BURR on threads of a PVC machined part

[Bader]

Hi everybody.

We are drilling a through hole (1/4") in a solid PVC rod (1" dia x 1.5" Length).

Our cleints wants us to make a thread of 1/8" NPTF in the middle of the part length so as to join the through hole.

PROBLEM: when we make the threads (tapping), there is BURR inside the thread (especially at the end) that is almost impossible to remove.
Since the part is for medical use, the burrs are NOT allowed.


How can we make BURR-FREE threads?
Any ideas on this from some one who has been machining Solid PVC would be welcome.
Is there any post machining method/process that can result in removal of burrs?

Regards,


Bader

[Tom]

There are significant recommendations on drilling and tapping of plastics in the "Plastic Engineering Handbook of the Society of the Plastics Industry."

The radii of the thread tips and valleys of the cutting tool would seem to play a role.

Beyond their recommedations I would experiment with chilling of the part and cutting tool.

[Louis]

Bader,

I have done some machining on PVC in the past, but never before for a medical end use, where loose particulate is usually a concern.

What will the part look like; I am a little unclear on from your description. Are your drilling straight through and then threading straight through, or just partially through?

Louis

[oldtool]

Bader

You might try a spiral tap. They tend to create a single chip and less burr. How are you feeding the tap currently? Could be to much pressure on the tap.

Good Luck

[Bader]

thank you for the info TOM...

Unfortunately, I dont have access to the book you mentioned. HOpe you notices i am not in USA !

Any one here who has 'practical experience' in machining solid PVC?

Bader

[Bader]

Dear Louis.

I am trying to attach a picture for the part to give a better idea.

The part is 1.5" long and 1" in dia. A through hold is drilled in the centre of the rod (1/4" dia) and than another hole is drilled from side (90 degree to the through hole) and it is THIS hole that needs to be threaded.


This has taken me one month but i just cant get rid of the burrs. Some one advised me not to CUT the threads but to form them by PRESSING. Any idea how that can be done or even if it is possible?

Thank you for all the help


Bader

[Will Call]

Bader,

I'm not sure it will work in PVC but you can try a roll form tap. They form the thread without cutting so there are no chips to worry about.

[Len]

Bader,
Try Will's idea, but pre-heat the tap. PVC does not really melt, but can be "persuaded" to flow at anything over it's Tg. You might try gently heating your current tap, rather than cooling it.

Len

[Bader]

Hi Will

You suggested to "try a roll from tap...." can you please explain what does that mean?


Bader

Len,

You mean heat the tapping-thread ?
seems like a good idea. have to be careful that it does not burn the pvc.
I wil try...

[Bader]

oldtool

we are using tap-drill and the feed is synchronized with the pitch. We also tried hand drill but the burr wont go away. what is a 'spiral tool' ?

Bader

[PDF]

I'm sure you've played around with your tapping speed until you lost hair...

Can you use a harder grade of PVC?

Now that you moved to a thicker part, you could try a bigger thread or a more coarse thread.

Maybe its just me, but the burr looks like it gets worse as the tap gets deeper. Maybe the heat from working the material worsen the burr... can you cool the part / tool?

Could you use some sort of threaded insert, and not actually tap the pvc?

That's about it for out of the box thinking for me today... I guess you could say I'm all tapped out. - groan -

[Bader]

Hi PDF

Yes, we did play out with the speeds. A lot.
We cannot change the any dimension as the drawing is supplied by the client.
I 'did' think of making a thread insert but again, the client wont agree as they have to go through the FDA process again.

cooling of the tap sound interesting in theory but i dont see how it can be done practically for machining 2000 pcs a month!

I am waiting for a post from some one who HAS done machining on solid PVC and made threads. It seems so easy and yet it has taken my sleep away since the past one month.

The drawing calls for dimensions as tight as .001" and i was worried about these from the start and never gave a though about the thread thing......


Bader

[Will Call]

Bader,

Here's one maker of the roll form taps and there are many more.

http://www.balax.com/forming.html

Do a search for thread forming taps or roll forming taps and you should find what you need.

Thread forming taps do not cut the thread. You drill the hole a little larger and the tap actually forms the thread when it taps typically giving you a much smoother thread without the chips or burrs.

Good luck and let us know how you make out.

[Louis]

Bader,

Are you using any lubricant or tapping dry?

[Tom]

All cutting of metal and plastic involves some "plastic deformation". What you want is to minimize the size of the area going through plastic deformation and heating, which will result in less smearing. Classically this is controlled through tooling rake angles and cutting feed rates. Cooling and lubricating fluids in metal cutting also change the interaction between the tool and the surface being cut.

How about cold water directed into the hole while cutting like metal machining lubricant?