Machine tapping PVC sheet with small blind holes

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Mark 2
Posts: 48
Joined: Tue Jan 24, 2012 11:11 am
Location: Seattle WA

Machine tapping PVC sheet with small blind holes

Post by Mark 2 » Tue Apr 09, 2019 4:23 pm

Howdy,

I've got a project that requires tapping several hundred 10-32 blind holes 1/2 to 5/8" deep in PVC sheet. Needless to say I'm not interested in using a tap handle. I have a versa-tapper but the results are not real good leaving ragged peaks on the threads. I've used both plain and TiN coated 4 flute bottom taps using WD-40 for lube.

Any suggestions as to more appropriate tap types as well as cutting lube?

Thank you, Dave

Mark 2
Posts: 48
Joined: Tue Jan 24, 2012 11:11 am
Location: Seattle WA

Re: Machine tapping PVC sheet with small blind holes

Post by Mark 2 » Tue Apr 09, 2019 4:57 pm

Belay that request!

3 drops of Transultex in each hole and it's a different world. The chips come out in the flutes and thread finish is dandy. Just had to get away from it for a bit and ponder. And then there it was: the answer.

Russ Hanscom
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Location: Farmington, NM

Re: Machine tapping PVC sheet with small blind holes

Post by Russ Hanscom » Wed Apr 10, 2019 8:12 pm

Did you consider a spiral flute tap? they are intended for blind holes.

Mark 2
Posts: 48
Joined: Tue Jan 24, 2012 11:11 am
Location: Seattle WA

Re: Machine tapping PVC sheet with small blind holes

Post by Mark 2 » Wed Apr 10, 2019 8:42 pm

After several dozens of holes tapped with the new TiN coated straight flute taps and oil based cutting fluid I still ended up with 5 holes trashed. Very odd as there was no indication leading up to the trashed holes that anything was going south. I pulled the plug on continuing the tapping. I am in fact ordering spiral flute taps for soft materials (including plastics) and will see what happens at that point.

Harold_V
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Re: Machine tapping PVC sheet with small blind holes

Post by Harold_V » Thu Apr 11, 2019 2:48 am

Mark 2 wrote:
Wed Apr 10, 2019 8:42 pm
I still ended up with 5 holes trashed.
In any particular order, or were they scrapped randomly?
I ask because there may be an obvious reason if they came one after the other.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

Mark 2
Posts: 48
Joined: Tue Jan 24, 2012 11:11 am
Location: Seattle WA

Re: Machine tapping PVC sheet with small blind holes

Post by Mark 2 » Thu Apr 11, 2019 7:40 am

These came in random order, not one right after another. That said the first trashed hole came after very many with no problem and with a plain tap of conventional 4 straight flute design. After changing to a TiN coated bottom tap the typical material extraction came up easily removed with compressed air while the tap was rotating. Then, out of the blue so to speak, and after many properly threaded holes the tap came up with flutes packed with material and requiring a dental pick to remove and then after only a lot of effort. Strange! I hesitate to do a partial hole, remove material from the flute, and enter again under power fearing that I'd not get the tap back in sync with the already formed threads.

Russ Hanscom
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Joined: Wed Mar 15, 2006 11:10 pm
Location: Farmington, NM

Re: Machine tapping PVC sheet with small blind holes

Post by Russ Hanscom » Thu Apr 11, 2019 9:37 am

Harold, should not this thread be more appropriate in General Discussions?

Harold_V
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Re: Machine tapping PVC sheet with small blind holes

Post by Harold_V » Thu Apr 11, 2019 3:55 pm

Russ Hanscom wrote:
Thu Apr 11, 2019 9:37 am
Harold, should not this thread be more appropriate in General Discussions?
Indeed, and it will get moved immediately. Thanks, Russ. I wasn't paying attention.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

Harold_V
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Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Machine tapping PVC sheet with small blind holes

