Drill Holders

[John]

That sucks about your elbow.

Rivet ladder? Why don't you come back out for some vacation time from all that wall slavery? Your elbow will thank you.

I ordered an A taper bit from McMaster Carr and they are the same as an SDS but with an A taper base. I am sure it will not work as well as a solid bit like I have been using but I will try it anyhow. I am going to have to make a solid one for my special needs.

[daniel banquo merrick]

I have a plan to do some drill tests in the garage to compare how fast various drills drill. My enthusiasm is low since I really don't want to spend the time pounding. I keep picturing a homemade trip hammer. I'll get to it someday.

I'm probably repeating myself (I'm old so it's allowed) but do try sharpening your carbide bits. My impression is that drilling time is cut almost in half compared to the factory points. A cheap diamond file will do the trick. I got a set of little flat diamond files for less than $10.

Something like this:
http://www.ebay.com/itm/6pc-DIAMOND-FLAT-FILE-Set-ceramics-tile-glass-/390088222449#vi-content

[John]

I have a plan to do some drill tests in the garage to compare how fast various drills drill. My enthusiasm is low since I really don't want to spend the time pounding. I keep picturing a homemade trip hammer.

I like the automated hammer idea. I picture a Rube Goldberg machine.

I agree with the sharpening being a key factor.

[daniel banquo merrick]

So the wife is working late so I went down to the garage and got out two brand new SDS bits and two brand new drill holders. I sharpened both bits to a similar shape. The 1/4" bit is a DeWalt made in Germany. The 3/8" bit is a Makita made in Japan. I whacked the 3/8" drill 40 times then hit the 1/4" drill 40 times, repeat, repeat, etc. After every 80 whacks I cleaned the holes. I used a heavier hammer than my usual Yosemite hammer.

After I got some depth, I started measuring the depth to the deepest part of the hole.

Turns out the 3/8" bit went 118% as fast as the 1/4" bit. I think a 1/4" hole is only faster because you don't have to drill so deep.

3/8" bit went 0.00321 inches per whack
1/4" bit went 0.00271 inches per whack

A 2.5" x 3/8" hole takes 623 whacks
A 1.25" x 1/4" hole takes 461 whacks

[John]

Huh. I wouldn't have had a guess for a winner on that one.

I would bet a 1/4" 5.10-type drill would beat the 1/4" SDS drill. I think the former would be the best contender for its class and the 3/8 SDS for its own.

Why am I envious of your granite block? I think I need one!

Another interesting experiment Dan. Thanks for sharing it.

[daniel banquo merrick]

I drilled two holes using the same, resharpened, 3/8" SDS bit and a sharpened 3/8" A-Taper bit. The procedure was the same as for the earlier test. The SDS bit drilled at basically the same rate (0.00324 inches/blow) while the A-Taper bit drilled faster (0.00386 inches/blow).

So we have:
1/4" SDS 0.00271 "/blow (100%)
3/8" SDS 0.00322 "/blow (119%)
3/8" A-Taper 0.00386 "/blow (142%)

The A-Taper bit is shorter, has shallower flutes, is held more rigidly in the holder and the holder is lighter. Any or all of these may increase the drilling rate.

[John]

That is really interesting. They could each be made of different materials too.

[daniel banquo merrick]

I've been cross posting this to StupidTalko, hope that's OK.

There has been some relevant research into the efficiency of drill bits. This paper is a very good source:
https://etd.lib.metu.edu.tr/upload/12608282/index.pdf
Chapter 3 beginning on page 62 is the good part.

I've been going over the thesis paper and have a few observations.

The blow to the end of the drill produces a stress wave or impulse that travels down the length of the drill bit. The efficiency of transmitting the impulse to the tip of the drill is key to understanding what is happening in my experiment. For hard rock like granite it is most efficient if this impulse is short. For softer rock, a longer impulse is more efficient. Short or long impulse can be envisioned as either time or length. Hitting the end of the drill causes a compression impulse traveling at a fixed speed. This compression wave has a length in both time and distance. The speed is a material property and is constant for steel. The speed or velocity is c=(E/p)^.5 where E is the modulus of elasticity and p is the density. For steel this is about 5000 meters/second or 16,000 ft/sec.

Small diameter drills have a longer impulse so are more efficient in soft rock. Fat drills have a shorter impulse so are more efficient in hard rock.

Any changes in the cross section of the drill cause a portion of the impulse to reflect back. The 3/8" SDS MAX drills have nearly constant cross section so very little of the impulse is reflected. The 1/4" bits neck down quite a bit so a lot of the impulse is reflected back towards the hammer where it reflects again and them reflects yet again at the neck down delivering a series of decreasing impacts to the tip. In soft rock these successive impacts can advance the drill but they do not in hard rock.

In hard rock, the length of the drill has very little effect for a straight shank drill. If a drill has a change in diameter, shorter drills will be more efficient.

