Worm Gear thread?

[steamer]

I've been reading up on Ivan Law's book on gear cutting about making a worm to mate with a standard spur gear, by interposing a 55/35 change gear cluster in the lathe train so that the pitch of the worm matches the standard DP pitch of various gears.   I am looking at getting a 40 tooth  16 DP gear to make a spindle dividing attachment for my Southbend....Anybody done that?
I have cutters to cut the cluster gears already...so the construction should be fairly straight forward....
Dave

[Maryak]

Hi Dave,

I had a play with worm gearing and rightly or wrongly, I came to the conclusion that it was probably just as easy to make 2 worms and use one as a cutter to free wheel hob the worm wheel, after gashing. From memory when cutting a helical gear in a mill the cutter is an undersize DP of the actual DP to allow for the helix.

Attached is my playsheet for DP worm gearing. E&OE coz we all know I'm not Marv 

Best Regards
Bob
 

[John S]

Yes did it years ago to make a dividing head that fitted direct onto the end of a Myford ML7 spindle.
I used the standard 40 T 20 DP gear out of the screwcutting set and worked out a train of gears to cut the worm, didn't have the Law book at the time, in fact it wasn't published at this point.

Worked well but be aware that you have to mount the worm at an angle to the gear equal to the helix angle of the worm so it sits in mesh correctly.

Ivans book explains this and the calculations to work the helix angle out.

[steamer]

Thankyou Gentlemen!

I will review and stew......

 

Dave

[jerry kieffer]

I've been reading up on Ivan Law's book on gear cutting about making a worm to mate with a standard spur gear, by interposing a 55/35 change gear cluster in the lathe train so that the pitch of the worm matches the standard DP pitch of various gears.   I am looking at getting a 40 tooth  16 DP gear to make a spindle dividing attachment for my Southbend....Anybody done that?
I have cutters to cut the cluster gears already...so the construction should be fairly straight forward....
Dave

Dave
       My personal method of making worms and worm gears is as follows.

(1)  I first machine and harden a single point cutter per the second attached photo.

(2) The cutter is then used to machine the teeth on the worm gear.

(3) The cutter is then used in the Lathe as a Lathe tool to machine the worm in threading mode per the first attached photo.

   The worm and Worm gear pictured is   32 DP and has  zero backlash at full depth.

Jerry Kieffer

[steamer]

Hey Bob,

I've looked into that option...I'm concerned about pitch error of the wheel...while a standard change wheel...while being cheap...is pretty accurately made ...even in the home shop with a dividing head.

John, yes per your description, the worm in this application must be mounted at the helix angle.   This introduces an error in the actual index position, but ...if the helix angle is small, say less than 3 degrees, it is so negligable...as to be ignored.

The helix angle should be a function of the OD of the worm....and the smaller the worm diameter, the bigger the helix angle from 90.

With an infinitely large worm diameter, the helix approaches 90.

Helix angle = ARCTAN(2 PI r/l)   Where l is the lead of the screw, and r the mean radius.....in the case of a single start worm, the pitch is the same as the lead.   The angle will be expressed in radians.

With a 1.5 inch diameter worm, the helix is less than 3 degrees from 90...so the error would be small.  (2.38 degrees)

So....with the cutting of 3 gears, and a gear cluster assembly, I'll have my worm and wheel.

Thanks!

Dave

[steamer]

Hey Jerry,

You posted as I was.  Thank you for the photo's!   More to ponder!

As I see it,  I still have to make the 55/35 cluster for the gear box.   That's not a problem.

As I still have the 16DP cutters to make the 47/37 transposition pair, I'll make the gears using them.

Thanks for the interest!

Dave

[Don1966]

Dave, I have a question regarding the Helix angle. When cutting the worm in the lathe, how and what do you use to set the angle and how accurate is it? How do you go about setting say 3.38 degrees accurately?
I know the cross slide has graduations and the tool must be square to the work, but is this good enough? I am trying to absorb some of this.

Don

[steamer]

With an essentially an ACME thread, the tool is square to the axis of rotation of the lathe.   The helix angle comes into play when setting the tool rotationally so that the top of the tool is normal to the path of the thread.   The most accurate way to set that is to grind the angles into the tool from the beginning.   Failing that facility...grind the tool symetrically from round stock and that will allow you to rotate the tool so that the tool is normal to the thread.....That's how I've done it in the past..and it's eyeballa metric.....in operation....though I suppose you could make a gage  or set up a protractor to set it...especially if you have a flat on the tool that is parallel to the top of the cutting edge.

Dave