Hobbing gears

I hate you John, You're too damned good

Of course, Gareth would throw it all away because of the wavy teeth on the first one :-)

Nice one!

Mark Rand RTFM

Good to see that the summer hasn't been wasted. Nice project - very, very nice. Can we have some more details on the electronic side?

Regards,

Marv

John Stevens> Hi all been busy this summer - back for a while,-

Well done John.

Fitch

I spied a pic of John's prototype hobber a few months back. ( He left the web address on another project so I looked at all his stuff)

Don Foreman has been helping develop an improved copy. I'll leave it to him to describe its design. Basically there is a "divide by" chip that is the guts of the thing. Count encoder pulses coming in, divide by setting on thumbwheel, send pulses out to stepper driver. Don's a REAL wizard at this stuff.

At this point, the first design is done, and parts to build it are ordered. As its a copy of John Stevenson's project, it should work.

Marv, I emailed you the schematic, board print, and parts list.

Karl

This is way cool stuff, John. I'm really impressed at this. Is the hobb a commercial item, or did you make it yourself?

Jim

================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ==================================================

Karl:

Thanks and regards,

Marv

Karl Townsend wrote:

That looks great! I sure would like the info on the electronics as well and anything else that might help in making one.

thanks,

Oliver

email to v8tr4atadvprodtekdotcom

I'll give a brief summary here. If there's interest in more detail, schematics, board layout etc I'll put up a web page later.

It's a very straightforward presettable divide-by-N circuit. John's friend had discovered a chip, CD4059, that can divide by any number between 2 and 15,999 or something like that. It has 3 decades of BCD countdown within. Three thumbwheel switches are used to "dial in" the desired number between 001 and 999 though the realistic range for gears would probably be more like 008 to 200 or so.

The CD4059 always divides its input by 2 before doing anything else, so it's useful to overcome that by double-pulsing it. Right now my adaptation of the circuit just exclusive-or's both channels of a quadrature encoder to get two pulses per nominal count. Also, the current version has an RS422/RS485 differential line receiver as input because Karl was going to use a counter with that output. This input device can also be used with single-ended inputs by biassing the unused inputs to about 2.5 volts with a couple of resistors. It has hysteresis and should provide good noise immunity.

The pulse doubling could be taken as far as 4 "real" counts per nominal count with one more 50-cent chip: generate pulses from all rising and falling edges of both quadrature channels and then feed a pulse-doubler with that to doublepulse the counter to accomodate it's churlish insistence in dividing by 2 before doing anything else. That would get you 8000 counts per turn from a 1000 CPT encoder with quadrature outputs, which most of them have.

Some attention would have to paid to steppermotor speed, though. With 8000 counts per spindle rev, divided by 20 for a 20-tooth gear, with spindle going at 100 RPM that's 666.7 pulses to the stepper motor driving the indexing head.

I also included a 555 oscillator to provide a manual "slew" for lining things up.

This could also be done with a PIC micro, but probably not noticably more simply or cheaply. After the line receiver there's only one xor gate (74HC86), the counter chip (about $2.50) and the 555.

The present board layout is single-sided thru-hole, very uncrowded, 3" x 4.5" including a bridge rectifier, filter and 5-volt regulator so all that is needed to power it is a little 9 or 12-volt transformer. I get small transformers for little gadgets like this by tearing apart surplus wallwarts.

This is 4000 "real" counts per turn. The pulse doubler produces a phoney extra pulse per pulse that the counter immediately divides out. It'd be 8000 if the encoder were geared up 2:1. 8000 was stuck in my mind because that matches up with a 200 pulse-per-rev steppermotor driving a 40;1 dividing head.

There is probably more than passing interest in this NG. I know that Karl was very interested in gear hobbing and has pursued it: I was interested enough to get the casting set for the Jacobs gear hobbing machine. The web page would be of great help.

Regards,

Marv

D> >

I would like to agree with this statement! I am knowledgeable enough with electronics to understand the concepts of what you have done but I am far from being able to make it without additional information. How about writing it up so someone with limited knowledge of electronics can obtain the necessary components and put it together. You would make my day! Rick

OK. There's a writeup, schematic, board layout and parts list at

Lovely - many, many thanks!

Regards,

Marv

D> >

Da Nada. I just fixed a discrepancy between the schematic and partslist, changed SN75175 to SN75173. Either part will work but the 73 draws significantly less current.

Looks like this thread is dead. so I'll scrub the webpage next FTP visit. Glad you found it of interest, Marv.

Many thanks for the information, Don, I took it all. If I have any questions, I'll contact you privately.

Thanks again,

Marv

D> >

I don't have any experience with hobbing gears, but I have a fairly good understanding of the counting and stepping circuits for indexing the workpiece.

I don't understand the relationship or function of the encoder on the mill arbor for cutting spur gears. Is it utilized for the table (power) feed rate?

Thanks to all the contributors for the very interesting additional info

WB ...............

It is probaly used to synchronize the rotation of the gear blank with the rotation of the hob. In principle, the hob will rotate the blank anyway, being a screw, but this way you do not have to gash the blank first, and you will get the correct number of teeth every time.

WB> I don't have any experience with hobbing gears, but I have a fairly good WB> understanding of the counting and stepping circuits for indexing the WB> workpiece.

WB> I don't understand the relationship or function of the encoder on the mill WB> arbor for cutting spur gears. WB> Is it utilized for the table (power) feed rate?

WB> Thanks to all the contributors for the very interesting additional info

WB> WB WB> ...............

Ole, Contary to popular opinion about the hob rotating the blank, although it do this the results are far from satisfactory due to backlash and the hob bouncing' in the cut. On one of the test gears I did to start rushed the job a bit being impatient and forgot to nip the blank nut on the spindle. The result was a gear with wavy and 'choppy' teeth. You are correct in that the encoder is there to syncronise the relationship between the hob and blank.

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