Advice please-tapping in cast aluminium

Please can I pick your assembled brains
I am finishing off making a Hemingway fixed steady for my MYFORD 7 and this involves tapping the finger adjustors in the aluminium casting for a quarter whit cap screw.
I am far from happy with the results in that the new taps did not cut cleanly leaving an ill formed thread and a floppy fixing. The metal just seems to tear
I used 3 in 1 oil as a lubricant and firstly tried a taper tap and secondly a second tap which gave a marginally better result. The casting was about 3/8 thick. I had a similar result on a thin piece of alum. plate with a 5 mm tap Last week and had to resort to using mild steel. The results using these taps on steel was fine and as to be expected.
I used the recommended size 20 drill and used thin oil and RTD grease as a lubricant.at different times
I can't believe everybody has these dire results and perhaps someone can suggest what I'm doing wrong please
Mike
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involves tapping the finger adjustors in the aluminium casting for a quarter whit cap screw.

a second tap which gave a marginally better result. The casting was about 3/8 thick. I had a similar result on a thin piece of alum. plate with a 5 mm tap Last week and had to resort to using mild steel. The results using these taps on steel was fine and as to be expected.

Perhaps won't help a lot given you have already got the taps but in soft materials like aluminium I use roll forming taps that swage the thread into place as opposed to cutting it. These form a far stronger thread but it's another set of taps to buy.
Oil also tend to rip threads in ally as it holds the chips, this is about the only thing I have found WD40 to be any good at. Or if you are a tight arse like me then make your own up out of 75% paraffin and 25% automatic transmission fluid, the red stuff. Works just as well and a damn sight cheaper.
Often helps to drop a tad down on tapping sizes as most tapping drills are chosen for 75% to 80% thread engagement which helps the tap in harder substances at the expense of not forming the whole thread. Tapping at core diameter in soft materials will give you the strongest thread possible.
This doesn't apply to roll form taps were you have to stick to published tapping sizes as with no material to be cut there is very little to displace to form the thread. -- Regards,
John Stevenson Nottingham, England.
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Please can I pick your assembled brains
I am finishing off making a Hemingway fixed steady for my MYFORD 7 and this involves tapping the finger adjustors in the aluminium casting for a quarter whit cap screw.
I am far from happy with the results in that the new taps did not cut cleanly leaving an ill formed thread and a floppy fixing.
What sort of new taps? Decent quality named brand HSS ones or cheapo carbon steel made in Itchifanni ones?
The metal just seems to tear
Aluminium castings need to be heat treated properly or they don't machine nicely. They just tear and gum up the tooling. Doesn't matter whether it's a milling cutter, lathe tool or a tap. So either the material is no good or the taps are no good.
I used 3 in 1 oil as a lubricant
Not an ideal move. Paraffin is about as good as anything for machining ally.
and firstly tried a taper tap and secondly a second tap which gave a marginally better result. The casting was about 3/8 thick. I had a similar result on a thin piece of alum. plate with a 5 mm tap Last week
Aluminium plate ought to be forged material, usually HE30, and should tap very cleanly. If your taps can't handle that then the taps are no good.
I tap aluminium castings very regularly. Usually to fit helicoil inserts in cylinder heads with buggered threads. It's the easiest material in the world to tap if the taps are sharp and the material is well heat treated. Find a bit of known decent material like a scrap cylinder head or some HE30 plate or bar and try a thread in that with paraffin as a lubricant. If still no joy then chuck all your new taps away and buy some proper ones.
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I have used a product called "tap-free" some- very thin, and turning th
tap slightly forward then back to clear the chips, and blowing out th chips with compressed air has worked for me. I like the home-made WD-4 also- you can adjust the formula, and does not dry out and leave a tack residue as WD-40 is wont to do-transmission fluid is a good penetratin oil, also. I have also used a engine treatment and penetrating oil tha is superior to A T F-it is called"marbles penetrating oil" I get it a my local Murray's(sp) - but you can find it in some auto parts stores Just my .0
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Mike D wrote:

involves tapping the finger adjustors in the aluminium casting for a quarter whit cap screw.

a second tap which gave a marginally better result. The casting was about 3/8 thick. I had a similar result on a thin piece of alum. plate with a 5 mm tap Last week and had to resort to using mild steel. The results using these taps on steel was fine and as to be expected.

Spend a few pence on actual cutting fluids rather than oil, would be a good start.
Kerosene works OK for aluminium, making this one of the few things WD-40 is really good for.
A proper tapping fluid for Al. would be better yet. It sounds like the castings are fairly pure Al. As such, any machining process will be akin to cutting warm bubble gum.
Remember, that if all else fails, there's always helicoils, or other forms of thread inserts to fall back on, provided there is room, otherwise a trip to awelding shop for a TIG welded filler, and back to the start.
Cheers Trevor Jones
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<snipped lots of stuff>

I use Isopropyl alchohol as a cutting fluid for Aluminium, whether milling, turnng, or tapping, and IMHO it is just about *the* best one I've used. Doesn't flash off that quickly, cools as it evaporates, and leaves no residue behind. I get really nice finishes with it too. Another advantage is that it leaves no residue unlike WD-40 or paraffin. I have a 5litre bottle I got from the local chemical suppliers that has lasted for ages, and it's handy for degreasing parts (use it in the ultrasonic bath) and cleaning machines too.
Next favourite would be straight paraffin, and then WD-40.
Peter
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Hi Peter,
Is there anything special about Isopropyl or will any alchohol (eg meths) do as well dyk ?
--
Boo

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wrote:

