Set screws for aluminum plate

[Matt-Aburg]

I have built a large fixture plate for my CNC router. It has about 650 3/8-16 holes in it. I originally purchased plastic setscrews because of corrosion concerns. Unfortunately, these setscrews are made out of PP and are a class 1A- 2A thread. After 4 turns into the plate the Hex strips and then they are scrap. I called Grainger today and was refunded in full and don't even have to return the warranty items.

Moving on, I talked to a rep and he suggested using Stainless setscrews. I have read that stainless still will react with aluminum. Other sources say to use zinc plated. Still other sources say the Zinc will corrode and be a big mess. (eventually).....

Here is my thoughts The Zinc plated setscrews cost about 4 times more than stainless. I could dip the stainless setscrews in lithium to reduce the contact with the aluminum plate. Note also that the setscrews (both) are class 3A...

Any thoughts or opinions would be appreciated, before I pull the trigger.

 

[RobinHood]

Are those through holes?

Are you moving the set-ups often? IE, are you constantly pulling out setscrews and replace them with other items?

Might be completely impractical: plug the unused holes with wax? Melt it out of the holes you need… should be resistant to coolant, corrosion and not mess up the threads?

 

[Matt-Aburg]

Are those through holes?

Are you moving the set-ups often? IE, are you constantly pulling out setscrews and replace them with other items?

Might be completely impractical: plug the unused holes with wax? Melt it out of the holes you need… should be resistant to coolant, corrosion and not mess up the threads?

These are tapped through. I would think that most will not be used. Ones that are most easy accessed, (front and sides) will be used more often.

 

[Former Member]

Silver Antiseize to prevent corossion or binding.

PM sent for a source that is way cheaper.

 

[phaxtris]

You would be better off using mild steel set screws than you would be using stainless, mild steel and aluminum is less corrosive than stainless and aluminum by quite a bit

Zinc plated as you know would be best

 

[Matt-Aburg]

You would be better off using mild steel set screws than you would be using stainless, mild steel and aluminum is less corrosive than stainless and aluminum by quite a bit

Zinc plated as you know would be best
View attachment 29900

I will look into this more this evening, I am in the middle of cutting housings on the router, and for sure I can say every hole is full of swarf !!! I will vacuum it first. My other alternative is to make a cbore and create custom 3D printed plugs as I have for the dowel locations. I am leery of this as it will remove some thread engagement.

What about just black oxide and the anti-seeze?

 

[phaxtris]

What about just black oxide and the anti-seeze?

Yes that would be closer than stainless, look up galvanic corrosion, that is the issue, the electrical difference between the two materials

 

[Matt-Aburg]

You would be better off using mild steel set screws than you would be using stainless, mild steel and aluminum is less corrosive than stainless and aluminum by quite a bit

Zinc plated as you know would be best
View attachment 29900

I have a (several chart)s that define it differently. I have attached one to debate.

 

[Former Member]

Interestingly enough it is common practice to use stainless steel fasteners to bolt aluminium pieces together for corrosion resistance, using anything else cause failure.

Our worse concern is stainless steel to stainless steel (ie nut to bolt) as a result we must use anti-seize.

 

[Matt-Aburg]

Interestingly enough it is common practice to use stainless steel fasteners to bolt aluminium pieces together for corrosion resistance, using anything else cause failure.

Our worse concern is stainless steel to stainless steel (ie nut to bolt) as a result we must use anti-seize.

So earlier, I mentioned lithium,. I meant white lithium grease. This would cause a slight barrier, and costs less. SS is the way I am leaning now for sure...

 

[Former Member]

A lot of new stainless steel fasteners are wax coated. The other alternative is dielectric grease (aka silicone grease).

 

[Chicken lights]

So earlier, I mentioned lithium,. I meant white lithium grease. This would cause a slight barrier, and costs less. SS is the way I am leaning now for sure...

