Cut a hob for a 10 pitch Acme gear

[dansawyer]

The project is to make a 360 tooth, 1/2 inch thick, 10 pitch Acme thread aluminum gear. The plan was to cut a 10 pitch Acme parallel groves in a 1 inch diameter tool steel rod, then cut 6, at 45 degrees, slots parallel to the rod axis with a mill to make a hob. Finally heat treat, quench, and temper. The plan was to then mount the gear blank on a vertical rotary table, set the mill to the tooth angle and cut the gear with the hob, rotate the table a degree, and repeat.
I have made a side mill cutter with a similar technique by cutting threads and then cutting the slots, as above. This process produced clean threads and clean cutting teeth. Contrarily. the cutting of the paralled Acme groves produced a very messy part. I concluded the difference was when cutting the threads the machine freeplay was loaded against the cutting direction. While cutting the groves there was no free play load and the cutter vibrated back and forth. This resulted in removing too much material from the sides. The resulting teeth would be too narrow and not cut clear gear teeth.
Any thoughts on this process would be appreciated? Thanks, Dan

 

[dansawyer]

Additional Information: The compound feed was turned to 90 degrees to allow feeding by using the lead screw. This meant the tool was subject to the runout in the lead screw. I will test by turning it back to 14.5 degrees and setting the apron with a dial indicator. That should remove one of the run out variables.

 

[benmychree]

Hob teeth that are not backed off will not cut freely, acme thread gear teeth are not appropriate for your application, although worm gear teeth may be, the forms are similar, I think it would make the most sense to use a regular involute gear cutter to cut the gear. a hob connotates being cut as a spiral, with backing off for cutting clearance, otherwise, it just rubs off particles of material, and the blank would need to be gashed with a gear cutter, as there would be no mechanism to rotate the blank at the proper ratio to mesh with a threaded hob.

 

[Jake M]

You may have found this, but there's a thread from a couple of years ago, and has some good links to other threads, there might be something useful in there.

Involute Gear Pitch for ACME Thread

I'm trying to figure out which involute gear pitch works with 8 and 10 TPI ACME thread? By "work" I mean for low pressure application similar to a lathe thread dial or other low torque application.

www.hobby-machinist.com

 

[dansawyer]

You may have found this, but there's a thread from a couple of years ago, and has some good links to other threads, there might be something useful in there.

Involute Gear Pitch for ACME Thread

I'm trying to figure out which involute gear pitch works with 8 and 10 TPI ACME thread? By "work" I mean for low pressure application similar to a lathe thread dial or other low torque application.

www.hobby-machinist.com

Thank you. I had not found that particular thread, but had found most of the data in the references.

 

[dansawyer]

Hob teeth that are not backed off will not cut freely, acme thread gear teeth are not appropriate for your application, although worm gear teeth may be, the forms are similar, I think it would make the most sense to use a regular involute gear cutter to cut the gear. a hob connotates being cut as a spiral, with backing off for cutting clearance, otherwise, it just rubs off particles of material, and the blank would need to be gashed with a gear cutter, as there would be no mechanism to rotate the blank at the proper ratio to mesh with a threaded hob.

Thank you for the correction on a hob. You are correct there is no spiral in this process. If a hob is threaded then this is not a hobbing process at all.

I have gotten as far as I am trying to make what could be called a 'solid, stacked cutter'. This cutter would be 6 to 8 Acme shaped ridges, there would be no threading, they would be parallel. The OD would be 1 inch. Using an end mill, slightly offset from center 8 passes would cut perpendiculay to the groves. This would create a cutter teeth. (I have created a side mill cutter using this process. )

The angle of thread in a 1 inch diameter 10 pitch thread is about 1.8 degrees. The gear blank would be mounted on a rotary table and the cutter in a mill. The mill head would be rotated 1.8 degrees so the cutter would cut threads at the same angle in the gear blank as the worm gear. The mill table would move the edge of the blank into the cutter and then back out. The rotary table would then rotate 1 degree and step and repeat.

This issue is cutting clean Acme groves. Next is to try using the compound feed at 14.5 degrees to cut a grove. I will post a pix.

 

[f350ca]

I've made gear cutters the way you describe. Think this one was 32 DP. Used O1 hardened it but didn't bother tempering as I was cutting bronze, for aluminum I wouldn't temper it either.
I used two gashes, one to give the cutter face on centreline, then another to give relief, then filed a little relief on the backs of the tips before hardening. It cut freely.
The advantage of using a hob like cutter is the centre tooth cuts the depth, but the teeth above and below shape close to the correct tooth form.
Im too lazy to draw it out, when tilted at 1.8 degrees will the top and bottom teeth of the cutter enter the other gear teeth correctly?






Greg

 

[dansawyer]

I've made gear cutters the way you describe. Think this one was 32 DP. Used O1 hardened it but didn't bother tempering as I was cutting bronze, for aluminum I wouldn't temper it either.
I used two gashes, one to give the cutter face on centreline, then another to give relief, then filed a little relief on the backs of the tips before hardening. It cut freely.
The advantage of using a hob like cutter is the centre tooth cuts the depth, but the teeth above and below shape close to the correct tooth form.
Im too lazy to draw it out, when tilted at 1.8 degrees will the top and bottom teeth of the cutter enter the other gear teeth correctly?


View attachment 422096

View attachment 422097

Greg

Thank you. That is encouraging. I will keep moving forward.