Gear Hob -First effort

[RichD]

I have a little project under way to make/adapt a mechanical counter to my X axis. Part of the job involves making a couple gears to transfer the revolutions to the counter. I've read a lot about gear hobs including the Hobbynut videos.

The cutter is an oddball DP because I already had the 2" blanks machined and I wanted to use 25 teeth with a 2" OD. I chose 25 because my dividing gadget (An adapted elcctric rotory switch) has a 25:1 ratio. Turned out has to be 13.4998 DP. So that's the cutter I'm making. I guess when you make your own cutters there's no reason to stay to a normal whole number DP.

Now don't ask me why I don't just buy a DRO. I just get these ideas and have to try em out.

I cannibalized a cheap HF measuring wheel with the resetable mechanical readout. I plan to make a new housing for it out of aluminum.

For the hob I used a 1" 8tpi B-16 stud for raw material. I think it has plenty of carbon in it for heat treating later.

So far the cutter is machined and I plan to heat treat it tomorrow. Some pictures below. Sorry about the low quality pictures...my cell phone was all I had.

 

[RichD]

The engraving was going well until the bit worked loose in the dremel chuck. I had to stone it down and start over. You can see some of the previous markings were too deep to hide...Oh well, it's just a cutter.

Rich

 

[Maryak]

Rich,

Nice Hob. :bow:

Best Regards
Bob

 

[black85vette]

Looking good. How did you grind the 60 degree tool to cut the grooves in the hob?

 

[RichD]

I already had the tool in my box. It is an ACME threading tool for 1" 5 tpi I think. The tip width was right for the width of the root of the tooth. My Father was a machinist and I inherited all his old shop tools. All I did was stone the cutting edge.

The 14.5° gear tooth pressure angle is always half of the cutting tool's included angle...in my case 29°.

I had read a book on generating gears using a standard rack form and thought I'd try it out. I used the specs for a 14.5° pressure angle 1 DP rack and drew it in AutoCad. Once you have a 1-DP rack, you can simply scale up or down to arrive at the DP you're after.

For Example:
DP-2 is half scale of DP-1 (or 1/2)
0.074075171 was the scale factor for my particular 13.4998-DP rack (or 1/13.4998)

With the rack scaled, I drew the hob and printed it out at 1:1 scale with all the necessary info attached. To double check my method, I ran the numbers through my homemade gear drawing spreadsheet and also through the WM Berg Gear software. Everything matched perfectly, except WM Berg uses slightly more clearance than my method, but my stuff comes from an old mechanical drawing book from ~1910 or so.

I did a simulated dry run of hobbing the gear in AutoCad and found that because my gear only has 25 teeth, the hob was "nicking" adjacent teeth. Going back over the book info I found that 32 teeth is the minimum number of teeth for a 14.5° pressure angle pinion without running into "undercutting". So that explained the nicks.

With commercial hobs, the hob is helical and the blank rotates along with the hob to stay in synch with the lead of the hob. The "nicks" I'm seeing get slowly generated into the proper curve below the pitch circle on gears with undercut teeth. On a straight hob you need to (or can if you want better results) simulate the generating process by performing partial indexing along with a matching move of the cutter.

If you know the degrees to move for one tooth you can divide that number by any number that yields a real world result in degrees, and advance that much extra for the first tooth on each pass or revolution. The number you divided by will be the number of "passes" you will use.

Before you cut each new pass you need to move the cutter a little bit in the same direction that you are indexing. How far to move depends on the number of passes you plan to take. Since you know the "pitch" of the cutter from one tooth to the next, you can simply divide that pitch by the number of passes. For Example:
pitch =.1
passes = 4
move the cutter .1/4=.025

Now it's important to remember that each "pass" represents a complete (or complete plus two teeth) revolution of the blank. You only do the partial indexing and cutter movement on the first cut for each pass. You return to normal indexing and leave the cutter in the new position for the remainder of the pass.

For low power applications like mine, undercutting is of no real consequence, but I do want the teeth to at least look nice, Right?

To improve the tooth form I'm going to do four passes. I plan to go all the way around at the standard 14.4° per tooth until full depth is reached and I believe I'll need to go past two extra teeth to get the adjacent teeth formed right too. After that I'll rotate an additional 3.6° (14.4/4), and then move the cutter .058" (the pitch of my cutter/4) in the direction of rotation.

Well, that's a long post but I was thinking out loud.

Rich

 

[Twmaster]

Rich, no worries about the long post. Very informative stuff. I cannot wait to see your hob in action.

 

[arnoldb]

That's a nice cutter you made there Rich - Well done!

Regards, Arnold

 

[RichD]

Had some time yesterday to harden the hob cutter and test it out.

I don't know whether it's my cutter design or just how they behave, but you get a lot more pounding with these straight-fluted square-faced hobs. I could only cut ~.020" per cut, but other than the pounding, the cuts looked OK.

I'll never choose to make a coarse tooth gear hob again as the cutter is too broad and the teeth too deep for my little X3 mill. I recently experienced some belt slippage on drilling an 11/16" hole so my approach was to keep the cutter speed a little high to prevent stalling out and more slippage. My new belts are due in soon.

I haven't finished yet but here's some more pics of my setup and progress.

 

[Maryak]

Had some time yesterday to harden the hob cutter and test it out.

