DIY BAM Friction test

I've been wanting to make some sort of a reliable and repeatable test set up for friction sensitivity of primaries and maybe some secondaries. I ended up deciding that the BAM friction apparatus would be the easiest to copy. I finally finished up with that so I decided to write it up here and maybe get some ideas on how to get more useful results.

I went with a NEMA23 stepper for movement and a 3d printed crank for the rotation to linear translation much like the original. It's all made out of rectangular steel tubing and some linear guides as it's clearly visible.

Initially I thought that the head of the bolt rubbing on the rough steel lower stage would be enough to set off some uNAP but that failed. The arm itself without any weight on the lever puts about 1.3kg force on the bolt head. The lever arm is over twice the length of the axle to bolt distance so that would give a mechanical advantage of 3:1 on the bolt head. I added a 1kg weight to the end of the lever arm. Still nothing just smearing of the uNAP on the lower plate. I added extra weight with my arm on the end of the lever(no way to know how much but definitely more than the 1kg) still nothing.

I'm guessing steel to steel friction just doesn't work for that low of a pressure for uNAP. When applying pressure with my hand on the lever i felt that there was some flex in the system. After all the lever itself is something like 15mm OD and 11mm ID.

I'm planning on moving the "hammer" part of the system a bit further to the right so it's more centered to the free real estate of the lower stage. Then perhaps I could rig up some ceramic tile as lower friction surface.

Do you think that lack of rigidity might be the problem?
Can anyone suggest what I can use for a repeatable abrasive?ceramic hammer rubbing on the ceramic tile if the bolt-on-tile doesn't work?
Would some sandpaper instead of the ceramic tile be more consistent? It would be easier.



[Edited on 31-10-2025 by ExcerptSix]

Beautiful work! Congratulations on the glorious contrapulation! I trust the rigidity (or lack of it) is not really a serious problem here, as the force in action is of a continuous nature and the magnitude does not depend whether a rigid or a flexible system transfers said force to the sample being tested. On an impact tester the rigidity plays a much more pronounced role since we have a sudden shock load that has to be transferred to a sample without a part of it being dampened by the soft character of the construction.
But anyhow....the apparatus shall give you a scale of sensitivity over a spectrum of substances and samples. It is not really important that you achieve the same absolute numeric values of sensitivity with your system compared to a test performed with a commercially manufactured friction unit. You shall create your own scale bound to the particular machine. The values shall be somewhat shifted no doubt, but the trendlines remain the same.....a more sensitive substance shall be perceived as such by both the improvised and professional testing unit. By all means you can use sandpaper of appropriate grit instead of ceramic tiles as the substrate. Consistency I trust is less influenced by machine design nuances and more dependent on your own dicipline and adherence to procedure when testing. Making sure the background conditions remain constant inbetween different samples, so that the results are influenced mostly by the nature of samples and the effects of the background fluctuations become minimised.