How to gring carbide scraper

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How to gring carbide scraper

This topic has 8 replies, 5 voices, and was last updated 10 October 2017 at 09:09 by Martin Dowing.

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7 October 2017 at 10:52 #320381
Martin Dowing
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@martindowing58466
Have made carbide scraper from an old file as seen on photo.

Got a handful of these blanks from a friend, so decided to make something useful out of these. They do braze nicely.

Cutting edge is 20mm wide, now the question is, how these kind of scrapers are ground. Should I make a large radius, like it is usually done with a traditional "old file type of scraper"? I have noted that dedicated inserts meant for scraper are square on cutting edge, without radius.

Another question is, should I leave 90 deg angle on the cutting edge or sharpen it to 80-85 deg or so. At the moment it seems to work fine, but probably there are recommended angles and for good reason so.

Finely do I need to struggle for mirror finish on cutting edge, not the easiest task on carbide, or a fine finish left by ultrafine diamondhone ex DMT is good enough?

Any other comments?

Martin

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7 October 2017 at 10:52 #8901
Martin Dowing
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@martindowing58466
7 October 2017 at 11:08 #320382
David Colwill
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@davidcolwill19261
All of the information I have seen says 5 degrees, also it seems to be the usual practice to flip it over and do the other side so that you get two go's. I sharpened mine on one of the cheapo diamond cup wheels and it gave a pretty good finish straight off but following the advice on a youtube video by Stefan Gotteswinter (he did a two part video on scraping which is well worth watching) I bought one of the lapidiary diamond discs which is about 3000 grit but I haven't mounted it yet so can't say how much improvement it will give.

Regards.

David.

Edited By David Colwill on 07/10/2017 11:09:17
8 October 2017 at 01:05 #320459
Mark Rand
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@markrand96270
95° is the recommended angle. That's 5° obtuse, rather than acute. A resin bonded diamond wheel is good for the initial forming of the wheel. After that a mirror finish will really improve your work and reduce the amount of effort needed.

The best way to achieve this is to use a disk of cast iron (aluminium will work, but be careful not to dig the scraper in to it). mounted on a motor or on the lathe. Said disk should have some 5 micron or 2.5 micron (better) diamond paste pressed into it using a disposable ball bearing. It doesn't need much. Half a gram will coat a 6" disk with enough paste to do the job. A good speed is about 300rpm, but up to 1500 can be used.

If you make a 'tool rest' that allows the scraper to point down at 5° from the horizontal you can then polish the scraper's face until you get the desired finish, use both sides of the scraper and work to keep the "parting line" in the middle of the carbide tip, so you are sharpening both edges the same.

If you can arrange a tool rest for the initial radius grinding, on a bench grinder or surface grinder, then having it 0.17 times the wheel diameter below the centre line will give you the desired 5° angle. Always grind the carbide towards the centre rather than towards the edge. this will eliminate chipping. the other trick with the initial radius grinding is to drill a hole in the tool at the desired radius back from the edge. this can then be stuck over a pin in the tool rest so you can pivot the scraper around and get the desired radius without too much head-scratching.

These are the methods I use to make and sharpen my own scrapers, which I've so far used to remove about three pounds of cast iron with, albeit with a Biax scraper to provide the muscle power. The final lapping with a diamond impregnated lap is quite vital. You won't believe the difference in ease of use and finish quality until you've done it. My lap is actually a piece of 5" dia cast iron that normally lives in the 4 jaw chuck of the Myford ML7 with 2.5 micron diamond paste on it.
8 October 2017 at 19:44 #320572
SteveI
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@stevei
Over on PM forum it has been documented that Sandvik expect you to put a radius and rake on the blakes as required by the job at hand. No one that values their time and the work piece or for that matter their sanity scrape with a flat blade.

For general scraping I use ~140mm, ~90mm and ~60mm radius from roughing to finishing. I also grind off the corners. Sharp corners have a habit of scratching your work.

I use 5° negative rack for cast iron. I vary this depending on how hard or soft the iron is. Grind each side so you get two goes at it before a touch up on the lap.

You would be well advised to follow Mark's advice about being sharp. It is critical. The sharper the blade the more you realise the benefit. I shape my blades with 600 grit and sharpen with 1200. I have recently also started using 2.5 micron diamond spray on a ceramic wheel after the 1200 grit to keep it really sharp.

Steve
8 October 2017 at 20:03 #320573
Watford
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@watford
Martin,

It would be good to get rid of the file teeth if you are going to do any serious scraping, or you are going to have some very sore hands.

