Small hole drilling in stainless steel

[WindyParticipant @windy30762]

I am drilling holes 0.020”diameter x 0.5”deep in 304 stainless steel and would like thoughts on ways to do this, some of the holes that I will also be drilling need to be 0.010”diameter x 0.5” deep in the same material. At the moment the drills are held in a pin chuck in a loose tailstock chuck and I slide the pin chuck by hand in a pecking action with coolant at the work piece. At about0.25” deep it starts to get difficult not to break the drill. Carbide drills have been tried but don’t think my lathe is fast enough. Also the drills that I am using need to be sharpened often, ideas on sharpening very small drills welcome. The way that they are sharpened at present is by hand with a fine diamond hone and magnifying glass. I did wonder if I used a mini drill mounted on the top slide if it would be better for the very small drills.
Any suggestions, articles to look at etc. welcome. Windy

Edited By Windy on 10/02/2010 00:07:45

Edited By Windy on 10/02/2010 00:09:11

[WindyParticipant @windy30762]

[chris stephensParticipant @chrisstephens63393]

Hi Windy,

My first thoughts are “you poor  s*d”

Second thoughts, are you making jets? If so, can you not counter-bore most of the hole and only drill the small hole through the last bit. 

Drilling Stainless creates problems, you need constant pressure to prevent work hardening the surface, but small drills don’t like much pressure. Speed is critical, don’t think going faster think going slower, let the drill do its thing and cut, not high speed polishing. Going too slow though puts too much strain on the drill. As I said problems.

I have drilled 0.8mm holes on my Bantam with its top speed of only 800RPM , not the many thousands that the books recommend, taking things very carefully. SHARP drills and flute cleaning are vital, but I wonder how well you can hand sharpen such small ones.

Carbide drills are  unlikely to be the answer, the smallest I could find in the J&L catalogue is 1mm, most makers start at 2mm. They are incredibly brittle and quite unforgiving of any “heavy handedness”

 

On the subject of small drill sharpening, you could not do better than make  DAG Brown’s four facet drill sharpener, it works a treat and not too difficult to make. There is a bit of a write-up of a version  at http://www.gadgetbuilder.com/DrillSharp.html

 

 “Thinks”, could our editor be persuaded to reprint the original articles in one of his “Best of ME” specials? I think it qualifies, as it is supposed to be one of the “most made” of all  tools from the magazine. 

 

 I am sure someone will suggest a marvellous way of doing the job, but the only other alternative that springs to mind would be to  use an EDM, that’s a “spark eroder” to you and me. Acquiring one of these would be impractical unless you are going to corner the market in small hole drilling, which begs the question , just how many of these holes are you contemplating drilling?

christephens 

[WindyParticipant @windy30762]

Thanks Chris for the reply, Dag Browns article is very interesting.

You can get some very small carbide drills that the electronics industry use but they are as you say very brittle.

What I am making is a set of converging-diverging nozzles for a test rig to check if my steam generator will be adequate for my turbine, a problem the full size steam record car had.

I do step drill the nozzles then ream with a homemade taper reamer 0.1044ӯ to 0.015ӯ a bit like an injector.

If the small hole is not dead centre with the step-drilled holes the reamer suffers it is also in hexagon bar.

If I could trust the hexagon bar to be concentric it would be reversed in the chuck and the 0.02ӯ would not have to be as deep?

There are only a few more nozzles to make then I will be having a test firing, have been told that the steam from the nozzles can be very noisy and I will probably need ear defenders?

Windy

Edited By Windy on 10/02/2010 03:46:00

[JohnFParticipant @johnf59703]

Hi Chris,

Have alook at your MEW inbox messages

Regards john

[Stewart HartParticipant @stewarthart90345]

Hi Windy this link may have some useful information.

 

http://www.homemodelenginemachinist.com/index.php?topic=8194.0

 

Stew 

[WindyParticipant @windy30762]

Thanks Stewart, a very useful link as was mentioned there are other ways to drill very small holes.

As one member of that link said (I need to drill 0.004″ holes, and I think the recommended speed is around 95,000 rpm to keep the drill bit rigid enough without having to worry too much about the pressures that need to be applied for it to drill thru the piece part, drill bit speed is the secret to small hole drilling.)

