Variable Pitch Threading (Lathe)

[PE @ IHS]

Has anyone tried cutting variable pitch (lead) threads using MC Lathe? I could not come up with any examples. The application is a feed screw that needs to be roughed out with grooving tools prior to being milled to size. Any experiences would be appreciated.

 

Peter

[McRae]

A short description of what I did for this was to use the z-x profile of the part, break the profile into small sections (.010), and then write a custom routine inside the post that would output the moves around the point. The variable codes need to be figured out inside of the post and spit into the code that way. I cna not think of any other way that the mastercam software could analyse a model and then calculate the variable lead.

 

A word of advise - don't tell the customer that the software will do it, get the order, and then beg for forgivness after the fact. The explicit feature that you want is not there.

 

Also, when attempting this, make sure that the maximum feed rate is not exceeded for the given spindle speed. This could be a problem. Clearance on the back edge of the tool must be analysed at the greatest pitch change. A grroving tool likely doesn't have the proper relief on the edge of the insert - so chipping may be an issue as teh material helixes along the flank of the insert.

 

Let us know how you make out.

[PE @ IHS]

Thanks for the reply. I think that I will encourage them to mill the entire thread instead of roughing it with a lathe tool. The machine is a twin spindle/twin turret Mazak lathe with live tooling, so it should be possible to mill the sucker.

 

Peter

[McRae]

Peter,

 

One more thing, there is not a relationship between the threading mark on the spindle and the c-axis zero of the milling spindle so when the machine connects the c-axis, there are no guarentees that the two will be coincident or in phase.

 

Mill the hell out of it and you won't be sorry.

[Philcott]

Peter,

 

What kind of cutter would you choose to use to cut the thread on the lathe? Would you use a thread milling cutter and approach the part with the (single tooth) cutter mounted parallel to your Z axis or an endmill ground to 60 degrees mounted perpendicular to z axis? I guess, depending on the part material you could even use a thread boring bar mounted in a live tool holder oriented parallel to z.

 

Phil

[McRae]

Phil,

 

This Feed Screw is more like an Augar than a Thread. There is a profile and a bunch of changes in pitch and profile cross sections as well as increasing and changeing numbers of starts/blends and even some places have little mixing tines on the root diameter.

 

Thread milling is only applicable to internal threads on a milling machine (or external knobs...) and on the lathe - a single point solution is much more efficient than a thread mill - besides, the accuracy in the thread would be better with the single point solution.

 

How are things in BC - do you still eat Alberta Beef?

[Philcott]

Andrew,

 

Things are going well for me in BC. Work has gotten quite busy once again. Lacrosse season is in full swing, hiking and camping weather is in it's prime, big salmon have showed up in local waters kayacks full of Scouts glide silently to island marine parks and I have a full tank of propane just waiting for the next cut of prime Alberta beef to hit the grill.

Hope the BSE scare doesn't put the pinch on Jack's sausage endeavors. He may need to push some salmon throught the horn instead of beef. I have enjoyed making my own game meat and salmon sausages in the past. You can limit the fat content and make some reasonably healthy food. I like my hot italian moose sausages the best.

 

Phil

 

Go Ottawa Go!!!