Drilling Plastic: Seeking Advice and Resources for Info

Greetings!

First time poster on this forum...hoping to get some help with a challenging task I am encountering at my new job.

I have recently taken a position at a paper roll service shop as a CNC drilling machine operator. Essentially, this machine is a modified lathe which has 180 drilling heads ganged together with a spline shaft, and is designed to drill holes into suction roll shells for paper machines.

Our company specializes in rubber and polyurethane covers, but we have had a bear of a time dialing in our drilling to give us good results in softer polyurethane (20-30P&J, or roughly 80-90A durometer). We have had problems maintaining hole size (usually dealing with hole sizes shrinking as much as 20%), as well as hole quality and appearance.

Has anyone on the forum had experience drilling/machining polyurethane? Our machine has some limitations, primarily with spindle speeds (2000rpm max), so feed/speed settings are somewhat limited. We have had mixed results with coolant vs. dry machining as well.

We have had promising results with some test drilling with brad point bits, but we worry about their longevity over a full drill job. Most of our drill jobs require bits to withstand upwards of 1000-1500 drill cycles or more...because of this, we have been reluctant to dive in with these bits for a full job (any drill breakage in a new cover is very bad).

Anyone have any advice, or sources of information we could use to help improve our performance with this material?

Thanks in advance!

Connor

What size holes are you trying to drill? Most plastics respond poorly to heat generated by running the drill too fast as plastics tend not to dissipate heat well. Also (especially on the softer plastics like polyethylene) the edges of the drill must be kept razor sharp.

**[I have recently taken a position at a...] Tread lightly and be polite..It is not uncommon that the shop has people who think they have tried everything and perhaps they have. You are the New Guy for a time.

I sharpened for a shop the used Onsrud router bits for drilling different kinds of plastics.

Made some polyurethane and rubber drills that were like a tube of hardened steel and drill rod with a cut out notch to expel slugs.OD was straight polished ..ID was .020 to .030 wall thickness under OD. Knife edge Point was tapering to the inside at about 30*. Id was drilled but with controlled feed to get best drill finish.. Then polished a little with a crocus stick rod. Polishing more to the deep. Had a friend who used my tube drills and a simple drill press to drill a fibrous rubber for a side job.

Also had success with punching holes with the tube drill just pushing through with no turning.

Not that hard to fab up a brad-point drill sharpening fixture to go with a simple bench grinder..just a bushing holder made with the drill in use.. bushing fixtured in front of grinder.. push drill into shaped wheel and turn just a little to make some clearance to center point.eyeball to make brad point same length over a pocket square.

Polyurethane punches well but needs a backer set-up so perhaps a second operation that would waste time on a production job. Punching with the hollow drill having the exit notch..

Found it can be good to gange change drills (on some jobs) even at 50% of the longest drill life can save downtime, rather than shutting down often to change the one that went bad. Changing all to know when the tool change will occur. Yes it pays to have inhouse sharpening with doing this..

I was part time working at a Polyurethane shop mostly running the Bridgeport.. A fellow came knocking on the door to make the owner an offer for the name and the product line. Said if owner would not sell then he would be run out of business. He was a big dollar fellow and paid a better than fair price. Yes the shop was sold.

Connor_U. said:

Greetings!

First time poster on this forum...hoping to get some help with a challenging task I am encountering at my new job.

I have recently taken a position at a paper roll service shop as a CNC drilling machine operator. Essentially, this machine is a modified lathe which has 180 drilling heads ganged together with a spline shaft, and is designed to drill holes into suction roll shells for paper machines.

Our company specializes in rubber and polyurethane covers, but we have had a bear of a time dialing in our drilling to give us good results in softer polyurethane (20-30P&J, or roughly 80-90A durometer). We have had problems maintaining hole size (usually dealing with hole sizes shrinking as much as 20%), as well as hole quality and appearance.

Has anyone on the forum had experience drilling/machining polyurethane? Our machine has some limitations, primarily with spindle speeds (2000rpm max), so feed/speed settings are somewhat limited. We have had mixed results with coolant vs. dry machining as well.

We have had promising results with some test drilling with brad point bits, but we worry about their longevity over a full drill job. Most of our drill jobs require bits to withstand upwards of 1000-1500 drill cycles or more...because of this, we have been reluctant to dive in with these bits for a full job (any drill breakage in a new cover is very bad).

Anyone have any advice, or sources of information we could use to help improve our performance with this material?

Thanks in advance!

Connor

Click to expand...

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put parts in the freezer and then machine when frozen. be aware plastic and rubber changes size as it gets up to room temperature

Brad points are probably a good way to go for this.

The thing to do is a test using just one drill and a sample of the same material. Repeat until failure. Then do a another test, repeat until failure, etc. If you plot the number of cycles before failure on a piece of paper it will make a curve like a hump. Using math you can then compute the mean number of cycles before a single failure of a machine with 180 bits, and other statistics as well, such as the probability of any particular failure. For example, you will be able to compute things like: if we 3000 cycles then there is a 18% chance that at least one of the bits will fail.

It's a kind of science called "operations analysis".

