View Full Version : Jig grinder for knee mill

12-03-2005, 09:29 PM
Hey everyone,

I posted a while back about buying a knee mill and adding a seperate z stage to which I could add a high speed spindle so I wouldn't be limited by the slow spindle speed of a typical milling machine. My goal is to grind ceramics, and this will require >15,000 RPM. Building a seperate stage will take time and I'd like to find an easier alternative in the mean time.

I was wondering what the consensus is regarding jig grinders attachments for knee mills. I read elsewhere that they tend to break down fairly quickly (bearing problems?). I am not doing production-line work, so I guess the longevity factor is relative, I just don't want something that is going to break down right away.

I'm also curious about runout in these types of units as I'd like to engrave on glass and I unsderstand that runout can be an issue with engraving glass.

I'm looking at the following auction, though items like this seem to appear on ebay on a regular basis...


I will use diamond burs. Coolant will also be used.

Any thoughts?


12-03-2005, 10:28 PM
You wouldn't need to build a seperate z-axis to use this item. There is a 3/4" shank on the back side that would fit into a collet in your mill head. You run the grinder with air and leave the spindle motor off. I would do a little more research into the grinder as how much air does it use (scfm) and do you have an air compressor that can keep up with it.

As for runout, you would need to contact the manufacture.

12-03-2005, 10:47 PM

I realize this item works in-line with the mill's spindle and won't require a seperate z stage... that's why I'm considering it.

Air supply shouldn't be a problem for me as I work at a university. The compressor is some behemoth tucked away somewhere in the building and supplies air to many different areas.

As for the air consumption vs. rpm output, they list a range:

43 lbs/in................53,500

57 lbs/in................59,000

71 lbs/in................64,000

85 lbs/in................70,000

I assume I can run it even slower with less airflow, but I imagine the torque rating will suffer.

12-04-2005, 12:33 AM
Being at a university that has a large compressor won't be a problem for you. I did see those figures and they compare air pressure in PSI to RPM but they make no mention of CFM used to maintain that pressure/RPM. You can run the grinder with less pressure but like you said torque will fall off rapidly. I did some checking and found the same model of grinder used for $795.00. If you can get this one for less than that, go for it.

12-04-2005, 02:55 AM
Thanks for the tip!