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Friction Coefficient Tests of 416 Stainless Steel on Cartridge Brass
by Dick Hatfield
1/30/04

Test Objective and Geometry

The objective of this test series was to measure the Friction Coefficient or Coefficient of Friction between 416 Stainless Steel and Cartridge Brass for various surface finishes on the Stainless Steel. The test was done by sliding a cartridge brass case head across a flat piece of 416 Stainless Steel. The flat piece of Stainless Steel was leveled and the cartridge brass was standing in the vertical position on the stainless steel. The friction coefficient to be measured was that between the brass case head and the stainless steel. Weights totaling 1200 grams were added to the cartridge brass to hold it in the vertical position and apply the normal force. A string was tied around the case-head extractor groove, extending to a small pulley, and down to a cup that was slowly filled with lead shot. When the case started to move, the experiment was stopped and the cup with its lead shot weighed. The weight of the lead shot and cup, pulling on the string, was the tangential force.

416 Stainless Steel Bar Stock


A one-inch diameter bar of 416 Stainless Steel two inches long was machined on both sides until a flat area about 7/8 inch wide resulted.  A fly cutter was used on both sides, but on one side special care was taken to machine the surface as smooth as possible limited by a hand feed on the mill.  Although a finish of about 60 RMS (Root Mean Square) resulted, there were a few spots where the feed was not perfectly uniform and a bit rougher surface resulted.  The bar was drilled and counter sunk for hold-down screws. Then it was mounted on a board and wedges were used to level the surface of the bar.

416 Stainless Steel Surface Finish

The 416 Stainless Steel surface finish was then prepared as follows:

A.      Surface as fly cut.

B.      Surface was sanded with 220 grit wet/dry for about 10 strokes to take any high spots off the milled surface.  The original fly cuts were still apparent.  (Note:  For all “sanding” that follows, a piece of plate glass was used as a flat surface and wet/dry sandpaper of the appropriate grit was placed on the glass.  The bar was wet with water and rubbed back and forth on the paper until all evidence of the previous cuts disappeared.   All sanding was perpendicular [+/- 45 o] to the axis of test.  The bar was then washed with soap and hot water and rinsed thoroughly and dried.)

C.      Surfaces was sanded with 400 grit.

D.      Surface was sanded with 600 grit.

E.      Surface was sanded with Crocus cloth.

F.      Surface was polished on a buffing wheel first with black polishing compound and then with white.

G.     The opposite surface (fly cut, but not as carefully as the main surface) was treated as in B above. 

I.      The surface was sanded with 600 grit and then polished with Flitz polish.

J.      The surface was sanded with 600 grit and then polished with Flitz polish followed by JB polish.

Table 1: Summary of Friction Coefficient Test Results

Test Series 416 Stainless Steel Surface Finish Range of Friction Coefficients Most Representative Single Value
B&G Sanded 200 Grit 0.318-0.469 0.37
A Fly Cut Tool 0.260-0.375 0.34
C Sanded 400 Grit 0.309-0.344 0.33
G Sanded 200 Grit (Lubricated 3-in-1 Oil) 0.360 0.31
D Sanded 600 Grit 0.284-0.292 0.29
E Sanded Crocus Cloth 0.260-0.279 0.27
I Sanded 600 Grit then Flitz Polish 0.165-0.230 0.19
J Sanded 600 Grit then Flitz then JB Polish 0.131 0.13
F High Polish with buffing wheel & rouge 0.108-0.140 0.12