Nahh, do it as a 2 stage process, get hollow core hss paper drills, there a internally sharpened Hss tube, it won't drill metal, but if used to cut the holes first in the rubber the results will be perfect, then go in with the drill to go through the rollers core. Will only work well on softer rubbers though! But hole will be dead nuts accurate to tube diamiter in the rubber! You want the depth of the hoolow bit such that the slugs stay on the roller, ie not all the way to the metal core, then let the drill shear the last little bit.

DMF_TomB said:

.
put parts in the freezer and then machine when frozen. be aware plastic and rubber changes size as it gets up to room temperature

Click to expand...

That is both unnecessary and impractical. and does nto work super well unless you are able to keep the urethane incredibl;y cold -50 or lower.

What are you doing for chip control, what size holes? How are you dressing your drills? Are you drilling with HSS/carbide? Is the urethane virgin?

We machine a fair bit of urethane, but don't do a ton of holes with drills. A single point bore job may be a better option after starting your hole.

vettedude said:

That is both unnecessary and impractical. and does nto work super well unless you are able to keep the urethane incredibl;y cold -50 or lower.

What are you doing for chip control, what size holes? How are you dressing your drills? Are you drilling with HSS/carbide? Is the urethane virgin?

We machine a fair bit of urethane, but don't do a ton of holes with drills. A single point bore job may be a better option after starting your hole.

Click to expand...

Some further information:

Hole Sizes: Typically in the range of .079 (1/16") to about .156" (5/32")

Drills: We typically have drills outsourced for sharpening, due to the time required for us to do it ourselves (not my decision. I would much prefer to do my own sharpening if I could..we have a great Giddings and Lewis sharpener just collecting dust in the corner, but the powers that be don't see the value in using it)

Chip Control: We have noticed that we seem to have much better chip control with brad points vs standard 118* or 90* tips. Our main concern with the brad point bit lies with durability...we have very limited opportunities for testing (again, not my decision), but we may have to find creative ways to sneak tests in.

Urethane: We are drilling into virgin urethane. Typically the urethane is laid over a composite base, then we drill through those layers into a pre-existing hole into a stainless steel or brass shell. Hard poly is pretty easy keep under control, but it's the soft stuff that gives us the headaches.

Freezing the poly is definitely not an option. Paper rolls are typically 15-30ft long and up to 75" in diameter, and weigh up to 80,000lbs...not too easy to put in the freezer

I think we will start by further exploring the brad point option, and seeing what happens from there. I will report back any findings of interest...I'm sure somebody someday may find this info useful!

Thanks!

Connor

Connor_U. said:

Thanks!

Connor

Click to expand...

I would give a single flute end-mill a try also, can you use through tool coolant? You may be clogging with chips.

Thru coolant is not possible with our setup (at least not without re-engineering the entire system).

As we drill on with more soft poly projects, I will post some results to this thread with info on what tools produced what results.

Thanks to all for the feedback!

Connor

You might try drilling as if it were steel. I'm talking about RPM and surface speed.

I did it with acrylic. Using carbide circuit board drills. Result was see through. I have a feeling you're not doing that, but might give some ideas.

Stan-

Is this material a semi-hard rubber? I didn't see how thick it was,but that small of a diameter you may want to try something like a punch, or as mentioned above a hollow tube... have you ever put studs in a snomobile track? There is a "drill" of sorts that you can buy that is hollow with about a 20deg chamfer leading down to a super sharp knife edge.

From the rubber holes that I've wrestled with in the past, I've always had to go with a larger sized drill so that it shrinks up to the size you need. Hole finish is a different story.. I have no advice for ya there.

Tap into the wealth of information that is already there. I do some work for a local roll cover shop. When they get used rolls in, sometimes they machine the old cover off. They custom grind HSS tool bits that do an amazing job taking the rubber off in thick strings. I'm sure your shop must do something similar. I would go talk to those guys and see what they are cutting with and have your drills sharpened based on what they have learned.

Brad point, YES; but not just brad point. I have a set of these LIPPED brad point drills that I purchased for wood work. I think they would be my first choice for your situation. The lips will help ensure that the hole diameter is correct as the OD is cut first.

The Lee Valley brand drills are excellent quality.

http://www.leevalley.com/US/Wood/page.aspx?p=42247&cat=1,180,42240

I do not know about resharpening: the lips make it a more complicated operation. Perhaps you could work out something with Lee Valley if the quantity is high enough. They may also have some recommendations if you discuss your problem with them.

All the usual disclaimers apply. I have no relationship with Lee Valley other then satisfied customer.

jscpm said:

Brad points are probably a good way to go for this.

The thing to do is a test using just one drill and a sample of the same material. Repeat until failure. Then do a another test, repeat until failure, etc. If you plot the number of cycles before failure on a piece of paper it will make a curve like a hump. Using math you can then compute the mean number of cycles before a single failure of a machine with 180 bits, and other statistics as well, such as the probability of any particular failure. For example, you will be able to compute things like: if we 3000 cycles then there is a 18% chance that at least one of the bits will fail.

It's a kind of science called "operations analysis".

Click to expand...