Planing spirals

[Sunset Machine]

About two years ago I needed to fix my lawnmower - the power drive transmission went kapooey. This is a little mower like you'd push around the yard, which is what I'm doing with it now. But I started a project requiring a worm and wheel, got as far as making the hob and for whatever reason put it on the back burner.

The next step was to cut flutes in the hob. I wanted to cut them helical (at right angles to the lead) and I guess that's what stopped me - no universal dividing head. The other day I ran across this picture:

  Planer-spirals.png (Size: 159.84 KB / Downloads: 115)

It looks like the right-end of the slanted bar slides on the table, guided by the tee-slot. A follower rides the bar and gives a twist to the work as it goes. Realize that the work and table are moving, the bar and cutter are stationary.

I've since found more documentation, and a word of caution that it won't cut a true helix. So that's my question to you math guys - why isn't it a true helix?

[TomG]

That machine looks like it's based on the Pratt & Whitney sine bar rifling machine. A rack attached to the tool follows a sine bar and in turn rotates the work at the required rate. It actually is a true helix.

Here's a pic of a similar machine that shows how it works.

Tom

[Sunset Machine]

Hot dang! Never thought about rifling, my planer has a 5 foot long capacity. Your picture, it looks like a pistol barrel is being done. On the left end, it's mounted in a quick change chuck with an automatic indexer. Is that about right? Thanks Tom.

[Sunset Machine]

A few days later...

I've been staring at that old picture of the planer/shaper attachment and it finally hit me - the path that the follower travels isn't parallel to the axis of the work. As the follower travels up the inclined rod, the distance between the work and the rod changes. It's actually following a curve rather than a straight line, and generates a poor helix. In contrast, the rifling machine is indeed a straight line with the path imparted to the work via rack and pinion gearing, generating a true helix.

[TomG]

That picture leaves a lot to the imagination. It probably is set up the same as the rifling machine, but you just can't see it in the pic. I can't think of a reason why not to do it the same.

BTW, I believe the rifling machine can do a barrel nearly as long as the sine bar (as long as the long leg of the triangle it creates). The indexing head and barrel supports are all adjustable to suit whatever length barrel is being rifled.

Tom

[sasquatch]

An interesting machine, and great restoration by the looks of it!!

[Sunset Machine]

Here's another image. It looks like the further away the sine bar is, and therefor the longer the follower rod, the more accurate the helix. The rod rotates in an arc but the radius of that arc changes (the longer the rod the less influence there is), unlike the rifling machine which rotates on the fixed pitch circle of that gear.

[TomG]

That's definitely not the same concept as the rifling machine. Not only is it an approximation of a helix, it is limited to only a few degrees of rotation.

Where are you coming up with these pictures?

Tom

[Sunset Machine]

The pics are coming from PDF files - "books" on shapers & planers. Here's one for a geared type, more of a proper design like the rifling machine. The helix angle is determined by the diameter of the gear, which is driving a worm to rotate the work. It would be quite a bit more complex to build, but I suppose could even be used to do some threading.

This "book" goes into better detail than most, better images and some setup formulas. I don't have a direct link handy, but the title is "Planing and Milling" by Franklin D. Jones, 1915 and found at books.google.com, 7.2 megs.

[TomG]

Thanks, I'll need to check that out.

Tom