On 2010-07-13, Ignoramus3537 wrote:
Making of encoder mounting plates requires several accurately drilled
holes. I already made one plate and it seems to work, but I had some
problems/issues with placing holes precisely.
The material is brass (it was a great idea to use brass).
I calulated all hole positions using a spreadsheet. See "Encoder
Mounts" sheet he
http://spreadsheets.google.com/pub?k...en&output=html
To fit it over a 17mm motor shaft, I had to drill a 43/64
hole. (17.065mm).
I do not know of any way how I could drill that hole precisely, with a
huge (comparatively) MT3 drill bit, as any drill bit would wander away
at least somewhat, I think.
So what I did was, I drilled the 17mm hole aproximately where it
should be. Then, with DRO, I located the center of the hole and then
moved to what should be a point (0,0), based on the calculated
position of the center of the hole. (kind of a backwards thinking
process).
With a mill, I would more likely have used a 2-flute center
cutting end mill and plunged through somewhat undersized (e.g. 5/8")
then used a boring head to take it out to the desired diameter.
But does the hole actually have to be that precise a fit? All
it needs to do is clear the shaft, I thought. Bore it a little
oversized, and turn something to be a sliding fit over the shaft and
with the right OD to center the encoder properly. You were talking
about getting a centering bushing from the encoder vendors, but why not
*make* one?
However -- if you want a drill which does not walk the way a
normal chisel-point drill bit does -- get one which has a split point --
which does a much better job following a staring hole made with a center
punch of appropriate size or a center drill.
For smaller holes, all I did was start them with a center drill and
then drill with a drill bit.
Again -- go for split point drills -- less wandering.
It actually seems to have worked, as the encoder works just fine.
When making that mounting plate, I realized that there is a lot to
precision drilling, and want to ask now if anyone knows tricks for
drilling precise holes. I have two more plates to make.
The required precision for locating encoder base is 0.01". (which is
not that bad).
Especially if you make your own centering bushing using the
lathe.
On a related note: to bolt the base to the motor, I had to drill four
holes for the mounting bolts. How can I precisely measure the distance
between holes. I tried using a caliper and it worked, obviously, but I
can not be totally sure how accurate I was. I would measure the
distance between two points in holes closest to one another, then
farthest, and average the two.
How precisely did you take the measurements? Assuming a digital
caliper and clean untapped holes all the same size, take the spurs on the
back of the caliper, place them in a single hole, and zero the caliper.
Then move one spur to the next hole, and expand so you are measuring
between two points as far apart as possible (the calipers will naturally
seek this position). This will directly read the center distance quite
accurately. Yes, the zero setting of the diameter of a single hole will
be a little undersized, because the spurs don't have a fully sharp edge,
so the chord takes up a bit -- but you have the same thing happening
when measuring between two holes so the errors cancel.
But it felt that there was a lot of
wiggle room in those measurements. SEM has a manual for the motors in
question and it specifies the distance, but based on what I drilled,
the distance is slightly wrong. (the motors were made 20 years ago).,
I was lucky that I drilled the holes slightly oversize. For the next
pair of plates, I would really like to drill to-size holes in the
right place.
Slightly oversized, with a centering bushing (again, made on
your lathe) allows you to get the encoder properly centered.
Good Luck,
DoN.
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