Re thread milling

[J Harp]

Don't want to derail DeHui's thread so I'll start another. When he mentioned a double start thread, Acme thread came to mind though he didn't say so.
When milling an internal two start Acme thread with the cutter shank parallel with the work, would a 29 degree cutter produce a correct thread form? Machinerys Handbood says the change in form is negligable when milling a 60 deg. thread, but I would think it would be a different story for an Acme thread.
On a non cnc mill how is the lead or tool advance provided?
These are strictly "would like to know" questions since I don't have a mill.

[macona]

You dont thread mill on a manual machine, plain and simple. It requires coordinated movement of all three axis.

[TGTool]

Well, as usual there can be exceptions. It is possible to make a fixture which holds the workpiece and is itself threaded with the lead that you want. Then a form cutter will machine the workpiece while it is rotated with a tommy bar or more sophisticated mechanism so it rises as it rotates to produce a thread. Someone on this forum discussed it here several years ago IIRC.

With a two start thread the lead is twice the pitch and of course you have to index the workpiece 180 degrees after cutting the first thread to then do the second start.

[Doc Nickel]

You dont thread mill on a manual machine, plain and simple. It requires coordinated movement of all three axis.

-I beg to differ.

I made a "milling head" for my lathe, that's much like a toolpost grinder. Using a custom cutter, which I also made, I mill 16 lead 32 tpi double-start threads in paintball gun barrel parts, in a single pass, on my manual Logan lathe.

Basically, I replaced a single-point cutter with a powered cutter, so I could do a single pass rather than multiple passes.

I'm sure you meant "not on a manual mill", though...

Doc.

[J Harp]

Thanks fellows for the discussion.
Yes I know you couldn't mill threads on a manusl machine without some sort of attachments. I also know that they were milling threads long before cnc came along, probably with purpose built machines for most production situations. Those old fellows figured out how to do many things mechanically.
On the Acme thread form question, it seems to me that if you couldn't tilt the cutter to match the helix angle then it wouldn't produce a correct thread form on the work, especially at high helix angles. I think it would be like sliding a board across a table saw at an angle to cut a cove, but I'm not able to work it out mathmatically, I'm a dummy at math.
Doc, I think you posted pictures of your thread milling attachment, but I forget the details.

[DR]

Makers of internal thread milling cutters will advise the minimum diameter threads that can be cut with a given cutter.

Obviously double lead threads present special problems in milling internal threads because of the rapid lead on each start. So, yes distortion of the thread form would be something that would need to be considered in the cutter's tooth form.

I believe some internal multi start threads were done on gear shaper type machines.

[lazlo]

-I beg to differ.

I made a "milling head" for my lathe, that's much like a toolpost grinder. Using a custom cutter, which I also made, I mill 16 lead 32 tpi double-start threads in paintball gun barrel parts, in a single pass, on my manual Logan lathe.

I seem to remember you had some great pictures of that setup too...

J. A. Radford has a neat thread milling setup in his "Improvements & Accessories For Your Lathe" book.