My First ACME Thread

[epanzella]

I had some time today between disasters so I decided to single point a 4tpi ACME worm gear that I'll need for a future project. I was amazed at how much tool pressure that thread generated and the tool was nearly ripped out of the holder on the first pass. Luckily 99% of the mess was cleaned up by subsequent passes. I ground a HSS cutter for this job. I had 30 minutes I could dedicate to this quick threading job. Fortunately it only took me 2 hours.

 

[benmychree]

Worm threads and Acme threads are not the same thing, Acme threads are 29 degrees included angle, worm threads are 30 degrees included angle, at least if they are circular pitch. Worm threads can be either diametral pitch or circular pitch (same as TPI, more or less) depth of tooth may also be different in Acme or worm threads.

 

[GeneT45]

Did you feed straight in or at roughly half the included angle? The latter should reduce the cutting force required. Still, 4TPI is big, so the forces are bound to be commensurate.

GsT

 

[epanzella]

Worm threads and Acme threads are not the same thing, Acme threads are 29 degrees included angle, worm threads are 30 degrees included angle, at least if they are circular pitch. Worm threads can be either diametral pitch or circular pitch (same as TPI, more or less) depth of tooth may also be different in Acme or worm threads.

Didn't know that. They look like ACME threads to me. I'll cut the worm wheel to match. Gotta work, right?

 

[epanzella]

Did you feed straight in or at roughly half the included angle? The latter should reduce the cutting force required. Still, 4TPI is big, so the forces are bound to be commensurate.

GsT

Yeah, I fed in with the compound @ 14.5 degrees then cleaned up with the cross slide. For 60 degree threads I just go straight in with the cross slide but I was being respectful of this large, blunt thread. It was STILL a lot of tool pressure. I started out at .005 per pass and after a few passes the cutter sounded like it exploded and shrapnel hit me in the face. The cutter was fine but it must have caught and sprung back throwing chips at me. I had safety glasses but it still rattled me. Went to .003 and then .002 as the cut got near the finish line. Lots of springs too. That's why it took 2 hours.

 

[jwmay]

Looks like a worm to me! Congrats!

 

[epanzella]

Bored and cut off. Broach the keyway and pop in a cupla set screws. FINITO.

 

[rwm]

We are using this same worm screw in our dividing head build. My lathe will not do 4TPI however. How are you planning on cutting the mating gear?

 

[epanzella]

We are using this same worm screw in our dividing head build. My lathe will not do 4TPI however. How are you planning on cutting the mating gear?

I have some Bridgeport R8 blanks suitable for an end mill holder, slitting saw arbor or whatever. I plan on cutting a 90 degree hole thru it along with a set screw. Then I'll stick in the same cutter I used for the worm to make the worm wheel. I guess that would make it a fly cutter but the bit will stick out the side instead of the bottom. I'll shim my spin indexer up on the mill table for the helix angle. (fingers crossed)

 

[rwm]

Hey, you should read over this thread:

Group Project: Dividing Head - The Build

If you get the hobbing working, I have some more material to cast brass/bronze blanks. Will be following the thread.

www.hobby-machinist.com

My trials and errors in doing exactly what you are trying to do. Bottom line was I needed a single point cutter to get the starter grooves and then a shop made hob to "run in " the gear. If you can machine a hob out of O1 I think that would work out well. In fact I would love to borrow it when you are done to perfect my gears!
BTW- You mean "lead angle" not "helix angle". The lead angle for 4TPI x 1" is 4.55 deg. That's what you shim your indexer to. Make sure you shim it in the correct direction! Easy to mess that up. I almost did.