How to avoid discolouring stainless steel?

[AndrewB]

I need to cut and drill holes in some polished stainless steel to make up some fittings for my boat. Previous experience has been that this tends to leave an area of discolouration around the hole, where the steel has got hot.

How can I avoid discolouring the steel, or is there a way to remove it?

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[boatmike]

Only use top grade HSS drills. Sharpen them regularly and dont run your drill too fast. You are heating the material under the drill point and work hardening it. With hand tools it often helps to start with a small hole and work it up to the size you want gradually

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[snowleopard]

use a sharp drill and low speed. make a dam around the hole with plasticene and fill with oil.

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[andyball]

recently tried a set of stepped hole cutters (tin coated) from axminster tools. which are great for up to 5mm stainless ....worked betterfor me than new cobalt or hss bits.

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[LORDNELSON]

I find new sharp cobalt drills go through stainless steel very fast with little heating. If they are "big" diameter (greater than 4mm) I would use a good size pilot hole and then increase to final diameter.

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[pampas]

BUY a drill of the size required called an armour drill fron a machine tool stockist. It has the special cutting angle and helix use plenty of cutting oil if the material is more than a 1/16" thick and constant pressure in a drilling machine. should solve the problem, sound as if you are hand drilling and work-hardening the steel.

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[silverseal]

I regularily drill stainless. Drill a 2mm pilot hole, expand by 2mm per time using sharp drill bits. You should get a clear spiral of swarf, each time , which means no heating of the piece and no discoloration. Use the slowest speed and oil to lubricate

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[tcm]

agree with most of the below, needs proper kit and coolant.

But don't use steel drills for final cut, or if you do, ream the hole with a non-steel tool such a a grinder bit, and perhaps countersink with another similar tool : the carbon in carbon steel leaves surface traces on the stainless steel which is why lots of so-called stainless becomes not-stainless when enthusistic diy-ers have a go at with hacksaw etc. And the grinding bit shouldn'[t have been used with carbon steel, for the same reason.

Not absolutely sure, but i beleive the discolouration effect may be related to the high-temp discoloration on olde single-skin chromium-plated exhuast pipes - s/s includes a fair proportion of chromium too


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[cliff]

Keep the temperature down,
Use sharp drills
DO NOT use "oil" as a coolant
DO use tapping and cutting fluid or proper cutting lubricant/coolant
If using HSS drills drill 1/16th pilot hole then step up to 1/16th below final size required and finish hole with NON Carbon steel drill or reamer or Tungsten carbide burr.
If possible use a drill press rather than by hand.
If by hand apply sufficient pressure to obtain a continous spiral of swarf.
If you do discolour the surface apply pickling/passivating paste to the discoloured area to remove the discolouration.


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[AndyL]

I think it might help to know how thick it is. Drilling thin stainless is most difficult. Small holes are better punched, and larger ones trepanned. If it is more than 5mm thick, then no problem, just flush with about any coolant (even water) and keep the pressure on. Most of the heat goes into the swarf.

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[cliff]

Clean water is fine but not ordinary oil as the oil prevents the cutting edge cutting efficiently and leads to localised overheating

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[William_H]

+Drill press is the way to go to get pressure and low speed on the drill. Of course you may not be able to get the job into a drill press. Of late drill presses are very cheap and well worth having. Intrigued about the comments don't use engine oil for lubricant cos I always have. If nothing else the smoke tells you the job is getting too hot so stop. Now contrary to the others I almost never use a pilot hole for holes up to 10mm. It seems to me a drill is designed to cut along all of its cutting edge not just the outside of the edge. Lastly don't hesitate to try sharpening your drill bits. Try first with large bits. Look at the cutting angle (not the vee shape of the cutting edges to the point) and take off metal in small bits you can use a smooth electric grinding wheel carefully(don't take off too much or let it get hot) or a oil stone can improve the edge of a drill bit not too badly damaged. I don't know about leaving carbon from HSS in the hole thats a new one on me I am assuming your discolouration is too much heat. good luck will

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[richardandtracy]

I bought a drill sharpening jig (see it at <A target="_blank" HREF=http://www.screwfix.com/app/sfd/sea/searchresults.jsp;jsessionid=BSM0YBNZ2JGOJCJO2C3CJ0Q?_dyncharset=UTF-8&q=d10738&n=&pn=1&pd=1&pi=1&cn=1&cd=1&x=13&y=9>http://www.screwfix.com/app/sfd/sea/searchresults.jsp;jsessionid=BSM0YBNZ2JGOJCJO2C3CJ0Q?_dyncharset=UTF-8&q=d10738&n=&pn=1&pd=1&pi=1&cn=1&cd=1&x=13&y=9</A> when they get their system up & running again) as I couldn't get consistent results hand sharpening. The jig is good & I can occasionally get good results - but it came with no instructions, and I don't know the knack of getting it to work every time. Can anyone help?

Regards

Richard.


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[AndrewB]

Many thanks for the tips everyone. I'm drilling pipe for use as stanchions, about 3mm thick, needs 24 x 8mm holes. Go slow and keep it cool seems to be the answer - I can see it's going to take time. I normally do step up from pilot holes. It would be good to have a press but I don't have room aboard.

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