3/8" SDS drills faster than 1/4" SDS


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Slabby D

Trad climber
B'ham WA
Jul 26, 2013 - 03:14pm PT
With HSS bits I've noticed that 17/64" drill significantly faster than both 1/4" or 3/8". I assume this is a result of a thicker bit transmitting more energy to the rock but offset by the additional rock requiring removal as you move to bigger bits. It would be curious to test what the optimal bit diameter is for maximizing depth.

Topic Author's Reply - Jul 29, 2013 - 11:41am PT

Yes, it is always a good thing to question data. You don't know me, my qualifications (ask if you are interested) or how I might be biased. Heck, I might have been bribed by the 3/8" drill cartel.

The usual way that experimental results are verified is by independent, hopefully objective, investigators trying to reproduce the results. Salamanizer has posted about his experience and similar conclusion but if you doubt our data, you should do tests of your own. If you are objective in your method and analysis, I have no doubt you will arrive at a similar conclusion.

a machine that hammered with the exact same amount of force on every blow

I'd love to see a hammer machine (I considered this but the 3/8" drill cartel isn't paying enough to fund it) and read about the results. A power drill might work. I'd love it if you or somebody else did this test. I don't have a power drill or hammer machine and don't want one.

One hole doesn't seem nearly enough to make a real analysis though does it?

A question deserves a reply. One data point is not enough for an analysis but I actually have 6 data points. I will get around to drilling more holes as soon as the cartel coughs up some more cash.

Seattle, WA
Jul 29, 2013 - 12:07pm PT
In a way, each hole is ~4-600 data points since you are estimating depth per blow. The may be effects from rock type, hole angle, fatigue etc that would change the results but I wouldn't discount this 40% difference on statistical significance...

Have you tried an SDS bit with the sds section of the butt cut/ground off to shorten the bit? Doing this was mentioned in some old threads.

Trying it in a roto hammer would also be interesting...

Jul 29, 2013 - 02:02pm PT
You can read this mind-numbing discource on the matter (all 170 pages of it) https://etd.lib.metu.edu.tr/upload/12608282/index.pdf or jump to the conclusions, (the reference to cross section reduction are talking about using smaller diameter drills but still in the same 18mm shank drilling machine):-

"Thinner drill bits are more efficient in softer rock (especially because of length effects), while the thicker drill bits are more efficient in harder rock. In medium rock hardness (around K=2x106 N/m), for cross-section reduction case, the efficiency is not dependent on thickness. When the straight drill bit is observed in medium rock hardness, it can be noticed that the efficiency of it is higher than the thinner ones.
Decreasing length increases the efficiency for bits with a cross-section change, especially for the ones with a cross-section reduction. As a result, for the shortest drill bits (250 mm), the thinner drill bits are more efficient than Φ18 mm even in medium rock."

Topic Author's Reply - Jul 29, 2013 - 03:46pm PT

Thanks for the link to the paper. I find this stuff very interesting. In the paper, when they refer to hand drilling they seem to mean a hand held drill such as a roto-hammer. I've looked but have never found any studies concerning hand held drills.

On a similar topic, there seems to be no studies of hand held hammer efficiency. One would think that some ergonomics researcher would have worked on this but apparently not. There are some studies regarding curved hammer handles.

I'll go over the paper you linked and look at some of the references it provides.

I hadn't thought about the hardness of the rock being such an important factor. In my mind I am thinking the natural frequencies of the rock and the drill will be important. My gut feel is that although my test block is very hard granite, it behaves somewhat differently than drilling on a larger mass of rock.
Brian in SLC

Social climber
Salt Lake City, UT
Jul 29, 2013 - 03:52pm PT
Interesting stuff.


Topic Author's Reply - Jul 30, 2013 - 01:56pm PT
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.

Topic Author's Reply - Jul 31, 2013 - 12:46pm PT
I'm thinking that the efficiency of a 1/4" SDS drill might be improved by grinding the change in diameter to a taper. I have no idea if this will work or what the minimum angle is to prevent the hammer impulse from being reflected back.

Probably need Ed Hartouni's help.

