Threading tool Question

[loose nut]

I have an internal threading tool, carbide tip and not a cheap one either. When I dug it out today I found that it has about 5 Deg. positive rake on it. Aren't threading tools suppose to be flat on top (form tool) to keep the profile accurate. Putting rake on it will change the shape of the finished thread or so I was taught. Any one know why this tool is made this way or is it a mistake.

[Scottike]

Try mounting it and check it with a threading fish, if it conforms to the pattern on the fish, it should be good to go. (Be sure to hold the fish parallel to the work, not the tool.)

[willmac]

If you grind a threading tool so that it has an accurate 60 degree (or 55 degree) angle and THEN grind a positive or negative rake it will not cut an accurate 60 degree or 55 degree thread.

However, if you do the trigonometry to work out in advance what the tool angle should be taking account of the rake you intend to use and then grind accurately to those angles, the tool will cut accurate threads. The problem with this is that sharpening a tool like this is problematic, not to mention that it is not that easy to make in the first place without a tool grinder.

The old circular form cutters have a similar issue - you have to know what you are doing when sharpening those so as to maintain the correct form.

If that is as clear as mud, let me know and I will try to explain further.

[rohart]

In my opinion, 5 degrees is neither here nor there. A bit of rake, and mounting the tool slightly below centre, when threading internally, is what I aim for.

If you mount a correct angled tool below centre, then you come across a similar geometrical error to the one you pointed out in your OP. But if the rake means that mounted below centre the tool surface is radial to the work, then the errors would cancel out.

Now that thinking was straight off the top of the old bonce, so I may be way out !

[loply]

In theory it could have any rake angle desired, it would just complicate the design of the insert.

When grinding your own it's more or less required to have it flat as there's no practical means of shaping it otherwise.

Having said that I've never really looked close enough to see if my carbide threading tool has any rake on it!

[loose nut]

When I checked it against the fish the shape of the bit was right on 60 deg. Or as close as my old eyes can tell so having rake on it will change the thread shape. Maybe not enough to matter as has been pointed out?????

[beanbag]

It would depend on the insert that goes into it. Most insert threading tools I know of use an insert with zero relief angle, and the holder tilts the insert downwards (negative rake). Then for the different thread pitches/diameters, they use different shims underneath to tweak the angles slightly. The insert itself has positive top rake.

Edit: Oh, you have a solid carbide bar. In that case, it would depend on how they ground the bit. Obviously the cutter needs some relief angle and the top rake will cut into that. Using a 3D CAD program, I came up with

1)take a 60 degree triangle, oriented vertically
2) cut down the sides with a 7 degree relief
3) Put a 5 degree top rake in it.

The resulting angle, when cutting a thread with infinite diameter, is 58.93 degrees. My guess is that if the company cared to make things exact, they would have tweaked the angles slightly to give you a 60 degree thread form.

One other option is to start with a 60 degree triangle, but with the top surface already tilted slightly downwards (this gives the relief). When viewed from above, the tip becomes more blunt (angle greater than 60 degrees). Then when you cut the top rake, it makes the angle smaller again.

[J Tiers]

When I checked it against the fish the shape of the bit was right on 60 deg. Or as close as my old eyes can tell so having rake on it will change the thread shape. Maybe not enough to matter as has been pointed out?????

Likely not very significant........ there will be a small error. But if it was an external cutter, and you use a lantern post, you can tilt down 5 deg and it should come out right. Since it is internal, you may be able to turn the boring bar 5 deg.

It also matters what the rake is like..... if an insert, there can be all SORTS of rake but the EDGE can be zero rake radially...... meaning it is made like a regular threading tool, with the center scooped out to provide rake. In that case the "internal" rake makes no difference.


But, when checking a threading bit that has overall rake, it matters how you check it. The regular "fish" probably does not apply.

A regular flat-top tool will be on-center, and will thus measure a 60 deg angle when correct. A line across the top surface will go through the work center.

But with a tool that has rake, the line of the top surface does NOT go through center. That makes the shape of the tool different.

For an example, a regular threading bit, if dropped well below center (negative rake) or raised up* a similar amount (positive rake) would cut a wider, shallower thread than the shape of the bit suggests.

Essentially, the tool is now tilted in the V-groove of the thread, and to cut the same shape, it has to be longer, i.e. to have a narrower angle. The width of the top of the groove is the same, but the distance along the line of the cutting edge to the bottom of the groove is longer. So the "V" of the tool must be longer and the angle narrower.

I am not sure there is a decent way to check that angle with the "fish".

* obviously if raised up the clearance would need to increase or the heel of the tool will rub

[JoeLee]

I have an internal threading tool, carbide tip and not a cheap one either. When I dug it out today I found that it has about 5 Deg. positive rake on it. Aren't threading tools suppose to be flat on top (form tool) to keep the profile accurate. Putting rake on it will change the shape of the finished thread or so I was taught. Any one know why this tool is made this way or is it a mistake.

I've seen some that are and some that are not. What I've noticed on fixed bars with flat top inserts or 0 rake is that the flat on the bottom of the bar is cut at a position to tilt the insert tip downward by 5 deg. Assuming you have a QC tool holder that is square to the compound rest. The rake angle varies a little with different size threads. My Vardex tool holders have different anvils to set under the insert to give the proper rake for any given thread. But the difference in most cases is not noticable.

http://www.vardexusa.com/vardex-pdf/...g_products.pdf

They also have a program you can down load that will give you all the correct dimensions and set up parameters for any thread.

JL......................

[Paul Alciatore]

In theory.....

When grinding your own it's more or less required to have it flat as there's no practical means of shaping it otherwise.

.....

There IS a simple, practical way of grinding it. First, while the top is still flat, grind the edge that will do the most cutting, that is, the edge that will be fed into the work by the compound, to the correct 30 degree angle with the proper relief. Next, grind the other edge about 5 or 10 degrees too wide, also with the proper relief as this edge also does a little cutting when you feed in at 29.5 degrees instead of the full 30. Now, grind the top rake keeping the first cutting edge horizontal and letting the second one drop below horizontal. Finally, use the gauge (fish) to gauge the second edge and grind it down until it just fits the gauge when the gauge is held horizontal to the original top of the tool. It will now cut a proper 60 degree thread. Or very, VERY close to it.

A one degree error will probably make little or no difference in any practical thread you are likely to be making.

[Mark McGrath]

The rake angle varies a little with different size threads. My Vardex tool holders have different anvils to set under the insert to give the proper rake for any given thread.

JL......................

The anvils used on threading tools are for altering the insert to suit the helix angle of the thread being turned and not the top rake.

[PixMan]

The anvils used on threading tools are for altering the insert to suit the helix angle of the thread being turned and not the top rake.

Semantics. He said rake angle, I'm sure he meant helix or lead angle.

[Evan]

However, if you do the trigonometry to work out in advance what the tool angle should be taking account of the rake you intend to use and then grind accurately to those angles, the tool will cut accurate threads.

Although the difference is small such a tool will only be correct for one particular diameter. The larger the diameter the less difference it makes. Also, a tool with rake will cut differently depending on the thread pitch since the depth will vary as a percentage of the diameter.

The only tool that will cut precisely the correct angle for any pitch and diameter is one with zero rake at the cutting edge (coplanar with the cross feed).

[Mark McGrath]

Semantics. He said rake angle, I'm sure he meant helix or lead angle.

It was top rake that was being discussed,so please don`t try and be smart.There are a
lot of people on here who don`t understand insert/tool geometry so although it may be clear
to you don`t assume it is clear to everyone.

[Evan]

To clarify to those who may not be familiar with the term rake and to what it refers, this is top rake: