Negative rake thread chaser

Re: Negative rake thread chaser

John K Jordan

Excellent!

Did you grind with the CBN wheel?

If you get time, a closeup of the thread chaser would be nice to see the angle you used, etc.

JKJ

Re: Negative rake thread chaser

john lucas

I roughed it on the CBN and then finished it on the tormek and strop. I have no idea what the angle is. I can measure it tomorrow but I just kind of eyeballed a negative rake. Since I wasn't using it as a scraper there isn't a burr so I'm not sure the angle is critical. I'm guessing it just provides a safer cut since you need to cut threads at or slightly below center.

Re: Negative rake thread chaser

John K Jordan

I wondered about the angle and how much difference it would make. I have made a number of small scrapers like this from Thompson steel:

They all seem to work. But just thinking about the range of possible upper angles, from zero degrees from horizontal (which is just a flat top with no negative rake) to approaching 90 degrees (where the edge wouldn't work at all). As the angle gets larger, the effect of the edge on the wood should get more forgiving but less able to remove wood. At some steep angle it would be so forgiving it wouldn't even work! I thought at some angle there might be a good balance.

I assume the lower angle doesn't have any effect on the scraping function.

JKJ

Re: Negative rake thread chaser

John C Lucas

I just measured. My 20 tip is 96 degree included angle. The 16 is 85. Both seem to cut good. When I get time I'll regrind the 20tpi to less than 90 but I don't think it will make much difference since I'm not raising a burr. Sometime when I be e time I'll make another chaser and grind it really sharp like 45 degrees. Might cut better but may not hold an edge long and since you can only sharpen the top surface on thread chasers it might not be a good thing

Re: Negative rake thread chaser

RonBauer

First of all I am a huge fan of NR scrapers. I do not use thread chasers so this may not apply. My NR's have a ~45 deg major bevel (the bottom), a ~20 deg minor bevel (the top ) for an included angle of ~65deg. I am not sure that the numbers are all that critical as long as there is a top bevel. I humbly disagree that there is no burr. I sharpen so there is a burr. Indeed, I follow Doc Greens' method for honing the bevels and then use a carbide rod to raise a fine burr. With these scrapers I cut incredibly fine shavings that are not much more than smoke. I just finished a spalted maple bowl that had the beginnings of "punkiness". Even those areas were cut clean. The finish surpasses any shear cutting/scraping done with a gouge and are much better at fairing the curve both exterior and interior. They are very forgiving and not at all grabby like traditional scrapers. I will lower the tool handle to present the edge in a type of bevel rubbing and get even smoother cuts. Unlike a traditional scraper the NR can be aggressively "pushed" into the cut for rapid stock removal. For those of you who are not so sure you can sample a NR performance using a skew laid flat which is a form of NR. It's not as good but it shows the potential of a NR. The only downside is the need for frequent sharpening but that time is mor than offset by a major reduction in sanding. All of this of course is only my opinion.

Re: Negative rake thread chaser

john lucas

Ron On my thread chasers you don't want a burr which is why it's polished off using my strop.

I'm not sure what woods you use but i can get a surface off a sharp bevel rubbing tool that is better than the surface I get with a negative rake scraper. On some woods it's close but on others like Cocobolo I can get a surface off my gouge or skew that is polished. I actually have to touch it up with some sandpaper to give it enough tooth to hold a good finish. This isn't always the case of course. Just like everyone else I have those pieces that require 180 or course grit to start with. In those cases I will often try the negative rake scraper. I do find them excellent for smoothing a curve.

When I was doing all those square ornaments I was switching back and forth between the negative rake scraper and my Hunter #1 hollowing tool. The Hunter tool used as a bevel rubbing tool gave a better finish every time. However it was very difficult to get the shape exactly a smooth arc. Consequently I would go back and touch it up if necessary with the negative rake scraper. 2 things. The negative rake left a surface that was slightly duller than the Hunter tool. The negative rake scraper didn't cut the edges of the 4 holes that appeared in the bowl as cleanly as the Hunter tool. The holes didn't matter because I ended up cleaning up the opening with a Dremel and small sander. However it was hard to sand the bowl shape with those 4 holes in them so As best I could I used the Hunter tool.