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Most economical way to Countersink Stainless steel plate

Tough material and a lot of holes

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John McNamara14/06/2015 06:33:32
1311 forum posts
113 photos

Hi All

I have a job coming up that requires countersinking 6mm stainless steel plate for an M6 countersink high tensile machine screw.

The specifications for the screw detail a maximum head diameter of 12mm with a 45 degree countersink chamfer around the 6mm hole. because of the variation in heads the finished width may need to be about 13mm The material is Asian as supplied by the laser cutter. 304 grade... he is not sure of the hardness, well one thing I know is it is tough to work on! ....

I do know that the cross drilled type of countersink cannot cope with the material. a good quality Weldon tool lost its edge after a few holes. The trouble is I need to countersink around 800 holes. Clearly the right tool will need to be sourced. Some of the holes are in plate that is too big for my mill, so I will have to use an electric drill, maybe in a movable drill press jig that is sitting on the may come in handy shelf.

Does anyone have a favoured tool for this type of job. Carbide will last longer but may chip when being hand driven? or HSS easier to sharpen?


John Gregson 114/06/2015 08:11:37
9 forum posts


Countersink with three cutting edges slow speed 125 to 250 and slow feed if you have, plenty of cutting fluid . Try liquid soap and oil this as worked for me


MalcB14/06/2015 09:23:45
257 forum posts
31 photos

For HSS 3 flute, the Dormer as as good as it gets but pricey. You may want to try CNCpoorboys which are cheaper with good service. As per John, slower spindle speed with relatively slow feed but not too low it burnishes the cutting edge. It's unfortunate that there isn't really such and economical way with stainless when doing larger batch quantities by hand or by conventional machining.

john carruthers14/06/2015 09:31:36
604 forum posts
176 photos

I ground a TCT masonry drill to 90 deg and use it dead slow, so far so good. I have used it in a hand brace but obviously wouldn't recomend that for hundreds of holes.

Michael Cox 114/06/2015 10:00:40
529 forum posts
27 photos

Just a thought but you could try a 90 degree carbide router bit.


Michael Gilligan14/06/2015 12:14:01
15457 forum posts
668 photos


For 800 holes; these indexable countersinks may work out to be cost-effective.

'Dmin' is shown as 0.25"


Edited By Michael Gilligan on 14/06/2015 12:16:25

Ady114/06/2015 12:21:08
3639 forum posts
514 photos

Masonry type drill with a manually ground tip

Slow speed(back gear best), high cutting pressure

Fine on a machine but hand drilling (as you well know) should be avoided whenever possible

I drilled a 12" set of digital caliper jaws, (4mm hardened stainless 6mm holes) and it cruised through

Edited By Ady1 on 14/06/2015 12:24:38

Ajohnw14/06/2015 13:13:08
3631 forum posts
160 photos

I'd guess that carbide of some sort will be the cheapest option for that many holes. I hate the stuff. Last time I did anything with it not able to use optimum conditions one hole drilled fine - same drill didn't on the next. That then took 2 drills to complete the job as the 1st one had done the damage. So as I see it if this happens you will ideally need carbide anyway.

The drills came from a toolmaker who was only allowed to use them for one hole in an exotic hardening material as any problems were too expensive. Wish I still knew him. They were still sharp. Very sharp high quality drills.

I don't think I would be too concerned about using carbide at the right speed in a hand arrangement for chamfering but that's just a gut feeling.



John McNamara15/06/2015 08:17:09
1311 forum posts
113 photos

Thank you for you thoughts everyone, always appreciated.

I guess I will have to buy a couple and see how they go, consensus being a 3 flute type. Ady1 and John Carruthers suggested a masonry drill I will try that first, If the finish is ok that will be the most economical method and I have a few in stock. I will report back with the result.



Edited By John McNamara on 15/06/2015 08:18:55

Edited By John McNamara on 15/06/2015 08:20:24

Ian S C15/06/2015 12:43:27
7468 forum posts
230 photos

The best counter sunk hole is done with a bit that is clearance diameter for the head, the counter sink is taken down so that the top is flush to the surface, and there will be a fraction of a mm parallel hole at the top of the couner sink. The only problem with a hand held drill is getting a slow enough speed, and enough rigidity to prevent chatter, and keep the pressure on, if it rubs, thats as far as you go.

Ian S C

Edited By Ian S C on 15/06/2015 12:47:41

John McNamara15/06/2015 14:20:26
1311 forum posts
113 photos

Thanks Ian

Agree re the small parallel section, also countersink screws are not as well controlled in manufacture, they vary in thickness from box to box, even within a single box you can get some that sit proud while other will be nice and flush. fortunately the job does not require a perfect height. "Life was not meant to be easy"


Ian S C16/06/2015 11:51:38
7468 forum posts
230 photos

In some cases with CS screws it's worth machining a few thou of the top if one is up a bit and it's bit of a hassle to deapen the countersink.

Ian S C

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