Importance of stock roundess

Importance of stock roundness

Newbie question...Does the roundness / tolerances of your stock affect the tolerances of a finished part? If so, why?

When machining with tight tolerances, it seems most people use ground stock...is this because of the tight(er) tolerances needed to properly hold the work? I could see this being more important if using collets or bushings.

If we had a mythical machine with zero runout, could we take a bent piece of stock and turn it perfectly true (ignoring deflection and other variables)?

Edited: For grammar. Admins...perhaps consider fixing 'roundess' in thread title to 'roundness' to improve search results?

jmille25 said:

When machining with tight tolerances, it seems most people use ground stock...
If we had a mythical machine with zero runout, could we take a bent piece of stock and turn in perfectly true (ignoring deflection and other variables)?

Click to expand...

Not so and yes Well at least as true as a straight piece
Nothing is perfectly true
Always some molecules off

Peter

Answer to the first question is no, stock roundness doesn't matter as long as it passes through the collet, and as long as the stock OD is larger than the finished part OD. Reason to use PG stock is if the finished part OD is the PG stock OD, then you don't need to machine the OD.

Answer to the second question is, if I can grab it with collet or chuck, I can make good parts. Bent stock is not likely to pass through a collet though.

Regards.

Mike

Swiss screw machines usually use precision ground material.

You can see in this video how the material slides inside a bushing.

jmille25 said:

Newbie question...Does the roundness / tolerances of your stock affect the tolerances of a finished part? If so, why?

When machining with tight tolerances, it seems most people use ground stock...is this because of the tight(er) tolerances needed to properly hold the work? I could see this being more important if using collets or bushings.

If we had a mythical machine with zero runout, could we take a bent piece of stock and turn in perfectly true (ignoring deflection and other variables)?

Click to expand...

Does the roundness / tolerances of your stock affect the tolerances of a finished part?

NO. The material being machined is brought into spec no matter what the original stock size was.
If your turning a major OD to 2.0000 from 2 1/4 stock, the stock could have a tolerance of +- and 1/8, makes no difference to the finished product.

When machining with tight tolerances, it seems most people use ground stock...is this because of the tight(er) tolerances needed to properly hold the work? I could see this being more important if using collets or bushings.

This is hard to answer really, it all depends on the application and setup.
Hot roll is much cheaper but when dealing with production in some applications the cost savings gets eat up quickly dealing with work holding issues
of inconsistent stock size and dog legs in material.
Again it all has to do with application, if your turning under the stock size and are using hard jaws in a chuck then the cost saving of hot roll is the better choice. Rough turn, then finish will deal with any movement of the material and make a good part, even if your working with tenths.

If we had a mythical machine with zero runout, could we take a bent piece of stock and turn in perfectly true (ignoring deflection and other variables)?[/

Absolutely, its done everyday all over the place.
It is common when working with hot rolled ht/sr shafting material to have a 1/8 dog leg in it, you just make sure to order large enough stock to be able to machine major diameter.
If your finish OD on a 6' long shaft is 5" and you order 5" 4140 hot roll HT/SR your going to be sad. It most likely will NOT be straight.
You would order 5 1/4 stock instead.
Why not just buy 5" ground stock?
Ground stock especially in larger sizes is expensive as a mofo.
Cheaper and better for the machinist (talking shaft work) to have some material to work with to control run out during the machining process so when its done you have close to zero run out. Its a bitch to rough a bunch of material of a shaft that has an area you cant machine and maintain zero run out, can be done but more of a pain in the butt versus being able to machine the whole shaft.

The above is just my honest opinion on metal working, others may disagree but it is what it is.

The lathe has not yet been invented that can work both ends of a part without reclamping.

Starting off with round stock assures that when you reclamp the part you will still be near the center.

jCandlish said:

The lathe has not yet been invented that can work both ends of a part without reclamping.

Starting off with round stock assures that when you reclamp the part you will still be near the center.

Click to expand...

Im not following what your getting at.

You could turn square hammer forged stock to a round size, flip and with use of the steady rest machine the other end round and have less than .002" run out.

Where the axis of rotation of the two clamping operations meet, which isn't a certainty, they most likely just pass near one another, there is going to be a kink.

I could be tiny, but it will be there.