I’d lean towards stainless fasteners, even just cheap grease should be fine. This is going to live inside a shop, not outside in the elements, right? Even fluid film should be enough to inhibit corrosion yet provide some lubrication

 

[phaxtris]

Stainless fasteners and aluminum are often used together because they are both corrosion resistant to external elements, quite often the reason aluminum was chosen in the first place, the electrical potential between stainless and aluminum is higher than it is between aluminum and steel, Wich makes the galvanic corrosion marginally worse

do not use dielectric grease, increasing the ease at Wich electrons can jump from one element to the other will speed up galvanic corrosion, you want to ideally use some kind of insulator

I Would go with steel in this cass because you do not need the corrosion resistance of stainless, so why pay for it? And given steel is marginally closer to aluminum electrically your slightly better off with the galvanic corrosion issue

Either will work, but your not loosing anything with steel plugs, and you would save a pile

Edit: here is a better chart, the further the elements are apart (numerically) the higher the corrosion potential, notice how much closer steel is to aluminum vs stainless to aluminum

 

[Aliva]

I may be missing something here but how long are these screws going to be in the plate?
If they are going to be used for a short duration for work holding, is corrosion a real concern? I would think SS with anti seize would be the way to go.

 

[phaxtris]

Galvanic corrosion happens suprsingly fast @Aliva, it can happen much quicker than rusting, and it happens in perfectly clean and dry conditions, it would not take long to destroy the threads in that plate, especially if a conductor like dielectric grease was used as a thread lubricant

 

[Former Member]

Stainless fasteners and aluminum are often used together because they are both corrosion resistant to external elements, quite often the reason aluminum was chosen in the first place, the electrical potential between stainless and aluminum is higher than it is between aluminum and steel, Wich makes the galvanic corrosion marginally worse

do not use dielectric grease, increasing the ease at Wich electrons can jump from one element to the other will speed up galvanic corrosion, you want to ideally use some kind of insulator

I Would go with steel in this cass because you do not need the corrosion resistance of stainless, so why pay for it? And given steel is marginally closer to aluminum electrically your slightly better off with the galvanic corrosion issue

Either will work, but your not loosing anything with steel plugs, and you would save a pile

Edit: here is a better chart, the further the elements are apart (numerically) the higher the corrosion potential, notice how much closer steel is to aluminum vs stainless to aluminum
View attachment 29935

I must say from the applications I have seen them used, the biggest issue is Galvanic corrosion between SS and SS and less so between the AL and SS.

 

[Aliva]

Galvanic corrosion happens suprsingly fast @Aliva, it can happen much quicker than rusting, and it happens in perfectly clean and dry conditions, it would not take long to destroy the threads in that plate, especially if a conductor like dielectric grease was used as a thread lubricant

I totally agree that galvanic corrosion can be a concern and should be considered in any application. But what is
" surprisingly fast". Are we talking hours, days weeks? The OP didn't mention how long these set screws will be in the aluminum plate, and I believe that is one important factor to take into account. I've removed steel set screws from aluminum motor sheaves that have be in place for months if not years and rarely had a problem. In some cases a little penetrating fluid was used to help with the stubborn ones. If these set screws are temporary, corrosion is a non issue. Again " temporary" may have to be defined.
The OP didn't really indicate what the set screws are being used for. Locating a part, some sort of clamping, or just plugging the threaded holes to prevent swarf. Masking tape can plug holes very effectively and cheap. Maybe contact other shops and see how they deal with this.

 

[phaxtris]

@Aliva I believe he stated the set screws would be used to plug the holes in the fixture plate, indicating some could be in there for years

You had no problems with steel in aluminum...that is why I recommend steel over stainless steel, it is much closer to aluminum than stainless steel is, and has less of a problem with galvanic corrosion as a result, plus it is much cheaper

The time it take for galvanic corrosion depends on a number of factors, the two main ones being electrical potential between the two materials, and the service conditions....is there a conductor present (conductive water, conducting grease, etc), noticeable corrosion can been seen in as little as weeks or months with the right conditions and the right materials

@Degen are you thinking of galling ? When stainless fasteners essentially cold weld themselves together ? This is not galvanic corrosion, there is no galvanic corrosion between two similar metals as there is no electrical potential, but galling is a real thing, and the reason thread lubricant is recommended with stainless fasteners

The op spent a lot of money and time on this plate, it would be a shame to pull some of the plugs out in a year or two and have the threads disintegrate, or even have the chance of it happening, in the end it's his choice, just want to provide the information as I have delt with this (and galling) many times before

 

[Former Member]

I would amost suggest making some simple threaded plastic rod, cut to length and slot. Simple and solves all the issues.

 

[phaxtris]

I recall seeing plastic redi-rod in the past, maybe the op could source some for his plugs