A CNC holder - material selection 2

[elinah34]

Hello,
I design a holder that on the one hand will have an interface to the CNC base plate and on the other hand will have a direct interface to the machined part.
I wonder which type of material the holder should be made of. It should be super precise (parallelism to the base plate, good flatness etc.) and be durable for avoiding dents and scratches that will damage it and spoil its high precision surfaces.
In addition it should be protected from oxidation, which can lead to rust and spoil its precision.
I thought about a stainless steel with high hardness such as PH 15-5 or maybe tool steel such as D2 etc, but I still think an elegant solution would be some other lightweight metal (for example aluminum) with a durable coating (such as hard anodizing), but I assume that high precision surfaces (for example 0.03 mm flatness) couldn't be achieved with the application of a durable coating.

 

[lucky-guesser]

"be durable for avoiding dents and scratches" good luck achieving that with aluminum. I was thinking tool steel as well, and having some sort of coating added after machining.

 

[elinah34]

You mean a coating to the tool steel?
What kind of coating? And how won't it affect the precision?
Thanks

 

[lucky-guesser]

You mean a coating to the tool steel?
What kind of coating? And how won't it affect the precision?

Yes, the coat the tool steel. Anodized, bluing (hot or cold), maybe electro-plate. I would have to do some more research on which would be best.

It would depend how accurate you need them to be, I believe these coatings would only add a few thou, not sure if that's good enough for you. You may want to machine it a few thou under and then have it coated. You may be able to machine to size after coating but not sure if that would be possible without ruining your finish.

I've made a set of clamping blocks for a mill before. We milled it to rough size with a mill, heat treated, ground to finished size, and then while machining you are always adding coolant/oil/solvents and always wiping things down so we didn't bother to have them coated. If you use the parts frequently they tend to not rust very quickly and you may not need a coating.

 

[elinah34]

Thanks

 

[EdStainless]

Tool steel with electroless Ni plate.
Normalize and Temper, rough machine, Q&T+T and then final grind.
The plate is a Ni-P chemistry and when you age it it will harden a lot.
It is thin enough that you may not need to lap to final dimensions.

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P.E. Metallurgy, consulting work welcomed

 

[elinah34]

EdStainless, thanks.
Why did you suggest this particular coating?
Is there any other alternative?

 

[mfgenggear]

Elnah
I would recommend A-2 tool steel, hard and nick resistant, no coating required. Very stable during heat treat. I used it a my preferred goto for fixtures. Arbors ect. Grinds flat or hold roundness and precise run out, and is very
Available. For small to medium size.

 

[elinah34]

Hey, what do you mean by "nick resistant"?
A-2 tool steel isn't sensitive to corossion?

 

[mfgenggear]

Elijah
A-2 is very resistant to corrosion, a tools enemy is nicks and ding, I remember stoning all the ground faces, to remove nicks and dings from ground surfaces.
It still needs to be correctly handle and coated with light oil during storage. As with all tools.

If you want bullet proof at no expense, engineering chrome. Ground. Soft core for toughness , extra hard surface, harder than
Case harden.
Next best is carburized steel

 

[EdStainless]

Ni+P plating is very thin and very hard when it is aged.
It is also very corrosion resistant.
The D tool steels, like D-2 have enough Cr that they have some slight corrosion resistance.
But they aren't that corrosion resistant.
On issue with plating is that the surface can slick.

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P.E. Metallurgy, consulting work welcomed

 

[mfgenggear]

Tool steels if lightly coated with oil it's fine.

 

[MintJulep]

Is there an experienced tool and die maker at your company, or at your preferred machine shop, or at the tool and die shop that will be making this fixture that you can work with?

Good fixture design is more than "use tool steel" and "make everything super precise".

Material selection is one of the last choices you should make, after you've quantified all of the other requirements.

 

[TugboatEng]

Increasing the strength of steel does not increase the stiffness. You can change the design of your clamps to prevent damage of the surface per MJ's recommendation. If you need accuracy you may consider a material with a low coefficient or thermal expansion such as Invar.

 

[mfgenggear]

It all depends what the tool holder is being used for. If stiffness is required then carbide is used. But now the price goes up , machining gets more expensive.

And absolutely discuss with and experience machinist who is going to use the tool and get input and review of the design.
Worst case scenario a design is made. Geometry wise which does not make the machinist happy.
And he will let you know.

The other unknown factor is this for one of kind special project or a 1000 pcs.
Making one tool is a very expensive proposition. Material ordered, sawed, rough machined, then semi finished machine. Heat treat. Hopefully low distortion Material and
Low distortion process. This part will make or break a po project. It could come out junk.
Inspect, finish operations, NDT, coating or plating if applicable.
So this tool very much needs to be engineered correctly.

 

[EdStainless]

To be honest, we made tooling, even for production out of whatever high strength steel we had that fit the size/shape.
Small items tended to be made from 4130, larger ones from 4335Vmod (we used a lot of it).
Sometimes you want the jaws softer than the parts that you clamp, in which case they need to be replaceable.
If you need rigidity, then you need more metal.
For very high precision we had machines that were temperature controlled.
The cooled cutting fluid was pumped through bearing housings and ways to keep everything stable.
We had tooling that was also cooled the same way.
Invar is too soft for tooling involving metals.

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P.E. Metallurgy, consulting work welcomed

 

[mfgenggear]

Good point Ed, very much like your post.
Standard 3 jaw are hard. And are a general
Holding chuck for general work.
Soft jaws can be 3 jaw style or pie jaws for
When minimum distortion is required to hold a complex geometry or thin wall ring or part.

Look at similar tools and have inspection take a hardness check. I suspect 40 HRc,
It all depends what it is used for and what it will machine.
PS most of all ready know but soft Jaws are machinable. To precisely fit the part. With the least amount of force. Very little if any out of roundness on the parts.

 

[enginesrus]

Either tool steel or something like 180ksi 4340 for tough dent resistant tooling. Most tooling uses hardened tooling points or buttons or pads to locate on.
What parts are supposed to be clamped to the fixture?
Rust?, then this is not a machine that uses coolant?
Rust is kept at bay by keeping oil or rust preventative on the fixture when not in use. If it needs it during use, more information on what the process is would be nice.
You said machined so I'm guessing its a CNC milling machine of some sort.