Re: [CAD_CAM_EDM_DRO] Off topic? Machining some inserts

On 02/10/2012 11:44 AM, Jack wrote:

> I am working on a project and ran into a stumbling block. I was trying to machine some parts on my junky 7X10 lathe but can't hold a reasonable tollerance in terms of concentricity. The part can be made out of any metal, brass or aluminum would be fine. (I was trying aluminum). The basic dimensions are 1/4" diameter by 1/4" long with a 2-56 threaded hole through. They might be a good candidate for a simple beginners CNC lathe project. I need 500 for the project. If made of brass they could be drilled #53 and I could take care of the threading.
>
> If you would be interested in making these parts for me please contact me via email and we can discuss.
>
> Thanks,
>
> jack

Jack:

Consider that a 3MT drawbar tool holder can hold a machinable block as
closely as your spindle can, as long as it is not removed and replaced.
Get a 3MT milling holder and a piece of drill rod, W1 is cheap and will
work well. I am using 1/4" for this example, but any size should work.

See http://www.speedymetals.com/p-3643-14-rd-w-1-drill-rod.aspx for one
vendor, if there is an industrial supplier locally, you can save the
shipping (which is EXPENSIVE!) I often use Fastenal (see
http://www.fastenal.com/web/home.ex) for stuff like this, their part
number for W1 is0987908
<http://www.fastenal.com/web/products/detail.ex?sku=0987908> and they
have oil hardening in stock when the water hardening is not available,
see 47766 <http://www.fastenal.com/web/products/detail.ex?sku=47766>.
Only take what is in stock, shipping can double the cost!

Grind, file, or mill (if you can), a "Weldon flat" on the rod and lock
the rod in place. Mount the holder with a drawbar (all thread, flat
washers and a wing nut will work.) Face the rod and /*_do not remove
the tool holder or the rod_*/. Centerdrill the drill rod and drill
through the drill rod for a #2-56 press fit hole, or drill and tap, but
remember, #2 taps in tool steel are tricky, feel free to go below 50%
thread so you don't break the tap. Countersink the hole to at least
1/8" so burrs on the parts will not cause problems. Mount a 2-56 stud
in the hole and secure it with Loctite red, /*_do not remove the tool
__holder_*_*or the rod*_/. After the Loctite is dry (24 hours), measure
and make sure it hasn't shifted too badly. It will probably be okay,
but the mandrel either makes or breaks _/*all*/_ the parts. If you
don't have a #2-56 stud, get a longish #2-56 hard screw (Allen or grade
8 or better) and cut it off with a Dremel abrasive disk after mounting.
It is often wise to turn a guide of root diameter a short distance from
the end to ensure you don't cross thread the mandrel, 1/8" is plenty for
#2-56 screws. In ideal conditions it will look like a dog point set screw.

Now, make your parts oversized and drill and tap the center. Thread
onto the mandrel with a flat washer (to protect the precision face) and
face one end, chamfer both the OD and ID, remove and reverse the part,
removing the flat washer. Face the other end to length, turn to the
correct OD, chamfer both the OD and ID.

Assuming the mandrel is good, this should be at least as concentric as
the mandrel.

You may find that your spindle bearing preload is out of spec. Reset
it. If the bearings are shot, you can safely assume that replacing them
is a wise idea, replace the nylon gears at the same time, they tend to
fail regularly. I have had good luck with Little Machine Shop, see
http://www.littlemachineshop.com/products/product_view.php?ProductID=1223&category=
<http://www.littlemachineshop.com/products/product_view.php?ProductID=1223&category=>
and
http://www.littlemachineshop.com/products/product_search.php?critFast=bearing+spindle&B1=Product+Search
<http://www.littlemachineshop.com/products/product_search.php?critFast=bearing+spindle&B1=Product+Search>.

You can face all of one end, lock the carriage Z axis (along the bed),
and face all of the other end, lock the carriage's other axis, unlock
the Z axis, and turn the OD on all parts. My best guess is that the
parts will be within a few thousandths or less UNLESS you remove and
replace the toolholder, then all bets are off. Always assume that
either a chuck or a taper is not concentric, you will sometimes be
wrong, but not often enough to be really hurt by it.

Make one part first and measure it to death, you may find you have a bad
spindle or bad ways!

Dave 8{)

--
/"Political Correctness is a doctrine, fostered by a delusional,
illogical minority and rabidly promoted by an unscrupulous mainstream
media, which holds forth the proposition that it is entirely possible to
pick up a turd by the clean end."/
(quoted from http://www.wattflyer.com/forums/showthread.php?t=30060)

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