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Thread-cutting oil

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Chris Bill18/03/2022 09:13:20
15 forum posts

Hi folks,

I bought myself a nice bottle of tap and die cutting oil figuring this would work for me on the lathe. VERY sticky stuff and tool performance DOES improve when used. However, it is so sticky that it seems to retain chips on the tool that I'd rather see fall off and not get gummed into the work piece. Any advice from this august forum (short of coolant spray - not ready to embark on that mess).



Hopper18/03/2022 09:20:18
6421 forum posts
335 photos

On steel, I always just use ordinary mineral oil, the same oil can I use to lubricate the lathe. Was taught to do this as an apprentice and it seems to work. Supposedly provides better lubrication of tool and chips than coolant. Coolant is just that, a coolant, not a particularly good lubricant.

On aluminium, WD40 works well. Cast iron and brass I do dry without lubricant.


Edited By Hopper on 18/03/2022 09:24:14

Kiwi Bloke18/03/2022 09:48:49
671 forum posts
1 photos

Stickiness is a problem with some compounds intended for low-speed, hand tool use, and at lathe cutting speeds, they tend to smoke unpleasantly. You could hunt for a lower-viscosity product, or bite the bullet and use a straight cutting oil - with a flow rate sufficient to clear the chips. Yes, it gets messy. Isn't that part of the fun?

Martin Connelly18/03/2022 10:47:43
2137 forum posts
222 photos

I use Rocol RTD but I don't use much. I have a 25mm paint brush that I just drip some onto and only smear a thin layer on. The brush is also on hand for clearing the chips off the tool tip.

Martin C

SillyOldDuffer18/03/2022 10:49:30
8699 forum posts
1967 photos

I use CT90, which is a thin tapping fluid. I was pleased to read Hopper's post where he says ordinary mineral oil will do because I do that and have always suspected it was a lazy bodge!

The fluid has up to three functions:

  1. Coolant - to remove heat from the tool so it wears less quickly
  2. Lubricant - encourages cutting rather than rubbing and helps swarf slide over and away from the cutter.
  3. Physical cleaning - washing swarf away

Suds is the classic all-purpose fluid. It's an oil or fat emulsion in water. Water is an excellent coolant and the oil provides lubrication. As the fluid is cheap (mostly water), it's flooded over the job and cutter and washes swarf off. Downside is the mess and needing a pump, tank, drain and recycling. Early suds were made from vegetable oils that went off, stank and were a serious bio-hazard. Modern suds contain disinfectants. I find suds rarely needed in a home-workshop - only for prolonged heavy cutting.

Mineral oil is a good lubricant, but an indifferent coolant and liable to smoke. I find it's good-enough for light and moderate cutting. I apply it with a brush or oil-can. Swarf is brushed rather than washed off. I've tried neat cutting oil, which is formulated to last longer by resisting the molecules being chopped up by heat and cutting, but for ordinary work I can't tell the difference.

Optimised cooling, lubrication and swarf clearance is important in production settings but home workshops are less fussy unless something heavy-duty is afoot.


Peter Ellis 518/03/2022 10:59:34
101 forum posts
11 photos

What is wrong with pork dripping ? I couldn´t find Boeshield or anything similar here, other than some pink spray intended for use on food industry equipment, that stained everything. Guy Lautard mentioned pork dripping and I had a kilo of that in the fridge and I haven´t looked back.

Graham Meek18/03/2022 11:06:54
478 forum posts
303 photos

I use a brush which has been dipped in a straight cutting oil, or one of the cutting compounds mentioned above . This is applied to the back of the work and helps stop chips adhering to the work. Plus the brush is ready to use should a chip lodge itself on the tool. Too much oil is just as bad as not enough, the brush will always apply an even film which is generally all that is needed, and the thicker compounds do not seem to present a problem when applied by brush.

I hope this helps.



Speedy Builder518/03/2022 11:22:27
2615 forum posts
212 photos

As an aircraft apprentice in the '60s, we used a block of lard - pork dripping was for toast !

Hopper18/03/2022 11:55:35
6421 forum posts
335 photos
Posted by SillyOldDuffer on 18/03/2022 10:49:30:

... I was pleased to read Hopper's post where he says ordinary mineral oil will do because I do that and have always suspected it was a lazy bodge!...

Mate, I am the king of the lazy bodge. I had a foreman once at a car factory in Zimbabwe who always said the best way to find the quickest and easiest method of doing any job was to give it to the laziest bloke in the workshop. I used to get all his special projects.

But mineral oil on screwcutting was official procedure in the apprentice training centre at Chrysler Australia, and at "trade school" aka tech college. I just can't remember when I did a couple of stints in the toolroom machine shop in my second year and onwards whether we still used mineral oil for screwcutting or just the suds pump. I suspect the latter but not sure after this much time. That would have been the quickest and easiest way and so would appeal to the average 17-year-old. Since then,all the lathes I used in various far-flung maintenance departments, construction sites and at home had no suds pump so mineral oil is the choice.

Edited By Hopper on 18/03/2022 11:58:10

Howard Lewis19/03/2022 14:59:24
6120 forum posts
14 photos

Bacon fat has been my lubricant of choice when using Taps or Dies, and for screwcutting, for a long time, on steel

WD40 or white spirit for Aluminium.

None for brass...

Occasionally Rocol RTD is used but the tin, after MANY years, is still only half used.


bernard towers19/03/2022 15:18:11
619 forum posts
109 photos

plus 1 for rtd money well spent and lasts forever

noel shelley19/03/2022 15:44:48
1353 forum posts
21 photos

ROCOL RTD Expensive but the bottle allows you addminister just a small drop where you want it so NO waste, a bottle will last years unless your screwing all the time ! Noel.

