More "rake " questions

[porthos]

how important is the angle of the rake (back, front and side). as a novice tool sharpener i would think that it is not very important but, what do i know. if it does matter; what will more or less angle do??

 

[Gaffer]

It depends on what the tool will be used for, but generally yes, it is very important to avoid rubbing and chaos. This is a great thread if you haven't seen it.

How To Grind A Hss Turning Tool

This is a discussion about tool grinding, which can be a dangerous business. I highly recommend safety glasses, leather gloves, dust mask and hearing protection. I also use an apron in case something comes apart and hits me. Please be careful and wear your safety gear. Lathe tools are some of...

www.hobby-machinist.com

 

[woodchucker]

It controls how aggressive or scraping the tool is.
you can get away with neutral to severe backrake.
The steeper you get, the more aggressive the cut, the faster and easier it cuts, but.. you can also wind up digging in(if your holder , x slide, compound) is loose, it could pull in a little take a bigger chunk or cause the part to rise over the cutter. Generally you want an easier cut, and not for it to pull in. With aluminum some like that shearing from the steep angle. I don't have negative rake on any of my HSS. I do on my WNMG (carbide trigon) cutter. When you have inserts that have no rake since they can be flipped over, the holder holds a negative rake (which in my opinion is not really negative since the inserts lip is presented at a positive cutting angle).

Side rake : for side rake you can get by with very little, as long as it doesn't rub. One of my favorite tools on the lathe is a half inch hss, with a minimal side rake and no back rake. There is a good chip breaker cut into it with a dremel cut off wheel, and that acts as the back rake and chip breaker. It's meant for hogging off lots of metal.

 

[rabler]

Well, getting the cutting edge on center (in a lathe) is probably more more critical. Personally I don't find that getting the grinds precisely at the target angle is critical, i.e., off by a degree or two doesn't make a big difference. But the angles do influence tool pressure, finish, and toollife, chip formation, .... A sharp (more acute) angle will need less pressure but dull faster.

 

[projectnut]

The standard cutting end relief for HSS tooling is usually around 7* to 8*. However, it will change depending on the material you're cutting. The side relief is almost always between 12* and 18*. I usually go for 15*. The back rake the one that changes the most depending on the material being cut. On aluminum it can be as high as 35*, and as low as 5* for cast iron.

Here's a link to a primer on the subject.

Basic Lathe Tool Grinding

 

[Iceberg86300]

Here's a snippet from a general machining textbook. I thought I had a far better resource but I'm still looking.

Rake angles. The rake-angle combination varies greatly according totool material, workpiece material, and cost. Thermally efficient cutting is improved with positive rake angles or at least positive rakeangles with respect to chip flow or true rake. It should be understoodthat a high positive rake results in a more fragile cutting edge andoften must be compromised for tool durability. As cutting speedsincrease, the rake angle has less effect on tool pressures.Therefore,in using tool materials that can be operated at higher cutting speeds, it becomes possible to use less positive rake angles or even negativerakes to increase tool strength and economize on tool maintenance.
Prismatic-insert and throwaway-insert tools are greatly simplified inuse and maintenance by using negative rake angles, although more
cutting power and increased cutting forces are required.Combining negative back rake with positive side rake allowssafer cutting through slots or keyways, thus placing the initial
impact loads on a portion of the cutting edge removed from the noseof the tool bit.

Sorry for any formatting issues.

Steve

Sent from my SM-N975U using Tapatalk

 

[porthos]

OK; the above helps. next question. sharp cutting point vs. a radius??

 

[Iceberg86300]

Ok, here's an excellent resource. Like, probably more than you'd ever want to know. After downloading just search for rake.

Not sure on policy for linking something like this so I'll just say I got it semi recently from pdfdrive but not the dot com or dot net ones b/c they haven't worked in months.

Shoot me a message if you need help.

Sent from my SM-N975U using Tapatalk

 

[projectnut]

OK; the above helps. next question. sharp cutting point vs. a radius??

The cutting radius is dependent on the speed and feed rate. A rule of thumb is that it should be a minimum of 1-1/2 times the tool moves for each revolution of the spindle. An example would be that if the tool moves .004" for each revolution of the spindle (chuck) the radius on the tool nose should be a minimum of .006". The larger the radius the more material can be removed per revolution. The smaller the radius (keeping the minimum in mind) the better the finish.

 

[porthos]

OK that makes things simple. all that i care about is the finish.