Hi Boo,
No nothing special about it, just that I had it in, tried it, and found it superb. I suppose that meths would work just as well, just might smell a little bit more than Isopropanol. It may work even better for tapping if mixed with a little light machine oil, but as mine is decanted and applied from a lab wash bottle (long hook type nozzle) I was too lazy to try this!
Peter
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Hmm, I also have glow fuel around...
--
Boo

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wrote:

I once used Isopropyl alcohol in the old plastic 3-in-one container that I use for cutting oil on the lathe. This was on an occasion when I was hacking off stringy steel as fast as I could remove it and a fair amount of heat was staying in the stock, rather than coming off with the swarf.
Since the part I was turning was loctited onto a mandrel, I didn't want it to get over 200C. I had already lost one part that way.
I found that cutting oil wasn't really soluble in it and I had to shake the bottle to get a dirty emulsion. A bit of Jizer in the mix helped the emulsion a bit.
The idea of using the alcohol was to try and gain some cooling by evaporation. It worked to an extent. I eventually gave up when I realized that I couldn't see across the shed and I started to worry that If I stopped the 'coolant' A glowing chip might cause the shed to explode.
It's probably a bit more benign on ally :-)
Since then I've got the coolant pump set up so I can flood cool if I have to. I just haven't got the screens and the wet suit needed to cope with the fallout.
Mark Rand RTFM
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Many thanks for all the helpful advice I tried John's Homebrew, it was much better but I am still unhappy with the results The taps were new via tracy tools the results give an oversize fixing 2 further questions Please
1 where do you get the other type of tap john mentioned? 2 how should the metal be heat treated?
mike
<DIV>Please can I pick your assembled brains</DIV> <DIV>&nbsp;</DIV> <DIV>I am finishing off making a Hemingway fixed steady for my MYFORD 7 and this involves tapping the finger adjustors in the aluminium casting for a quarter whit cap screw.</DIV> <DIV>&nbsp;</DIV> <DIV>I am far from happy with the results in that the new taps did not cut cleanly leaving&nbsp;an ill formed thread and a floppy fixing.&nbsp; The metal just seems to tear</DIV> <DIV>&nbsp;</DIV> <DIV>I used&nbsp; 3 in 1 &nbsp;oil as a lubricant and firstly tried a taper&nbsp;&nbsp;tap and secondly a&nbsp;second&nbsp;&nbsp;tap which gave a marginally better result.&nbsp; The casting was about 3/8&nbsp;thick.&nbsp; I had a similar result on a thin piece of alum. plate with a 5 mm tap &nbsp;Last week and had to resort to using mild steel.&nbsp; The results using these taps on steel was fine and as to be expected.</DIV> <DIV>&nbsp;</DIV> <DIV>I used&nbsp;&nbsp; the recommended size 20 drill and used thin oil&nbsp;and &nbsp;RTD grease as a lubricant.at different times</DIV> <DIV>&nbsp;</DIV> <DIV>I can't believe everybody has these dire results and perhaps someone can suggest what I'm doing wrong&nbsp; please</DIV> <DIV>&nbsp;</DIV> <DIV>Mike</DIV></BODY></HTML>
------=
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J&L industrial supply,
http://catalogs.shoplocal.com/jlindustrial/index.aspx?storeid 39181&circularids30&jlpass2&firstname=&lastname=&loggedinlse&itemcount=0&mode=test&pagenumber2
or just search for 'thread forming'
Stu G
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Many thanks for all the helpful advice I tried John's Homebrew, it was much better but I am still unhappy with the results The taps were new via tracy tools the results give an oversize fixing 2 further questions Please
1 where do you get the other type of tap john mentioned? 2 how should the metal be heat treated?
mike
Sounds as though the taps are no good. It's best to stick to known brands like Dormer, Blue Point etc. If you live anywhere near Slough there's a little shop in Farnham Common called Sert Tools which has boxes of hundreds of second hand taps, many as new, for a quid or so each. Most of my stock has come from there. If that's not an option then J&L online as has already been suggested.
Aluminium heat treatment will be done at the factory. It's not something you can do yourself.
You seem to be posting in html. If you could stick to plain text it would be appreciated.
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Thanks for the extra info Mike ps think I have killed html now
Please can I pick your assembled brains
I am finishing off making a Hemingway fixed steady for my MYFORD 7 and this involves tapping the finger adjustors in the aluminium casting for a quarter whit cap screw.
I am far from happy with the results in that the new taps did not cut cleanly leaving an ill formed thread and a floppy fixing. The metal just seems to tear
I used 3 in 1 oil as a lubricant and firstly tried a taper tap and secondly a second tap which gave a marginally better result. The casting was about 3/8 thick. I had a similar result on a thin piece of alum. plate with a 5 mm tap Last week and had to resort to using mild steel. The results using these taps on steel was fine and as to be expected.
I used the recommended size 20 drill and used thin oil and RTD grease as a lubricant.at different times
I can't believe everybody has these dire results and perhaps someone can suggest what I'm doing wrong please
Mike
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-- Regards,
John Stevenson Nottingham, England.
Visit the new Model Engineering adverts page at:- http://www.homeworkshop.org.uk /
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On Wed, 14 Feb 2007 08:58:29 GMT, John Stevenson

Tubal Cain's tables recommend 5.3mm for 1/4 Whit, to give 65% thread engagement.
Regards, Tony
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I don't use Aluminium very much*, but in general I usually drill in at least 2 stages - you get a hole much closer to nominal size that way. However, I also agree with Tony that you don't (normally) need to go for such a high thread engagement. So for a 1/4" BSW, I would go something like 4.5mm - 5.2mm for a 71% thread engagement.
*Because it's sticky stuff (tends to tear on screwcutting) and less strong than steel, and most of my uses are not particularly weight-sensitive. Which probably says I don't know enough about it to be using the right alloy!
David
--
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