Why scrape when you can grind?

[michiganbuck]

I think that the waviness of a poor grind, and the scallops of a scraping job are not deep enough for Oil grooving..that the oil grooving should be at 45* (or what angle) and about .003 or so (or +) deep, and 1" (or what) apart.. because the scraper scalps may only be .0005 or so deep and not deep enough for insuring/spreading a film of oil.

Grinding is just about never as flat as scraping.

But I am not a scraper hand so this is just my opinion.

 

[lucky7]

With all due respect to Bridgeport, their flaking on top of their knees is dumb. Catches swarf and grit and causes premature wear. Flaking should be on the saddle, not the knee. But then salesmen wouldn’t see it and say ‘wow, no wear’…

 

[johansen]

I dont think it would be that much work for whoever wants to... Take say a 6 inch by 1.5" ground and flat surface, lapped to ensure good global flatness, cut your oil groove as standard, (diagonal across 80% of the plate) then take a 1/16” burr and grind a cross hatch pattern on .25" centers, out to 1/8th inch to the edge of the way surface. (This will limit you to about 70 to 80% contact with ample oil delivery, with 16 points per square inch of contact)

Then compare its stick slip with a scraped surface with the same single oil groove (no extra grooves).

For the mating surface, use the same surface plate you scraped them on.

 

[johansen]

because the scraper scalps may only be .0005 or so deep and not deep enough for insuring/spreading a film of oil.

Its not just that, its the natural rise and fall of the scraper's hills and flat topped mountains (after some wear) that make a sort of kingsberry wedge to push oil up to the point of contact.

I forget where i read this but it was some long document regarding the geometry of the oil delivery groove. You do not want a simple ball end mill. More like a ball end mill, then kiss the top of the groove with a bigger ball end mill so you get a 10 or less degree chamfer to make a wedge for the oil to be dragged out of the groove by its own viscosity. I think it was some document regarding the limits of box ways and how fast your rapids can be.

 

[boslab]

The National health service ( I kid you not) bought grinding machines to finish the ball on hip implants, they went super finish I read, that’s when the problems started, beautiful surface on the ball, perfect fit into the cup, problem there was the surfaces virtually “wrung” like slip gauges, the ball started tearing lumps out of the socket by cold welding, cohesive forces at an atomic level ( slips will weld if left long enough ) two ground surfaces of the same material apparently rip each other to bits , fascinating.
Scraped can be “ flatter” than ground though not “smoother” I read, scraped holds oil where ground sheds oil besides I do like how scraped surfaces look,
Especially “ Moore” scraping, it’s like calligraphy
Mark

 

[EPAIII]

So, what did they DO? Scrape the balls? And the sockets? I want to see a video of that! Or just stop with a coarser grade of abrasive? Cut oil grooves? Dimple them with small hammers?

Seriously, what was their solution?

The National health service ( I kid you not) bought grinding machines to finish the ball on hip implants, they went super finish I read, that’s when the problems started, beautiful surface on the ball, perfect fit into the cup, problem there was the surfaces virtually “wrung” like slip gauges, the ball started tearing lumps out of the socket by cold welding, cohesive forces at an atomic level ( slips will weld if left long enough ) two ground surfaces of the same material apparently rip each other to bits , fascinating.
Scraped can be “ flatter” than ground though not “smoother” I read, scraped holds oil where ground sheds oil besides I do like how scraped surfaces look,
Especially “ Moore” scraping, it’s like calligraphy
Mark

 

[michiganbuck]

Good to know that one can sometimes grind a part to .001 and better before scraping. The wheel choice and grinding techniques need to be such that the wheel will not place surface stresses in the part. The part should be plate checked and the error mapped and the to-be-taken amounts marked on the wheel-facing side. Then the wheel facing side indicated as in set-up to know that only the error will be taken and the setup secure so the part can't move. . *Consider that a strong magnet can suck down and distort a part so as to make it still in error, consider that grinding can heat the part and that swelling can make the part still in error, any stresses put into may make the part wish to distort over time, and if the part is surface burned during grinding deep stresses may be put into the part that can not be scraped out.
*The bottom line on grinding is that only an experienced/seasoned grinder hand should attempt pre-scraping grinding, or the part may finish less than desired.

note: I am not a scraper hand, but have done some scraping.

 

[michiganbuck]

I should add grinding is a roughing operation and not as close as scraping. Done wrong it can do more harm than good.

The likes of a cast iron surface plate with .008 wash out can be Blanchard ground to .001, and then scraped in for near zero.