Thread: Scraping and Struggling.

Hi BT,
The way I'm seeing it now, you dont have any alignment issues until you machine the face with the spigot on it. (other than the bevels to the face)

Give it a go, if you cant get all three faces as good as you want you wont lose much height machining enough off to start over. I think I'd start by machining the bevels a little deep to ensure the flat faces meant, then scrape the flat face until the bevels meet(?). Sure sounds easy enough

On the bevels you could over scrape the middle 80mm so its touching on the ends.(the same way they have removed the center section of the dividing head and the other table.) You can also machine the middle of the flat face of course. That will make your life a little easier.

Stuart

Bob
I would think about this job overnight.
There have been a few suggestions, however, if these are digested into the brain overnight, a simple solution often presents itself.
Thats been my experience
The job is too good to mess up !
regards
Bruce

I skipped over a few replies since I have been up for...let's see...18 hours now. The order of scraping starts off with a known, quantified surface reference. You then transfer all that flat goodness to piece one by scraping it with reference to the master. Then you can use that newly flat component to transfer the quality flat to the other machine elements.

Bob, It is apparent that I need to ad some water soluble Canode spotting ink to the package soon to be enroute. Others may differ, but I still like it best for scraping.

Next month I shall have a nifty Czechoslovakian granite surface plate available for a special forum price. Kindly advise if you'll be coveting it. I shall also be in PER sometime in Feb for tuition. I scrape for coffee.

Greg

Hi Greg,

Originally Posted by Greg Q

The order of scraping starts off with a known, quantified surface reference. You then transfer all that flat goodness to piece one by scraping it with reference to the master. Then you can use that newly flat component to transfer the quality flat to the other machine elements.

But is Bob really interested in it being flat? or does he just want it to match what he has?(with any luck at all its pretty flat anyway).
He could check the other table he has(the one the dividing head normally goes on) against his master, then check his dividing head to that table But this assumes he wants to scrape both of those, if he doesn't want to scrape them ATM isn't the best he can do to match the dividing head as it is, there is no movement after all?
If at some future date he chooses to scrape the table and the dividing head he will of course have to rescrape the new base.

Stuart

Originally Posted by Stustoys

Hi Greg,

But is Bob really interested in it being flat? or does he just want it to match what he has?(with any luck at all its pretty flat anyway).
He could check the other table he has(the one the dividing head normally goes on) against his master, then check his dividing head to that table But this assumes he wants to scrape both of those, if he doesn't want to scrape them ATM isn't the best he can do to match the dividing head as it is, there is no movement after all?
If at some future date he chooses to scrape the table and the dividing head he will of course have to rescrape the new base.

Stuart

You may be right Stu, I'm too shattered to honestly digest it all, although I imagine its pretty straightforward. I wonder why I can still type though? At any rate; matching mating, fixed components to each other can be cheerfully done absent any third party flat reference as you so rightly point out. I do the same when scraping woodworking tool components into a more intimate relathionthip* with each other.

*Did I listhp just then?

Greg

I think Greg needth thum coffee. Or maybe thleep.

I think you're right Stuart, I retract all my waffle about references. Bob I wish I had something helpful to say about the alignment problem but it's well beyond my experience. Where's the other Phil when you need him? The un-handlebared one.

I heard that (insert smiley here)

Bob, it would be interesting to see the fit of the bits you currently have (I think I've picked up that you have a mating pair and want to match up a 3rd part). Given the precision involved in getting three surfaces to match up exactly, I'm leaning towards the idea that only two of them are defining the mate (and they are probably the two wedge surfaces). After all, only two planes are required to define a line.
If you can blue up one part and see what transfers over, that may reduce your work load a bit - there's no point in scraping the large surface beautifully flat if it is really only in clearance. (That is, clearance enough that when the bolt is tightened the surfaces contact with minimal deflection)
If on the other hand all three surfaces are scraped in I can only ask the question "Why have those cunning chaps put a redundant surface in there?"

Michael

Hi Guys,
About a 50/50 chance I'd say
I would have thought your coffee could get you well past 18 hours, maybe its losing it effect?

As I'm seeing it there isn't an alignment issue until Bob starts on the second face.

The scraping on non moving parts doesn't need to be better than about 5(?) points per inch right?

The reasons I can think of that might be why there is a third surface are
Increased rigidity,
Would the bevels themselves be wide enough to ensure alignment(in roll) on their own?
and maybe it would remove any height changes from different torques on the hold down bolts if only the bevels were used.

Of course they could all be wrong.

Stuart

I'm really going to throw a spanner in the works now -
Looking at the photo in post 9, the section that Bob is trying to replicate does not look to have been scraped. Possibly a filed or fine machined finish is adequate - the parts do not not move relative to each other after all. The only requirement is that the contact is solid and the two parts locate together properly.
Have a 3rd surface in there would certainly provide a more rigid mate, but I also wonder how to achieve that in practice. The other part of Bob's mating pair is the one with the scraping marks. It suggests that that is scraped into the section, rather than the other way around. If you consider that scraping typically removes tenth's of thous, chasing the perfect fit-up could be very time intensive. The slightest bit of wear and there goes the location.
Having thought about it a bit more, these are the two ways that I'd be approaching the problem. As Bruce suggests, it's the sort of thing that really requires a bit of hands on to consider (and I'm not making that trip either...).

Attachment 192833

Michael

Originally Posted by Pete F

Sorry Bob, I'm completely confused, normal I know



Pete

Same here, we were discussing flat, now scraping angles has come into the conversation.. What does the overall part look like, including it's mating part... How big are both parts?

Whoops. Just realised a mistake in my previous post. Option two won't work as drawn. You have to start too loose (clearance on the sides) so that it is contacting on the top and then slowly machine the top of the plug down until it is a good fit.

Pete & RC, my understanding is that Bob wants to create a form the same as the far right photo in post 9, to mate with the part shown second from the left. He has started with a flat surface and then wants to use that as a datum to put bevels on the sides & cut T slots. while he started talking about scraping a flat, using the mating part was suggested as a form gauge and things have gone on from there. (Hence the need for angles)

Michael

Michael
Thanks for that correction, as I was concerned & thought I had overlooked something.
This job of Bobs is one I cannot get off my mind.
I even woke up at about 3 am this morning trying to figure a solution.
Like you said it would be a lot easier if we were over at his place seeing it all in front of us.
However my latest thoughts are to machine the 2 angles so that say a feeler gauge of .002" or thinner would just fit between the 2 mating flat surfaces.
Then handscrape the 2 angles so that the 2 flat surfaces come closer together & get some blue showing, at which time all could be mated accurately, on all 3 mating areas.
regards
Bruce

Originally Posted by Michael G

while he started talking about scraping a flat, using the mating part was suggested as a form gauge and things have gone on from there. (Hence the need for angles)

Michael

He is going to have to use the matiung part as a form tool...

All three surfaces are going to have be scraped in at the same time... To me it looks fairly straight forward.. Pointless scraping the big flat area first as that is the last bit that will touch the mating part when you start scraping the sides....

I would be machining/scraping the base section first, then using that side as a datum for the machining of the top...