Mike
8 October 2017 at 21:10 #320588
Martin Dowing
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@martindowing58466
Many thanks for up to date comments.

These are very informative and not something one could guess.

Meantime, while scraper was still sharp (now it is blunt and needs sharpening) I had a go on steel plate, which will later become female slide of dovetail. Male will be cast iron. Exercise was just for training. Blade was square and I have noted that idea was a horrible one. It will never be square again. After 4 iterations formed surface tends to "catch" surface place lightly and contact area is growing… but still I am far away from nice patterns of contact points. Surface got shiny with pattern of mirror shining facets, with scratches here and there from corners of square tool.

I know, it is unusual to scrap steel, is it OK to use 5deg negative relieve, like in case of cast iron, or perhaps it is better to make it neutral or slightly positive?

One more question,

This diamond paste is to be applied regulary or once impregnated cast iron lap is going to last for long?

And yes, I will get rid of file teeth.

Martin

Edited By Martin Dowing on 08/10/2017 22:06:35
9 October 2017 at 08:35 #320651
SteveI
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@stevei
Martin,

For steel a positive rake is what the text books say. (Although that depends on the type.) For steel you should be able to scrape what looks to the eye like little chips rather than "dust" like with cast iron. (The cast iron "dust" looks a little like a chip under magnification.) Which is one way to find out if your scraping steel or cast iron if your not sure.

As your grade of steel is unknown try and experiment a little 0° – +5°. – +8°. This is where having a modular scraper with a detachable blade comes in. You are going to waste a lot of your carbide blade altering the rake angle. My hand scraper is designed to attach a BIAX style blade. I have a selection of blades with different radius and rakes and bends for different tasks some home made some BIAX. Once roughed in the blade only needs touch up. BIAX blades are not inexpensive but the concept of a modular mild steel blade with carbide tip is efficient. I do not know what grade of carbide BIAX use but I am increasingly of the opinion that it is better than the Sandvik blanks. You can of course do perfectly good work with the Sandvik blanks.

I use a tennis racket grip wound over the end of the scraper to keep my hands from complaining too much. I also use a flat angle grinder flap disk plastic backing screwed in to the handle to spread the load on my stomach and have the scraper held in both hands at the business end and the handle end pushed in to the stomach. Guide, position and lift with the hands and push with the body. Your file wont have much flex in it. A mild steel scraper does. You may find the work easier with a little flex.

Gently run your sharpened carbide blade across a finger nail, it should produce finger nail chip/dust. That is the absolute minimum state of sharpness you want. If your not sharp you will get chatter marks in the scrapes and it will be harder work.

What are you planning to scrape?

Steve
10 October 2017 at 09:09 #320811
Martin Dowing
Participant
@martindowing58466
Steve,

Thanks for your comments. I already realise need to equipp myself in several scrapers, however must begin from something.

Perhaps at the moment I may grind 5 deg relief on my existing blade through entire face of cutting edge. So used one way it will work for cast iron and used another way it will work for steel.

At the moment I am scraping slide of little gadget which will allow me to mount bigger version of Dremmel on cross slide. It will have dovetail based adjustments and I want this dovetail to work smoothly.

This will be used for internal grinding.

This is just begining. I intend to learn more from the art. As a training job I have bought worn second hand saddle for my ML7 lathe which I intend to restore and allign it to very good accuracy, including dovetail. Btw, my existing saddle is working fine and it is together with bedways scraped by professional – this job is just to learn.

I know, there will be a need to purchase in engineering company *60 deg reference prisms* suitable for the job,

Btw, what is in your opinion best material for these? Cast iron or 55+/-5 HRc steel bar? Should I make a "matching pair" or one would do?

Anyway I will begin with flat bearing surfaces mating with bedways. Then there is some of a bit more complicated metrology to master for correct diagnosis of dovetail. Then I will proceed with scraping and if successful I will go into more serious project like restoring an old machine to its former glory or converting Asian cheapo into proper precision machine. At this point I will probably kiss goodbye to $2500 and purchase a Biax.

In unlikely situation of myself becoming profficient in scraping to professional level I may attempt to make some money in machine restoration business albeit I am aware of my limitations – being 48 you are learning slower and even if you master quality job, your *speed* might be not competitive. In any case I am not pressed for money much and such activity would be rather productive spending of my free time (and I have a lot of it) on something useful with rewards being a secondary issue.

In any case this skill will help me greatly with my metalworking hobby.

Martin
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