Mimosa1974 Posted April 14, 2017 Report Share Posted April 14, 2017 I've made a pair of radiator stays for my TR6 from 3mm thick stainless flat bar. The bars are mounted at either end, so 4 holes need to be drilled. I'd like help please to understand why, in the process of drilling, I have completely blunted a new COBALT drill bit, which started off cutting well but then gave up at the start of the third hole, and utterly destroyed, and by destroyed I mean severely blunted the cutting tip and worn away the circumference of new TITANIUM drill bit (!). I understood that both will cut stainless but the COBALT was the best (hence me cutting through the first two holes relatively cleanly) but it quickly gave up on the third hole and just started scorching the metal. The size of both bits used was 8mm, and the drill was a conventional hand-held type. The reason I made the stays was because I've fitted a 'pull' electric fan and need about 5mm more clearance between the crank 'nut' and the face of the fan. Of course I have removed the old fan and it's adaptor plate. Has anyone got an idea why these expensive drill bits failed so dramatically? Oh, and lastly, well done to the TR Register for winning the 'Most Outstanding Website' at the National Car Club Awards 2017. I'm not surprised and it's richly deserved. Dave Quote Link to post Share on other sites
RogerH Posted April 14, 2017 Report Share Posted April 14, 2017 Hi Dave, did you use a coolant/cutting fluid? Please say yes. Did you use light pressure? please say yes. Did you use a pilot hole - 3mm or so? please say yes. Have a look here http://www.ttp-hard-drills.com/vid-stainless-steel-drilling.html Roger Quote Link to post Share on other sites
mike3739 Posted April 14, 2017 Report Share Posted April 14, 2017 Hi Dave, did you use a coolant/cutting fluid? Please say yes. Did you use light pressure? please say yes. Did you use a pilot hole - 3mm or so? please say yes. Have a look here http://www.ttp-hard-drills.com/vid-stainless-steel-drilling.html Roger Roger has all the all the answers Quote Link to post Share on other sites
Dave Larnder Posted April 14, 2017 Report Share Posted April 14, 2017 Dave Were the drill bits Chinese? if so I am not surprised. What speed were you putting the pilot through at and what for the finished size? Dave Quote Link to post Share on other sites
Lebro Posted April 14, 2017 Report Share Posted April 14, 2017 Agree with all above statements. Stainless (some grades anyway) is much harder to drill than mild steel, & needs to be done slowly, with plenty of coolant / lubricant, & as Roger says start small, & build up the hole size gradualy. Bob. Quote Link to post Share on other sites
Mimosa1974 Posted April 14, 2017 Author Report Share Posted April 14, 2017 Roger, guys Er. ............. no, no and ......... (you've already guessed) ....... no. What a chump. What a shame that the sites I went on made no mention of speed, pressure, pilot holes or coolant/cutting compound. But it all makes sense of course - now you've reminded me. The bits were indeed German. So what speed and pressure should I have been using? As slow and light as starts to cut I guess. Someone nod in agreement please . I forget that I'm not a teenager anymore and should be thinking more like the 'grey beard I'm becoming. I'm off to the iron mongers. Dave Quote Link to post Share on other sites
Ur1328 Posted April 14, 2017 Report Share Posted April 14, 2017 Slow and steady wins the race! Quote Link to post Share on other sites
RogerH Posted April 14, 2017 Report Share Posted April 14, 2017 Hi Dave, have the revs low enough so that high temps are not generated = 300/400rpm ish Apply enough pressure for the drill to cut, but back off once cutting to maintain the cut but not dig in. Anything will act as a cutting/cooling agent - engine oil would be quite good. There are specialist coolants but that is the problem -m they are specialist and probably not in your garage. As per my link above stainless steel contains stuff that wants to be something else (Chrome, vanadium etc) and heat is the magic ingredient. As well as your special cobalt drill there are also correct angles that the drill should be sharpened at. However most standard drills will work. Roger Quote Link to post Share on other sites
