[grf]

hi everyone. i m trying to make some gear cutting hob 1.5 module and i am having a bit of a problem. i followed some instructions i found online but i am stuck. i made a lathe tool that has 40 degrees tip and when i try to create the necessary rack form all hell comes loose. i tried it on a mini 7x12 lathe and on a C6 with the same results. when getting 2mill deep the lathe starts either vibrating or making a high pitch sound and some times just grabs in and stops. i tried various speeds and angles on the tool with always the same result. the best behavior i had was at around 100 rpm but the lathe did not have torq that low. stock diameter is 20mm and the depth of cut 5.1mm
any ideas?
please help.

thanks
Greg

[JLSD]

Hi,
Don't take this as discouragement, but I'd reckon this to be work for a rather bigger machine. As a friend of mine would say "think of a friend you haven't used yet, get him to do it!
The olde worlde way, setting the topslide at the flank angle, (so each cut is only root + 1 flank) may help if you're not already doing it.
Another thing to remember is that hobs need radial relief. Traditionally done on lathes with a cam-operated topslide synchronised to advance the cut, then rapid retract at each flute. Nasty clanking things!! Only mention this to save you getting to the point where you've made a "thing of beauty" to find it won't cut worth a damn!
Hope you find a way,
Regards,
John

[grf]

thanks john
relief is something that is done later on the mill by the instructions but i have not reached that point yet. i do not understand how this guy is doing it
[ame="http://www.youtube.com/watch?v=8UBxNPbg0ls&feature=PlayList&p=91C71B14714 2B698&index=0&playnext=1"]YouTube- Home made Gear cutting Hobs part4[/ame]
his lathe is not bigger than the ones i tried.

[Mcgyver]

Hi Greg,

the problem is the width or depth of cut. If you've got the 40 angle and are plunging, think of the linear distance along the two edges and tip that is in contact with the work....imagine trying to take a cut that wide with say a standard knife tool on such a light lathe - now way! Try taking it buy moving left and right sightly, take a bit from the left side, a bit from the right or trying opening it up to depth with a parting tool. A broad form tool cut just isn't easy, even on a heavier lathe.

Your right to go slowly....if you can get on a heavier lathe for this job all the better. what DP are you doing?


just for prosperity, he's incorrect to call it a hob. A hob has the V form in a helix around the cylinder and is used in a universal mill or gear hobber where the work is at an angle by the helix angle to the cutter....picture a horizontal mill where the table can pivot. Here's an image of a hob, note how the V form wraps around in a helix.: http://en.wikipedia.org/wiki/File:Mi...ngInvolute.jpg ...in use the shaft the cutting is on is angled to the work and both the work and cutter rotate in synch. Putting the relief on those was the clanky bit John mention


The cutter he's made in that video will produce a gear tooth form of facets approximating an involute...which is fine, i've done it lots of times, but just be aware its not hobbing and the gears are an approximation; ie they'll noisy and rough for higher speed stuff.

here's a good description i found helpful

http://www.helicron.net/workshop/gea...g/gear_cutter/

[grf]

thanks for the reply. i read a lot about hobs and gears lately and the second link you post is the one i was based on trying to make the "hob". i know it is not the correct way to do it but it is very difficult to find cutters that are not sold as a set (a lot of money that i do not have). i am trying to make a 1.5 module gear (17DP i think). this means that my depth of cut is 5.11mm. if i did not see the you tube video with that tiny lathe i would have given up by now. i thought to try and feed from the left only and modify my tool to cut only from one side (20 degrees on side and less than 20 the other) so i can give it some top rake but i do not know how to keep track of the pitch ( now i am doing that using the compound) and also my 7x12 does not have a lock on the saddle.
i will try to make the cut first with something like a parting tool and see what happens. thanks again and please feel free to help a newbie lol

[Mcgyver]

hey, its correct if it works for you, I was just advancing the learning on what a hob is - in case you find your self seeking employment at a gear plant. "Gears?, I didn't know you could make gears, I thought you had to buy 'em"

when you it say the saddle doesn't have a lock, do you mean in the long direction, the carriage? that would a high priority mod imo

17 DP is pretty large. could you come at by carving out the V with smaller tools and then have a V shaped 40 degree master that you used a gauge to get the right width?

Don't give it top rake. while your instincts are correct that the more acute angle will reduce cutting force, a form tool should have zero rake or it won't cut its form.

There's no easy answer, pressing light duty equipment into performing all kinds of acrobatics is just the home shop ingenuity. How about this:
1) Make a block the exact width of the pitch and a clamp that fits the ways.
2) Lock the carriage - by the carriage lock if it has one, by a clamp on each side if it doesn;t
3) With a parting tool, gash or remove the centre of the V
4) remove the left hand clamp
5) place the exact width block on the left side of the carriage and jam the clamp up against it.
6) tighten the clamp and remove the block
7) move the carriage over to the left until it hits the clamp
8) repeat
9) set the compound over to one direction, cut one side of the V and start stepping over with the block again doing it for all
10) slew the compound over the other way, take cuts until you get the first V way exactly as you like then proceed as above

by creating the block of gauge, you've an easy way to step over the carriage the exact pitch required for the V's

[grf]

yes that is the one. the carriage does not lock so if i try to push side ways the carriage moves and it is the end. at the moment i am using it by putting the feed screw at neutral and engaging the feed nut but it is really sloppy set up.

next weekend i will try these two
1
i will make the same tool but from much smaller tool steel
try to make the cuts to the correct depth and step.
if it works i will use the bigger tool to form the total of the width and hopefully it will do the job as it is gonna be half ready.
2
try and make the parting tool a bit narrower than the 3mm it comes and then try and cut some material before using the form tool

if none of the above then i will mill the step blocks and set the compound in 20 degrees and cut sideways.

when you say that these gears are more noisy and not for high speed what do you mean? what speed is considered to be high 100, 1000, 10000,20000 rpm?

[RICHARD ZASTROW]

grf, I see on my chart that a 1.5 module has a whole depth of 3.250 mm. Where does the 5.11 depth of cut come from?

Is the 40 degrees the result of 20 degree pressure angle on the flanks?

Mcgyver is correct, the tooth form will not be correct if the tool has the correct angle but has a positive rake. The tooth will be thick at the tip.

Gear hobs are made to compensate for the lead angle as it rotates through the work as well as the gash angle of the hob. That's the angled cut you see on the hob that is the cutting face of the hob in the picture of the hob.

Dick Z

[grf]

5.11 is comes from http://www.helicron.net/workshop/gearcutting/involute/
it is for the tool not to use the tool. this is the depth that i need to cut with the 40 degree form tool so the cutter has the correct shape to cut the 3.25 if i understood correct the instructions. the tool i was thinking about is gonna look more like a right hand tool that is going to have a 20 degree angle so when fed into the stock it will cut 20 degrees and because it will be fed side ways 20degrees the other side is going to be forms at 20 degrees. i do not know maybe i am wrong. the tool is not going to have a hook shape but the cutting edge is gonna have relief both on the side and the top to help a bit with the cut.

[grf]

VICTORY!!!!!!!!!
yes i did it and i am very excited about it.
no no fancy tricks i just thought that my milling machine is much more durable than my lathe so i putted the lathe tool in the vice and the O1 in the head and there you go. it did manage to bend and jump a couple of times but nothing that stopped me. the cut went much faster too (at 170rpm the lowest i could get but plenty of torque)