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camel back straight edge not as flat as I would like.

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bmikkalson

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St, Paul MN
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Manyl ight passes and cbn cutter. I very lightly clamp the part. But on the surface plate it hinges in opposite corners.

The surface quality it great.but the mastor tells a different story.
http:// [URL=http://s28.photobucket.com/user/bmikkalson/media/Mobile%20Uploads/IMAG0510.jpg.html][/URL]
On the last cut I have the clamps just snug. And take off' just a a dusting. I am going to experiment with some shims tomorrow. A and the 3point if I have some jacks.

I was hoping not to scrape all this down. Or does cast iron move this much?
 
It could be flexing in your setup, you have it clamped to a thin flat bar with quite a distance between centers. The cutting tool will leave a nice surface when working the large flat area of a cylinder head. But on that thin surface the cutting tool could be flexing the bar giving you the twist you see in your SE? At least it sounds reasonable to me when I think about it.

Charles
 
Ben you will want to mill all sides of the SE before scraping the flat. Also has the SE been High Temp Stress Relieved? Wring it, etc. I would recommend you machine it, stress relieve it and machine it again before scraping it, or it will move or never settle down while your scraping it or scraping a way. Rich
 
Hi, the set up is a 2 in bar and the plate is 3/4 thick. I'm not certain if it's my set up or the SE is moving after. I have the clamps barely holding it on and taking .001 cuts. ????? Idk
 
Ben you will want to mill all sides of the SE before scraping the flat. Also has the SE been High Temp Stress Relieved? Wring it, etc. I would recommend you machine it, stress relieve it and machine it again before scraping it, or it will move or never settle down while your scraping it or scraping a way. Rich

The SE has been anealed I have done machining to 3 of the 5 sides. Just for kicks, I'm gonna throw it out side and see how much it moves.
 
Hi, the set up is a 2 in bar and the plate is 3/4 thick. I'm not certain if it's my set up or the SE is moving after. I have the clamps barely holding it on and taking .001 cuts. ????? Idk
.
we would use trim pads. that is blocks bolted down and marked up with blueing and a .001 taken off
so it is machinned and then checked with .0001 indicator attached to spindle to confirm within .0002
these trim pads freshly machinned being flat are then used as support for part
........ then your part is put on and where you have a clamp use a indicator and make sure it does
not go down as clamp tightened, we use .0010 and .0015 and .0020 shims from starrett feeler gage
stock the difference if .001 and .0015 on opposite ends is .0005. after machinning check with indicator
and then release clamps and check again it often goes up after finger tight clamps are loosened.
also finish cutter needs to be sharp. often we change inserts if we are getting waves .0003 where
passes overlap
........ if we supported part at 3 spots it would be at Airy points
http://en.wikipedia.org/wiki/Airy_points
.......... if supported not at exact Airy points part will sag from its own weight.
 
.
we would use trim pads. that is blocks bolted down and marked up with blueing and a .001 taken off
so it is machinned and then checked with .0001 indicator attached to spindle to confirm within .0002
these trim pads freshly machinned being flat are then used as support for part
........ then your part is put on and where you have a clamp use a indicator and make sure it does
not go down as clamp tightened, we use .0010 and .0015 and .0020 shims from starrett feeler gage
stock the difference if .001 and .0015 on opposite ends is .0005. after machinning check with indicator
and then release clamps and check again it often goes up after finger tight clamps are loosened.
also finish cutter needs to be sharp. often we change inserts if we are getting waves .0003 where
passes overlap
........ if we supported part at 3 spots it would be at Airy points
Airy points - Wikipedia, the free encyclopedia
.......... if supported not at exact Airy points part will sag from its own weight.

This is a very good idea, it solved how I am going to mill the table. . One question is these airy points called the three point way ? I have learned that 3 points about 1/3 separated. Is exact placement critical ?
 
Look at the book I gave you Ben during the class.... The hinge points ....I use and have used 30% for years and not the 25%.