Topic Author's Reply - Aug 2, 2013 - 07:56pm PT
I know most people are bored with this but it seems interesting to me.

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

I took Salamanizer's advice and got a grinding wheel. He said Silicon Oxide but I ended up with green Silicon Carbide. Whatever, 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.

Credit: Banquo

Trad climber
Nothing creative to say
Aug 2, 2013 - 08:33pm PT
A 2.5" x 3/8" hole takes 623 whacks

Going to name a route "623 whacks" this weekend. established with a rotohammer!

3/8" Drill Cartel

Trad climber
The land of Fruits & Nuts!
Aug 2, 2013 - 09:04pm PT
Now that is interesting. I would have never thought the taper of the drill shaft itself could have any effect on the energy transfer enough to have a significant effect on drilling speed. 21% faster is hardly a nominal difference.

Silicon Carbide is what I meant. The "green wheel".

Are those SDS bits in those drill holders? How does that system work? I had an idea about a drill holder where the bit was inset in the handle shortening the length of the exposed bit. This in theory would shorten the overall length of the drill system and give more control over the drilling itself.
Kinda like a bullpup design for drilling.

Your holders look like they have drill bits that are inset deep in the handle like I'm talking about. Cool!!!

I also thought to have an SDS compatible holder where the bit was held in with a magnet. In theory you could easily replace a bit on lead with one hand and no tools. I'm not an engineer though. But a similar system is used to hold the bit on an apex for power drills and screwdrivers every day.

Aug 2, 2013 - 09:36pm PT
I'm interested my man. I've only hand drilled to replace some bolts on Nightcrawler a few years back with Greg Barnes. I've put up a few FA's recently that are fairly bold and should most likely have a few bolts added to them so the sane can enjoy them as well. If you ever finish the prototype I'd buy one off you for sure. As far as this line goes:

On a similar topic, there seems to be no studies of hand held hammer efficiency. One would think that some ergonomics researcher would have worked on this but apparently not. There are some studies regarding curved hammer handles.

I was a carpenter for a long time. I've never see fully curved shaft on hammers like you do on ice axes. Some hammers like the Stileto have a very slight curve at the butt section the helps to set nails up high. If framers haven't started using them I'd say there is a pretty good reason.

Social climber
san joser
Aug 2, 2013 - 09:37pm PT
Banquo will sell you one of his fancy drills, you know. He makes several kinds including one with a deep recess that will swallow most of a 6" SDS bit and leave just enough sticking out to drill a 2" hole. I've tried that one and it works well. The only caveat is that particular drill is too deep for a 4" SDS 1/4" bit.

So the compromise is his standard SDS drill that doesn't swallow as much of the 6" bit so you can still use the 4" SDS.

I think he's off to SoYo this weekend, so he may not respond until next week.


Topic Author's Reply - Aug 2, 2013 - 11:25pm PT
My drill holders:


I'd be happy to make one for whoever will cough up some cash. Cragnshag and others are paying $80 for the SDS holders and $50 for the A-Tapers. I can make them pretty much any length.

I've found some really awesome foam handle grip material. A 12" piece of the stuff is priced:

1 - 4 $26.56
5 - 9 $14.61
10 - 24 $11.95
25 - 49 $9.30
50 - 99 $5.14
100 - 249 $4.36
250 - 499 $3.89
500+ $3.56

Geez, that's $2 an inch if I only buy one. I need to get up to the 50 price which means I'd have to sell about 150 to 200 drill holders. Won't happen.

Topic Author's Reply - Aug 9, 2013 - 07:48pm PT
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 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.

Credit: Banquo

Trad climber
Aug 9, 2013 - 08:07pm PT
This is super cool man. Keep it up.

Topic Author's Reply - Aug 9, 2013 - 09:12pm PT
I am certain the length, diameter, shape and mount type (A-Taper or SDS) all affect the drill rate. I also believe the ergonomics, shape and weight of the hammer will affect the drill rate.

One thing I haven't tested is different SDS holders. If somebody has a Petzl, Fixe and/or Pika holder they would loan me or trade for that would be cool.

