milling flat surfaces

[billyboy]

i have heard that after surfacing/face milling a piece of cast iron/steel that it should be laid to "weather" or season for a while, i have an idea this is for any distortions in the steel to settle, if this is so then how long is a while? and does this distortion effect all metals?

bill

[toolmakerjim]

i have heard anedocatoal statements about this and freeezing/thawing etc.
my cousin Bill Large the muzzleloading barrel maker used to bury material in his garden to season, that supposedly was in the '50s. where i worked with him it was straight from the rack to the saw, no seasoning. the difference was probably due to the advances in metalurgy since he used killed LaSalle Stressproof with Tellurium.
all barrels required straightening after machining, not during in process as some people have suggested.
my experience has been that if you want something flat irregardless of material set it up using double back 3-M tape and machine equal amounts off both sides.
hope this helps some...jim

[Forrest Addy]

There's a lot of hokum about metals "seasoning" as though they were wood. Seasoning was actually effective in the harsh winters and hot summers of the Northeast. Iron castings were left in the yard for a couple of years and sure enough they were far more stable than raw castings still warm from the sand or that were cast and aged in milder climates.

It was the thermal cycling that did it. The micro-structure of the metal was influenced by changes in climatic temperature promoting changes that reduced the residual stress in the metal. These changes don't have to be wrought slowly over the seasons. They can be greatly accellerated by thermal cycling the material between furnace and freezer; thus two years of seasoning can be compressed into two days of scientific thermal cycling.

Nowadays custom knike makers routinely "cryo" treat to acheive the last refinement in enhanced edge properties to their fancy knives. Cryo treatment is necessity in high precision gage materials where millionths of an inch are involved. ANSI 52100 is a chrome steel alloy once (and maybe still - I've been out of touch with that part of industry) commonly used in the making of gages and gage blocks and some manufacturers thermal cycled their rough-machined heat-treated parts ten times before commencing finish work.

It doesn't matter if the metal rusts. All that matters is the metal sees in its pre-machining stabilizing treatment greater extremes of temperatures than is found in its working environment.

The machining process has an effect on part stability. The rubbing of the cutting edge over the work imparts a cold worked film of metal under compression. The spreading effect of these compressive stress if unopposed by similar machining on the opposite side frequently leads to distortions expecially in long clender work like rules, scales and straight edges. Different metals cold work in varying degrees and the nature of the tool (carbide/HSS; positive/negative rake; keen edge/honed edge; /dull...) has its effect on distortion as well. For this reason HSS is preferred where low residual machining stresses are desirable.

When seeking the ultimate refinement of stability and accuracy the whole spectrum of concept, materials, designed, and processing has to be carefully considered in advance before committing to hardware.

[This message has been edited by Forrest Addy (edited 01-31-2005).]

[rsr911]

being a car buff and former drag racer I know all about buying bodies from the south and motors from the north. It might be hokus pokus but my northern '69 Windsor block repeatly survived 10 second blasts down the 1/4 with 600+ NA HP on tap where many newer block failed. Forrest has it right, it's all in the seasoning LOL.

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-Christian D. Sokolowski

[Norman Atkinson]

Hi Billy Boy!

I machined the headstock casting for a Westbury- not Dore Westbury. By the next week, the split bore had closed sufficiently to cause trouble. I did the same with the Quorn t&c base castings- and they closed sufficiently for PGMS bed bar to need reaming.

I'm an amateur. The much respected George Thomas changed his Mk1 Pillar drill castings for the same reason. The write up is in Model Engineer and later in his book.
George refers to the closing up of his bores after slitting by anything up to 2 1/2 thous. One he quotes made a loud screaming noise! So did I!

In the olden days, castings were put out to weather. Then they were brought in- and rough machined- and then put out to weather again prior to final machining.

Some steels will do exactly the same trick.
You could, as my father did, normalise the castings in the blacksmiths fire.

Norman

[SGW]

Maching a surface will machine stresses into the surface skin. Leaving the material alone for a while afterwards may or may not help. To really fix it you need to do a thermal cycle, as others have said.

[Wayne Sorce]

I'm interested in learning more about stress/movement in various materials after machining I would like to use delrin ,aluminum or even a clear plastic.The device would be 3 to 4 inches sq.X .500 thick.It would need to have as little movement as possible after machining and at the same time be of a fairly light rigid material.It would not be stressed by any attachments but stand by itself.It would possibly be used in all weather conditions. ie:extreme cold and heat. Thanks in advance, Wayne