Re: [CAD_CAM_EDM_DRO] Beginners screws ( was ACME ( was Re: leadscrew,

On Tue, 20 May 2003 16:45:52 -0400, you wrote:

>OK,
>just uploaded the file of DIY linear guide.
>It is done for somebody with limited machining capabilities, but with
>burning desire to learn CNC, like myself.
>Alex

Couple of comments, if you would.

1) that angle is not precision. None of the stuff is, unless it's
specifically made that way. I had some 4 inch aluminum angle that was
off a good 1/8 inch at the end of the 4 inches... So your bearings
might not ride squarely. Square tubing might, and I do say *might*
be more square. It's also more rigid, I suspect.

2) On the bottom (l/r) rail, the forces will twist the angle
counterclockwise, which might be something you want to look out for.

3) The inside of an angle is not a controlled dimension (and seeing
what I found out with the 4 inch stuff, sometimes the outside is not
controlled either....) It is neither necessarily smooth nor even
parallel to the side, and that's not considering the fillet at the
angle.

There's a simpler method I've seen that was used for a sliding table
for wood saws. You do need to make an accurate 45 degree angle.

You take two square tubes, put them on a 45 degree angle, and these
become your rails. You take an aluminum angle, and mount it on the
bottom of the table, inside the rails. On this angle, you take
another piece of aluminum angle (or steel, if you want) and mount it
so that the edges are 45 degrees to the vertical. The angle is
rotated to match the sides of the rails, and there's perhaps an eighth
of an inch gap between the angle and the rails on each side.

From those edges, you put the bearings in, so that they will ride on
the surfaces (one top, 45 degree angle, but flat against the rail,
ditto bottom).

The adjustments might be easier to do, and it seems mechanically
simpler.

I've tried the same kind of idea on a circular rod, but the bearings
do not ride well, too much pressure on one contact point.

Harvey

>
>
>----- Original Message -----
>From: CL <datac@...>
>To: <CAD_CAM_EDM_DRO@yahoogroups.com>
>Sent: Monday, May 19, 2003 10:47 PM
>Subject: Re: [CAD_CAM_EDM_DRO] Beginners screws ( was ACME ( was Re:
>leadscrew,
>
>
>>
>>
>> alex wrote:
>>
>> >Thank you, Ross - I'm more or less sure now what to do.
>> >Will place sprocket directly on the shaft of stepper, then pull carriage
>in
>> >a loop by the chain.
>> >
>> You might want to slide that sprocket in a little further so you can at
>> least get a smidget of support on the end of that motor shaft with an
>> outbound bearing....
>>
>> It still is not enough, but that would be better than nothing.
>> Most steppers are not made to take much of any side load on their
>bearings.
>>
>> Ultimately, you need to put the sprocket between two bearings on a shaft
>> that is driven direct by the stepper with a Helical coupler.
>> Or, mount the motor to a short section of square aluminum tubing with a
>> bearing on each side. One bearing can be right on the motor shaft, the
>> other on an extension with the Sprocket.
>>
>> That obviously takes more work.
>>
>> Then again, if motors are cheap enough, just replace the motor when it
>> goes "south".
>>
>> Chris L
>>