[DMF_TomB]

For specialized applications requiring slots parallel to 0.00001" or 10 millions of an inch. We used an capacitance gage traveling the slot to measure and graph it out full size.

We used phenolic blocks sanded flat on a piece of sandpaper on a granite surface plate. To these block we used double sided tape and applied 400 grit and 600 grit sandpaper to. We change sandpaper often. We could easily use micron paper 10 to 100 times finer and get a surface that you can see your reflection in if we wanted too. In fact the reflection of zebra striped lines and checking the reflection quality is also a way of checking surface quality. Also we work in a temperature controlled clean room as dust can seriously effect readings.

Now the surfaces we got were just short of a mirror finish. When assembling the equipment even fingerprints and even drops of water will affect the slot width. I am guessing this is on the order of 10 to 100 times finer than scraping.

take you scraped block and measure it with a gage that can show 10 millions. you will see how flat it is compared to sanding. also we were working with titanium as iron and steel would loose surface quality due to oxidation within 24 hours.

now the true test of gage block quality is when clean, you can wring them together and they will stick to one another. I believe gage blocks are lapped similiar to making glass lenses by an abrasion process.

do your scrapped objects stick to one another ?

[Mcgyver]

The use of abrasives as you refer to hasn't much to do with the subject of the thread; that is using a dremel to effect localized material removal in lieu of scraping. Besides, it’s still apples to oranges, lapping doesn’t do what scraping does and techniques used for gauge block manufacturing aren’t of much use where scraping would be used. I guess one could argue if a fine enough abrasive was used the dremel technique wouldn't have a depth of cut anymore than that of scraping....this would be very difficult though as it is the geometry of the scrapers cutting edge that controls depth cut. With such a fine abrasive that its taking a tenth or so, it'd think it would be quite time consuming. Scraping btw is probably quicker than the uninitiated assumes it to be; the work on that cube for example would be measured in hours not days or weeks to its to highest standard of accuracy one likley able to do in a machine or home shop

We used phenolic blocks sanded flat on a piece of sandpaper on a granite surface plate. To these block we used double sided tape and applied 400 grit and 600 grit sandpaper to. We change sandpaper often. We could easily use micron paper 10 to 100 times finer and get a surface that you can see your reflection in if we wanted too. In fact the reflection of zebra striped lines and checking the reflection quality is also a way of checking surface quality. Also we work in a temperature controlled clean room as dust can seriously effect readings.

Now the surfaces we got were just short of a mirror finish. When assembling the equipment even fingerprints and even drops of water will affect the slot width. I am guessing this is on the order of 10 to 100 times finer than scraping.

Finer in what sense? Its not difficult to put a mirror finish on something with abrasives, but try to keep it flat and parallel over a substantial area and it can be challenging. There are ways to do so but they are not practical for the making of shop tooling. Do you make your 8" angle plates or set up your machine tool bearing ways to 10 μin’s? Really though, they are not competing techniques and ideally the engineer knows a variety of techniques to apply as the situation demands

also we were working with titanium as iron and steel would loose surface quality due to oxidation within 24 hours

gauge blocks are made of tool steel are to millionths of an inch and seem to hold up ok.

now the true test of gage block quality is when clean, you can wring them together and they will stick to one another. I believe gage blocks are lapped similar to making glass lenses by an abrasion process.

Do your scrapped objects stick to one another ?

there's no claim to make anything to gauge block accuracy, nor is the thread about gauge blocks…..but how accurate is scraping? Depends on the accuracy of the surface plate of course, but a surface well done won’t create much movement on a 10th’s indicator’s needle (so it’s not far off 10 millionths). At that point, being with a 10th of thou over a large area, you are approach the accuracy of the surface plate, at least quality tool room ones likely to be found in our shops. Light pieces will wring with a bit of oil and as you can see from the photo’s its reflective. Still, all that is irrelevant as scraping is different than lapping as is different from the dremel idea of this thread

interesting work you are doing re the parallel slots. When you gauge it to 10 μin, what do you do - map the high spots and concetrate the abrasive action toward them?

[RICHARD ZASTROW]

Just to add to Mcgyvers post, #102: Generally, we don't want the surfaces to "wring" together. The objective of most scraped surfaces is to provide a surface, as accurate as is REQUIRED FOR THE APPLICATION, that allows movement without "stick-slip".

Normally that will include lubrication which the minuscule hollows of scraping provide.
ex. oil fills these hollows.

If the surface is "gauge block quality" the mating surfaces would "wring" together and stick.

I realize there are "plastic" surfaces that slide well and flat air bearings whose surfaces would function well at super smooth, flat etc conditions.

That's overkill and impractical for a general machine shop environment.

Maybe practical for a Cat Herder LOL

Dick Z

[JerryFlyGuy]

Kinda late to the part [re: the overall thread] and it's been an interesting read. I'm curious as to how large an area can be scraped accuratly? Does it require an extra large surface to do it accuratly?

Curious more than anything

J

[handlewanker]

Hi Jerry, scaping is done to produce a surface that is........flat ,round, oval, whatever.

The reference surface is what will reveal the scaped surface imperfections, and if you have a large area to be scraped or rescraped, you need a correspondingly large area, be it a table or straight edge to show up the imperfections of the surface your're trying to get to a similar surface condition.

A lathe bed is something that will try your skill due to the various planes that won't allow flat table application, but must be true to one another as the design dictated.
Ian.

[JerryFlyGuy]

Ian, so are you saying that if I've somethng that is 4ft long I need a 4ft long reference? Can not a smaller reference be used over a larger area w/ the same result?

[Oldmanandhistoy]

What variation between high and low are you getting with abrasives?

I can achieve less than +/- 0.002mm (best I can measure with the equipment I have).

I say don’t knock it until you have tried it, using wet and dry paper belt material and a light touch minuscule amounts can be removed very accurately.

Btw I have learned a great deal about scrapping from you on the zone and would just like to say thank you for your time posting.

John

EDIT: In fact it is a lot less than 0.002mm as I can detect almost no movement on a 0.002mm DTI

[Mcgyver]

thanks John,

I say don’t knock it until you have tried it, using wet and dry paper belt material and a light touch minuscule amounts can be removed very accurately.

fair enough, I haven't tried it....its the uncertainty of the depth of cut that concerns me. You say there is no movement on a .002mm indicator; well that's pretty good then if there is less than .002 mm between highs and lows.

scraping has kind of self regulating depth cut built in by the included angle of the cutting edge. I'm thinking there isn't this same regulation with a say a die grinder in that depth of cut will vary, I'd think, quite a bit with dwell. then again if you're getting less than .002mm high to low across the work maybe it doesn't although i dont't understand why....