The shape of the hammer influences the duration and shape of the pulse.

The drill holder certainly influences the pulse.

For most efficient drilling in hard granite, we would want a short, sharp impulse. Any changes in the drill section (Diameter, flutes, slots) will tend to dull and/or reflect the impulse.

It would be good to do tests comparing different drill holders.

Using my granite block (~100 lbs) probably effectively makes the rock somewhat softer. The difference might actually be more than the 18% I got when drilling in a large mass of rock.

We should test some straight shank 1/4" drills. I think the 1/4" drills that John makes are straight shanked.

[John]

Wow, I was surprised to see such a long paper on the subject.

This seems to support how the flutes, since they are not really necessary for most hand drilling, would be better if shallower and therefore more material could transfer the energy to the rock.

I can send you one of my drill holders with bit if you want to test a straight-shank bit. Give me your address to ship it to.

[daniel banquo merrick]

Google Daniel Merrick to get my address or:
www.danielmerrick.com
I'm frighteningly public.

I also want to grind down the shoulder on a 1/4" SDS bit to see if I can reduce the reflection of the hammer pulse and thereby improve efficiency.

I am also trying to figure out the interaction between the holder and the drill bit. The reduction from the holder to the bit is also subject to impulse reflection.

[John]

Got the address, I will get you a drill soon.

[daniel banquo merrick]

I tried grinding a 1/4" SDS bit down so there is a tapered transition from the SDS part to the drill part. I simply mounted the bit backwards in an electric drill and spun it while I worked it on the bench grinder. It didn't take long and came out pretty good. Its pretty hard steel and the lathe won't cut it.


   
1/4" SDS bits. The left one is ground down to create a smooth transition in diameter.

I took somebody's advice and got a grinding wheel. I ended up with green Silicon Carbide. It works well and I am sure I got the bits sharper than before.

Using the newly extra sharp drills and a Black Diamond hammer instead of the Omega Pacific hammer I found that the ground down 1.4" SDS bit drilled 21% faster than the bit with the shoulder. I really find this hard to believe and think I need to find a volunteer to do a blind test.

Looking back to the first test, the plot I made used a trend line with the intercept set to zero which isn't right since the drilling rate isn't linear when starting a hole. So I recalculated everything and here are the three tests so far:

Test 1, Hand sharpened bits, OP hammer
1/4" SDS 0.00279 "/blow
3/8" SDS 0.00326 "/blow (117%)
Theory: The shoulder in the 1/4" bit reduces impulse efficiency and smaller drills are not as efficient at the stone face.

Test 2, Hand sharpened bits, OP hammer
3/8" SDS 0.00324 "/blow
3/8" A-Taper 0.00386 "/blow (119%)
Theory: The A-Taper bit is shorter and held more rigidly.

Test 3, Wheel Sharpened bits, BD hammer
1/4" SDS 0.00323 "/blow
1/4" SDS ground down shoulder 0.00390 "/blow (121%)
Theory: The in the standard 1/4" SDS bit the shoulder reflects the hammer impulse reducing efficiency.

Test 3 doesn't seem consistent with Test 1 so I want to repeat #3 and have somebody else do the drilling and not let them know which bit is which. However, I am pretty confident that it is good data. I hit one 20 then the other 20 and kept cycling so my hits should have been pretty consistent. I think the difference between 1 & 3 is sharper bits and the way I hold the two hammers. I find I naturally choke up more on the OP hammer. Also, the paper we have been looking at has a section where the author discusses the shape of the hammer and how it affects the impulse and efficiency.

[John]

That is quite a find. I have to admit, I don't really understand why the shoulder would help or hinder performance but it is an interesting feature to focus on.

One thing to remember about your new grinder to never let hardened metal get too hot for too long or you can unintentionally ruin the hardening. Grind/dip/grind/dip works. Use machining coolant in the dipping water so you don't rust everything around you too.

I am going to buzz out some holders and bits soon and get one off to you to be tested in the lab. Looking forward to seeing the results. I should have given you one when we saw you last weekend. Duh.

I do think the bits look cooler with the smooth lines which is very important.

[daniel banquo merrick]

Cross post from StupidTalko

Got done with my work early today so I repeated the 1/4" SDS comparison above. I mounted the two drills in identical holders and positioned the foam grips so that I couldn't tell which was which. I mixed them up (I can hide my own Easter eggs) until I really didn't know which was which and then drilled two holes. 20 blows to one then 20 to the other and repeat until I got to 480 blows. When I was done I pulled the bits to see which was which. The bit ground to a taper was once again faster.

This time it was 27% faster.

The tapered drill went 271 blows/inch or .0037 inches/blow. This was with the BD hammer.

I'm still pretty amazed.

[John]

I think your drills look good with their new accessory belts on. They have a flattering slimming effect.

27% is significant. Wow.

I got a chance to try the A taper 1/4" rig and the bit bent! I don't know what to think about it yet.