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View Full Version : Can steel or brass be successfully pressed into Delrin?



tony ennis
01-27-2009, 06:27 PM
Or nylon, PVC, etc?

The metal piece would be an electrical contact with a tiny bit of pressure on it. It would be faced flush to the Delrin it is pressed into, so there'd be no moment arm or other type of leverage.

NickH
01-27-2009, 06:31 PM
I think if you go for slight negative clearance and a knurled part pressed in you might be laughing,
Regards,
Nick

Spin Doctor
01-27-2009, 06:32 PM
Do you mean as in a bushing or pressed into the surface? If into the surface, pretty much no

Evan
01-27-2009, 06:52 PM
Delrin has about ten times the coefficient of linear expansion with temperature that steel does. If it warms up the steel part will fall out.

SGW
01-27-2009, 07:53 PM
It might take some experimentation, but you ought to be able to press something with a good sharp, straight knurl into a properly-sized hole so it would stay.

Another option would be to thread the hole and the part and screw it in.

barts
01-27-2009, 09:01 PM
Delrin has about ten times the coefficient of linear expansion with temperature that steel does. If it warms up the steel part will fall out.

Normally this would be an issue, but the Young's modulus of Delrin is only
4.5x10^5 as opposed to steel's 30x10^6, or roughly 60 times as flexible.
This implies that a lot of interference can be used, enough so that it will
swamp any temperature effects.

- Bart

nheng
01-27-2009, 09:21 PM
If you make a more definitive tongue and groove in the parts, the delrin will press over it like a tire onto a rim. Even if it loosens, it should stay on. Den

lugnut
01-27-2009, 09:50 PM
So, can he use a little loc-tite to help hold the piece in place?:confused:
Mel

Evan
01-27-2009, 10:15 PM
Normally this would be an issue, but the Young's modulus of Delrin is only
4.5x10^5 as opposed to steel's 30x10^6, or roughly 60 times as flexible.
This implies that a lot of interference can be used, enough so that it will
swamp any temperature effects.



Delrin or acetal isn't the best choice for this sort of fit. Is anybody else actually speaking from experience? I am. I use acetal a lot. I built a camera ball mount and used a press fit of aluminum over acetal. It worked fine until it was cooled below freezing and just fell apart. Acetal isn't very flexible which is why it is used for gears and cams in lawnmower engines. 60 times more flexible than not very flexible at all is still not very flexible.

gellfex
01-27-2009, 10:30 PM
I do a fair number of delrin press fits, usually a piece of precision stainless rod acting as an axle to another component, with no knurling or other treatement of the rod. The trick, since delrin is way more elastic than metal, is to aggressively undersize the hole, but not so much the rod will ream itself a new hole when pressed. I often do >.005 under. I will qualify this with the fact that my stuff is never in extreme conditions.

I just shipped a piece with a 1" dia hex broached AL bushing pressed into a 1/2" thick delrin sheave.

K Barton
01-27-2009, 10:34 PM
In the comanay that I work for we used to press delrin busshings over .3125 dia stainless rods. there was about .015" press in this appliaction, once pressed into place they would hold positon. These bushing/rod assemblies were used as cup locateing positioners on fruit handling weight sizers. On products that we cuttently manufacture we are using UHMW belt tensioning rollers which have a press of about .060" to hold position of part onto a bearing o.d. of 2.047"

Ken

barts
01-28-2009, 12:09 AM
Delrin or acetal isn't the best choice for this sort of fit. Is anybody else actually speaking from experience? I am. I use acetal a lot. I built a camera ball mount and used a press fit of aluminum over acetal. It worked fine until it was cooled below freezing and just fell apart. Acetal isn't very flexible which is why it is used for gears and cams in lawnmower engines. 60 times more flexible than not very flexible at all is still not very flexible.

The yield strength of acetal is 10ksi, E is .45 mpsi, and CLTE is 5x10^-5
The yield strength of 6061T6 35ksi,E is 10 mpsi, and CLTE is 13x10^-6

For long term stability, press fit should be limited to 2.5 ksi or below to avoid creep. As nheng pointed out, a tongue and groove ("snap fit") in the mating parts will prevent loosening; in general press fits in plastic should have some sort of mechanical keying. Knurling works, since localized creep occurs in the Delrin when the high spots are causing the parts to lock together.

http://www2.dupont.com/Plastics/en_US/assets/downloads/design/DCI385.pdf
http://www.kmsbearings.com/pdf/Delrin%20Acetal%20Design%20Guide.pdf

tattoomike68
01-28-2009, 12:22 AM
Sounds like he needs a gheto shock mount of some kind.

yea you can shove a sleave in a plastic bush tight as hell and it will stay.

what are you working on? a pickup truck or snowmobile?

Evan
01-28-2009, 12:26 AM
If it works it works then. Delrin is more compliant but the elasticity of Delrin is non linear over time. Delrin will withstand a certain amount of deformation when the stress is applied slowly but apply the same stress quickly and it may shatter. It's also temperature dependent.

In my milling machine I used acetal springs I designed to preload the Z leadscrew with a very high spring rate coaxial spring that goes from zero to 100lbs over 1/8 inch of compression. Very low compliance, about the same as a steel spring in the same volume.

Most delrin gears and similar items that have bushings are injection molded over the bushing.

darryl
01-28-2009, 01:25 AM
Another issue to consider is the conductivity of the contact material. Brass is not particularly a good conductor, so it can and will heat as current passes through it. The contact point in particular is subject to becoming a relatively high resistance, so if any considerable current must pass through it, it will become an issue. As soon as the contact heats and loosens in the hole, the problem will become worse.

