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What are the typical tolerances achieved, as forged, on a round, 6.5in dia. round part, when forged in 1020 steel. See drawing for final tolerances.

RE: Forging

As always, it depends quite a bit on the details.  Due to the size of this part, it would have to be hot forged instead of cold forged/extruded.  Hot forging is usually performed with flash, which is then subsequently trimmed, but it can also be performed in closed dies with no flash (usually on horizontally-oriented mechanical presses).  When forging with flash, there are multiple tolerances that must be added up (die mismatch, trimming, etc.).  With closed-die forging it is simpler.  I would estimate an outer diameter tolerance of + 0.075 / - 0.035 using the guidelines from the FIA Forging Handbook and DIN EN 10243-2 Steel die forgings - Tolerances on dimensions - Part 2: Upset forgings made on horizontal forging machines.

RE: Forging

Great and thanks for your input. A followup question: if it is hot forged, and there is flash and it's cleaned, can a closer tolerance be held in the cleaning process, i.e. +or-0.050?, or do I miss-understand the cleaning process. A second question: what is the most cost effective process to produce this part? Cast, forged, bar stock, and if cast, what type? I know this is a lot to ask, but you seen to know your stuff.

RE: Forging


No matter what manufacturing process is used, a part of that size will need some type of secondary machining operation in order to achieve an OD tolerance of +/- 0.050", as well as achieving the desired shape on some of the diameter transitions.  Cost depends a lot on the number of pieces required: for 10's of pieces, machining from bar will be lowest cost.  Casting will likely be cheaper than forging under most circumstances, but will not have the same mechanical properties, fracture toughness, or surface finish.  Feel free to post some additional details about the application, and I'm sure we can give you some useful feedback.

RE: Forging

IVP, thanks for the great input. The part is a piston for a mud pump. The entire OD will be coated with Urethane. So, the strength of a forging is needed. The customer is currently machining from bar at a high cost. Quantities are 3600 per sized of 4 sizes per year. So, a forging, nearest possible to net shape, requiring little machining is best. What are your thoughts? The 4 sizes are max. OD of 5, 5.5, 6.125 and 6.5 in.  

RE: Forging

>>>So, the strength of a forging is needed.<<<
No requirement stated so far leads to any such conclusion.

You have not stated a tolerance on the linear dimensions controlling the axial size of the various steps.  You have not stated a tolerance for concentricity or colinearity.  Your drawing does not state a standard by which to interpret it.

Forgings of that general shape and size tend to be hollowed out somewhat to reduce the shipping weight, if nothing else.

My first guess at the cheapest process for producing the stated quantities and sizes is to buy circles flame-cut from thick plate by a steel distributor, edge-clamp them at the bottom to a plate, and let a CNC vertical mill gnaw on them, orbiting end mills, basically unattended, to produce the central hole and all of the reduced diameter steps, then flip the part onto a pilot in the center hole, clamp it with a stud in the pilot, and machine the OD last.

That's based on WAGs about the missing tolerances; a proper drawing could change my recommendations.


Mike Halloran
Pembroke Pines, FL, USA

RE: Forging

I would recommend you look at dutile iron for this application and have the raw parts cast and machined after casting. If you need really high strength then have it heated-treated into ADI.

The bar stock approach is not a bad starting place depending on how much the CNC lathe time is priced. This is not a particularly good machining center part. A CNC lathe will remove the material faster and have a faster cycle time.

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