Centerless stock is spec'd to tenths or millionths.

jCandlish said:

The lathe has not yet been invented that can work both ends of a part without reclamping.

Click to expand...

Hmm, and I thought that was what those carbide facedrivers I make where for. One clamp, total part turn.
Bob

Between centers work needs no consideration for the outer surface geometry other than it's presence.
With both ends and all between readily accessible with correct drive type.

Peter from Holland said:

Not so and yes Well at least as true as a straight piece
Nothing is perfectly true
Always some molecules off

Peter

Click to expand...

Something to consider if turning a piece of stock that is bent is the internal stresses set up by it getting bent. I have had this happen to me.
The material can keep moving because of the internal stresses even though you are machining it true.
Years ago when doing Mil spec work the material requirements called for oversized stock machined down due to surface stresses from the production of the material.

To the OP...

Are you writing a term paper ?

digger doug said:

To the OP...

Are you writing a term paper ?

Click to expand...

Nope, for whatever reason I found myself pondering possible sources of error and was curious to know if I was overlooking something...so I figured I would ask people with a lot more experience than myself.

jmille25 said:

Nope, for whatever reason I found myself pondering possible sources of error and was curious to know if I was overlooking something...so I figured I would ask people with a lot more experience than myself.

Click to expand...

Well then post up the specific problems you are having and we shall
have a go at solving them.

digger doug said:

Well then post up the specific problems you are having and we shall
have a go at solving them.

Click to expand...

Sorry, should have been more specific...it was more of an intellectual exercise than anything else. Sometimes questioning the basics helps me to build up a mental model of what's really going on. So for the rare occasion when I machine or fabricate something for my own use and run into issues...I might be a bit more prepared.

As always, I appreciate the knowledge everyone has to share!

John Welden said:

Swiss screw machines usually use precision ground material.

You can see in this video how the material slides inside a bushing.

Click to expand...

i wonder how companies can put "swiss" in their name just like that. where does "swiss" actually come from?

dian said:

where does "swiss" actually come from?

Click to expand...

This lathe.





And Moutier.

Using a typical manual lathe, as found in a beginner's shop, within reason the roundness of the stock does not matter. A three or four jaw chuck, when tightened, can actually dig it's jaws into the metal to some degree. Aluminum, brass, and soft steel will yield somewhat and you can see the jaw marks on it when it is removed.

In a collet, you are gripping over all or most of the surface so the high spots will determine the actual areas of grip. A collet will not dig in, but it can compress any local highs to a certain degree.

In either of these cases, you will get a firm grip on the metal and that is all you need to machine precise features. If the part needs to be turned around so the other end can be machined, then you will usually grip it on one of the freshly machined surfaces that were made while it was first mounted. A collet will ensure the best accuracy at this point.

I said within reason and that can be a generous thing. I have machined stock with irregularities that were easily visible and had no problems. Almost any hot rolled or cold rolled stock can be machined with no problems from the grip. If it looks round, it probably is OK.

As the video that was posted above, there is a point where better stock is desirable. Since you are asking this question, it is a reasonable assumption that you are not there. When you get there, you will have more experience and will know that you need better stock. This will not happen tomorrow or even next week or next month. So, make chips and don't worry about this for a while.

camscan said:

For precision turned parts then you need precision ground stock but forgeneral,run of the mill,commercial turned parts then standard stock is the norm.

Click to expand...

No... it is juist what John said when he posted this originally. A swiss style machine uses tgp or ground stock because it needs to slide through the bushing. If there is excessive slop between the bushing and stock you will get some crap parts, if not breaking tools.

camscan said:

Sorry but I dont agree. I stand by my original statement,it all depends on precision and consistency required. Check a batch of stock and see how very little,if at all, it varies. I do not know what tgp means so cant comment. I have run sliding heads(Swiss) in my time and only ever used ground stock when necessary. Just consider the additional cost when not necessary.

Click to expand...

The precision of the stock is not related to the precision of the parts, except when regarding a swiss style machine. And that is only because if your stock is rattling around inside the bushing, you will get inconsistent results. On a 'regular' lathe, you can take a piece of beat to shit hot rolled and still make a "precision" part, they are not related.

edit: tgp = turned ground and polished