I've made several switching systems where the contacts were brass, and in hindsight copper would have been better. One switch setup was on my lathe, and both sections basically burned up. The pressed-in contacts got hot and loosened up, then the plastic actually melted away from the contact.

In almost every case I've seen where brass is used in contacts, there's a coating on it. Maybe it's silver, I don't know, but I would advise against using brass by itself for contacts. Even in the lowly wall switch where the conductor material is brass, there's a contact point of some other material spotted onto the brass pieces. Not so bad where you make a secure direct connection to the brass via a tightening screw, as in switches and outlets, but you're suggesting light contact pressure, so beware.

On the choice of plastic material, I would not recommend pvc or any of the other softer and/or low plastic temperature characteristic plastics to hold contacts. I do use pvc a lot for this type of thing, but mostly because I have lots of it. It's not ideal.

Circlip
01-28-2009, 04:58 AM
Why don't you tell us what you're thinking of making Tony? (or would you have to kill us) After all the posturing about its chemical and mechanical properties, if its a one off for home use, the parameters change from needing thousands off. If the hole generated is too tight for a press fit, you stand a chance of the plastic splitting in use. You could always heat the bit you're trying to push in, and a REALLY clever way would be to contact someone with ultrasonic facilities.

Regards Ian.

Doc Nickel
01-28-2009, 05:47 AM
Is anybody else actually speaking from experience?

-Yes.

There's close to 100,000 paintball guns out there that use a smooth 4mm stainless rod about 20mm long, pressed at right angles into an 18mm Delrin rod about 50mm long. This is the "bolt", which is driven by a small pneumatic ram that snaps it back and forth up to 20 times per second.

It's very common for players to shoot 800 to 1,200 rounds per game, and 4,000 to 8,000 per day. The entire marker can, with decent maintnence, live to see a half-million cycles or more.

The oldest examples of this brand are pushing fourteen years old, and I know for a fact they're used in weather ranging from 100F to below freezing. I've been building and repairing this sort of marker professionally for eleven years now.

I even designed and built (http://www.docsmachine.com/gear/dyna.html) my own version that cured a factory misalignment problem with the gun.

And I have never once heard of a bolt coming apart. I've never had to warranty- either fix or replace- one of my own, and I've made and sold over a hundred of them.

If that's not enough, there's two inline pressure regulators for paintball guns, and one "LP" pneumatics reg, that use a Delrin primary piston. The two inline regs are designed and rated for CO2 use- I'm sure I don't need to note how cold something passing CO2 can get, especially where a phase transformation is involved.

In this case, since Tony hasn't given us any specifics other than that it's an electrical contact, presuming indoor and low-voltage use, I'd have no qualms whatsoever about recommending a simple press fit.

Doc.

Evan
01-28-2009, 06:33 AM
That sounds like a good application. You have the CLE working for you in that case. Since the bolt will become colder in use it will grip the steel rod tighter even on a hot day.

The situation would be different if it became hotter in use.

Doc Nickel
01-28-2009, 08:00 AM
You missed the "100F" part. My first four prototypes of that bolt were taken to a tournament in Huntington Beach, California. The weekend of the event, the coolest it ever got was 95F. They were coming very close to setting record highs.

Later that year, the same team shot the same guns in a (considerably smaller) local event, during which we got the first freak snow of the season.

We use a great deal of Delrin. Virtually every paintball gun, especially the higher-end ones, use a Delrin bolt: bolt with spring-loaded detent (http://www.docsmachine.com/galleries/2ndgenfast3.jpg), bolt with no detent (http://www.docsmachine.com/galleries/delrinbolt.jpg), and a bolt-and-hammer (http://www.docsmachine.com/impulse/impulse6.jpg) combo. (The latter hammer was eventually switched to aluminum thanks to peening problems in the groove.)

We also use it in internal valves (http://www.docsmachine.com/tech/eq3.jpg) (the poppet to the left of the grooved aluminum cylinder) whole bodies and grip frames (http://www.docsmachine.com/spartan2.jpg), and lots (http://www.docsmachine.com/galleries/sterlingpump3.jpg) of pump grips (http://www.docsmachine.com/galleries/paphanpair.jpg). I have no photos but there's several guns that have most of the moving portions of their spool-valve internals made out of Delrin.

The only problem we've had with the material is if the portion that actually seals the breech- the bolt- has to have an active O-ring (as in, part of a spool valve) then there will inevitably be scoring due to dirt and grit, which will lead to leaks. I can name two very nice markers that failed in the marketplace because of this.

But apart from that, it's great stuff. Again, if we can presume Tony's use won't involve subfreezing weather, boiling-hot days, or fifty-amp service, then he'll have no trouble at all with a light press fit of his contact.

Doc.

Circlip
01-28-2009, 08:02 AM
But But But you're being Practical Doc!

Doc Nickel
01-28-2009, 08:17 AM
But But But you're being Practical Doc!

-Someone has to be.

While I do note that Tony hasn't given us any real specifications, I doubt I'm wrong in presuming the part won't be used in subfreezing or broiling-hot conditions. Therefore arguments of expansion rates and so forth are pretty much entirely meaningless.

Doc.

Evan
01-28-2009, 08:46 AM
You missed the "100F" part.

No, I didn't miss it. You missed this part: "Since the bolt will become colder in use it will grip the steel rod tighter even on a hot day"

100F isn't that much hotter than room temp and a 4mm hole doesn't change much in size.

NickH
01-28-2009, 09:00 AM
I suppose we could wait for the OP to clarify requirements if he wants to and then get into a fight that's relevant to his needs ?
:D
Nick

Your Old Dog
01-28-2009, 10:35 AM
If I were going to try that, I'd size it to allow me to use some 5 minute epoxy as it remains somewhat plyable and may grip knurling better. Heat it up and see if it stays.