I have to stop reading this as My blood pressure is sky rocketing.
 
class

This is a very good idea, it solved how I am going to mill the table. . One question is these airy points called the three point way ? I have learned that 3 points about 1/3 separated. Is exact placement critical ?
.
If you took a class on rebuilding machines and scraping maybe you should review your class
notes and any other info you got from the class
...... if you took the class from Richard he has probably 100x more experience than i have at precision
machine rebuilding
 
Once you get within ~.002-.003 all over on a large area (if you can get that close) on a turret mill, that is about as good as it gets unless the machine is brand new and everything is "perfect". That barely covers the tooth mark depth for a lot of situations. Your part looks that close, .003" anyway. From what I can see from my computor screen anyway :D

Even if your part has stress in it, that is not the problem today if you have already made several passes and still get the same results. It will move the first roughing pass, a little more the next, but by the time you are merely skimming .002 at a time, it is not moving "right now" though it might again tomorrow. But the point is, when it moves, the marks won't be the same you "keep" getting off the machine.

What machine are you milling it on? It is somewhat unusual for a turret mill to make a casting hollow. The way they wear, usually they face slightly convex.

As Richard notes, if you are going to invest the time to make this a precision tool (meaning you are going to have to scrape it) then by all means rough machine it and then have some form of qualified stress relief done. I used to make and sell SE's and I would get them from the foundry, rough machine them "all over" and take them to Elmira Heat Treat for thermal stress relief. At that point, they usually came back about .005-.007 out of flat ~3" x 30" long. At that point, I sold them because you can imagine doing a further light machine and another stress relief could add 50% or more to the cost. Also, most people want different angle configurations, etc. so it was pointless to charge everyone for something only a few would use. For myself, and I have a planer, it was seldom worth it to even mount the casting back on a machine at that point. Knocking even .007" off the rocking point(s) as a rough scraping operation and getting down to ..002 (where it would reliably come off the machine without significant time and effort) went faster than the machine set up and time, and puts less stress back in the casting.

Every time you machine a casting, it puts some stress in it.

All these reasons are why precision surfaces and alignments are scraped. If you have a really good grinder and perfect process control, you can grind. But that puts stress in, too. And the average job shop grinder is still going to yield a surface that will blue up about like you show, though the low and high spots will only be a few or single tenths different. This will be a combination of thermal issues, stress in the casting issues, machine imperfections, foundation on which the machine sets, and operator control/setup/inputs including how it is mounted to the magnet and residual magnetic influence. That blue will show 50 or even 10 millionths, if you are using it thinly.

You can probably have that part hitting all over in less than 10 minutes with a scraper, except there will be some holes, and the bearing quality will be coarse.

Point being, the reason people scrape at a given point is not because they love doing it and like to give so much money to the chiropractors, but because at a certain point scraping is just way faster and more efficient that trying to machine any closer. If you have a way grinder and maintain it every day and keep good process control, you can get within tenths on really big areas and just flake for bearing/oil/knock off the few obvious contact points and everything else is flat. In the small shop, you are probably going to have to scrape when you get to within .001 on a small casting, and .003-.005 or so on something "large" off a turret mill.

You are closer than that, though it will probably take .003 - .005 on that to get flatness and get under all the residual tool marks. That is very quick work with a biax and probably still faster even by hand than to keep trying for "better" given your machine, existing set ups, and experience so far.

smt
 
on the big cnc gantry mill i use, we have to do a lot of maintenance and calibration checks to get it
to mill flat within .0005" per 40"
.
we occasionally get stuff needing repairs or calibration adjustments because it will mill over
.0010" per 40" off......... i agree with some machines getting anything better than .002" per 40"
might be impossible on those particular machines
 
Stephen unless I am mistaken he is using a head milling machine specifically used for facing engine heads. It is similar to this one and you can see the bar he has mounted it to between centers.

Charles
 

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Look at the book I gave you Ben during the class.... The hinge points ....I use and have used 30% for years and not the 25%.

I have to stop reading this as My blood pressure is sky rocketing.

Haha, the book I have shows a hand sketched hand with big thumbs. Lol. I question if 30 or 25 percent is really measurable with the eye. The problem I have is that it is not hinging at 30. So I need to figure that out first.
 
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