View Full Version : Hardened Hob
OK, I read an article of a machinist using a "hardened hob" to get a very highly polished finish on a die. And I mean highly polished, like mirror finish.
So, what is a hardened hob, what is a hob, and how would it be used?
Forgive me if this is a stupid question, but I just need to know.
This page might answer some of your questions. The machinist is making a rotary table and needs a worm gear and a hob to create it with, and a worm shaft. The hob requires hardening which he also shows:
http://homepage3.nifty.com/amigos/index_table/making_of_the_idex_table-e.htm
Tinkerer
11-29-2006, 12:20 AM
Are you sure he did not mean a burnishing tool. Hard metal sphere comes in various sizes on a shaft used to polish (burnish) the surface.
The hob in the pictures I linked is not a thread so can't be used for burnishing without setting and unsetting it for each tooth/gap pair as well as maintaining the correct helix angle. The hob I made was from a spare lead screw I bought for the purpose, but is otherwise identical to the image, and I suppose a section of it could be used for burnishing the worm wheel although burnishing is something a thread dial gear sees a lot of in normal use.
Burnishing a spur gear vs a worm wheel looks to me to be a bit more effort.
Rustybolt
11-29-2006, 06:29 AM
OK, I read an article of a machinist using a "hardened hob" to get a very highly polished finish on a die. And I mean highly polished, like mirror finish.
So, what is a hardened hob, what is a hob, and how would it be used?
Forgive me if this is a stupid question, but I just need to know.
In this casre a 'hob' is a hardened male representation of a cavity that is going to be made. Using a very powerful four post press, the hob is literally pushed into the die material creating a cavity. The finish on the tool is the finish in the hole.
In this casre a 'hob' is a hardened male representation of a cavity that is going to be made. Using a very powerful four post press, the hob is literally pushed into the die material creating a cavity. The finish on the tool is the finish in the hole.
I'm not quite sure that is what this writer was describing. Perhaps he used the wrong "word" for how the finish was achieved. Regardless, the die was made, the internal finish was brought to a highly polished state at a very exacting size. The final product had to be hard to begin with or hardened afterwards. How would one go about cutting an internal surface, say with a reamer, and then getting the surface polished without distorting it in some way?
The term "hob" does not just refer to a cutting process. It also is used to refer to the process of making a mold using a highly finished plug to make the female impression in the mold material through direct application of pressure, as Rustybolt explained.
Molding dies are very commonly made with aluminum alloys, usually a variant of 7075. It's hard to imagine but even hardened materials will flow like fluid with enough application of pressure. Of course, the hob must be even harder.
As for polishing after the impression or cut is made many possibilities exist. Some of the lesser known ones include electropolishing which is an electrochemical method that takes advantage of the fact that microscopic projections of material are preferentially etched with the right electrolyte and current. Electropolishing is capable of producing a mirror finish.
JCHannum
11-29-2006, 09:05 AM
I'm not quite sure that is what this writer was describing. Perhaps he used the wrong "word" for how the finish was achieved. Regardless, the die was made, the internal finish was brought to a highly polished state at a very exacting size. The final product had to be hard to begin with or hardened afterwards. How would one go about cutting an internal surface, say with a reamer, and then getting the surface polished without distorting it in some way?
Ballizing is one method of doing this. It is similar to the hobbing process described. It entails pressing a ball bearing through the reamed hole to finish it to size. The surface will be quite smooth, and, depending on the workpiece's material, may be workhardened.
TGTool
11-29-2006, 09:55 AM
We used the process Evan describes to make parts of molds for powdered metal parts, in this case electrical contacts. The contacts themselves were to have a pattern of small square pyramids standing up from the surface, so the mold had to have the female equivalent of little square depressions. These components were HSS, so the pattern was pressed into the ends of the annealed blanks by hobbing. Then the remaining part was machined, the parts hardened and ground, and the pattern polished with diamond compound.
I don't know either why two such different machining processes came to have the same name. Probably origins in unrelated places or disciplines.