I have some old Rawl and Star holders I could test but they hardly seem relevant. I had a Pika but sold it to somebody.

Trad climber
Aug 9, 2013 - 09:48pm PT
I'm pretty sure I have a Pika holder somewhere but, embarrassingly, cannot seem to find it. It's not in my pin rack box and I don't know where else it would be. All my hardware is just gathering rust.

Topic Author's Reply - Aug 12, 2013 - 02:56pm PT
I ran a very simple impact FEM model to show how the impulse bounces back from the shoulder of the bit. In the animated GIF, compression is blue and tension is red. An impulse is applied to the left end and the right end is fixed.


Edit: The file loads slowly so be patient.

Topic Author's Reply - Sep 4, 2013 - 08:12pm PT
I managed to get a copy of an old paper that is the only one that I can get my hands on that discusses how a percussion drill does its job. "Basic Studies of Percussion Drilling" was published in January 1959 in Mining Engineering on page 68-75. In the introduction the author states that although materials and equipment have improved, rock drill bits have undergone no major modification since they were first used in the 1860's for drilling blast holes in mines. Looking at his discussion, I would say that other than developing carbide tips, not much has changed since 1959 either.

The main area of interest to me is the investigation of how the drill tip penetrates the rock and breaks it up. They figured this out with high speed photography. As the tip impacts the rock, the rock below the tip crushes into a fine powder. The tip penetrates the fine powder and the powder expands outward in its space causing chips to flake off on each side. For a given volume of rock, it takes more energy to pulverize the fine powder than a corresponding volume of chips. This means we want to minimize the powder and maximize the chips. You can minimize the powder by having a sharp drill and by having a drill tip with a small included angle. If the tip is too sharp, it dulls easily and if the included angle is too small the bit will chip. The optimum angle is probably a function of the rock and the bit material. Carbide is very brittle so it probably needs a larger angle than HSS. On the other hand, carbide is very hard so it can be kept sharper. The paper states that the angle must be 90 degrees or less to result in any chips at all. Carbide bits are very close to 90 degrees so not very efficient.

Percussion drill bit penetration sequence.
Percussion drill bit penetration sequence.
Credit: Banquo

The tests were done on a flat surface rather than in a hole so they could photograph the process. In actual use, the bit is in a hole and the chipping might be limited by the space available at the base of the hole.

Size and depth of single blow drill craters. I believe these are 1.5 i...
Size and depth of single blow drill craters. I believe these are 1.5 inch drill bits which is probably what they use for blast drilling in mining.
Credit: Banquo

The width and depth (or volume) of the crater is a function of the included angle of the tip, the sharpness of the bit, the blow energy and the rock. The rock factors might be the crushing strength, the shear strength and the grain size.

My conclusion is that part of the reason a 3/8 drill is faster than a 1/4 inch drill is because the larger hole size allows more chipping and less pulverization. This could be checked by collecting the crushed rock from a 3/8 inch hole and from a 1/4 inch hole then analyze the samples for particle size. Very boring work but I did this particle size analysis in soils lab as a student.

Indexing is what the author uses to describe the amount of rotation between each blow. He states that indexing can be too small and reduce drilling efficiency but cannot be to large. Maximum indexing is one half the angle between the cutting edges of the bit which is 90 degrees for the flat tip bits we use. If you use a cross tip, it would be 45 degrees. I really don't know what the minimum rotation is but for the single cutter bits we use but it must be 90 degrees or less. I generally rotate at least 15 degrees per blow. 30 degrees might be better but my wrist will only twist so much.

It may be that a 1/2 inch drill is faster than a 3/8 inch drill. Need to check that.

Unfortunately, the paper I have is only a summary of research. I'd like to see that original paper which was published by Drilling Research, Inc in "Collected Reports" vols 1-6 1949-1954. There is a copy of this at Stanford but I don't believe it can be checked out.

The 1954 vol 6 pp 9-18 seem to be what I want. If anybody is at Stanford, perhaps they could get me a photocopy or scan. Also 1953 2nd qtr pp 14-18.
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