Newsgroups: rec.crafts.metalworking
Subject: Re: Finish on precision ground stock
From: jmorton@euler.Berkeley.EDU (John Morton)
Date: 27 Sep 1994 17:29:33 GMT

In article <pwwUguy.hayesp@delphi.com>, PAUL  <hayesp@delphi.com> wrote:
>John Morton <jmorton@euler.Berkeley.EDU> writes:
> 
>>The Starret ground stock I have always found to be quite good.  They offer
>>+/-.001" on the thickness.  I'm not sure about straightness, but I have never
>>seen a piece that was visibly not straight, nor edges which were badly out
>>of square.  Starrett specifies 35 um. finish, I believe.
> 
>Let me elaborate...The steel companies get oversized low carbon CRS and slap
>each of the 4 sides on a magnetic chuck grinder and grind it to dimension. The
>squareness of this material (1 se to the next) is never better than the
>squareness of the original material was. I can agree that Starrett does about
>the finest job you can find. It was my intention to point out that this shou
>ld be taken into account for the particular purpose you have in mind. as for
>flatness, visibly straight is a term that carpenters use!

What, so I'm a carpenter?  :-)  I'd like to point out that you can easily
see deviations from straightness of less than .001" by holding the piece
against a surface plate or a straightedge.

You are correct in saying that the purpose will guide your selection of
stock, and one should certainly not buy ground stock expecting to use it
as parallels, for example.  In the shops where I've worked, Starrett ground
stock was just a good way to get small amounts of tool steel.  There are
enough good surfaces to put you well on your way to the finished part.
However all these jobs went into the furnace, where you lose whatever
flatness you were fortunate enough to get.  The finish grinder is what
makes the part.  (I had overlooked the fact that mild steel ground stock
is available - I've never used it.)

BTW I can suggest a wonderful project for any shop with a surface grinder:
a set of 1/16" thick parallels in every 1/16" of width from about 3/8"
to the height of your milling vise jaws (~ 1 3/4").  I use mine constantly
when using the mill, and wonder how I got on without them.  The fun part
was quenching them.  It is very difficult to avoid distortion in a 
slender piece, so I wound up quenching them between ground steel blocks
the size of bricks.  I made a rack in the furnace to hold all the pieces
in such a way that I could grab one with tongs and slap it onto a brick; 
then my accomplice would slap another brick on top of it.

This quenching method is very effective - you could touch the parts within
a few seconds, and they were fully hard when tested.

John Morton					University of California
jmorton@euler.berkeley.edu			Mechanical Engineering
{decvax,cbosgd}!ucbvax!euler!jmorton		Machine Shop

Newsgroups: rec.crafts.metalworking
Subject: Re: Finish on precision ground stock
From: jmorton@euler.Berkeley.EDU (John Morton)
Date: 29 Sep 1994 17:28:16 GMT

In article <244@landau.win.net>, Joe Landau <jrl@landau.win.net> wrote:
> 
>In article <369kpt$dca@agate.berkeley.edu>, John Morton (jmorton@euler.Berkeley.EDU) writes:
>>In article <pwwUguy.hayesp@delphi.com>, PAUL  <hayesp@delphi.com> wrote:
>>>John Morton <jmorton@euler.Berkeley.EDU> writes:
>
>>BTW I can suggest a wonderful project for any shop with a surface grinder:
>>a set of 1/16" thick parallels in every 1/16" of width from about 3/8"
>>to the height of your milling vise jaws (~ 1 3/4").  I use mine constantly
>>
>Is it feasible for a rank amateur without a surface grinder to make a set of
>parallels?   

My approach to this would be to start by evaluating what you are able to
produce with whatever you've got.  What this means is measuring between
surfaces with your best measuring tool.  It is impractical to try for
better parallism than you are able to measure.

If you have an accurate milling machine in good condition you may be able 
to produce pretty good parallels.  Use indicators to monitor the movement
of the knee, and measure both ends of everything.  A worn mill tends to
send the table over a hump as it travels, which would cause your part
to be thinner on the ends.

You also cannot get a parallel part without either a parallel work surface
(i.e. the bed of the vise, or better, the mill table itself, working
against an angle plate) or shims down to .0005" to make corrections.

The big drawback to these parallels is that they will not be hard.
And if you harden them they will no longer be straight, and too hard
to cut accurately except with a grinder.  Oh well.  You can get pretty
cheap parallels (which are only pretty good).  I suggested the above
project because I have never seen a set like that for sale.

John Morton					University of California
jmorton@euler.berkeley.edu			Mechanical Engineering
{decvax,cbosgd}!ucbvax!euler!jmorton		Machine Shop