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beanbag
11-09-2009, 03:28 PM
Drilling holes that are going to be tapped.
Should I use exactly the drill size recommended, or slightly over or undersize?
Reason being that I am a cheapskate and would rather buy a standard size drill than a metric one for the hole.

Evan
11-09-2009, 03:48 PM
Drilling holes in plastic can be an interesting experience. Because it is a lousy conductor of heat it will expand inward and grab the bit tighter than you can imagine. A lot depends on how fast the hole is drilled, the size of the hole and depth as well as the type of drill bit used.

For larger holes a spade bit is the best choice followed by boring to true dimension. For small holes the bit should ground for zero or almost zero rake on the cutting edges similar to a bit for brass. Peck drilling is how to prevent overheating giving time for the bit and work to cool each peck. Compressed air to blow out chips, cool the material and cool the bit is a very good idea. If you follow these rules you will end up with a hole very close to the nominal bit size or very slightly undersize.

I have also modified drill bits to allow for deep hole drilling in plastic. By grinding the flutes down about half an inch behind the tip of the bit the contact area is reduced. I leave a 1/4" land of original flutes each half inch and then grind down another portion. This makes a bit that still tracks straight but creates far less heat.

beanbag
11-09-2009, 03:54 PM
The hole is 3mm and 15mm deep if that helps. Going to be tapped with a metric size. I have a choice of using flood coolant (but more of a drizzle than a blast) or mist/air blast.

Evan
11-09-2009, 04:59 PM
Use air instead of flood. That isn't a challenging hole to drill but watch out when you tap it. Don't power tap or you may end up with a permanently embedded tap. Hand tap nice and slow and you won't have a problem. Believe it or not you can easily break a tap in plastic.

Limy Sami
11-09-2009, 05:08 PM
The hole is 3mm and 15mm deep if that helps. Going to be tapped with a metric size. I have a choice of using flood coolant (but more of a drizzle than a blast) or mist/air blast.

What size thread you tapping?

airsmith282
11-09-2009, 08:49 PM
I have never had a problem drilling delrin or nylatron or even teflon, just use a slow speed and slow feed and back it out often and you wont have any problems at all ,

beanbag
11-09-2009, 09:26 PM
my mistake, the thread is 3x0.5, the hole is 2.5mm.

I tried looking up the sfm for delrin online, but failed. I'll try to find it in a machining handbook, but I've heard that it's a really high number. But I thought for plastics you are supposed to use low speed and high feed???

Black_Moons
11-09-2009, 09:53 PM
'SFM' ratings are for max production vs non excessive tool wear typicaly, What speed you should actualy machine it at could be much less or more, depending on your requirements and how you are cutting it.
HSS and carbide can mow through plastics at high SFM without dulling, The workpeice suriveing is another matter, So basicly just pick SFM to not melt the work.

the slower the cooler, and the higher feed the more heat you tend to take out with the chips and less rubbing of the work.
Rigidity of clamping and work and toolbit and desired finish will affect how fast or slow you can feed.

(Watching your plastic all of a sudden start bending while machineing is not fun, Neither is watching it slip outta the vise, or the vise deform it from the clamping pressure)

Expairment on some scrap, go carefuly with the real deal. If its production, try uping the feeds and speeds with each part or couple of holes and see how it goes, back off when it starts looking worse.

tattoomike68
11-09-2009, 10:03 PM
my mistake, the thread is 3x0.5, the hole is 2.5mm.

I tried looking up the sfm for delrin online, but failed. I'll try to find it in a machining handbook, but I've heard that it's a really high number. But I thought for plastics you are supposed to use low speed and high feed???

yes drill it slow and keep the feed up and peck drill. the low speeds on most drill presses is fine. Most of the time its not the drill that gets hot its the huge chipload that gets hot and welds the bit into the plastic.


Drilling plastic makes a big volume of chips. Thats why you peck drill.

Iv seen 6 spindle screw machines run large plastic and we had gangs of 5 yard dumpsters just to haul off the chips.

you are just tapping a little hole, just go for it.

Ken_Shea
11-09-2009, 10:42 PM
Your going to see a multitude of suggested methods for machining/drilling plastics, just as I did , they are not all wrong nor are they all correct in every operation. What to use, slow rpm and high feed or higher rpm and drill fast, the idea is to reduce heat to stop the gluing tendency of plastics, both methods will help in that regard, my thinking is higher rpm and faster feed, get the job done and it does not have time to get hot. That worked for me.
BTW, I did drill to size for tapping.
Flood coolant really works well, just a big mess, use air.

The key for you here I believe was already mentioned, try a sample piece and see how it goes.
I enjoy machining plastic, you just have to watch the heat, also use cutters designed for plastics, what a difference that makes, however, did use regular 135 degree drills for the drilling.

Scishopguy
11-10-2009, 02:29 PM
I would not recommend high downfeed rates with delrin. It tends to autofeed violently, causing the surrounding material to chip out and fly into your face. I second Evan's point about regrinding your drills to a zero rake cutting edge. This adds heat so be sure to peck drill and use air to cool things. Same is true for acrylic (Plex or Perspecs).

gellfex
11-10-2009, 05:32 PM
I drill & tap LOTS of delrin in all sizes from 3/8 down to 2-56. I use a few drops of cutting lubricant like Safetap or Tapmatic rather than a flood coolant, and you'll quickly get a sense of how much you can feed. I don't peck that much, a proper feed speed and perfect spirals will emerge. The self feeding is mostly a problem of larger size drills or opening up holes with pilot holes bigger than the drill's web. I keep a fractional index with 0 deg rake cutting edges for safely opening up holes and counterboring.

For a drill like you're talking anywhere from 600 to 2000 rpm would be fine. You'd have to really go crazy to get a seized drill like you would with polycarbonate, this stuff cuts like a dream. It's low friction keeps the cutting relatively cool compared to thermoplastics like acrylic polycarbonate or styrene.

As for size, boldly go under, the stuff is elastic and soft relative to metal and you'll want the most thread you can get. I'll often use a drill for upwards of 80% threads, like a #31 for 8-32 or #37 for 6-32. My chart shows a #40 would give you a 75% thread on that M3, you could easily go down a size or 2. And I power tap everything, even deep holes.

Delrin is a miracle material, enjoy.