Replace spindle with pencil grinder?

There are several threads on changing the motor or spindle on the nomad to get increased spindle speeds.

What about adapting the head to hold an air powered die/pencil grinder? I’ve been looking around and there are a lot of options for something like this. Speeds up to 60K or more, rated at .1 HP

Seems like something like this with a front exhaust could work pretty well.

Possible downsides are

  • Cost of grinder and suitable air source. This is not an issue for me as I would love to have a pencil grinder around for other stuff.
  • Continuous use? cheap die grinders are probably not up to an hour or more of continuous use. Do high end grinders handle continuous duty?
  • Spindle speed would be manual control and not that constant? ie. drops in speed as load increases and there would be no feedback loop.

I’m new to CNC and will stick with the 10K max of the nomad for awhile. However, my interests are in the direction of micro-machining and I’m already starting to feel like I’l end up limited with only 10K RPM. So I’m thinking about possible future directions.

What are your thoughts?
Is there an obvious fault/problem with attempting to use a pencil die grinder?

-Dave

The biggest problem is the die grinders are notorious for runout.

That should have been my first “downsides” - I did think/wonder what the runout would be.

Cheap pencil grinders are going to be a “get-what-you-pay-for” kind of thing. After a little more research, what I’m really looking for is an air motor (don’t care if it comes in a pencil format or not).

Found a cool Ingersoll Rand air motor catalog -

IRITS-0409-044_Air Motor Catalog.pdf on this page:

they have motors specifically designed for milling applications, ie can handle the side loads. However on first scan the closest thing is a 20K rpm that they want you to run at about 12K for max power. So not a great improvement.

Also probably not cheap to get your hands on one.

I’m going to keep looking for other air motor options. Might luck out with repurposing some other tool. Or maybe find used dental handset or such.

Will also look for run out specs on mid-range pencil grinders.

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Imagine that, they make just the sort of thing I’m looking for:

I asked for a price, probably costs almost as much as the Nomad! (ha)

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Take the run-out specs with a grain of salt. You get, at best, what you pay for. Like any similar tool, the collet has a lot to do with it, and the spec for the spindle needs to be checked for HOW it is specified. I have one from a reputable tool house (not a brand name, but whatever they were giving the house label) that has specs of 0.010mm (0.0004") TIR for the spindle. This is true, if you consider the OUTSIDE of the spindle, as this is controlled by the bearings. The collet seat is out by roughly 10 times that, and isn’t quite parallel to the rotational axis, either. Throw a collet into the mix, and it barely useful as a handheld tool.

I have been through a number of pencil die grinders of varying quality over the years. The lifetime of even the better ones is not outstanding (on par with of lower than a standard die grinder at best… not a lot of room in there for the vanes) even with proper lube. Even with an exhaust tube, there will be lube mist to deal with and a little leakage at the bearings, which you must consider.

There are air spindles made for this type of service that will be a lot better choice by the time you get to the price of a pencil grinder with decent runout and life that doesn’t make an oily mess.

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I don’t have a Nomad but I have a cheap air grinder and I am amazed by the volume of compressed air it takes to run. In addition to the VERY noisy operation, the compressor continuously working for possibly hours on end. My wife complains of my shop compressor turning on from time to time, if I used a die grinder in my shop for long periods of time, I would find the compressor and the die grinder parked by the side of the road with a sign free on it. I don’t know that it would be much of an improvement.

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I’d be curious how crazy it would be to couple a Foredom or the like to the stock spindle.
Perhaps swap out the spindle bearings for higher grade/rpm rating and see how it works out.
More torque and speed using existing runout of stock spindle. Plus motor/heat/RFI is kept away from stock electronics. Not sure the duty cycle, but a lot of folks swear by them.
Would be a cool experiment anyhow

SR model is 1/6 HP to 18k rpm
TX model is 1/3 HP to 15k rpm

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Interesting idea…
I think the issue is that the stock spindle bearing aren’t good for much more than 10K. Thought I read that somewhere where someone was thinking of just putting a bigger motor and or changing the drive pulley.

You could probably do the flex shaft and the existing spindle would be fine as long as you don’t go to high with the final spindle speed.

I don’t know how well it would work to try to drive the belt at 10K and get the spindle up to whatever the existing ratio is (maybe 3 : 1).

Might be able to hit 30K - HP from the flex drive would be reduced by the belt ratio.

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I’m pretty sure @edwardrford mentioned at least considering a Foredom back in the early days of the project. He did use a Chinese Foredom clone: https://wiki.shapeoko.com/index.php/Spindle_Options

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It would be a cool and simple experiment.

If I didn’t have mine housed in an enclosure and had more time I’d give it a go.
As is I hardly find time to run my Nomad on a reg basis. :man_shrugging:

Ran a 60krpm pencil grinder on the shapeoko, runnout seemed fine but power was lacking and it did indeed take a HUGE amount of air. You’d also want an inline oiler to ensure bearing life and that might get messy