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Posted by on April 20, 2008, 8:22 pm
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> Someone moaned about the amount of politics and calendars:-
>
> I may have let slip (to the point of monotony) that I have been rebuilding=
an
> Ebay basket case Hardinge HLV for the last 18 months.
>
> At the start of March I started to work on improving the removable, harden=
ed
> dovetail way bed. Having removed the bed, I measured its thickness in a nu=
mber
> of places. The micrometer showed that there was at least 3.7 thou of wear.=
At
> least, because the micrometer doesn't measure to the bottom of the roughne=
ss.
>
> I could have sent the bed away to be reground. Trouble is that I'm cheap.
> That's why I didn't just wait for a lathe to turn up in better condition t=
o
> start with...
>
> I reasoned that I might be able to grind the bed on my =A375 J&S 1400 grin=
der.
>
> Because the bed is38" long and the table travel on the grinder is only 24"=
,
> the bed had to be ground in two sections. I got a bit of a visible furrow =
at
> the point where the table stopped at the end of the cut in the middle of t=
he
> bed, but even after the roughing cuts, the bed thickness is now even to wi=
thin
> a couple of tenths outside of that narrow furrow, which the carriage will
> ignore. I had to remove 6 thou to get rid of all of the wear marks.
>
> Ok. That did for the thickness of the bed. Now I can think about the
> dovetails. The dovetails have a 1/8" wide, unmachined, vertical section at=
the
> top. I assume that I can measure from that to the dovetail on the opposite=
> side to get an indication of the wear.
>
> While thinking of that, it was assumed that the way to grind the dovetails=
> would be to make a sine table. The old 6"x12" Humphrey's electromagnetic c=
huck
> that I inherited off father could be pressed into use for that. A 15" long=
bit
> of 1 1/4" square bar was milled into angle iron on the Myford and hinge pi=
ns
> turned on the ends. A couple of lengths of 2" square were milled and groun=
d
> into L shapes for the sides. Holes could be bored in these for the hinge p=
ins
> before final grinding to get the height of the side at the exact level of =
the
> bottom of the hinge pin. Something like this:-
>
> =A0 |-----------------------------------|
> =A0 | =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =
|
> =A0 | =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 |---| |
> |-------|_______________________|___|_|
> | =A0/- =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 |/-|
> | =A0_/ =A0|_________________________/___
> | =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =
=A0|
> |______________________________________|
>
> Boring the holes means that the boring head that I got some years ago need=
ed
> an adapter to fit the Myford headstock mandrel. Did that, got a set of tap=
s
> from RDG to finish the internal threads, having had problems with getting
> these right before.
>
> In the meantime, the junction box on the mag chuck needs sorting.The screw=
s
> holding it to the chuck have waled out heads and one of the screws holding=
the
> cover on was sheared off before I was born by the look of it. No problem. =
Bit
> of 3/8" silver steel for the cap head screws and 3/8" brass for the
> countersunk screws. drill out the busted screw and run a 5/32"BSW tap down=
all
> the holes to clean them up..
>
> A jig had to be made to hold a tenths dial gauge at a variable, fixed heig=
ht
> and slide along the bed, measuring the variations in width. Just a bit of
> sawing, milling, grinding, drilling, tapping and Loctiting. Job done.
>
> The jig shows that the vertical bits at the top of the dovetails seem to b=
e
> parallel within =A0a tenth, but that the dovetails have a reasonably stead=
y 12
> thou slope compared to the vertical bit. Bugger. The jig needs modifying s=
o
> that it can measure one dovetail against the opposite dovetail. If I make =
a
> push tool to fit the boring head, I can use it with the Myford dividing he=
ad
> to make a ball turning tool and make some ball ended pins to go on the jig=
to
> bear against the dovetails. Haven't got any 1/2" round HSS. Never mind, ma=
ke a
> 3/8" to 1/2" adapter to hold the 3/8" HSS that I've got.
>
> To cut off the bit of round HSS to the right length, I can use one of the =
thin
> cut off disks I got for the grinder. Bugger, the wheel arbours won't grip
> something that thin. Never mind. Slice off a bit of 95mm bar and turn up a=
> spacer to fit the wheel arbour.
>
> Now, What was it that I was trying to do...
>
> Mark Rand
> RTFM
Mark Hi, I admire your determination and totally agree that "one thing
leads to another" and while interesting can lead to us "not clearing
the swamp of aligators" <VBG>
One question I must ask though is why do you need to measure the
dovetail to dovetail surface distance? I can't get past thinking the
important issue is to get them as parallel to the top and each other
as possible when re-grinding and as parallel to the bed axis as you
can. I can't remember if the HLV bed is located with dowel pins or not
so that a little adjustment of the axis would be possible when setting
up. Surely, for grinding you can set up from your new top surface and
the small vertical surfaces once you have mapped the wear on each
dovetail face from its' adjoining "unworn" vertical surface to ensure
that you grind suficient off to cover the double grind setup? If I
remember correctly (long time ago) the HLV has a two part bed under
the headstock so wouldn't the short piece under the far left (from
front) give you the unworn figures to work out the minimum grinding to
remove all of the wear. Alternatively couldn't you grind the headstock
part of the bed first to just clear all wear and that will surely be
enough to clear any lesser wear on the tailstock end.
Another comment would be why you need to measure the dovetail angle to
such accuracy? If the bed surfaces are worn then surely you will be
grinding the saddle as well to match. As long as the angles you grind
on the bed and saddle are exactly the same it shouldn't really matter
if it is 59 or 61 degrees?? Couldn't you use a simple 60 deg (ish)
fixture for both operations?
Mark, I'm not looking to start any deep discussion just looking to
understand what it is that I've obviously missed when thinking about
the geometry. I have a very old flat top dovetail bed lathe that I
would like to restore in the future and would like to learn from your
experience before I decide to start and mess it up.
Best regards
Keith
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>
> I may have let slip (to the point of monotony) that I have been rebuilding=