SillyOldDuffer19/03/2022 15:59:57
8699 forum posts
1967 photos
Posted by Peter Ellis 5 on 18/03/2022 10:59:34:

What is wrong with pork dripping ?

Biohazard. Like bacon fat and similar it rots. Full of bacteria, some of which are harmful. Before antibiotics minor machine room injuries often resulted in amputations and sepsis.

Milk being an almost perfect water/fat emulsion makes an excellent cutting fluid. Not recommended though because it takes less than a day to go off and the stench is unbearable!

Fans of ye olde workshop receipts should be made to use milk for a month because it makes the disadvantages of rotting animal products obvious! Unlike dripping, where a microscope is needed to see the nasties.



JasonB19/03/2022 16:15:39
22764 forum posts
2656 photos
1 articles

In my first year at school the old boy metal Working teacher had us use Tallow, I think he also used it to slick back his white hair. But being rendered fat it did not seem to rot.

He retired and the new younger teacher had us all use Trefolex but always felt that was a pain to get out of blind holes as it is a bit thick and seemed to hold onto the swarf.

These days I use CT90 or Dormers Super Cut which have a nozzle on the bottle so easy to apply a small drop to the tap or end of rod being threaded being a lower viscosity it's easily cleaned off.

Mike Poole19/03/2022 17:32:13
3344 forum posts
74 photos

Tallow was the regular cutting lube for steel conduit when I did a spell on the wiring gang as an apprentice. Trefolex is my favourite and I probably have enough for a good few years supply.


bernard towers19/03/2022 18:43:44
619 forum posts
109 photos

sorry Noel haven't got the strength for the last bit!!!

bernard towers19/03/2022 18:43:44
619 forum posts
109 photos

sorry Noel haven't got the strength for the last bit!!!

Bdog50724/03/2022 08:16:04
4 forum posts

Good morning all.

Another vote for CT90 here. They also make a cutting compound which is excellent stuff, particularly when machine tapping harder materials, and a smear along the length of a hacksaw blade makes for lighter work when sawing, and extends the life of the blade considerably.



DMB24/03/2022 09:37:11
1312 forum posts
1 photos

Always try to avoid blind tapped holes like the plague.

I find that there is generally, no need for a blind tapped hole, so ignore drawing, poke tapping drill right through and tap generous depth for what is required, not necessarily all the way through. Good example being the clamp screw mount, part no. 22 and the adjustable fences, part no. 11, both page 99 in Harold Hall' s book, " Milling, a complete course" where he describes in detail the construction of his tool and cutter grinding jig. I drilled the tapping size holes right through. No messing about trying to remove tap swarf from the bottom of a blind hole, just take a large breath, mouth close to hole and blow - HARD! Also no risk of winding tap in too deep and busting it in the work. I either use my special thin oil mix for small holes or Trefolex for larger stuff where it's easier to wash out with paraffin. Don't use any form of sludgy jollop, Trefolex pork dripping,etc., etc in small holes - very difficult to wash out. Downside of what I've done above, is an unsightly hole in the tops of the fences, but does it matter?

Now go to page101, "Swivel Base" Part no. 31, has an M3 blind tapped hole 8mm deep, x4 off. I actually had to do 8 off - 4 wrong edge and 4 correct edge because I failed to check drawing one more time! Boy, did I swear! I thought that my error wouldn't be very obvious by re-doing those holes on the opposite edges rather than re -positioning the 6 off M6 through holes in the top. Point is, 8 blind tapped tiny M3 holes in mild steel and no busted tap. I used the thin oil jollop.

Guy Lautard's recommendation of 1 part Turpentine, not Turps substitute, 2 parts White Spirit, 3 parts Olive Oil. I use a tall narrow Salad cream bottle with a paper measuring strip up the side. Shake well and decant into small plastic squeeze bottle with pouring spout, wife's used hair colouring jollop bottle. This is my favourite concoction for tapping, or just drilling, small holes. Ali and like alloys, paraffin or WD40. Always Trefolex on dies because it's easier to wash out of the dies and off the work. Lastly, don't buy 2nd hand taps and dies - very likely to be blunt, will year rather than cut and taps could snap under the strain.

Edited By DMB on 24/03/2022 09:38:18

Edited By DMB on 24/03/2022 09:41:46

Nigel McBurney 124/03/2022 09:47:53
1000 forum posts
3 photos

The first place where I worked 1958 to 1964 used soluble oil on the capstan ,so tapping,threading on that machine was all done with the soluble with no ill effects,tapping by hand ,tapping attachments on drilling machines and lathes we used a mixture of tallow and flowers of sulphur which worked well on most materials usually applied with a toothbrush,the commercial Trefolex is similar though probably the factory made tallow mix was no doubt cheaper. All the centre and instrument lathes used soluble oil in a glass jar and a brush,ordinary oil was sqirted from an oil can when reaming steel and sometimes tallow when reaming cast iron. later on in my life at another company had a vast auto lathe shop these used neat cutting oils which lubricated cutting tools for maximum production but the oil is awfully messy if used on conventional lathes, I found on milling machines soluble and mineral cutting oils were used on milling depending on the material being cut. though soluble oil was preferred,no doubt cheaper and easier to keep machines clean.At home I use Trefolex,and both soluble and paste forms of Rocol they were all found very cheap at autojumble,these work well but are sticky and will cause chips to stick to taps and dies,well i advise remove the chips and the goo with a toothbrush, after all we are in the light end of engineering and expect a bit of mess,seems to me that the younger generation wants do all sorts of things but expect not to make a mess or get dirty.

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