 

To do it this way I would have to make a high speed-drilling spindle.

 

All the nozzles have now been done and have just the test stand to finish.

Hopefully next week there will be a test session to check pressures on combinations of nozzles, temperatures, speed of pumps and quantities of fluids used etc.

I have found that keeping a video record of testing has been very useful as sometimes with all the action of bench testing which is only for a limited time I can miss things that could cause problems when in competition.

Windy

 

 

Edited By Windy on 14/02/2010 10:05:27

[mgjParticipant @mgj]

I think the aero people spark erode.(they do these very small holes in the hot/power  turbine blades to allow cooling air to be bled through

 

Your problem is that very small errors in dia at such speeds are quite large % errors, on dia which are huge area type errors. Then mass flow goes to hell in operation and you have no idea where you are, and even if you do, its very difficult to replicate a nozzle.  If you are checking a steam generator Mdot is bit critical!

 

Noisy – hopefully. If your Delaval nozzle is right and a fraction over expanded, you should achieve the speed of sound in steam at that temp and pressure.

 

Hope it all works. 

 

 

 

 

[mgjParticipant @mgj]

oops – better than the speed of sound. If you have your sums right it will be sonic at the throat.

[Geoff TheasbyParticipant @geofftheasby]

Rolls Royce certainly spark erode the holes in turbine blades.   I had a chance to look round their Barnoldswick factory some years ago, and it was explained to us.

Of course, in this case, the holes are very deep, so it’s like gun drilling for the size.

 

Regards

Geoff

[WindyParticipant @windy30762]

Hi Meyrick, was told it would be supersonic by a friend who has just done nozzle tests on his full size flash steam generator.

We are both going down similar paths one miniature the other full size.

Ear defenders required, hope the neighbours are out its bad enough running the piston engine.

More later.

 

Windy

 

Edited By Windy on 14/02/2010 14:48:32

Edited By Windy on 14/02/2010 14:50:50

[mgjParticipant @mgj]

Its a little while since I dealt with con/di nozzles, or did the maffs for htem. but yes, you want it so that the outer cone is jujust allows overexpansion.

 

Ideally you want it perfect, but thats impossible because of changing conditions, so just overexpanded costs a lot less than just underexpanded. And for top performance that expansion cone angle is critical. 

 

Sadly in all the moves in the army i have lost my propulsion notes from RMCS Shrivenham, and con/di nozzles are the bread and butter of rocket motors. So I cannot help you in detail. The maths of course is the same- its just a hot gas or fluid going through an orifice and PV^gamma on one side = PV^gamma on the other, and you are changing HP at as low velocity as possible (feed it with a big pipe) into LP and HV on the other.

 

To get the best velocity you want every bit of pressure on one side, and absolutely the right cone angle on the other. Still – I don’t suppose you’ll see the overexpansion shock cones like you do do on a jet efflux on reheat. , but they will be there just the same.

 

I’m sure if you Googled Delaval nozzles the formula would appear. Its not a difficult calculation – you just need the formula which I don’t have any more. 

 

I assume you are going to superheat?

 

(and for those who have difficulty with the theory, or hte maths,  – I’f you don’t know and understand the figures, how do you know how to correct the errors? Without a lot of hassle and failure. )

 

 

[WindyParticipant @windy30762]

Sorry about shooting off at a tangent from small hole drilling.

Amazing that something that Gustaf de Laval in 1888 did for use on an impulse steam turbines is now used on the majority of rocket engines today.

One of my 1911 books shows a gas turbine design not too different to a modern one.

It seems that a lot of mechanical and theoretical ideas are very old but with modern materials become a reality.

I have a great deal of information on the delaval nozzles for impulse steam turbines and am using that as a basis for the design of them.

At the moment the flash steam generator has to be proved adequate.

 

Windy

Edited By Windy on 14/02/2010 20:17:49

Edited By Windy on 14/02/2010 20:21:49

[mgjParticipant @mgj]

Every rocket motor from a tiny little anti tank missile (which for its few milliseconds of life produces more thrust than a 747 at takeoff power)right up to the Saturn engines. And every jet engine too – dimensions and cone angles will be different of course, because M dot, density and mach nos are different, but if you know one, you know t’other because the theory is exactly the same.

[Author]

Posts