Viability of using a flex shaft on a CNC router

[Thermosman]

I was wondering if it's viable to use a flexible shaft on a CNC router to transfer the rotary motion from a stationary spindle motor to the head of the router. There are some flex shafts on mcmaster carr which have ball bearings and are rated for high load, but I am unsure if these would be strong enough for the large radial force applied to the end of the shaft when milling. Additionally, would they be able to transfer enough torque. Also, is there a good way to interface the end of the flex shaft with the collet on the spindle motor, which would likely be a er20 collet chuck, and to put another collet chuck on the operational end of the shaft. Also, how much longer than the maximum distance between the head and motor position would their have to be to prevent the shaft from bending at the head too much?

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[wizard]

I suspect speed would be the issue here. As you know flex shafts transmitting considerable power are possible. However these are generally used in low speed grinding applications.

On top of that the life span of these shafts is not infinet and eventually break no matter what bend radius percsustions you take. Of course everything breaks over time but i can see CNC being especially hard on the shafts. Remember the shaft has to compensate for changes in vertical as well as lateral position and frankly im not sure success can be had without a spline drive. The only other option i know of would be an extremely long shaft that would form an upside down U.

I supose you could check with Foredom or similar manufactures for design assitance. They do have fractional HP drives. My guess is that they would decline to support such application.

[awerby]

It would be a poor idea. Flex-shafts make sense as a jeweler's hand tool, where the torque requirements and duty cycles are low and frequent maintenance is expected. But if you want to run ER-20 tooling, which goes up to 1/2" diameter (requiring lots of torque) and have it last through long duty cycles without tearing it down and re-lubing a lot, then no - it's far from optimal. The reason these are used at all is to avoid having to carry the weight of a heavy motor, but this is less of a problem for an automated machine than for a person. And even jewelers, when they want very high speeds, forego flex-shaft tools for micro-motors that are hand-held, since flex-shafts can't handle the RPMs.

Even routers that were designed for intermittent hand use would be much better than flex-shafts for this application. And a real 3-phase spindle run from a VFD inverter would be a lot better than that.

[elfrench]

An overhead arm driving the spindle with a round belt would be a better solution.

[wizard]

An overhead arm driving the spindle with a round belt would be a better solution.

There are likely other ways to do so mechanically, in fact I was just imagining a mechanical solution where the belt sits above the gantry driving a spline shaft on the spindle assembly / gantry saddle. Mechanically it isn't anything that hsn't been done before but I still have to ask my self why? These days we have all sorts of motor technologies to choose from, finding a suitable electrical motor to mount on the Z-Axis sholdn't be a problem for any design. If not electrical maybe an air turbine spindle.

I'm just wondering if there are design considerations we have not seen that may be a factor in the question about flex shafts.

[jgro]

Carvewright uses a flex shaft. So, it can be done. CarveWright.com - 3D Carving CNC Router Systems

jgro