Angular groove in round tube

[Chewy]

I need to make two angular grooves across from each other on a round tube. Think of a helical gear or square tooth thread, very course, maybe 2-3 threads per inch. The grooves are at the end of the tube and only about 3/8-1/2" long. I tried to draw a sketch but it got too confusing. I am making a very tiny hand impact driver for itsy bitsy screws. They make large ones that I have for 3/8 square drive sockets and bits.

The tube size is about 1/2 " with a 1/4" hole. Need two grooves about 1/8" wide by about 1/2" long at an angle of around 45 degrees. The grooves need to be perpendicular to the center of the tube, so just milling a slot at an angle won't do. I would need to slowly feed in and turn at the same time. Think of a rifle land that goes through to the outside of the barrel.

The only way I can think to do this is to set the lathe for 5 threads (that is the lowest I go) and hand turn the chuck for a partial rotation over and over again. Then index 180 degrees and do the same to the other side. I don't have a helical setup like the You tube guys have. I have a rotary table and spin indexer. A jig setup that advance and rotates would work. Just haven't figured out how to build one.

 

[RJSakowski]

Tom Lipton, OxTools, did a You Tube video on cutting a helix on his mill. It requires an indexer, an encoder and a servo or stepper to drive the indexer. The encoder tracks the x feed and drives the servo whiuch is connected to the indexer to cut the helix.

 

[Huub Buis]

The "threading" lathe setup to get the right pitch is OK. If you make the groves the same way you cut threads, than the groove will start at the same position and be OK.
To cut the grove, you could put an pencil driver in the tool post and slowly mill the groove.
To cut the second groove (multi start thread) you could set the top slide half the pitch back and it will cut the second groove at the right position.

 

[RJSakowski]

Not to minimize your effort but there was a tool called a Yankee screwdriver that used a helix similar to what you described. It might be a good starting point for your project. https://www.ebay.com/b/Yankee-Screwdriver/104039/bn_57336523

 

[Chewy]

RJ, I am familiar with Tom's helical setup. Not wanting to spend hundreds of dollars and months making something for an experiment. Also, I have 3-4 of the Yankee screwdrivers. I used them a lot as a kid. They require pushing force whereas the impact driver you turn and press, creating a preload and then rap with a hammer to create shock. I used an air hammer many times to loosen stuck bushings and parts.

Hhub is more on the idea. Maybe mount a Dremal on the tool post with a 1/8 end mill and do a partial; rotation. Put the tube in a square collet block in a 4J. Cut and rotate 180 degrees. Leave the threading dial locked in so it starts in the right place. Hadn't thought about the Dremal, I was thinking of a form tool to cut the groove a little at a time. Something on the order of a trepanning tool on it's side.

The tool is only going to be about 4" long. The screw head diameter is .075. The screwdriver bit will be made from a piece of 1/4" O1 and hardened.

 

[Eddyde]

I think RJ was suggesting you use the barrel from a Yankee, to make your impact driver.

 

[Chewy]

If he was, my mistake. My apologizes. But that would put the tool about 4 times too big. The whole tool is only designed to be 1/2" diameter and 3-4 inches long with a spring inside and a solid top.

 

[RJSakowski]

Given your largest pitch is 5 tpi, I fail to see how that will meet your design criteria of a 45º helix angle. A 1/2" diameter shaft would require a pitch of something like .6 tpi.

 

[Chewy]

40-45 degrees would be fine, but how do I get there? Have a PM 1228 lathe, no DRO. Mill has DRO. Thought about doing it by eye, but the taper needs to be smooth. No dips or flat spots.

 

[Eddyde]

But that would put the tool about 4 times too big. The whole tool is only designed to be 1/2" diameter and 3-4 inches long with a spring inside and a solid top.

Can't you cut it to whatever length you need?

Otherwise, one approach on the mill might be; mount the rotary table so that its axis (A) is perpendicular and centered to the spindles axis (Z). Thus the wok will be parallel to the table. Map out the helix as waypoints. set the end mill to the finished depth. Move the table one increment X axis and rotate the A axis one increment to reach the first waypoint, then repeat until complete. If you have enough waypoints, say .01"& .1° increments, the result should be fairly even. You can then use a riffling file to smooth it out.
I have done 2 dimensional curves using this technique on flat pieces, my DRO has a function to automatically map the waypoints. The results are surprisingly good. No reason it wouldn't work on the helix.