Rough threads - what went wrong?

K

KKrum

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I recently cut an M5x0.5 thread in salvaged mystery steel (the counterweight shaft of a junky telescope mount) using a carbide insert on my mini lathe. I could tell from the ragged chips that it wasn't cutting right. It left burrs on the threads. The part ended up fitting okay, but I'd like to do a better job next time.

I know you can't tell me what actually went wrong, but I'm hoping you can add to my list of things that may have gone wrong...
  • Speed and feed - I know carbide likes to run fast
  • Tool height - maybe a hair too low, creating negative rake
  • Lubricant - I used Chinese mystery cutting oil
 
It's always good to post close up pictures, as well as full lathe pictures. Helps us help you.
 
I'm going to go with speed. I have no idea if that was the problem but it is often one, for me at least. In order to thread something that small with carbide you'd need your lathe running so fast that no mere mortal could stop the carriage before it slams into the Chuck. So you probably running too slow... because you had to.
 
KKrum said:
View attachment 400923
My lathe is bolted to a purpose-built bench. The bolts run through a 4x4 under the plywood top.
View attachment 400924
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Can you show a close up of the tool as well?

I see what you mean by the threads, they are a bit rough looking.

You kept the half nuts always engaged? Even when retracting from the cut? What depth of cut did you use for each pass?

Trying to get you to describe how you threaded. Don't know if it's a problem, but if you describe what you did, we can hopefully narrow down the possibilities.
 
WobblyHand said:
Can you show a close up of the tool as well?

I see what you mean by the threads, they are a bit rough looking.

You kept the half nuts always engaged? Even when retracting from the cut? What depth of cut did you use for each pass?

Trying to get you to describe how you threaded. Don't know if it's a problem, but if you describe what you did, we can hopefully narrow down the possibilities.
Click to expand...
Half nut always engaged, backed out with the cross slide and returned it to zero, advanced cut with the compound. I don't remember the depth per pass.

I got the major diameter to (IIRC) 0.002" under. (I don't have a metric micrometer.) I touched off with the tool, zeroed the cross slide, then hit the minor diameter in 2-3 passes. I got the minor diameter from this chart: https://www.newmantools.com/tech/threadmf.htm. I tested the mating part without removing the work from the chuck. It didn't fit, so I did 1 or 2 more very light passes and it did.
 
KKrum said:
Half nut always engaged, backed out with the cross slide and returned it to zero, advanced cut with the compound. I don't remember the depth per pass.

I got the major diameter to (IIRC) 0.002" under. (I don't have a metric micrometer.) I touched off with the tool, zeroed the cross slide, then hit the minor diameter in 2-3 passes. I got the minor diameter from this chart: https://www.newmantools.com/tech/threadmf.htm. I tested the mating part without removing the work from the chuck. It didn't fit, so I did 1 or 2 more very light passes and it did.
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Thanks for the explanation. Sounds like you have the basic idea, which is what I was "testing" for. The compound angle looks close, but it is hard to tell. How did you set the compound angle?

Typically one takes a scratch pass (a few thousandths of an inch) and then checks with a thread gauge, just to make sure one has the gears right. Then a deeper cut, sort of getting lighter every cut thereafter. At the end of the process, one is only taking off a thousandth at a time until you get the fit you need. I was just checking that you didn't try to cut the thread all at once!

How do you set the tool height? Many people make a height gauge for their lathes. It's important to get the tool height right.
 
SLK001 said:
You should probably be using a very sharp HSS cutter for your threads. It will cut much nicer.
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Have to agree, but I've cut decent threads on a mini-lathe using sharp carbide.
 
A very simple height gauge can be made with a length of tube of a dia that will stand up ok, both ends squared off, new (sharp) centre in the head stock, stand the tube up on the cross slide and scribe around it using the centre point, flip it and scribe again. set the tool height to that line.
Scribe both ends so it doesnt matter which way up it is.
 
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