CNC Owners

I like 2 separate fixtures. I often have a cue (or anything) in the 4th axis fixture gluing up while I'm turning stock with the spinner. Things are simpler if you aren't using a program to spin the stock (or disengaging/swapping belts kind of thing), only controlling the cutter. My 4th axis is a positioning table. It is very accurate, but it can't spin fast enough for turning. That may be a consideration, depending on the equipment you source. If there is ever a problem or you want to tweak one fixture, you can keep working with the other if you have a little down time.

The disadvantage of 2 separate fixtures is you need more footprint, have to set up two different fixtures, and have some extra cost. IMO, Sherline equipment for the spinner (about $700) is a great option.

If you are going to turn on a CNC, keep in mind diameter error will be double the positional error of the machine. When I final turn a shaft or a butt, I'm typically within .002 at both ends. Minimal sanding. If a machine has .005 positional tolerance, a tip or joint diameter might be .010" off.

I'm happy with .001 to perhaps .002 which as Kelly pointed out is actually twice as accurate. The router machine I just built achieves that consistently. Just have to watch it closely when cutting down to final size on things. It's tricky putting that wood back on.

You will end up paying exponentially for the last bit of accuracy which isn't necessary dealing with wood IMO.


JC

cnyncrvr said:

Hi Gents. I wanted to ask what you all consider an acceptable tolerance for your CNC machines to hold. I ask because I am currently considering designing and building a CNC router specifically for cue work. My problem is that I tend to do every thing OVERKILL so much so that its to the point of almost being comical most of the time. I know that the tighter a machine needs to be the more expensive the materials used in its construction are and I really would like to avoid spending the money trying to build a machine that can hold tolerances with in tenths of a thousandth if holding tolerance with in say one thousandth or two thousands would be considered acceptable by the lions share.

Also on a side note, do you think it would be better to have have a 4th axis rotary indexer for stuff like points, windows, inlay work and a separate spinning station for basic turning and tapering or would having a 4th rotary indexer be enough to accomplish both tasks?

Click to expand...

Precision/repeatability is far more important in a CNC than accuracy. Precision is like hitting the edge of a bullseye with an arrow at the same exact spot every single time. Accuracy is hitting inside the bullseye, but at different spots every time. So this means removing as much backlash and flex as possible. This would mean C5 precision ground ballscrews if you go overkill. Steppers OK but servos are better, but more expensive, and probably not necessary for cutting wood. The wood will move more from the shop climate than the tolerances of the machine. The Chinese ballscrews are C7 but still .002"/ft positional accuracy, and if you order with double nut, repeatability is be very good.

To build a rotary axis that can both do indexing/positioning AND standard turning would not be easy as gear reduction is needed for enough resolution, which slows down the rotational speed. You'll need a servo and a reduction that can handle the servo speeds.

As long as you are designing this machine I’ll throw this out there:

Early CNC lathes with indexing capabilities used an electric clutch that engaged
a servo motor for indexing functions. It would disengage for normal turning and a
normal electric motor did this work. The clutch was similar to the clutch on your cars
air conditioning pump.

Just an idea

I am not a fan of having one machine for turning and inlay.

I want an inlay machine to be as accurate and repeatable as I can afford. With a recut fixture on the rotary. Don't want to wear it out tapering.

I want a taper machine with a dead on drive center and cheap to replace parts. I put a lot of wear on my taper machines.

Larry

Terry C said:

As long as you are designing this machine I’ll throw this out there:

Early CNC lathes with indexing capabilities used an electric clutch that engaged
a servo motor for indexing functions. It would disengage for normal turning and a
normal electric motor did this work. The clutch was similar to the clutch on your cars
air conditioning pump.

Just an idea

Click to expand...

OR you can get one of these, but COMPLETE OVERKILL:

http://www.theinturn.com/

Directly answering your question ... I would expect no less than .001 repeatably on all 3 axis.