How to finish a countersunk hole

[Wm.Traynor]

I have been cutting countersunk holes in flat, bar-steel and the bit has left a rough finish. So far I have found pointy stones for Dremels on the web. It is really difficult to find very coarse lapping paste, on its own in small quantities. All I have is #220 and it will take a month of Sundays, to lap the screw into the hole,I reckon.
Anyone got any suggestions please? I am making washers for the take-down bolts of a rifle (long story).

[Blr243]

I’ve been thinking long and hard but as yet nothing is jumping at me. How thick is the flat bar. What diameter is the first hole. And what diameter is the large part of the countersink section at the top surface of the flat bar If you are useing conventional steel cutting bits maybe finish with a blunt bit rather than a nice sharp one It will overheat and prob sacrifice the bit but may help tidy up the surface

[Noisydad]

Have you tried valve grinding paste?

[Wm.Traynor]

Have you tried valve grinding paste?

Is it coarser than the #220 I have tried?

[pomemax]

Wm. Traynor .
Rip a very small piece of cloth and use it under the countersink bit
do you mean you throwing Up a bur all round or just a rough surface if its all round your cutter is blunt

[Wm.Traynor]

I’ve been thinking long and hard but as yet nothing is jumping at me. How thick is the flat bar. What diameter is the first hole. And what diameter is the large part of the countersink section at the top surface of the flat bar If you are useing conventional steel cutting bits maybe finish with a blunt bit rather than a nice sharp one It will overheat and prob sacrifice the bit but may help tidy up the surface

The flat bar is 3mm thick. The "first hole" is 1/4". The diameter of the large part of the countersink is 9mm.
The countersink bit is fairly new and is what is used to chamfer case mouths. I did wonder what sort of tool should have been used in the first place, after I saw the finish. This thing is a Kunnings product and is not "high-end"
Do you think I should cut another 6 or so countersinks and then try it on the washers to see if that smooths the finish?

[Blr243]

Maybe some pics of your tools and product might help. I’m not sure I’m getting a clear picture

[Am88]

You need a better bit mate, I'm a boily by trade but supervise now and with a good quality countersink bit the boys countersink 20mm steel plate to accept 5/8" countersunk bolts, no burrs or roughness of the countersink.

Cheers

[straightshooter]

Inexpensive countersinks are usually not very good.
Easiest way is to grind a broken drill to the same angle as your countersunk screw.
By eye will be accurate enough.
Use a fine grinding wheel and provide a substantial clearance angle.
If using a drill press then there is no need for both cutting faces to be equal, near enough will be good enough.
Clamp the work to the drill table to prevent chatter.

[Die Judicii]

Yeah, a pic of what you've done so far would help.

My question is,,, if your bolt or whatever that will be going in to the counter sink will cover the rough area that you are concerned with,,,
why fret your frilles with it anyway ?
If its covered, it will be out of sight any way. ( or is it really bad ) ??

Another issue that can be causing it is incorrect drill speed, so maybe check that.

[Wm.Traynor]

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The "washer". Look carefully and you will see dark patches around the countersink. They are the rough bits.

[Wm.Traynor]

Die Judicii
Not worried about appearance mate. Concerned that the roughness might give a false indication when the bedding is tested for "rock" at the fore end tip and at the tang. This washer is for the tang, BTW.
I had the drill on the slow setting.

[Wm.Traynor]

straightshooter,
Yep, it's a cheapie. As for grinding a drill, thank you for your faith in me but I am Notoriously Bad

[pomemax]

They are called pressure ridges in you cutting face I take it you useing a hand dill on real slow and pushing fairly hard . rip some rag over half the hole and do the same again .
but dot push as hard hold the washer in a vice if you have one ,horizontal if you dont have one pin it through holes with pannel pins to a board

[Wm.Traynor]

I'll give that a go pomemax

[Wm.Traynor]

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I put about 1 sq cm of very coarse abrasive paper between the C S bit and the washer, which was secured in a vice and I can't see those pressure ridges that pomemax spoke of.
Thank you very much mate

[Die Judicii]

I reckon Pomemax hit the nail on the head.
?? using a hand (pistol drill) on slow speed.

In a case like this the hand drill on low ,,,,,, is still too fast.
A pedestal drill would be far better, and can achieve much lower speeds. Your pressure ridges are a classic sign of "speed too high"

I don't know how you went about it,, but if I were making these I would do the counter sink drilling (any drilling) first and foremost (in the plate) then,
cut out your washer shape/outline last.

[straightshooter]

The real culprit in cheaper tools is inferior steel combined with the lack of a positive rake angle applied to the cutting edge.

[Wm.Traynor]

I don't know how you went about it,, but if I were making these I would do the counter sink drilling (any drilling) first and foremost (in the plate) then,
cut out your washer shape/outline last.

That hits the nail on the head too, mate