Waldi Posted April 14, 2017 Report Share Posted April 14, 2017 Some stainless steels are "work hardening", all 300 series are (304 and 316 being the most common of them). So when you "work" it by cutting, bending, drilling it will get hard easily. There are special drills for SS, with different cutting angles. It may be difficult to drill an area that is work hardened. Waldi Quote Link to post Share on other sites
ChrisR-4A Posted April 14, 2017 Report Share Posted April 14, 2017 +1 for Rogers comments, Also are you using a pillar drill or a hand drill, much more difficult with hand drill. You need enough pressure to keep the drill cutting without making it stall and don't let it just spin on the surface as this quickly surface hardens the SS and then the problem gets worse. After a pilot hole you could try drilling through half way from each side avoiding heat build up. Chris Quote Link to post Share on other sites
Mimosa1974 Posted April 14, 2017 Author Report Share Posted April 14, 2017 Guys. Many thanks for the feedback; especially the bit about 'work-hardening' on stainless I it's tacked incorrectly, which makes so much sense because that's exactly what happened. Yes, I'm using a hand-drill but I must say that using the engine oil and a pilot drill and graduating to the larger hole is the way ahead. Whilst I'm not sure exactly what speed the drill is rotating at, it becomes obvious when its right because the bit really bites the metal easily and makes quick progress. Every day's a school day. Thanks again for the very helpful advice. Happy Easter! Dave Quote Link to post Share on other sites
john.r.davies Posted April 15, 2017 Report Share Posted April 15, 2017 Opinions on this style of "Step drill", please? Obviously not for deep holes, but fine for brackets and the like? John Quote Link to post Share on other sites
Lebro Posted April 15, 2017 Report Share Posted April 15, 2017 I have one similar to that, only it has a round shank, not hex. I find it excellent for cutting most materials including mild steel. Bob. Quote Link to post Share on other sites
Waldi Posted April 15, 2017 Report Share Posted April 15, 2017 I have a set of these but with a round shank, perferct for making nice round holes, in mild steel up to 2 or 3 mm. Think my set of 3 was only 10 -20 euro in Holland Quote Link to post Share on other sites
AlanT Posted April 15, 2017 Report Share Posted April 15, 2017 I made a lot of "penny washers" about an 1 inch diameter, 1/8 thick, with a 3/8 hole, for TR bumper irons and similar mountings. Very tough marine grade of Stainless. I drilled in a lathe. The 1/8 pilot holes at 300 rpm took ages. Cleared out to 3/8 at 48 rpm. Pulled a big spiral swarf and this was quite quick. Ran drip coolant using ordinary soluble oil. Ordinary drill that belonged to my Dad. Keep cutting and keep cool. The idea is the heat leaves with the swarf. Quote Link to post Share on other sites
Richard71 Posted April 15, 2017 Report Share Posted April 15, 2017 Opinions on this style of "Step drill", please? Obviously not for deep holes, but fine for brackets and the like? John I use them all the time, great tool! Quote Link to post Share on other sites
TR5tar Posted April 15, 2017 Report Share Posted April 15, 2017 Opinions on this style of "Step drill", please? Obviously not for deep holes, but fine for brackets and the like? John I've also got a set of these, and they work fine for what I use them for, which is drilling silver and bronze (so soft metal). Darren Quote Link to post Share on other sites
stillp Posted April 15, 2017 Report Share Posted April 15, 2017 The markings on those step drills are important. Without them it was all too easy to drill a bigger hole than you'd planned! I used to put a stripe of Tippex on the next step up. Pete Quote Link to post Share on other sites
hman Posted April 15, 2017 Report Share Posted April 15, 2017 Been using them for years to fit camera systems on Buses. They are the only safe way to drill through fixed panelling when your not sure whats behind them. Anybody who's tried drilling through thin sheet with a twist drill knows what I mean. H. Quote Link to post Share on other sites
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