Thread milled blind holes: Best practice to use standard threads in thread milled designs?

[tbraxton]

Creo Parametric 7

I have multiple designs in process that will have thread milled holes in them. What is the best practice for standard thread milled features in Creo? The standard tapped holes are not out of the box configured to deal with these features.

Is anyone aware of the existence of an open-source custom hole chart for thread milled holes for UNC/UNF threads? Would custom hole charts be capable of dealing with thread milled holes?

I am open to other solutions (UDFs) etc. if there is an advantage to using them.

[Patriot_1776]

'Sup Tom!  The thread callout should be the same regardless of whether it's cut with a tap or thread milled.  If it's say, a .750-10 UNC-2B, it's that regardless of the method used to machine it, and so the initial milling for the minor diameter should have the same diameter as if you drilled the hole with the standard tap drill.  If you DEVIATE that, and make your own minor diameter, and pitch, etc., then that's a whole 'nuther story.

So, simple answer is just call out what the thread IS (use the standard Creo hole), and then the burden is on the Machinist to make it conform to the standard...as it should be.

[tbraxton]

I have standard holes in the model now. The issue is that there is a drill point created when using a standard hole that is problematic in the design, hence the use of thread milling in a blind flat bottom hole. 

Another way to ask the question is in Creo 7 is it possible to eliminate the drill point when using a standard hole feature. I want a flat bottom hole.

[StephenW · Accepted solution]

My quick fix is to edit the drill point angle to 180 and it creates as a flat bottom.

[tbraxton]

That works, I would not have expected the 180 angle to work on the drill point for the shape.

[Patriot_1776]

'Sup Tom!

Ah, I get what you mean now, I replied before you posted your second post.  For thread milling the machinist doesn't care what he uses to create the minor dia (drill or mill) and the thread callout will be the same, but if you need to have a flat bottom it needs to be specified in the callout (or shown), and as Stephen showed, you can make the angle 180 to take the drill point out.

Question:  Why are you thread milling such a small diameter thread?  You could mill the minor dia (no drill point) and use a bottoming tap and it would be a lot faster/less work (no CNC programming to interpolate the toolpath).  Usually you do thread milling on larger threads (1"+) or ones where you're not doing a UNC but a UN thread with a specific threads per inch.  Just curious.

Best of luck with it!

[tbraxton]

I am not doing the programming. The part is made in a 5 axis machine and the CNC guys want to thread mill the holes.

One other recent design that required thread milling is an injection molded bore in polysulfone that receives a solenoid valve with 4 radial seals along the bore axis. We did not have room for an unscrewing mechanism in the mold and the threads are milled post molding. In that one we actually modeled the threads in the parts as we initially planned to mold them in. The bore features all must be within 0.05mm TIR of the bore axis and so far they are meeting that on the parts.

[Patriot_1776]

Interesting.  Well, if they wanna cut chips, let 'em cut!  Always good when the parts meet spec, Eh?

Thought I'd ask, it was unusual milling holes that small, at least from when I've had to do them in the past.  I've been out of the plastics world for the last 8 years, but I remember that you wanted to use thread-forming self-tapping screws instead of thread-CUTTING because the formed threads were stronger because plastics are so notch-sensitive if they're cut with a sharp object.  So, I'm wondering if you couldn't mold the id of the hole such that you could insert a hot thread-forming "tap" (with the speed/feed rate of an actual tap so the pitch was correct), that would simply displace the semi-molten plastic, then back it out and you'd have stronger threads.  Dunno if that's even a thing, but, it'd be cool to try something like that.