PDA

View Full Version : Milled surface for scraping?



J Tiers
02-26-2009, 09:57 AM
I have an item which I have to scrape-in which was milled (no by me) to approximate shape. (It's new, meaning it was cast a while back, but never scraped or flattened before). The part is about 50mm wide, by a half meter or so, and needs scraped on two sides.

I had heard that milled surfaces were good to scrape, but I am not finding that. The surface is (ok, now that would be a "was") somewhat "washboard", with "ripples" about 1.5mm apart, that go clear across in the 50mm dimension. The depth isn't large, possibly three or 4 thous.

I have NO idea how the ripples were made, supposedly the thing was milled to flatten it and take off the casting "rind". It seems to be heck to get these ripples out. I would have re-cut the surface, but I don't have the travel to do it in one pass.

The first thing I did was to shovel off the surface to get under that rippling, because it was not going to give a decent surface.

Even though the surface appears to be smooth now, those darn things are still showing up in the markings. it is as if they hardened the cast iron and it is still affecting the "cut" of the scraper. (Yes, I HAVE diamond-honed the scrapers).

Anyone had this and have a suggestion besides shoveling off the surface even farther?

Jpfalt
02-26-2009, 11:35 AM
Go to a scraper ground with a nearly straight front profile and scrape in the direction of the crests of the ripples. I find that levels out the surface in a hurry.

lazlo
02-26-2009, 11:41 AM
Jerry, when I resurfaced that beater cast iron surface plate I found on Ebay, I facemilled it mirror smooth. It wasn't hard to get the scraping going with a mirror-honed scraping blade, but if you don't have a hone, you might want to scratch the surface up with some medium sand paper.

At the first scraping class in Georgia, a lot of folks were complaining about scraping past the Blanchard grinder marks on the Durabar blanks. I'd try knocking those down a file or stoning, or even sand paper if you have to -- you're going to be scraping the whole surface anyway, right?

Top is done, bottom was a roughing pass (the plate had some serious gouges in it):

http://i164.photobucket.com/albums/u15/rtgeorge_album/SEHInserts019s.jpg

Spin Doctor
02-26-2009, 12:06 PM
In general milled or planer/shaper surfaces are much easier to get the scraper to bite into than ground surfaces. One reason is the ground surface is slicker. Another IMO is untempered Martinsite in the surface material. As to the ripples in the surface, I suspect they are a signature of the machine the piece was milled on. You're just going to have to knock them down. No way around it

Mcgyver
02-26-2009, 12:13 PM
file them out. sometimes for when really roughing it, I'll use a file rather than a scraper. go at it the same way, spot it and file the blue....might hasten things if you have to down 3 or 4 thou

J Tiers
02-26-2009, 10:43 PM
Well, I did as mentioned, shovel it off with the scraper (yes I have a hone... Glendo, with 1200 + wheel ), although I think there is something going on since I KNOW I am below the marks in some places, which are smooth and mark smooth. Other washboard spots showed up intermittently while I was scraping heavy to get the surface to mark all over.... the ends were marking light, as was one part of the side... if I get smooth marks over an area, and later get some washboard marks, it suggests the surface was not so smooth after milling...

I had thought the surface was just hit with a slab mill, and a fine feed. But the slab mill would have left a flat surface across, with the table possibly causing a little lengthwise "cupping" or whatever if it sagged. This has a bit of a twist. Maybe the part relieved after milling.

And, a bigger slab mill wouldn't have left such a fine washboard.

What sort of milling leaves a washboard with the peaks only a mm or so apart, and several thou deep? I've used a slab mill, and it didn't leave that sort of mark.

Jpfalt
02-27-2009, 01:43 PM
If the slab mill is not ground to equal radius on all flutes or the arbor system runs out at all, you can get a washboard just like you can with a face mill with a tooth hanging out a little.

Back to my prior comment, go to a scraper with a large front radius and scrape in the direction of the crests. It will take off just the peaks.

oldtiffie
02-27-2009, 02:07 PM
I think that the slab-milling cutter has been either blunt or eccentric or on a bent arbor or the bearings in the arbor supports are either too far apart or worn.

The end result is that the cutter will "rub" and climb over the job and eventually one tooth will "snag" and get "pulled in" when it gets under work (rub) hardened surface.

Horizontal mills are notorious for "wash-boarding" - same applies when using side and face cutters, form/gear cutters etc. You can often see it. It has a "thrum - thrum - thrum" sound which is a dead give-away.

Breaking the work-hardened surface will cause the job to "ease stresses" and "relax" (and deform). It broken progressively, it will ease stress and relax/distort progressively as well.


I'd have faced it on a lathe or fly-cut it on a mill to get under and rid of the work-hardened surface to "ease"it all in one go. It may haved needed another "follow up" cut after that before scraping.

Forrest Addy
02-27-2009, 02:37 PM
On a challenge oncw, I scraped in (rough scraped mind you, jsst a few spots) a hunk of cast iron from a sawed face. As a face it was already pretty flat but the saw striations took some time.

In any case, unless the scraped surface forms a lubricatedbearing A few vestigail tools maks wont impair the fit or function. I've seen may a field joint scraped for fitup and alignment that still had tool marks on it.

Anything you have in your home shop has to be first rate of course if you doon't5 want visiting yokels to snigger at you. Me, I'm secure in my slovenliness and ignore the japes.

J Tiers
02-27-2009, 10:44 PM
Its a straightedge...... I was being secretive because the person who I got it might read this forum, and I didn't want to seem to be complaining.

I'm not complaining, I'm asking two things... 1) what sort of milled surface is the one everyone says is great for scraping, and 2) what on earth made these deep and narrow marks? (I'm not gonna ask, see above reason).

I've seen the washboarding on my own mill, but the key is that these were SO close together, and SO deep, relative to their width.

I'd have not thought anything of it if the stripes were 2 or 3 times as wide...... I probably wasn't clear that these were so close together and deep as to resemble the marks from a roughing cutter, but going the wrong way....so it is NOT possible that it really was a roughing cutter.

I have them almost all the way gone, but obviously they need to be PAST 'gone" for the purpose intended...... so I have more work to do.

Spin Doctor
02-28-2009, 11:50 AM
Having seen the washboarding effect on mills that have gears trains (I suspect that some of it comes from gear train resonances) I feel your pain. Thats one reason that anything finished we would always finish grind if possible. Also I was wondering if the cutter used had a wiping insert that is set about .001" higher than the cutting insert (it would look like a reverse hand insert laid one its side). These can be notorious for rubbing.

As an aside if you ever get the chance to look at the milled surface on a late model 4.0 liter straight six cylinder head from a Jeep the surface will not be a smooth milled surface. It will actually look like a scraped surface with very wide scaping marks. This is done for gasket seating. Older ones were finished on a broaching machine with a tool set about 15 feet long