Cutting Tool Generatin -- new name

[philbur]

Hi all, I'm trying to run the demo version 3.34 using the tutorial. The tutorial appears to be for version 2.0. The screen layouts are not the same, particularly the left hand panel in the "Spur Gears" window. For example I am currently stuck trying to find "add slave to project". Any guidance would be appreciated.

Here's a shot showing the left hand panel of the Demo:



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Here's the tutorial window. Compare the left hand panel with the Demo above:



Phil :)

[philbur]

Art,

I think the single lip would basically be a Vee shaped engraving bit with the involute profile where the the straight cutting edge is on an engraving bit. One point about a single lip engraving bit is that it has effectively zero cutter thickness as there is no need for a second cutting edge on the back side, the face of the cutter runs through the rotational axis, which is not possible on a 2 lip cutter.

The second advantage is the conical shape imparts maximum possible strength/stiffness. The cutter is going to need reasonable stength because it has to cut over a relatively long cutting edge.

To get a small cutter to cut on two edges is going to require concentricity much better than 0.01mm. This level of concentricity is normally achieved on a precision grinder. Most people are not going to manage that kind of precision on their CNC mill, so even though it may look like a two lip cutter in practice it will only cut on one lip.

For your "spade" shaped 2 lip cutter the cutter thickness must be less than the gear tooth root width which for small module gears is going to give a cutter thickness significantly less than 1.0mm. This is not going to be very strong for what is effectively a form cutting tool. Also a simple "spade" shaped 2 lip cutter must, by concequence of it's design, be a negative rake cutter.

One possible process for a single lip conical cutter might be to use your program to cut an involute profile lathe form tool. Use it to turn an offset involute (conical) profile on the end of suitable bar stock, the offset in order to provide the necessary relief angle. Then face the conical section in the mill (orientated correctly to ensure the correct relief) to produce a conical, single lip "D" bit arrangement. Then harden and temper.

John Stevenson showed a hand formed two lip spade shaped cutter on the HSM site, maybe he can say something about it's form and function.

Johns Spade bit:


Phil :)

[philbur]

Correction to the ramblings:

I don't think the length of the parallelogram major diagonal and the included angle is enough information to fully define the parallelogram geometry. However if you make an initial guess at the tool thickness, which must be less than the major diagonal (see next paragraph) then that gives you enough information to check the "feed rate" clearance for the none cutting edge. then you can adjust the tool thickness input to give a reasonable compromise between tool thickness and not cutting edge clearance.

With a couple of quick sketches it seems that the tool thickness must actually be less the the length of the major diagonal x sin(machining angle as measured from the horizontal)

PS: The known angle is not the included angle its the angle from the horizontal!

My head hurts!

Phil:)

[philbur]

John's video also says "What's your problem":

http://www.youtube.com/watch?v=fps0OR1eF_s

It would be extemely interesting to see that cutter up close.

Phil:)

[philbur]

Hi Art:

I had a second attempt. This time I cut the flats manually and then just ran the program for the volute with safe Z's inserted for each pass.

I thin the graphical presentation of the cutter thickness in the volute profile needs to be the diagonal thickness of the cutter. My second attempt was still to thick to sallow the cutter to rotate without rising some way out of the tooth profile as the diagonal passed over the tooth flanks.

The final Z height when cutting the involute seems to be way high. I had to run it again with Z0 2mm lower than the top of the stock. This may have something to do with me doing the flats manually and getting resetting of Z0 wrong, but I don't think so. The low side flash came of 2 passes earlier than the high side. Looking at the code as the end mill passes the cutter tip on the last pass the end mill should at least have a Z value that is half the stock diameter plus half the tool thickness and it is not.

Cutting the involute by progressively lower Z passes (with the same small DOC as the flats) seems to result in deflection of the cutter on the last pass. If the end mill came in from the side the risk of deflection would be greatly reduced and the need for constantly varying the Z height as it follows the involute profile would be unnecessary.

Phil:)

PS: I think a better graphical presentation of the cutter tip would be your diagram 3, where the circle diameter would be equal to the involute width at the cutter tip location (at the root).

[philbur]

Hi Art:

How does one know the correct total depth of cut when using the cutter to produce a gear.

Also the root profile on the cutter will have zero clearance, not sure how much of a problem this may be. Using a slitting saw to pre-cut the slots to depth would help.

Phil:

[philbur]

Hi Art:

Trouble with grinding the relief angle on the end is it has to be reversed either side of the axis of rotation.

Seems the maximum theoretical diagonal tip thickness for all spur gears is approximately 23%. Anything thicker is wider than the involute at that point.

The reason I ask about the depth of cut is that the cutter doesn't fit to the full depth of my reference gear. So the cutter used to produce my reference gear must have had a different full depth of cut, if you see what I mean.

Phil:)

PS: All the commercially produced gears I have to hand have a much more rounded root. See post 12 first photo.

[philbur]

Here's a cutter ready for use. 5mm diameter - 1.5 module - 25 teeth, hardened and tempered.



Phil:)

[philbur]

Art:

Oops, possibly my mistake on the Z height. Just discovered tonight that I had make the cutter from 6mm stock, but the code was for 5mm stock.

Anyway I carried on with cutting the gear, just to confirm how the cutter would work. It didn't. The brass blank ate the cutter, so I rechecked the hardness on the tip with a file. Seemed pretty good so I checked further up the cutter with a scriber. It was somewhat soft. I need to take more care with the heat treatment. I didn't expect the brass to be such a challenge, although it is of unknown origin.

Anyway I'll redo it all all tomorrow.

Phil:)

Still too high eh? Thats amazing, I told it to go to 1.6 times the depth just in case.
Ill revisit that math to see where I dicked up..

Art

[philbur]

To improve cutter strength how easy would this geometry be to produce code for. Its kind of a first step to a conical cutter but keep two cutting edges.

Instead of the cutter having two flat faces (with a thickness of less than 1mm) give the cutter two curves faces by profiling instead of facing the thickness, if you see what I mean. Or maybe it makes the maths for the involute to complicated. The input would be a diagonal width (Edit: of the none cutting edges) at the tip as a % of the root width (always less than 100%).



Phil:)

[philbur]

Art:

So the only issue might be finding a company that will provide single lip engraving cutters with a 40 degree included angle? I'm sure there are companies out there who will make engraving cutters to order.

Alternatively I don't think most people care what standard pressure angles are use commercially. So if a 30 degree cutter can produce matching gears with a 15 degree pressure angle then most HSM'ers would be satisfied with using a stardard off the shelf cutter.

Phil:)

Edit: This opens up many possibilities with respect to the use of home made multi-tipped rotary cutters with a simple rack profile produced on a manual lathe/mill. Then you use it to make multiple passes, indexing the blank gear a fraction of a tooth on each pass, moving the rack profile cutter the corresponding fraction. Even I could write the code for that. Looks like I've got my own development project. Oops again! I already have the home made cutters, previously use for manual gear making using a different but not dissimilar process.

[philbur]

But a small diameter end-mill rotates at much higher rpm (say at least 10 and maybe 50 times) and has more than one cutting edge (say 2 flutes). So the number of facets per minute with and end-mill is order of magnitude 20 to 100 times that of a fly cutter.

Phil:)

PS: The flycutter does cut both faces at the same time. so 10 to 50?

PPS: Maybe not so simple, the fycutter is much stronger so DOC can be higher??