# Lathe tool geometry for turning threads in Aluminum



## Mark Stonich (Feb 7, 2017)

A 6" vise is overkill for a RF45 style mill. A 4" is more appropiate for that size mill & IMO 5" max. I have a 5" GMT vise on my PM45 & it's slightly too big. Not enough Y axis travel to make use of the 5" full capacity. Better to save your money rather than getting something too big & most importantly the weight. I take my vise of the table quite often, a 6" is still light enough for me to be carried by hand but I'm glad I have a 5". I also have a 4" vise as well. I prefer the 5" though.

But those GMT 6" Premium vises are pretty nice. I'd love to have one but don't need one on my current mill. But if you plan on upgrading to a full size knee mill in the future than the 6" will be perfect.


Here's what the 5" looks like on my mill.




I couldn't even complete this cut without my bellows & DRO scale getting in the way. Not enough Y travel & the 5" vise is not even maxed out.




Here's what a 6" vise looks like on another PM45 (gt40's)
View attachment 253544


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## mikey (Feb 8, 2017)

A threading tool needs a 60 degree included angle at the tip for an Imperial thread if the mating part has that thread form. The material you're threading has no bearing on the tool, only thread form and tolerance specs. 

Aluminum is relatively soft but at threading speeds you won't run into chip welding (built up edge or BUE) if you use cutting oil. Even in aluminum, you will find that a sulfur-based oil works well because threading is a high pressure application. 

If you wish for the tool to cut easily, increase your relief angles to 15 degrees and it will breeze right through aluminum. If you plan to cut with the compound only (called the step over method) instead of feed straight in with the cross slide then a 5 degree side rake works really well and cuts cleaner threads. You do not usually use back rake on a threading tool, although there is no reason you couldn't. If you plan to use the compound and then clean up by feeding with the cross slide for the last pass or two, forgo the side rake.

Over the years, I've cut a lot of threads in aluminum, steel, stainless and Delrin. My current tool has a standard 60 degree included angle with 15 degrees of relief on both sides and zero side or back rake. It has about a 1/64" flat at the very tip and this tool cuts clean, easy and trouble-free. 

There is no reason you can't use 7075 instead of 6061. 7075 is harder, stronger and lighter than 6061 and I like using it when strength and light weight are factors.


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## willthedancer (Feb 8, 2017)

I wouldn't back rake that tool, just stone it bright on top, and use the compound at 29 and a half degrees for your infeed. Kerosene or mineral spirits will be good cutting fluids. Either alloy will give a nice part. The 7075 is a slightly firmer alloy, and will have slightly less tendency to tear and also stack up.


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## Mark Stonich (Feb 8, 2017)

Thanks guys. Seems like I was overthinking this. 

Mikey, is 15º side clearance what you use for threading all metals or just softer stuff? 

Can I assume that, with this geometry, helix angle isn't an issue?


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## savarin (Feb 8, 2017)

I cut all my threads now at 90' as I have a solid plinth and rarely use the compound.
All my threads have turned out better since I started doing this.
I must admit I dont measure the clearance angles except by looking to see there is enough.
What AZ trike are you building?


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## Mark Stonich (Feb 8, 2017)

savarin said:


> I cut all my threads now at 90' as I have a solid plinth and rarely use the compound.
> All my threads have turned out better since I started doing this.
> I must admit I dont measure the clearance angles except by looking to see there is enough.
> What AZ trike are you building?



Eliminating the compound for some work is somewhere down my long To Do list. I saw a video on doing it. Yours?

I wouldn't build any of the AZ trikes. I'm aiming for about 1/2 the weight. I have some experience building custom lugged and lugless conventional bike frames and I've built several very light recumbents. Including a 24 lb. one with a 60" wheelbase and a 23 lb. SWB one with heavy "Big Apple tires. I'm not that into trikes, except for winter. But we're both 70 now and someday we won't be able to cut it on 2 wheels. If I wait till we need them, I probably won't have the eye/hand coordination needed for TIG welding. I'll build a 2nd set of forks for both trikes. Heavier, so we can add hub motors if needed.


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## mikey (Feb 8, 2017)

Mark Stonich said:


> Thanks guys. Seems like I was overthinking this.
> 
> Mikey, is 15º side clearance what you use for threading all metals or just softer stuff?
> 
> Can I assume that, with this geometry, helix angle isn't an issue?



Yeah, I use 15 degrees for just about everything. I find I get less tearing of the surface. 

As for the helix issue, we are using a solid HSS tool held flat in a tool holder. We cannot alter flank angles like folks do with an inserted tool. I suppose there is some rubbing on the flanks but if we use a 29-1/2 degree angle on the compound and feed with the compound to cut the majority of the thread, then we can clean up any of the defects caused by rubbing by feeding the last thou or two with the cross slide. 