Post by Harold_V » Thu Apr 11, 2019 4:08 pm

Mark 2 wrote:
Thu Apr 11, 2019 7:40 am
These came in random order, not one right after another. That said the first trashed hole came after very many with no problem and with a plain tap of conventional 4 straight flute design.
Trouble shooting is one of my quirks. You just detailed the reason for the failure. Conventional taps are also known as hand taps. They are NOT suited for use in power tapping, as they don't direct the chip away from the cut. The reason you damaged threads was exactly as you suggested, as the flutes, once packed, don't allow the developed chip anywhere to go, so heat rises and the resulting thread is destroyed. In metals, the tap would have broken instead.
After changing to a TiN coated bottom tap the typical material extraction came up easily removed with compressed air while the tap was rotating. Then, out of the blue so to speak, and after many properly threaded holes the tap came up with flutes packed with material and requiring a dental pick to remove and then after only a lot of effort. Strange!
I strongly suspect that you experienced much the same problem. The packing was the result of heat generated by continuous tapping. At some point, the tap would be hot enough to soften the material to the point of fusing, and that sounds like what you experienced.
I hesitate to do a partial hole, remove material from the flute, and enter again under power fearing that I'd not get the tap back in sync with the already formed threads.
A wise decision, but you should have coupled that thought with the use of a proper tap. While I'm not a huge fan of spiral flute taps, for soft materials they serve the purpose nicely. Instead of packing the flutes, the developed chip is intended to follow the spiral flute and be ejected, so there is no flute packing. That works nicely in softer materials, but in tougher materials, it isn't uncommon for the tap to break. They are relatively weak as compared to other designs (like gun taps). However, you still must consider that if the tap gets over heated (easy to do when running a tapping head), you still run the risk of fusing.

Your thinking was spot on in regards to removing the tap and re-entering the hole. That's reasonably safe to do with metals, but plastics are generally too soft to take the risk.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

Mark 2
Posts: 48
Joined: Tue Jan 24, 2012 11:11 am
Location: Seattle WA

Re: Machine tapping PVC sheet with small blind holes

Post by Mark 2 » Thu Apr 11, 2019 5:40 pm

Harold,

Excellent deductive work! Spiral flute taps are on their way.

Can you, or anyone with knowledge on the subject, recommend an appropriate cutting fluid for this application in PVC? I selected a wax listed as appropriate for plastic but am now thinking that it may be too viscous and could compound the problem instead of improving it.

Thank you! Dave

Harold_V
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Joined: Fri Dec 20, 2002 11:02 pm
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Re: Machine tapping PVC sheet with small blind holes

Post by Harold_V » Fri Apr 12, 2019 1:30 am

I'm not convinced the viscosity of the wax you speak of will be troublesome, as the tap will heat, likely resulting in a fluid condition of the wax at the point of cut. However, before scrapping something of value, might be a good idea to do a few test holes first, doing the exact thing you'd do on the product you're tapping. I'd also suggest you keep watch on the temperature of the tap, which may have a tendency to overheat. Many of the plastics cut with considerable friction. That won't hurt the tap, but if the plastic softens while being cut, that will be troublesome.

I haven't had to tap any PVC, so I can't speak from personal experience, but I have had considerable experience with a wax for tapping aluminum, and it yielded excellent results, even in cast aluminum, which tends to be less than user friendly (soft, and loves to tear). My experience came in my apprenticeship, with the wax marketed by Johnson, which we'd apply with an acid brush. It stuck well to the tap and left little residue in the hole. Two or more holes could be tapped between application, so it was a great medium for us. We tapped with tapping heads.

Armed with that thought, might be a good idea to give the one you selected a go before worrying. It may prove to be a good choice. I'd enjoy reading about your experience, good or bad.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

Mark 2
Posts: 48
Joined: Tue Jan 24, 2012 11:11 am
Location: Seattle WA

Re: Machine tapping PVC sheet with small blind holes

Post by Mark 2 » Fri Apr 12, 2019 5:32 am

Thanks, Harold, for your thoughts and advise! I'd come to the same conclusion about tapping test holes in the same manner as production prior to the customer's parts and will use the wax first. As of yet I don't have an alternative if wax (or something else!) doesn't work. I will also slow the rotational speed down as much as possible in an attempt to lower heat build up. I will post results later. Best regards, Dave

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