Estimated coefficients of friction used in the Rifle Chamber Friction Analysis

Estimated/Measured Coefficient of Friction (u) Comments Maximum Plastic Strain
(%)
Cartridge Case Wall Radial Thinning (inch) Maximum Bolt Face Load (lbs)
0.55 Very rough chamber, rough reamer finish with tool marks 14.39 0.006054 4435
0.41/0.37 Rough chamber finish, 320 grit finish (200 grit) 13.42 0.006328 4386
0.35/0.29 Smooth chamber, 600 grit 12.87 0.006046 4639
0.25/0.27 Smooth chamber, crocus cloth or smoother 11.10 0.005415 4477
0.19/0.19 Polished chamber Flitz 9.69 0.004858 4565
0.11* Probably not possible: Polished chamber, polished brass with grease 7.92 0.004481 4787
0.01 Friction this low is probably not physically possible and would be dangerous 14.17 0.009328 (case head expansion that could cause the case head to rupturing) 7656
0.00 If Case Head Separation (Hand calculation
f = pressure * area)
    8785

Cartridge Brass (7mm Rem Magnum case) Surface Finish


Four 7 mm Winchester Mag cases were used as the brass source.  A special fixture was made that held  a case in a vertical position on center within an aluminum tube.  The tube could be chucked and the face of the case head machined smooth as noted below.  This tube was also used as the center of the assembly that placed weight on the case to perform the friction test. The cases head's surface finishes were prepared as follows: 

#1. Face machined off smooth and then tumble polished in a standard vibrator case polisher.

#2. Face machined off smooth and then polished with 0000 steel wool while turning the lathe but was not tumble polished.

#3.  O. D. of head turned to 0.520”.  An recess cut was made to an I.D. of 0.450” giving a 0.035” wide ring on the face of the case.  This ring area was O.0529 in2 which gave a loading on the case of 50 psi with the 1200 gram mass of the assembly.   The face was polished with 0000 steel wool while in the lathe but was not tumble polished.

#4.  Duplicate of number 1. 

Test Assembly


A test case was mounted vertically in the tube/weight assembly.  The total weight of the assembly in each trial was 1200 grams.  A light fish line was looped around the extractor groove of the case and attached to a cup that would hold weights and establish the “pull” on the case to determine the force required to break down static friction. Both the case contact face and the steel surface were cleaned with lacquer thinner before each test run.  Lead shot was slowly added to the “pull” cup until the static friction between the case and the steel was overcome.  This test was repeated several times for each surface finish of the 416 Stainless Steel.


Test Procedure


The string is around the case head extraction groove and extends over the pulley and is tied to the shot cup. The platform under the cup is to limit the sliding so that the case head will not slide off the 416 Stainless Steel surface when it starts to move. The friction coefficient is the force it takes to make the brass slide (weight in the cup) divided by the normal force of the brass against the 416 Stainless steel. The weight on the brass (weight of the assembly) was 1200 grams. In the beginning, a large number of trials (5+) were performed for each combination.  After a few trial sets, it became obvious that there was a fairly wide range in the break-away (B-A) force for a given test as there seemed to be some spots on the steel that were rougher than others.  It also became obvious, especially as the steel was sanded smoother, that the break-away force was fairly constant until the test using a polished steel.  As a result many of the tests included only 2-5 trials.

Test Results

A -- Fly cut surface finish on 416 Stainless Steel for three cartridge cases

 416 SS Fly Cut

Case #1 Friction Coefficient

Case #2 Friction Coefficient

Case #3 Friction Coefficient

1

0.266

0.358

0.333

2

 0.265

 0.360

0.355

3

 0.259

 0.353

0.369

4

 0.266

 0.391

0.399

5

 0.277

 0.392

0.353

6

 0.235

 0.405

0.356

7

 0.245

 0.410

0.331

8

 0.293

 

0.363

9

 0.287

 

0.352

Ave

0.267

0.381

0.357

Comment:  Several trials with Case #1 were made to develop technique before the “official” runs shown above and there may have been some material left on the steel that was not removed with lacquer thinner that was removed with the subsequent “sanding” and water wash.  Hence the lower values in this set as compared to later ones.    Note that most of the Case #1 trial values were lower than other cases for a given trial.  Case #2 was fresh brass and not tumbled.  It was polished with 0000 steel wool in the lathe.

