reamed hole vs. drill rod size

[BobsModels]

K

As to your question about speed, my experience is down to a #80 drill ie about .3 mm. For any hole in the range of #70 or less I use about 100rpm or less watching the hole with loupe. I take real light cuts and you can see the bit cutting. The reason for the slow speed is I cannot control the feed rate for running a drill at super high speed as charts call for, I just work harden the material and polish the flutes. I need to be cutting - at least for me otherwise I get a mess. The slow speed gets me cutting and produces a hole.

The setup must have zero run out at those sizes, any deflection will certainly snap off the bit.

The other issue is drill bit quality. I use PTD or Dormer in that size. If you look at suppliers less expensive under a microscope you will instantly see why to spend some extra money on drill bits. To start the holes that small I use 5/0 center drill but just nick it with the tip. I have used the ball method and spotting drill, both work but it is what you get accustom to.

I have never tried something as small as .2mm ie about .007" I think. I have seen a .001 hole that was drilled with a real special tool, I think it was a spade bit custom made.

Bob

 

[IanN]

Any advice on drilling 0.2 ~ 0.3 mm holes will be appreciated.
:
I am also looking for a reliable and simple way to gauge the jets, as if the drill has a problem it makes a slightly larger hole than intended, and at these sizes (less than 15 thou diameter) a thou makes a big difference to area and gas flow.
:
Can anyone tell me what speed to use for 0.2mm dia?

Hi Steamchick,

Addressing the points you raise in turn:

Drilling small holes - I posted links to a couple of videos showing how I do it:

1) Rotate the work, not the drill

2) Pick up the centre with a graver

3)Hold the drill in a pin-vice in your fingers

The above method has been used by clock and watchmakers for several hundred years

Gauging small holes - in your case, the gas flow rate for the hole is the important parameter here and this will be dependent on hole area and shape (“roundness”), depth of jet (friction) and any belling at the jet entry and exit (resulting in turbulence). If you do a lot of this stuff you can build a small test rig to assessed the hole area quite easily in both absolute and relative terms (for comparing a new jet against a known existing one) by fitting it in a tube and measuring the pressure drop across it when connected to an air supply (I have an adapted domestic pressure cooker surplus to wife’s requirements connected to a car foot pump and use a water filled manometer)

Last point - drill speed - there is no such thing as “the correct speed”, the most effective speed will depend on tooling and machine as much as material or anything else

The speeds you will find in data books are derived for optimum economy in industrial settings (high speed = high tool wear but high production rate, low speed = low tool wear but low production rate. High production rate = low costs, high tool wear = high cost, so there is an optimum trade off)

Any reasonable speed will cut - for very small holes “as fast as you can” is a good speed. My lathe has three speeds plus three back gear speeds, fastest I can go is about 600RPM so everything below a few mm gets cut at that speed.....

All the best,
Ian

 

[petertha]

Back to the subject of correcting drill rod, I believe there are 2 tasks. Correcting the slight oversize to some reduced diameter issues. And correcting what are often times an eliptical shape into something approaching circular, since I've measured many are +/- 0.0005 at ~ right angles on the same portion of shaft. I've spent too much time making lapping tools. They are OK for the last 0.0002" or so but if theses laps are introduced on raw stock, it doesn't go too well. You need coarser compound to do the cutting & generally that also consumes the lap itself.

So I gave this method a try fully thinking it will be yet another way of how NOT to do it. But I'm actually very impressed. In about 15 minutes I've made myself some nice sections of 'true' 5mm shaft stock for my rocker axles that are within a tenth of target dimension & desired finish. And its a very controlled process.

I got the idea by (carefully) mic-ing varoius wet/dry papers in the 600, 1000, 1200# range. I was impressed with how consistent they are across the sheet & across other sheets of the same vendor. For example the 600# I have measure 0.0075". Amazingly for some reason my 1000# thickness was quite close to 600#, within tenths, otherwise I would make a separate lap for each grit. So I worked this dimension in as an allowance, both annular (surrounding the stock) and between each lap face. Hopefully the sketch makes sense.

So I started by milling a block of aluminum, cut in half so it gave 2 symmetric pieces. Clamped them together face to face in the vise & drilled the oversize hole (0.0075 annular = 0.015" diametric). It would have been much better to ream the hole for a better finish but I didn't have the right size. So I chose the next closest numbered drill I had. There will be some buffering with the paper itself I guess. Then separate the 2 lap halves, position in vise & simultaneously mill off the face allowance from both. make a small transition radius on the hole corners The idea was to place 2 strips of abrasive paper clam-shelling the stock. If everything is dimension-ed reasonably close, by the time you fully squeezing the assembly together, it should be getting close to target dimension. You could spin the stock in the lathe, or as I did, just grip it in an electric drill. The benefit is the 'tool' never changes shape or wears out by lapping action. Fresh abrasive is a postage stamp area of wet/dry paper.

Well it worked out way better than I expected. You can actually feel a bit of vibration initially, which are the eccentric bumps of the shape. I used 600#, then 1000# mic-ing every so often. A longer section of shaft (3-4") means you are probably lapping more consistently across any one area. I figure 5 seconds of lap-sanding was about 0.0001" so it was actually quite easy to sneak up on the target dimension & finish simultaneously. I tried 2 strips of paper & also a single wrap just to get a sense of how much of the paper was in contact & cutting.

