Practice scraping

[Erik Brewster]

What about starting with a surface that has been ground on a surface grinder? I have several machines that really need cleaned up and was thinking that a surface grinder would straighten and flatten out to create a good surface to start with. Tim

You could certainly scrape from a surface ground surface. It's a litter harder to get a "bite" on the first pass with the scraper.

But why would you scrape after surface grinding? The grinder should leave everything nice and true. You could flake it after to get the oil pockets and be done really quickly.

 

[Richard King]

I read this before and didn't have time to comment. You need to tell us a couple of things first. How accurate are you planning on scraping the machines to will be scraping? You would first need to know how accurate the is the surface grinder you are using.
As you will not be able to get the plate any better then the machine it ground on. That is why we scrape so we can get parts better then can be machined or ground. I have a wonderful little booklet called "Why are they Scraped" and will share it with anyone who wants a copy. Just email me for a copy. Rich Richard@Handscraping.com For those who worry about Copyright I have a permission letter from the writer so I can make copies.

 

[astjp2]

We use the Thompson grinder to sharpen the BT's (dies for punching out aluminum parts) and it is well used but seems to make a straight surface. I was thinking that I could take a piece of 4340 and grind it smooth and flat to use as a precision surface. I dont have a surface plate at my home shop so this would be used to make a straight edge. Would I need to get it heat treated to harder than annealed? Tim

 

[4GSR]

I was thinking that I could take a piece of 4340 and grind it smooth and flat to use as a precision surface. I dont have a surface plate at my home shop so this would be used to make a straight edge. ..... Tim

Tim,

My first "surface plate" was a piece of 1-1/2" A-36 plate about 9" x 12" surfaced ground flat. Still have it here somewhere. Not much good for scraping and fitting, but for other needs worked good.

A ground surface does not "mark" good when taking impressions for scraping. You could scrape the ground surface to accept marking compound, but you still need a master to check it against.

 

[Richard King]

If you ever look at a cast iron plate or cast iron straight edge they are ribbed so they won't bend. A flat bar will follow the contour of a concave or convex part. You might make your machines better using your ground bar, but more then likely with out a solid re-enforced scraping plate or straight edge with ribbing you could possibly make your machines worse. But nothing ventured nothing gained. I have found over the years if a man gets an idea in his head he should give it a try. Tell us long will you make it, how thick and how close do you want to make your machines when your done? Lets talk more about it and attach some pictures as you make your scraping gage. Rich
Below:
Cast Iron Plate.....................Straight Edge top left .... small plate to show ribs ......24" straight edges... triangle cast iron and ribbed camel backs

 

[astjp2]

I have some a2 and some 4340 that is 1.5 square and 16-20 inches long, I can put either on the grinder. I was thinking that I would like to hold .001 from end to end, I don't think it is unreasonable if I am patient. Our machines at work have to hold .0002 with box ways. Tim

 

[Richard King]

You will have to be careful how you push down on it and not bend it into a depression After you grind it lay it on a surface plate and hinge it to see where the Airy or swivel points are. When it is flat the should be about 30% from each end. One more thing as Erick mentioned. a super flat finish will not hold the blue and rub of easy You should 1/2 moon or scrape square cuts in a checker board patern so the blue can have some cavities to hold the blue. When you apply the blue I recommend a soft foam roller like you can buy from a hardware store. 1" x 4". Apply it so you can still see the steel under it. If it's so thick you can't see through it, it will smear and you will get a false reading. A test you can make is blue up your surface plate lay 2 pieces of .0015" or .002" feeler gage on the extreme ends and set your gage on the feeler gages, hold down on it and slide it back and forth a couple of inches and then turn it over and see if any blue has transferred to the bar. Rich

 

[astjp2]

[SUP]This is what I am trying to fix, mill was free to me but I need to get it into a workable condition...[/SUP]

 

[Richard King]

That's Nasty.

You should take it to work and fly cut or mill the ways first. Trying to scrape that would take lots of hours even if you had a Biax Power Scraper. If the machine you using to fly cut is decent you could probably hold .001". Then scrape off the machine marks to give it a smooth surface and some oil pockets. I would also be sure the ways are getting lots of way lube too with a gravity feed oil cup or a pressure pump lube like a BiJur or LubeUSA. Rich

 

[astjp2]

That's Nasty.

You should take it to work and fly cut or mill the ways first. Trying to scrape that would take lots of hours even if you had a Biax Power Scraper. If the machine you using to fly cut is decent you could probably hold .001". Then scrape off the machine marks to give it a smooth surface and some oil pockets. I would also be sure the ways are getting lots of way lube too with a gravity feed oil cup or a pressure pump lube like a BiJur or LubeUSA. Rich

I was thinking of getting a chi com one shot, the bijur pumps are 500+ without lines, distribution blocks or nozzles. I wish there was a chart with the size of nozzle for each way, gets confusing with 10-20 lube lines running everywhere....