B -- 416 Stainless Steel sanded with 220 grit for about 10 strokes.  The original fly cut marks still visible.

416 SS Sanded 220 grit

Case #1 Friction Coefficient

Case #3 Friction Coefficient

1

0.288

0.460

2

 0.323

 0.450

3

 0.363

 0.460

4

 

 0.483

 5

 

 0.490

Ave

0.325

0.469

Comment: As noted above, this test was an attempt to knock the high points off the fly cut surface.  It may have cleaned up the steel a bit hence the higher Case #1 values.  Case #3 had a fairly narrow ring (0.035”) for the contact surface and may have caught on the sharp grooves left by the 220 grit sanding.

C -- 416 Stainless Steel sanded with 400 grit until no evidence of previous marks.

416 SS Sanded 400 grit

Case #1 Friction Coefficient

Case #2 Friction Coefficient

Case #2 Friction Coefficient

1

0.325

0.342 

0.346

2

 0.304

 0.348

0.338

3

 0.317

 0.335

0.340

4

 0.290

 0.369

 

5

 0.307

 0.328

 

Ave

0.309

0.344

0.341

Comment: It was noted that a bit of brass would often rub off on to the steel.  This could be wiped off with a piece of paper towel and some lacquer thinner.  To see if it had much of an effect, in the Case #1 set above, the steel was cleaned between trials 2 and 3.  Doesn’t seem to have an effect.

D -- 416 Stainless Steel sanded with 600 grit until no evidence of previous marks.

416 SS Sanded 600 grit

Case #1 Friction Coefficient

Case #3 Friction Coefficient

1

0.275

0.294

2

 0.285

 0.308

3

 0.292

 0.326

4

 

 0.282

 5

 

 0.279

 6

 

 0.263

Ave

0.284

0.292

Comment:  In this family case #3 trials 1-3 were performed before the case #1 set.  Then case #3 trials 4-6 were run in preparation for set D1 below to establish a baseline.

D1 -- Lubrication test:  Using Case #3, the surface of the steel was lightly swabbed with 3-in-1 oil.  Break-away force was 376 grams.  Once the static friction was overcome, the assembly slid quickly the full length of the steel.  Without the oil, the dynamic friction was enough to make the assembly slide much slower. Since this was essentially the same value as without the oil, no further tests were made, but see test set G.

E -- 416 Stainless Steel sanded with Crocus Cloth until no evidence of previous marks.

416 SS Sanded Crocus

Case #1 Friction Coefficient

Case #4 Friction Coefficient

1

 0.274

0.260

2

 0.294

 0.260

3

 0.270

 

Ave

0.279

0.260

F -- 416 Stainless Steel polished with buffing wheel and first black compound and then white compound.

416 SS High Polish

Case #1 Friction Coefficient

Case #4 Friction Coefficient

1

 0.113

 0.143

2

 0.103

 0.137

3

 0.108

 0.140

Ave

0.108

0.140

Test assembly did not break-away with a sudden motion as in all of other tests.  The assembly would suddenly start to creep rather than slide fairly quickly indicating that the static and dynamic friction were nearly the same value. It is probably not possible to achieve this level of polish in a rifle chamber with a bore mop and a polishing compound.

G -- Special Test:  The steel specimen was turned over and sanded with 220 grit until 50% of fly cut marks were removed.  Then case #1 was used and three trials made.  Then the steel was swabbed lightly with 3-in-1 oil and the test repeated.

