Custom (25°) Threading Tool for a CNC Lathe?

[cwilliamrose]

Count me clueless when it comes to CNC tooling.

I have need to turn, or more accurately, have turned a 25° thread in some tiny brass parts. The CNC shop I have found that is interested in this job can't find a source for a threading tool with this oddball thread angle. My question is -- how do you do this custom profile when using a CNC machine? With my manual machines I would just grind a tool bit as best I could, set it up as best I could and go for it. Since I need hundreds of these, doing them manually is not a good option for either my time, my back, or my sanity.

This is what the part looks like;




The part is 3/8" long. It is a miniature brass insert for use in wood. The internal thread will be either #4-40 or #6-32. The body of the insert is smaller than anything I have found that is commercially available. The external thread is a copy of what is used in the larger inserts that are commercially available.

Any ideas?

 

[mmcmdl]

CNC's would use a form tool . Can't believe a shop would even look for a commercially available insert . Have you looked into rivnuts or nutserts ? Not sure Keenserts are sold that small but that would be another option .

 

[RJSakowski]

A CNC lathe shouldn't need a form tool to cut the thread. As long as the angle of the tool is less than the flank angle, the CNC lathe could follow the thread flank. A grooving tool with a width narrower that the root of the thread could make that thread.

 

[mmcmdl]

A CNC lathe shouldn't need a form tool to cut the thread. As long as the angle of the tool is less than the flank angle, the CNC lathe could follow the thread flank. A grooving tool with a width narrower that the root of the thread could make that thread.

Not following along on this but it is early . A cnc can generate the rear 12.5 degree angle in the P cycle . It won't on front angle thus the tool must be ground .

 

[RJSakowski]

Not following along on this but it is early . A cnc can generate the rear 12.5 degree angle in the P cycle . It won't on front angle thus the tool must be ground .

I take it that is because of the helix? Couldn't the tool be ground to provide the clearance needed?

 

[mmcmdl]

The tool itself generates the thread shape , no different from a manual lathe . A form tool ground to the finish shape of thread could do this in one cycle . A smaller tool ground ( with 12.5 angles on each side ) could also be used . You would need multiple offset threading cycles to accomplish this which wouldn't make sense . The P cycle only denotes the angle at which the tool is fed into the workpiece .

 

[whitmore]

Brass is grabby, and how do you HOLD it for making that thread?
I'd consider a mill cutter, not a lathe tool, and advance the work with a master screw,
supporting the brass in a steady rest (block bored to guide the floppy brass rod)
both entering and leaving the cut, with enough cutaway to clear chips.
I'd envision short lengths of brass rod (10 inches?) getting the exterior
cut, then parting off sections and drilling/threading the interior as
separate pieces. However you HOLD the rod to the master screw,
there's gonna be waste brass for that attachment and inlet rest.

 

[cwilliamrose]

Have you looked into rivnuts or nutserts ? Not sure Keenserts are sold that small but that would be another option .

I have used rivnuts but not in wood. They have a bad reputation due to spinning once installed and exposed to use in service. The nutserts seem to need open space behind where they are installed for expansion which is not always going to be the case in my application.

I didn't know about Keenserts but I see they aren't recommended for soft materials like plastic -- and I assume wood. We did test Helicoils in wood buy they were easily pulled out of the wood because they have very little engagement. There was a wood version of Helicoils announced but it never got to market -- the main difference was the diamond profile coil wire was wider on the OD. Must have been a bear to produce as a coil!

The E-Z Lok brass inserts are bullet proof in wood but the bodies of the available ones are too large for our application, that's why I designed smaller ones for #4-40 and #6-32 screws.

 

[mmcmdl]

If the OD thread could be a regular thread such as a 1/4-20 , these could be knocked out on a DSM Harding in a a heartbeat . Drill and tap ID , thread with a diehead and part off . Why the weird thread that only screws into the wood ?

 

[cwilliamrose]

If the OD thread could be a regular thread such as a 1/4-20 , these could be knocked out on a DSM Harding in a a heartbeat . Drill and tap ID , thread with a diehead and part off . Why the weird thread that only screws into the wood ?

Those "knife threads' displace very little wood. Using a standard thread would require running a tap into the hole and that would remove wood fibers and weaken the assembly. Even these special threads can be damaging if driven into brittle wood or into a small pilot hole;



With fresh wood and a proper pilot hole these inserts go in very clean and they will not pull out, even under crazy loads. I have one as shown above installed in a 1/4" spruce test board faced with 1/16" mahogany plywood which has been subjected to a 181# on-axis load for ten days without a hint of failure. That same insert (#6-32 threads) broke the heads off of two #6-32 stainless screws when they were torqued well beyond a reasonable level and there was no damage to the wood or the insert.