Rack Cutting

[Anorak Bob]

While I was away Michael G asked me if I had any additional information pertaining to the cutting of racks on a mill. Additional because Michael was pondering the hows a couple of years ago. http://www.woodworkforums.com/showthread.php?t=163644&p=1589223#post1589223

I had a look and found some information in a Cincinnati milling treatise that may or may not be relevant. A skim through Google Images resulted in the discovery of photos of a Brown and Sharpe rack indexing attachment that may provide MG with some inspiration - http://www.practicalmachinist.com/vb/antique-machinery-history/brown-sharpe-milling-attachment-239525/

Others may have information or rack cutting experience that they may be able to share. I have yet to cut a gear let alone a rack.

BT

[PDW]

I used my shaper. It was simpler than making a rack cutting attachment. Funnily enough one came up for sale on fleabay recently but I didn't bid on it - I think it was 50 taper and while convertible to 40 taper, too much hassle given I've cut 1 rack in 30+ years. PDW

[Michael G]

Thanks Bob. Interesting stuff - may still be filed in the too hard basket but currently sitting in the future projects file (why? - because I'd like to see if I can) From your earlier post I printed out the picture of the stack of gears and while in Scotland I tried working out the number of teeth. I got some but some I missed. Most interestingly though only by a few teeth and probably because I assumed that they would be similar gears to those for the D/H, so 48 instead of 49 and so on.

Michael

[steran50]

HI,
I was actually just wondering today how would you cut a Rack on a Milling Machine, now I know, Thanks Bob. I would think that one of those attachments wouldn't be cheap to buy.

[Michael G]

By coincidence there is one on US ebay at the moment -
http://www.ebay.com/itm/Brown-and-Sharpe-No-22-Rack-Gear-Cutting-Attachment-/251383573864?pt=LH_DefaultDomain_0&hash=item3a87a0ed68
Looks to run on a 50 taper so no good to me, although the weight is something too.

As the pictures will eventually disappear, here the main ones for posterity.
327172 327171 327170

Michael

[Pete F]

I've only ever had cut gears and haven't had a reason to cut a rack, but is there any reason the table isn't simply moved along by the appropriate distance according to the Circular pitch being cut? I have a DRO on my mill, but failing that I would set up a temporary equivalent so any errors weren't cumulative.

[Michael G]

No, that's how it's done.
The doosaflicky that Bob posted is the linear version of a dividing head so that the circular pitch movements are more exact and regular (similar to cutting a gear on a rotary table vs. dividing head - they can be done on the RT but requires a more meticulous effort to get it right). A DRO would make things far easier but this is old school pre DRO stuff - black magic to the CNC generation
The pictures that I posted for the cutting device are because on a normal horizontal mill the cutter is orientated in the wrong direction to cut a rack more than a few inches long (runs into the arbor support or the machine). Strictly speaking you could sharpen an end mill to 29 degrees and use it in a vertical head but that becomes a fraught exercise as the cutter is now long and thin.
I've used a few horizontal (disc type) cutters lately and they work beautifully for slits and other deep but narrow cuts so much prefer to do it this way.

Michael