416 SS Sanded 220 grit

Case #1 Friction Coefficient

Case #1 Friction Coefficient (Lubricated)

1

 0.297

0.308

2

 0.323

 0.304

3

 0.334

 

Ave

0.318

0.306

I - Flitz Two: Al proposed that the test with Flitz applied as in H above was not typical of a chamber that had been reamed then polished with Flitz. He proposed that the test bar be sanded with 600 grit and then a Flitz saturated rag be used to polish the sanded surface. The test bar was sanded as in D. I.e. wet or dry 600 grit sandpaper was laid on a piece of glass and wetted and the bar rubbed across (perpendicular to the test drag direction) some 200 times. The surface took on a nice smooth appearance, but a 10X glass showed the definite grooves of the sandpaper. Then a piece of soft cloth (old Tee shirt) was stretched on the glass and a blob of Flitz placed in the middle. The bar was then rubbed across the Flitz some 500 times. Every 100 strokes the Flitz would be redistributed. The result was almost a mirror finish. Under a 10X glass, the edges of the grooves of the 600 grit were rounded. The bar was washed with strong detergent twice and well rinsed in water. All four of the cases were tumble polished for a bit over an hour. This did not give a polished brass, but did clean up the test surface and assured that there were no rough edges on that surface.

416 SS Sanded 600 grit then Flitz

Case #1 Friction Coefficient

Case #2 Friction Coefficient 

Case #3 Friction Coefficient 

1

 0.209

0.166

0.160

2

 0.216

 0.170

0.165
3 0.231 0.178 0.169
4  0.243 0.190  

5

0.252 

 

Ave

0.230

0.176

0.165

The action in these tests were very similar to F, the polished steel. In many cases the test assembly would begin to creep and move either a tenth of an inch or two or until the weight assembly reached the support surface. There was not a sudden break-away with the assembly sliding until stopped. It is interesting to note the increasing test load for each test. Examination of the test bar showed that a bit of some material (undoubtedly brass) would "burnish" onto the test surface as a particular test set proceeded. Attempts wipe this off with the lacquer thinner and a clean rag did not seem to make the burnish disappear. However, after the first five trials with Case 1, the case and the bar were vigorously rubbed with the thinner and a rag and two additional trials with B-A forces of 276 and 271 resulted. Although everything was kept clean as practical (in my garage) some dust particles did get on the surfaces and one or two longitudinal scratches resulted. 

J - JB Test: After the completion of Test I, the test bar was washed and polished with JB bore cleaner in a manner very similar to the Flitz test. I.e. a JB saturated cloth over a glass plate. The polish consisted of about 500 strokes. Visually the bar seemed a bit shiner than when polished with Flitz. The 600 grit grooves were still plainly visible. Again the bar was washed and rinsed.

416 SS Sanded 600 grit Polished with Filtz then with JB

Case #4 Friction Coefficient

1

 0.135

2

 0.128

3

 0.131

Ave

0.131

Case #4 was chosen as it had not been used in the I set and was a duplicate of Case #1. The same creep break-away occurred. 

Comments

This was an interesting series of experiments with some of the results a bit surprising.  It appears that the coefficient of friction between the brass and steel is essentially constant at about 0.33 (average of  A:C1, A:C2, A:C3; B:C1;C:C1, C:C2, C:C3 ) for steel surfaces until a fairly fine surface is achieved at, for example, the 600 grit sanding.  The next two surface finishes (600 & Crocus cloth) show about a 15% drop in coefficient.  The break of two weeks between the first tests and test H seemed to have an effect on the brass that made the coefficient climb even though the Flitz finish appears to be finer than 600 grit and similar to a Crocus cloth finish.  When the steel is polished another drop is seen of about 55% with a very different break-away effect.  Finally it was a significant surprise that the oiled surfaces did not seem to have any effect.

All of this is to help in estimating bolt face loading in a firearm.  It is obvious that there is a lot going on at the microscopic level with “mountains” in the brass catching on “valleys” in the steel and vice versa.  One wonders if the brass may be forced into the steel “valleys” a good deal more vigorously by the cartridge case internal pressure and the real friction under firing dynamics may well be quite a bit higher.  That is a much more difficult experiment to run.

 E-mail Dick Hatfield
 

One well conducted experiment is better than a thousand opinions.

Last Updated: 02/09/2013
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