Speed/feed recommendation

PackardV8 · Feb 25, 2018

Automotive machine shop with new 3-jaw chuck with finger jaws to hold aluminum pistons by the oil ring groove. We're going to be cutting .050" - .070" off the flat top 4" diameter pistons.

What spindle speed and cross feed speed would you select? How much cut per pass?

FWIW - the tops of new pistons show a definite visible fine thread pattern, as they were probably cut as fast as possible on a CNC production lathe, so mirror finish is not necessary.

Some pistons show evidence of the center having a slight depression as if having been touched with a tool in a tailstock prior to lathe tool cross-cutting. Our assumption was so there would be no chance of leaving a tiny pigtail thread of aluminum in the center. Thoughts on that?

jack vines

Limy Sami · Feb 25, 2018

You don't say how many you have to do, so I'm assuming ''enough for an engine or two''

Personally I prefer to hold pistons in tall soft jaws - bored to fit.

I've never used special jaws that fit in the ring grooves and on the face of it, do NOT like the idea.

IIRC you're an old school type machine guy, pistons can be notoriously fragile so I'd play safe and be gentle, run around the 200 rpm mark (<> 200ft min) ...........with a DOC of .015 max & .004'' ish per rev feed, ......which should me 4 cuts to do the job.

Pistons are usually a high silicon Alu, which is abrasive so an Alu specific inset or brazes carbide tool, honed to about a .020 rad - larger rads tend to increase the cutting forces - which with flimsy parts you don't need.

PackardV8 · Feb 25, 2018

Limy Sami said:
You don't say how many you have to do, so I'm assuming ''enough for an engine or two''

Personally I prefer to hold pistons in tall soft jaws - bored to fit.

I've never used special jaws that fit in the ring grooves and on the face of it, do NOT like the idea.

IIRC you're an old school type machine guy, pistons can be notoriously fragile so I'd play safe and be gentle, run around the 200 rpm mark (<> 200ft min) ...........with a DOC of .015 max & .004'' ish per rev feed, ......which should me 4 cuts to do the job.

Pistons are usually a high silicon Alu, which is abrasive so an Alu specific inset or brazes carbide tool, honed to about a .020 rad - larger rads tend to increase the cutting forces - which with flimsy parts you don't need.

Hi, Sami,

Thanks for the reply.

Agree, the "tall soft jaws" are great for old-school tall-deck cylindrical pistons. However, today's very short slipper skirt pistons don't hold well in them, thus the fingers in the grooves.

We're anticipating doing only one set of eight per week.

jack vines

Limy Sami · Feb 25, 2018

Ah' ......I'm with you now Packard, slippers are a different kettle of fish, and as you're doing '' a set a week'' ring groove jaws sound a good option, ......however I'd be extra careful chucking the pistons, ...and build in enough meat a couple of re-cuts of the jaws, for IMHO any wear will cause both gripping and concentricity / wobble problems.

packardbill · Feb 25, 2018

Perhaps you could make a rod same diameter as wristpin, only longer, with a hole in each end to bolt to faceplate....I don't know how the top of the piston would indicate with the tail of the slippers bottomed to the face plate. You could always shim one side. PB

Limy Sami · Feb 25, 2018

Another option is to use a Keats Angle plate like this, RDGTOOLS KEATS ANGLE PLATE FOR ROUND BAR WITH CLAMP FOR MYFORD LATHES | eBay

packardbill · Feb 25, 2018

That Keats plate is a GREAT idea. I hope it made Keats rich.

Another thought on my idea is to use a 4-jaw with removale jaws. Bolt the through bar to the jaw mounts,shim the slipper tail if needed. If the through bar is exact size or .001" smaller, you don't need to bear down on the clamping bolts, just snug them so you don't deform the slipper ends. Make light cuts like SAMI recommended. I'm also wondering about increasing speed as you face toward the center, you may have to, for finish. Not a problem for variable speed lathe or maybe even a vfd, but belt/cone might be a problem.
Will you be coating the pistons?...Can't offer any knowledge here, just my own inquisitiveness. PB

Clive603 · Feb 25, 2018

Fixture on the milling machine is the way I'd tackle the job. But most of the piston work I see is opening up valve cut-outs for BMW saloon car racers.

Fixture usually pays for itself in time saved and confident accuracy after 20 or so gotta be careful items.

No fan of piston work in the first place and absolutely no fan of spinning up such relatively delicate, and usually expensive, parts in any sort of lathe chuck.

Clive

Tonytn36 · Feb 25, 2018

DO NOT use the 3rd ring groove to hold the piston! That is not the proper way to clamp one.

That groove is machined with diamond tooling to a flatness of 10 microns or so and a straightness of 5 microns. These are required for sealing.

Proper clamping is done with a clamping clevis and pin through the pin bore and against a plate to the datum counterbore machined into the bottom of the piston. You clamp it the same way it is loaded when in operation and your tooling looks similar to the top end of a rod. Do not over-tighten, you may crack the lower pin boss. 700-800lbs max axial load on the pin depending on the pin boss thickness and whether the lower end of the pin bosses are machined or not.

Also, check the crown thickness before machining. If they are performance parts, there may not be enough material to remove without risking burn-through. Order the correct compression height parts if possible. I would advise against going any thinner than 0.178".

PackardV8 · Feb 26, 2018

These jaws are industry standard. I checked around and most shops who do the work find this the most cost-effective method of cutting flat piston tops.

But yes, cutting valve notches is a mill op.

jack vines

Tonytn36 · Feb 26, 2018

PackardV8 said:
These jaws are industry standard. I checked around and most shops who do the work find this the most cost-effective method of cutting flat piston tops.

But yes, cutting valve notches is a mill op.

jack vines

That isn't a piston manufacturing standard, that may be an uninformed aftermarket standard, but it certainly isn't a manufacturers standard by any means.

Limy Sami · Feb 26, 2018

Tonytn36 said:
That isn't a piston manufacturing standard, that may be an uninformed aftermarket standard, but it certainly isn't a manufacturers standard by any means.

I know I said earlier that was an option, but I agree with Tony and don't like the idea much at all, .........if they're slipper pistons as in short, I'd still put them in tall bored soft jaws do I was gripping as near the top of crown as possible - where I had something solid to bite down on.

FYI Either brew your own from Alu, or get Dillon to quote you Special Tongue Grove - SOFT JAWS - Dillon Manufacturing, Inc.

PackardV8 · Feb 26, 2018

Tonytn36 said:
That isn't a piston manufacturing standard, that may be an uninformed aftermarket standard, but it certainly isn't a manufacturers standard by any means.

Sorry for the lack of specificity. No, it's not how production facilities turning out new pistons do it. It is in fact the engine remanufacturing industry standard; especially for those remanufacturing diesel engines, where after milling the block deck, the piston height has to be set within a very few thou.

FWIW, today, we just cut .050" of the top of two sets of pistons. Worked like a charm, turning @ 500 RPM, with a slow cross feed.

jack vines

MortyNTenon · Apr 19, 2018

Must be why they already make jaws to do this job. Sometimes simple is good enough. I have put a fitted shaft through the pin hole and clamped it down to the table on a vertical mill also, but usually when I get to make some sort of chamber. Clamp down a few at a time.

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Speed/feed recommendation

PackardV8

Hot Rolled

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Diamond

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Diamond

packardbill

Cast Iron

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packardbill

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Clive603

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Limy Sami

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