possible problems, the preload on the spindle bearings might be
too light, allowing the spindle itself to shift even while the head stays perfectly aligned.

above all have patience, alignment can take forever...

 

[BaronJ]

Hi Guys,

I haven't found any comment in this thread that mentions chuck run out ! Apart from the fact that three jaws chucks very rarely repeat accurately and can have several thou runout to add into the mix.

Head tilt will also play a large part in producing a taper even if the head is perfectly in line with the bed. Turning between centers is one way of getting parallelism but it tells you nothing about bed twist.