The bottom line is that I don't worry about the helix angle. I worry more about hitting tolerances and coming in on size so my nut fits properly. I can't even guess how many threads I've screwcut and never had an issue caused by neglecting the helix angle. Dunno if others worry about it, though.


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## mikey (Feb 8, 2017)

savarin said:


> I cut all my threads now at 90' as I have a solid plinth and rarely use the compound.
> All my threads have turned out better since I started doing this.
> I must admit I dont measure the clearance angles except by looking to see there is enough.
> What AZ trike are you building?



Savarin, do you think your threads are better because you have more solid support for the tool or because you're feeding in with the cross slide? Not being a dick about this because I've seen other comments like yours. Given that feeding straight in results in a more uniform chip because you're cutting on both sides, breaking the chip is more difficult and can lead to chip flow issues. It also leads to more flank wear on the tool. All of this theoretically "should" produce a rougher thread.

I am commenting on this only because I used a Sherline lathe for many years and always cut threads by feeding straight in. Like you, my tool is solidly supported on the cross slide and I never had issues cutting nice, clean threads by feeding straight in as long as my tool was sharp and I paid attention to my depths of cut. I did wonder how much better feeding with a cross slide would be so when I bought my Emco lathe I compared the thread quality between it and my Sherline. I found that under 20 power magnification there is a definite improvement of the surface finish when feeding with the compound BUT this was not visible to the naked eye and there was no difference in fit or function between the two. 

There is some controversy about which way is better. At least for myself, I know there is a difference and that feeding with the compound produces cleaner threads. I also know that the difference is marginal and either way works fine for all practical purposes. I also think that having a solidly supported tool makes a big difference when feeding straight in ... hence, my question.


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## 12bolts (Feb 8, 2017)

Charles, you really need to get to bed sometimes........



Cheers Phil


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## Bill Gruby (Feb 8, 2017)

I have the same set-up as Charles. A solid plinth only allows for threads to be cut at 90 degrees. Another myth busted, you can actually cut the threads from any angle. It's just harder on the tool.

 "Billy G"


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## willthedancer (Feb 8, 2017)

The only time I have concerned myself with the lead relief angle is when I cut multiple leads, and it becomes significant.


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## savarin (Feb 8, 2017)

mikey said:


> Savarin, do you think your threads are better because you have more solid support for the tool or because you're feeding in with the cross slide?


Considering how flexible the standard 9x20 is in its normal configuration I am convinced its the vastly improved rigidity. I also diamond hone the bit both sides and top for a polished edge. I think this helps as well.


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## mikey (Feb 8, 2017)

I suspect you're right. I also think a "plinth" would make turning better, too, which is why mine is under construction. But my compound is tight and I'll use it to cut threads with.


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## Mark Stonich (Feb 9, 2017)

savarin said:


> I also diamond hone the bit both sides and top for a polished edge. I think this helps as well.



Savarin,
What grit diamond hone do you use for finishing HSS bits?


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## savarin (Feb 9, 2017)

Mark Stonich said:


> Savarin,
> What grit diamond hone do you use for finishing HSS bits?


I use these
https://www.bunnings.com.au/eze-lap-diamond-fine-stone-sharpener_p5766728
the fine first then the extra fine.


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## Mark Stonich (Feb 9, 2017)

Amazon has a great price on a 3 pack of these for $16.56 USD
https://www.amazon.com/EZE-LAP-PAK-...ie=UTF8&qid=1486644563&sr=8-1&keywords=Ezelap

They also have 4 & 5 packs but I already have 100 & 200 grit.


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## Ken Mach (Feb 11, 2017)

I've always threaded with the compound at 90 deg. Parallel with the work. The trick I have used is to advance the tool ahead slightly with each roughing cut eg. .005 doc and advance .002 that way you can control the chip so it doesn't gum up in the thread and scuff the back side. When you reach the root diameter you can move the tool back a few thousands and clean up the back side of the thread then move the tool ahead until you reach the desired pitch diameter. This method works extremely well for acme, buttress and square threads as well as 60 deg threads. It also allows you to cut some pretty coarse threads on small machines.


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## Highsider (Feb 11, 2017)

If you had a 60 deg. included angle before grinding the top rake, it will be great for thread cutting.  To avoid staining aluminum, never use Chlorinated hydrocarbon cutting fluids. Use regular cutting oil, or better yet synthetic cutting fluid specifically for aluminum.  It's more expensive but works so much better.


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## pstemari (Feb 11, 2017)

The bad thing about plunging in straight from the side with small lathes is that you'll tend to chatter.  With that technique you effectively have a form tool, and form tools are not friendly to less-than-totally rigid lathes.

Feeding with the compound, most of the cutting is done by the left side of the bit and you have half the force trying to bend the tool and bit.


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