Backlash Question

OK, I’m about six months into the “Flying by the Seat of my Pants”
design of my small router / PBC drill mill.

Motors are Vexta PH268M-E1.5 (NEMA 23)
Driver is HobbyCNC 4 Axis
Power is 24VAC 10 Amp trans + rectifier & caps ~ 30VDC
Software is TurboCNC 4.x
All this seems to work as I haven’t “Smoked" Anything.

I have developed processes which allowed me to make linear slides and
leadnuts from polyurethane casting resin which have no (perceivable by me) play or backlash.

I’m Prototyping / Proofing the frame –
(3/4 MDF, Max expected size 20”x – 20” y – 12” z, with a 12" sq table on the X Axis) Using 3/8” x 16 TPI Threaded Rod and ½” round slide rails

In working on the frame it looks like I’ve come up with way to mount the
leadscrews with no backlash.

NOW the question occurs to me. . .
Should I adjust things so that I have just the SLIGHTEST bit of backlash?

Is a SMALL bit of backlash a GOOD thing to have?

With no backlash on the leadnut or leadscrew / frame will the machine be too
TIGHT? (The leadscrew DOES turn freely by finger force in the frame, no binding)

With no backlash am I increasing the chance of snapping bits if the machine
were to “Kick Back” for some reason?

(Am I obsessing ?)

Too much time “Thinking” not enough time “Doing “

Quote:

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Originally Posted by scratch_6057

With no backlash am I increasing the chance of snapping bits if the machine
were to “Kick Back” for some reason?

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It can't kick back unless it has backlash. The only reason you'd want any backlash at all would be to prevent binding. The less the better, with none being best.

Gerry is absolutely right, no such thing as good backlash.

Mike, how did you go about measuring your backlash? You might want to try a dial indicator, maybe even a tenths indicator. There is almost always a little bit lurking there somewhere.

Best,

BW

Quote:

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Originally Posted by BobWarfield

Gerry is absolutely right, no such thing as good backlash.

Mike, how did you go about measuring your backlash? You might want to try a dial indicator, maybe even a tenths indicator. There is almost always a little bit lurking there somewhere.

Best,

BW

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Perhaps no backlash is a bit of Exaggeration / overstatement on my part,
I should have emphasized the fact that I could not FEEL or PERCEIVE any
backlash.

I’m certain that there is some backlash in things as they are setup now,
and more will creep in later.


I don’t have the access to the tools your suggesting, as I’m no machinist by
any stretch of anyone’s imagination. I’m a computer / network support person
and in a former life I was an Aircraft Structural Mechanic, (before things got
to the point where you need a 150,000 dollar piece of equipment to diagnose
a problem on a 20,000 dollar car I maintained my own cars) have some Tech
School Training in electronics and an interest in robotics. So I guess I’m just
another one of those dorky, neardy, dirt under the fingernail, gear-head
types.

I found some Nylon spacers at the hardware store . . .
Nylon Spacer 0.995 – 0.997 OD
0.376 Thick
0.328 ID

And tapped them for the 3/8 x 16 TPI threaded rod ( 3/8 x 16 OD = 0.364 )
Being nylon they gave a little while being tapped and contracted back when
the tap was removed which leaves a thread that grips the threaded rod.

Then, in the ¾ MDF frame material I used a “1 Inch” Fostner Bit (actually
0.999 OD) in my drill press to cut “bearing pockets” for the spacers and
before removing the MDF from the clamp, I coated the walls and bottom of
the pocket with “ZAP-A-GAP” a gap filling CA glue, (crazy glue type stuff)
letting it soak into the MDF, and pored a curing accelerator on the glue.
This formed a plastic type of coating on the inside of the “bearing pocket”
firmly bonded to the fibers of the MDF. Then once more just to clean up the
inside of the pocket, I ran the fostner bit in just barely touching the bottom
of the pocket.

SIDENOTE : If you try this DON’T try to take a shortcut like I did and think
that you can apply the CA to the pocket then use the spacer mounted on a
piece of the threaded rod chucked into the drill press to “FORM” or distribute
the glue into the pocket! ! ! It won’t work 8-( ! ! ! Don’t ask how I know this.

NOW without pictures, this might be hard to do but I’ll try, imagine everything
is lined up on the 3/8 leadscrew and you’re looking at a Z (vertical) axis. Lets
see if I can work out a verbal picture.

Two 3/8 nuts “jam locked” alongside one another

Outside of ¾ MDF Frame Member
Inside of MDF - Pocket - Inside MDF

Nylon Spacer (Nylon Spacer fits inside Pocket)

Two 3/8 nuts “jam locked” alongside one another

Lead Nut

Two 3/8 nuts “jam locked” alongside one another

Nylon Spacer (Nylon Spacer fits inside Pocket)

Inside of MDF - Pocket - Inside MDF
Outside of ¾ MDF Frame Member

Two 3/8 nuts “jam locked” alongside one another

The 3/8 “jam locked” nuts serve only as a retaining mechanism, they do not
force the spacers to bind inside the pocket.

As far as the Leadnut and linear slides are concerned they are cast from a
thermoset polyurethane compound (Alumilite “RC-3”) which has a viscosity of
110 ( ~10W30 Motor Oil at 90 deg F) and sets up to a Shore D hardness of
74.

As I’ve about run out of time (story of my life) I’ll post more on the Alumilite
castings later. Now that I have proven (to myself at least) that I’m heading
in the right direction I guess I should start a build log, look for it coming soon,
after I clean up my shop and charge the battery on the camera.