Saturday, February 7, 2009

Tapping ACME nut.


The short story is: I re-tapped the acetal nut for the X axis to an ACME thread.

The longer story involves much pain. :-)

To re-tap the nut to ACME thread involved making an ACME tap. (off-the-shelf ACME taps cost ~ $400 which is a bit much for a one-off). Making an ACME tap seemed very simple: Just cut a piece of thread rod, grind a taper onto it, and then grind an edge. No worries, right?

So being a doofus, I actually did grind the taper, but in a way that involves destroying my finger tips along the way. I made a simple jig with bearings that allow me to rotate the rod against the grinder. The idea was to let the rod spin against the grinder, ensuring that it would be ground evenly and giving an accurate taper.

In practise, the rod spun much too fast, and in reaching out to slow the spinning rod, I found out that rolled ACME thread has fine burs along the thread edges. Some of them relatively large. As I can measure by the many fine cuts in my finger tips....

I started grinding an edge on the rod as well, but I really need a diamond cup grinder to do the postive rake. After mulling it over, I slapped myself upside the head, and just milled the edge onto it. The ACME rod I have isn't ground, nor hardened, so milling it is fine. I just used a carbide boring tool to cut the postive rake.

Once I'd actually managed to wake to using the mill, it all went fairly fast; Just cut the edge, polish it up a bit with the diamond lap, then bung it on the lathe and it's done.

Well, almost. The amount of plastic being removed is fairly large and the taper on the acme isn't as shallow as it should be, so machine tapping it mostly out. I used lathe power to get the tap started, but then using a spanner to turn the nut by hand to cut the thread.

The end result is fairly good. It's a relatively loose fit, so I'll need to add an anti-backlash nut at some point, but the tap itself worked well.

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