Large Workpiece Machining

[brengine]

We have some components for our machine that in the past have always been cast iron. We recently tried starting with a solid chunk of P20 steel, and machined everything. We are very plesed with the results, and the component life is at least double the cast version.

Since we start with a solid chuck, there is a ton of machining spindle time required to manufacture these. The initial block is ~36" x 36" x 18" thick. We literally machine away probably 1/3 of the material. There is a large pocket in the center ~24" x 24" x 6-8" deep. Some times the entire face of the part gets machined down 4" except for maybe a 10" x 10" area.

My question is...how can we reduce the machining spinde time? Is there a laser (or other) cutting process that can go through 18" of material? Or some type of band saw? Or is there a manufacturer out there that can supply us the raw material closer to the final dimensional size? Or ???

Thanks,
Ken

 

[mrainey]

A steel forging sounds likely.

Software For Metalworking

http://closetolerancesoftware.com

 

[ajack1]

Why not just cast the part in P20 or something very similar?

 

[arunmrao]

Good suggestion by ajack 1. I have cast die molds in P20 material and saved considerable material and machining hours for the customer. The parts to be formed were in plastics or fibreglass. Additional advantage being the short turnaround time to produce a die mold.

I have not failed. I've just found 10,000 ways that won't work." — Thomas Edison
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[KENAT]

It sounds like you've changed 2 things, the material grade and the process. Do you have any idea which is causing the improved performance or is it a combination of both?

Have you tried casting the part from the improved material like ajack & arunmrao mention.

Else my first response was the same as mrainey though this is a pretty big piece.

Some suppliers will supply pre-cut material - for a fee, you may want to shop around.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[looslib]

Depending on volume, I would try the cast or forged P20 block. Either process should be able to get you close to the final shape.

"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

 

[Albigger]

Steel forging, as has already been mentioned.

We've used this process in the past to save considerable machine time (although I haven't specifically done P20, we have done some decent size blocks, 50x50x13" thick).

--Jay

 

[BillPSU]

If you decide to change to either a casting or a forging you will have the pay for the tooling for either process and you will have the leadtime to make the tool. The casting tooling for the cast iron will probably not work with the steel as steel has a much larger shrink factor and the risering the casting mold will be necessary to avoid shrinkage problems.

 

[Albigger]

Just to clarify, I think looking at open die forging would be the most economical for you. The shapes that you specified in your opening post seem simple enough that it should be able to be done or at least come close with open die.

 

[arunmrao]

BillPSU,If it is a casting ,tooling cost is very marginal . This is because polystyrene (popularly known as thermocol) material can be used.

It is very quick and economical,disadvantage can't make multiple castings.

I have not failed. I've just found 10,000 ways that won't work." — Thomas Edison
_____________________________________

 

[BillPSU]

arunmrao, the gating and runner system and contolling porosity and shrink will be a problem. Finding a foundry will also present an issue. The number of foundries that can melt and pore this material with a large enough flask will also present a problem.

I know that poring this casting can be done. Another question is where bregine is located?

 

[MikeHalloran]

Only 33 pct stock removal? Guys who mill aircraft spars from billet (>95 pct removed) would laugh at you.

If you're not using CNC machines to do this work, that would be a good place to start. A medium size horizontal machining center with a strong spindle and chip conveyor should be able to do a part like that overnight, maybe unattended once you get the process stable.

Whether CNC or not, most machinists tend to baby their cutting tools so as to maximize edge life.. at the expense of machining time, for which they get paid by the hour, so you can see the incentive.

The thing to do is go through each part cut by cut with your cutting tool supplier and maximize the metal removal rate. You can probably cut the machining time in half, and maybe go beyond that.

If chips aren't hitting the ceiling, you're not pushing hard enough...



Mike Halloran
Pembroke Pines, FL, USA

 

[mechvhstech]

Not to beat a dead horse, we ran into P-20 machining a few years back and for productivity gains and as a cost cutting measure switched to PX-5 (aka modified P-20-RC 29–33 prehardened ) that machines about 30-40% faster), we also changed our tooling to increase tool life -> yes we did the above an Okuma HMC with box ways.
Hope this helps.
Vic

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Mech Rulz

 

[DDRSS10]

Try plunge milling, I've had some amazing results with that method.

 

[david91784]

I agree that plunge milling is the way to do the job. Most horizontal mills have higher trust on the z axis...so more removal rate. By plunging there is much less vibration especially on deep cuts, which allows for longer tool life. Side milling, especially on deep pockets shortens the life of the cutting tool and puts more stress on the machine tool at the spindle bearings...much more vibration. If the pocket deepth is extreem, program the plunge cut to back away from the x or y axis about .100 to allow more chip clearance at the body of the cutter holder. The cuts will still be parallel and vibration will be even less. The result will be fast removal rates with little down time for tool changes. This will be cheaper than buying a forging and having to cut through the slag skin on the forged pocket. Delivery time of a billet is much quicker than a forging.

 

[dicer]

If by saying plunging they are talking about drilling with carbide insert drills with through spindle cooling then yes that is a faster way to go.
Everyone is mentioning forging, but you said it used to be cast iron? Why not cast steel? There are many heavy equipment parts that are made of cast steel and hold up very well.