Single point threading speed for aluminum

[o0norton0o]

If you're going to cut thread at 1,700 RPM, I would thread away from the chuck. If it was me and I was threading toward the chuck I would take shallow cuts and go at it at about 75 RPM.

That's what I had planned to do, but chickened out. I think when I made these before, I turned them at 450 rpm towards the chuck and it was scarey as hell. It worked but the threads came out rough IMO.

I’ve cut perfectly good threads in 6061-T6 at 100 rpm using an HSS tool and some lube. It would help us to know your equipment and procedure. Lathe size? Aluminum alloy being threaded? RPM? Threading tool HSS or carbide? Approach angle of tool to work piece? DOC for each pass? Pics of mucked up threads?

Tom

I have a small metric atlas MK2. I've replaced the plastic gibs with metal, so the rigidity is better than previously. I also added a QCTP and just changed to new half nuts so the carriage/lead screw interface is pretty tight. I was threading with the compound feed at 29.5 towards the headstock at 55 rpm. At first I tried the carbide triangle, and once I had gone too far and ruined the piece I tried the HSS to see if I could get a good thread result from the piece I had ruined... At 55 rpm, I did NOT get a good result with HSS either. I did use 20wt oil to lube the cutter and piece....

 

[o0norton0o]

So,.. If I have the guts to set up a reverse spinning chuck with an upside down cutter going away from the headstock.... what speed should I use? and should I now be angled at 30.5 since the cutting edge has changed sides relative to the direction the cutter is moving? Any other comments are appreciated

 

[WobblyHand]

It's time to show us some pictures. Please show a picture of the compound of your lathe overhead, so we can see the angle. And so we can see the cutting tool. Basically your set up. I can't think of a reason why your threads would come out mucked up, unless there is a setup issue. If you are using 6061 and WD-40 with a decent tool, with 20 TPI threads, they should come out reasonably even at low speeds. Mine do, and I'm really a beginner.

As a quick check, make sure your cutter is on center-line. You can do this fairly easily, by gently trapping a vertical scale between your work piece and the cutter. Slowly tighten the cross slide. Is the scale vertical? No? Then you are not on center-line. If the top of the scale is pointing towards you, the cutter is low. If the top of the scale is pointing away from you, the cutter is above center-line. Adjust cutter height to be on center-line.

 

[OTmachine]

And take the last couple of thousandths with the cross slide instead of the compound to clean up both sides.

 

[Stephen S]

If you're going to cut thread at 1,700 RPM, I would thread away from the chuck. If it was me and I was threading toward the chuck I would take shallow cuts and go at it at about 75 RPM.

I agree. I have an on older Clausing Lathe (mid to late 1960). whin I thread I put in back gear take light cuts. If the tool has a positive hook of more than 5° that could cause issues. In another post W-D 40 on aluminum works great.
When I single point thread. On the last pass, I make 1 to 2 passes without changing the dials. I also use a tread file to smooth the threads.

 

[o0norton0o]

Pictures will have to come later, I have to go out for a few. Thanks for all the comments so far

 

[WobblyHand]

If you are going to cut thread at high speeds towards the chuck, you need a CNC, or a really good proximity stop, or a special threading tool that disengages the half nuts automatically, or you are a 15 YRO video gamer. Threading away from the chuck is a lot easier on your nerves/pucker factor. But honestly, at the moment, it is unclear if the OP's setup is correct. So @o0norton0o please show us your setup.

The reason I had bad single pointed threads was because of the wrong compound angle. You want the compound set to 60.5 degrees away from the spindle axis, or equivalently, 29.5 degrees towards the tail stock starting from being perpendicular to the spindle axis. Many beginners get caught by this - including me. Symptoms of "not getting it right" include rough nasty looking threads. This is why I am asking for pictures, it's a really common issue. If correctly set up, and the tool is at center-line, you will have much nicer threads, and your cutting tool will have longer life. I've not found it necessary to thread at very high speeds, especially if using a HSS tool. Actually, I'm a bit of a chicken, and probably thread slower than I should. My threads come out "good enough".

 

[Smithdoor]

I adapt a high tech motorcycle fork damper to fit my Norton Commando forks. I have to make a few different parts. I use the appropriate cutting speeds for those parts. The trickiest part I modify is the aluminum fork damper cap with it's integrated controls. The stock modern fork cap is wider than the Norton fork, but there's plenty of meat to turn it down and re-thread it to the Norton fork tube size threads (about 1-1/8" diameter x 20 tpi) I reduce the diameter of the fork cap at the proper calculated speed (about 1,700 rpm) but I slowed the rpm's down greatly to do the threading. The result was horrible and not a usable part at all. I can replace the fork cap nut pretty cheaply on ebay and give it another go, which is the plan, so,...

I have a question about threading aluminum and the choice of turning speed. As the speed of cutting moves toward the optimum speed for the best result, is there a mathematical progression in the increase of the quality of the cut? If I try, 450 rpm /900 rpm /1350rpm /1700rpm will I get a steadily increasing quality or is there a threshold based on the alloy or some other quality of aluminum that dictates the quality of threading. Is there something particularly about single point threading that requires consideration?

For example: I might be totally comfortable cutting down the diameter at 1,700 rpm's but not so comfortable threading at that speed. What are the solutions.

1) Suck it up and get comfortable at the 1,700 rpm speed cutting going away from the headstock with an upside down cutter where there's inches of safe space to stop the carriage.

2) Find a speed where things happen slow enough for you to feel comfortable and accept a slightly lower result

3) Something else?

*Feel free to mention anything that you feel I might be unaware of. Thank you in advance for any help

I agree with (

stupoty​

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I think the rpm for threading is dictated by your comfort level in not having a massive lathe crash)

At this point speed not problem. It needs to slow so you can do the work.

If doing blind threading use reverse so cut ends up outside the part.

Dave

 

[Huub Buis]

In general, I do all threading (steel, aluminum, plastics) at 300 RPM using the CNC. A large pitch is done at a lower RPM because my feed rate is limited to about 500 mm/min.
If things get difficult (very small pitch or very large pitch) I switch from an insert to a HSS tool. I can't say that a higher RPM gives a better finish. That is probably because my lathes or parts can't handle the high cutting forces caused by a high RPM so I never thread at the advised speed.

A right hand insert threading tool can handle the cutting forces only when used as a right hand tool. If you use the tool as a left hand tool, the clamping system (bolt,clamp) has to absorb all the cutting forces. Also the clearance angle of the cutting tip is made for a right hand helix angle.

Using the threading tool upside down will prevent tool dig in (and breaking the tip) when the cutting forces get to high. This would be my preferred way for threading a large pitch. Beware that not all chucks can be used in reversed direction.

To see what gives the best results using your lathe, stock and setup, just try different settings.

 

[o0norton0o]

I'll post a picture even though I already said I am feeding at 29.5 degrees just so we're all clear. The whole notion of the 29.5 feed angle is to have the cutter working on one side so a 60 degree cutter approaching squarely from the that angle would cut primarily on one side of the cutter only. I get that because I'm a woodworker. I know a lot about angles...

The threads you see are backwards. After I ruined the part. I switched to the HSS and threaded left handed away from the headstock just to see if I could get abetter result with the HSS at 55 rpm, no longer risking the crash. I think the last time I made these, I threaded at 450 rpm and they came out looking a little rough, but they worked. It was also scarey as hell threading toward the headstock at that speed. Hence, my modification posted here previously about spinning the chuck backwards, flipping the cutter upside down, and cutting right hand threads away from the headstock...