This is not a good way way of removing too much stock. Its kind of like lapping but without all the mess. Digital photo's tend to exaggerate machining marks so hopefully you can see the contrast of the finished rod & the aluminum tool. Drill holes are nasty looking surfaces under magnification once you split them open.

 

[petertha]

Forgot some of the final shots

 

[Steamchick]

Well done! This is very close to a tool I used in 1960s when I was being taught to re-grind car crankshafts. The lapping tool had a series of different sizes of interchangable jaws - for various journal sizes. Just like you have made. They were mounted in a mole-grip like contraption, which was clamped onto the crankshaft journal and a slight to moderate pressure applied. After a half dozen passes with crank rotating 60 - 100 rpm.and 600 grade emery tape, the journal was mic'ed and the reflections of light were observed to watch the dozens of flats from the crank journal grinder dissappear. I'll explain the crank journal grinder was an adjustable v-block rest with the grindstone opposite to one face. The whole stone etc. Was mounted on a double hinge arrangement so it could float around the crank as the shaft rotated at single rpm. But this design of crank grinder developed tiny flats - dozens- instead of a true circular crank journal. The final lapping was to take the tops off the corners of the flats to make the journal round. Alll this at 0.0001in. The lapping tool was hand held to resist torque and move the emery back and fro along the journal. You felt it go slack within a few rotations of the shaft as the emery wore away. A final lap with finest grade emery polished the journal. All thoroughly cleaned aftwerwards. I have done the same using mole grips and emery on my model crank journals. Well done for finding and old methodology and having the success you deserve. I suggest that if this shafting is to run in plain bearings, you initially rub a light smear of molybdenum grease on it during assembly. This will act as an anti-scuffing compound until the lubrication gets into the surface of the material (at a molecular level, the lubricant fills pores in the surface of ground metal).
Regards,

 

[Steamchick]

I should have said "as the emery wore away the peaks". I have also had to fight the spell checker that decided I wanted to write about "lapwings", etc!

 

[Steamchick]

Thanks all for your help. I'll try the graver to create the centre - now I recall my grandfather teaching me this technique to centre wheel hubs for my Scalextric cars when I bought custom tyres and made wheels on his watchmakers lathe. Nearly 60 years ago. Thanks for the memory!

 

[L98fiero]

Hi,
For centering work pieces for small drill I now never use Slocomb pattern cente drills. I use a spotting drill which give a much finer starting point, so you could try the smallest one you can find - they're not too expensive and the Chinese ones are adequate for brass and mild steel.

TerryD

I agree, never use a Slocome/combined drill and countersink to start holes unless that's all you have. The only way I use them is with just a spot no bigger than the size of the drill part. If you drill down to the 60° portion, the lips of your drill will contact there before the center producing an 'off round' hole and probably resulting in wander. If you use a spotting drill with an angle larger than the drill you will be using, the chisel point of the drill will contact first and center the drill generally producing a straight hole. Start the hole with the shortest new drill you have and work your way up to the longest you need. You can even go so far as to drill the first part of the hole undersize and redrill with an endmill that's the correct size to ensure location. That's the process I've used to produce 1/8" holes, 6¼" deep in aluminum and 3/16" holes 8" deep in aluminum nickel bronze and Kirksite, all on a turret mill. Regular twist drills are not particularly stable as there's little support from the lands of the drill but starting the drill correctly goes a long way to getting the results you want, even gundrills won't drill straight if they aren't started properly.

 

[Rustkolector]

Petertha, Steamchick
Watch this How Its Made video on manufacturing light aircraft engines. In the very beginning they show similar use of the tools you discuss for polishing their aircraft engine crankshafts. The outside cylindrical hone does much the same thing and we are now discussing the gray area where lapping and honing are somewhat the same process. One is probably better for sizing and the other for finish, but with careful use either will do the job equally.
Jeff

 

[peterl95124]

Slocumb appears to be a brand name, not a specific drill type,
but based on what I see with a google search, this appears to be
the type of drill for preparing the end of a rod for support by a
tail-stock center, or for tapering with an offset tail-stock center.

these types of drills are not appropriate for starting a hole,
at least for me they are too fragile, and the 60-degree angle is wrong.

use a 120-degree "spotting drill" as that's exactly what they are
designed for. they are both very rigid and very sturdy.

HTH, YMMV, etc...

 

[petertha]

Watch this How Its Made video on manufacturing light aircraft engines. In the very beginning they show similar use of the tools you discuss for polishing their aircraft engine crankshafts.

Interesting video clip, thanks! Just curious, anyone recognize the measurement gauge? I'm curious what kind of resolution they are achieving.

I messed around with different kinds of lapping tools like this squeeze clamp style with replaceable brass or aluminum split lap collars. They 'work' but as mentioned I would say best suited to final finishing (like 10ths). A big disadvantage is you can only slide them on if the shaft is open to the same diameter. If you are turning a part in a lathe & still have a larger diameter chunk associated with using a tail stock center, only a split lap can get access to the smaller diameter. In the end its still kind of a 'by feel' process.

 

[Murph]

A neat trick the antique camp stove guys do for gas jets is to drill a larger hole, and silver solder a section of hypodermic tubing into the hole, and cut it off flush with a cutting wheel