I don't know whether it's my cutter design or just how they behave, but you get a lot more pounding with these straight-fluted square-faced hobs. I could only cut ~.020" per cut, but other than the pounding, the cuts looked OK.

I'll never choose to make a coarse tooth gear hob again as the cutter is too broad and the teeth too deep for my little X3 mill.

Rich,

Yes they sure do pound or mine and obviously yours do. I wonder If I left too much land on the teeth, (1/32") :-\

If you won't use a coarse hob in future, what do you intend to use in its' place. ???

Best Regards
Bob

 

[SAM in LA]

Rich,

Thank you for the detailed tutorial on making and using a gear hob.

This forum and its members are like a social encyclopedia of machining knowledge.

SAM

 

[RichD]

If you won't use a coarse hob in future, what do you intend to use in its' place. ???

Bob,
What I meant was I'll choose to utilize gears of a higher diametral pitch (smaller teeth) whenever possible. For example I could have used 50 teeth instead of 25 and the teeth would have been half as big and a whole lot easier to cut with the size mill I have. Because my dividing setup is rather crude I was trying to keep it simple (one complete turn of the knob for each tooth at 25:1 ratio).

Oh well, it's not like I'm getting paid to make them
Regards,
Rich

 

[arnoldb]

Rich good progress Thm: .Having the coarse teeth on a hob nearly bit me as well on my current build.

I've been thinking (a decidedly dangerous process!) that for future hobs I'd rather try and mill the teeth with a dovetail cutter, so that I can make more teeth around the periphery of the hob (e.g. instead of just 4 - make 8). For hobs for spur gears, increasing the diameter of the hob will also allow more teeth to be cut, even with a normal end mill. Just a thought though; I'll try it in future; just don't know when though...

Regards, Arnold

 

[RichD]

Rich good progress Thm: .Having the coarse teeth on a hob nearly bit me as well on my current build.

I've been thinking (a decidedly dangerous process!) that for future hobs I'd rather try and mill the teeth with a dovetail cutter, so that I can make more teeth around the periphery of the hob (e.g. instead of just 4 - make 8). For hobs for spur gears, increasing the diameter of the hob will also allow more teeth to be cut, even with a normal end mill. Just a thought though; I'll try it in future; just don't know when though...

Regards, Arnold

Well, I think the dovetail cutter undercutting as a final step after initial milling might be worth a try. That would give you quite a front rake to the cutting edge and continue towards the root of the cutter teeth. That's a good idea Arnold.

I've been toying with a spreadsheet that helps figure out the gearing required to cut a helix using an endmill vertically. I'll attach it here and you guys can tweak it for your use if you want.. Another spreadsheet I use for gears and other oddball stuff is included There is some additional columns in the gear design file where the numbers may not make sense to most. Those numbers I use when drawing gears in AutoCad. They're mostly radiuses and half-tooth values that you don't need for machining.

View attachment GEAR.xls

View attachment MILLING A HELIX.xls

 

[Maryak]

Bob,
What I meant was I'll choose to utilize gears of a higher diametral pitch (smaller teeth) whenever possible. For example I could have used 50 teeth instead of 25 and the teeth would have been half as big and a whole lot easier to cut with the size mill I have. Because my dividing setup is rather crude I was trying to keep it simple (one complete turn of the knob for each tooth at 25:1 ratio).

Oh well, it's not like I'm getting paid to make them
Regards,
Rich

Rich,

All is revealed and understood - Thanks. :bow:

Best Regards
Bob

 

[RichD]

Slow Progress...
Last night I spent some more time on the gear cutting job. I had initially cut in increments to .070" depth indexing between each cut. Got tired of that and changed my method to completing the cuts to total depth before indexing (seemed like less work). Here's a picture of the full depth 0.1598" cut compared to the previous shallow cuts. Had to slow down the mill and the feed as the cuts got deeper engaging more and more of the hob teeth. Going to try and get some more done today.
Rich

View attachment Pdr_0176.jpg

 

[RichD]

Got a little further along today. I had kind of gotten ahead of myself with cutting the gear before figuring out how I was going to mount them. I also got the bright notion that I could part off two gears out of the one .550" thick blank. I had originally made the blanks with a .250" hole which was a stupid way to go. That made for a flimsy mandrel.

In order to hold the blank for enlarging the hole and parting, I had to make a hollow piece with a counter-bore to press the finished gear into. Sort of reverse engineering but it worked out well and is still concentric. The steady rest was at it's limit 2.375", but it sure makes parting off a breeze.

The last picture shows how the gear will install onto the X axis feed dial.

Regards,
Rich

View attachment Pdr_0183.jpg

 

[Deanofid]

It looks like it came out very well, Rich. Nice job.

Dean

 

[Twmaster]

That's very slick Rich. I did not quite understand how the cutting edge is formed other than the milling to create the gap and edge.

 

[Noitoen]

Very nice work

For perfect backlash "free gears", you could use the trick they do in older printers where they make 2 thin gears (the big one) and mount one spring loaded to remove backlash. (I can explain better if you want)

 

[RichD]

Thanks to all for the kind words.

My motorcycle has hte backlash free gears between the crankshaft pinion and the clutch basket so I am familiar with that. I won't bother with that this time though.

The next phase of work will involve mounting the mechanical digital counter and making some kind of housing for it. I also need to make an input shaft adapter to make with the counter and the other gear.

Regards,
Rich