Lathe: checking for taper with new bed

[ddickey]

@VicHobbyGuy I believe it is because of push off. If you are taking a test cut over say 6" with a super sharp HSS tool on a large >2" piece of 6061 aluminum and you measure a slightly larger TS end take a spring pass, I bet it will clean up only the last part of your piece. You would not want it smaller.
Find tolerances here.

https://pearl-hifi.com/06_Lit_Archive/14_Books_Tech_Papers/Schlesinger_Georg/Testing_Machine_Tools.pdf

Book is shown @10:15

 

[VicHobbyGuy]

Thanks, but I still don't see any reason why it makes a difference.

 

[GrifterGuru]

Any amount of negative taper (I.E less diameter at tailstock than at headstock) is always going to be less than ideal when and where it comes to machining within a specific tolerance. It is the difference between certainty of hitting a diameter "on size/within tolerance" and "what the fudge happened there"

1.6 thou' over 16"** (which is what 0.0001" per inch equates to) is never going to be good. Sure, anything upto 1 thou might be a good goal to work towards if at all possible, but with a flexi min 7x? who can tell how good you can get since each lathe is a distinct and separate beast despite ostensibly being made to be "the same".

I have to re-shim the headstock on my SC3 and I plan to use the "shim the vee" method described by Ted Hansen in his book. How much shim I use, depends on how much taper I am seeing once I re-measure.

** = You can get up to 16" envelope using a centre in the spindle and a steady mounted at the end of the bedway, possibly a tad more, depending on the centre you use and how you mount the steady on the bedway.

 

[SouthernChap]

@SouthernChap If you're testing your lathe for taper and it is cutting a smaller diameter on the tailstock end then that is unacceptable.

Okay, feels like you're pulling my leg. Can you provide an explanation that is relevant to the members of this forum.

Depending on the direction of bed twist, surely the taper could go in one direction or another.

If it is larger then there is a tolerance that is acceptable.

Acceptable by who's standard exactly? And for which particular application?

Oh and before you answer please bear in mind that you'll need to show us all your 'Lathe Alignment Police' badge.

 

[GrifterGuru]

Depending on the direction of bed twist, surely the taper could go in one direction or another.

@SouthernChap Absolutely well and truly spot-on.

 

[SouthernChap]

Any amount of negative taper (I.E less diameter at tailstock than at headstock) is always going to be less than ideal when and where it comes to machining within a specific tolerance. It is the difference between certainty of hitting a diameter "on size/within tolerance" and "what the fudge happened there"

1.6 thou' over 16"** (which is what 0.0001" per inch equates to) is never going to be good. Sure, anything upto 1 thou might be a good goal to work towards if at all possible, but with a flexi min 7x? who can tell how good you can get since each lathe is a distinct and separate beast despite ostensibly being made to be "the same".

I have to re-shim the headstock on my SC3 and I plan to use the "shim the vee" method described by Ted Hansen in his book. How much shim I use, depends on how much taper I am seeing once I re-measure.

** = You can get up to 16" envelope using a centre in the spindle and a steady mounted at the end of the bedway, possibly a tad more, depending on the centre you use and how you mount the steady on the bedway.

Well yep, obviously you can't put material back; a blown dimension is a blown dimension.

I suppose the point I was trying to make to our absolutist forum member is the majority of cuts (and not a small majority) on a mini-lathe by mini-lathe users will be taken much closer to the headstock (eh, most cuts taken by most people, that's the reason the most common wear on a used lathe is where it is).

The poor OP does not need to be told that his lathe alignment efforts are 'unacceptable'.

The abovementioned 'majority' is why I described the actual distance between centres for most mini-lathe owners as less than 14". Most people will be using the tailstock and that's some used up space right there.

 

[GrifterGuru]

Well yep, obviously you can't put material back; a blown dimension is a blown dimension.

I suppose the point I was trying to make to our absolutist forum member is the majority of cuts (and not a small majority) on a mini-lathe by mini-lathe users will be taken much closer to the headstock (eh, most cuts taken by most people, that's the reason the most common wear on a used lathe is where it is).

The poor OP does not need to be told that his lathe alignment efforts are 'unacceptable'.

The abovementioned 'majority' is why I described the actual distance between centres for most mini-lathe owners as less than 14". Most people will be using the tailstock and that's some used up space right there.

True enough...

Even with a 16" bedway, once a chuck is on and the tailstock is in place, you really only have an accessible envelope of 12 1/2" if you are lucky..

you immediately lose 5+ inches just using a chuck and tailstock live centre, let alone the loss when using a chuck and jobber drill.

a 7x12 would get you a net comfortbale 7" envelope and a 7x14 gets you a net comfortable 9" envelope with live tailstock centre.

As to "acceptable accuracy"... I could see that being the case in a production environment, but in a hobbyist "shop", that relaxes to a point of "accurate enough", though what that means is different to each person.

Don't even get me started on the "Swing" and how much of that is actually usable once you figure the cross slide in, or the cross slide and compound!!

 

[ddickey]

Okay I give up, lol. Yes, a mini lathe with a taper inclined away from the operator is fine if you're okay with it. My comment wasn't to condemn but to educate or think about what you're seeing.
It is a tolerance standard that would be used in industry or anyone who wants their machine tool set up as close as possible to those standards. I tend to geek out when it comes to those standards and lathe setups.

 

[GrifterGuru]

Okay I give up, lol. Yes, a mini lathe with a taper inclined away from the operator is fine if you're okay with it. My comment wasn't to condemn but to educate or think about what you're seeing.
It is a tolerance standard that would be used in industry or anyone who wants their machine tool set up as close as possible to those standards. I tend to geek out when it comes to those standards and lathe setups.

Which is fair point.

That said, everyone is different. For me? I would rather spend the time to get the thing set-up as true and straight as possible with as little taper as possible, even within the confines of the pee poor bedway, known for lack of rigidity that a 7x has. Yes, a bigger or better built machine will better take adjustments due to more mass, but that is the way things are.

 

[SouthernChap]

My comment wasn't to condemn but to educate or think about what you're seeing.

And that's the problem with internet forums and all text based communications generally. It's easy to sound more 'assertive' than one intends.

One of the fantastic things about this forum is that many members on here take a bit of extra effort with beginners (and I've been the recipient of such extra effort) by providing more comprehensive detail with their answers. It can help the beginner fully understand the point the more experienced person is trying to make, and tends to end up sounding more 'gentle'.

It is a tolerance standard that would be used in industry or anyone who wants their machine tool set up as close as possible to those standards. I tend to geek out when it comes to those standards and lathe setups.

There's the thing though, the OP has a mini-lathe. Given the possible work envelope and the likely precision requirements, such standards are at best, more than likely gilding the lily, at worse, a potentially demotivating distraction.

I'm sorry I suggested you might have been willy-waving, that turned out to be unfair, but think about the best teachers or mentors you've had. They will have been the ones who pushed you out of your comfort zone and stretched your skills to help them grow, but will have done it with an awareness of your needs at that point in your learning.