MDS Dimension Change With Humidity

[rsterne]

I have seen posts about the dimensional stability of MDS as used in hammers, so I wanted to do an experiment.... I started with a piece of MDS that has been sitting in my shop for over a year.... It was just over 1" in diameter and about 2" long.... The diameter, measured with digital calipers, was 1.0415".... I let it soak in water for 24 hours at room temperature and remeasured it.... The diameter increased 0.002" to 1.0435".... I am currently baking it in a toaster oven, at 250*F for an hour and will remeasure it when the drying is completed....

Bob

[oldpro]

 I have had them lock up completely in the bore thats why I run them fairly loose.

[Motorhead]

Odd thing ... in the 100's i made & sold sizing them at @ 1.052" for the m-rods 1.058" tube I.D.  Only have had @ 4 that were brought to my attention that oddly grew in size over time enough to bind slightly.
Those @ 4 hammers ALL had the aluminum weighting core that was installed under a slight give or take .002" press fit.  Never have seen or heard of such size growth on the lighter 100% MDS versions however.

In the last few years making the MDS hammers I did reduce there O.D. to 1.050" and never heard another peep of such issues.

The material works, that i have no doubt of .... how machined, press fit of weighting cores etc .. IMO is more at root of potential troubles than the instability of the MDS material used alone.


JMO,
Scott

[rsterne]

After drying in the toaster oven for an hour at 250*F, and then allowing another hour to stabize to room temperature it measured 1.0425".... This represents a total change of only 0.002" over the three measurements.... While I have no doubt that if you fitted a hammer fairly tight in a tube and then it was exposed to high humidity for a prolonged period of time it might bind.... if the hammer has the usual 0.005" or more clearance in the tube I can't imagine it being a problem.... The information I have found for the worst case for nylon is 0.7% increase in diameter from a newly cast (and therefore dry) part to prolonged immersion in water.... That represents 0.007" on a 1" diameter.... but after a piece of MDS has been sitting around in a shop for a prolonged period before machining, it would have already absorbed some water.... The literature says that the post machining change in diameter of a part due to humidity changes should not exceed 0.5%, or 0.005" on a 1" diameter.... My findings would indicate that is likely twice what may be common....

All I can say is that unless an MDS hammer is fitted extremely snug in a tube, I cannot imagine a problem.... and there is no reason to fit it "tight" with most trigger sears that project at least 0.030" or more into the tube.... I can see a hammer that is made to a given diameter and had a metal core pressed into it expanding over time as the nylon "relaxes" (yields) from the continuous stress of the pressed in core.... Perhaps this is a more likely scenario than growth from humidity?.... The growth of the MDS is proportional to the dimension of the material.... so a relatively "thin" shell of MDS over a metal core (1/4" thick or less) should grow LESS when exposed to humidity than a (1") hammer made completely from the MDS....

Bob

[Machinist]

I think I only have .001" clearance on the MDS hammer I made.  If it binds, I know why.

Years ago I made some airgun stuff from acetol mixed with Teflon.  I wonder how that reacts to moisture?

https://www.interstateplastics.com/Acetal-13-Af-Blend-Rod-ACEAE1.php?diameter=0.250&dim2=3&src=adwordspla&utm_source=adwordsfroogle&utm_campaign=adwordsfroogle&utm_medium=na&utm_content=plastic+rods-ACEAE1&keyword=GOOGLEPLA-rods-ACEAE1-0.25-3.00-0.00&gclid=EAIaIQobChMIp-O0o4Pt2gIVjGSGCh123wxNEAYYASABEgJECvD_BwE

Steve

[nervoustrigger]

Perhaps this is a good time to mention be mindful not to overtighten fasteners and deform the tube.  Quite a few designs have a fastener at the rear of the air tube, threaded up into the receiver.  In many cases it’s not hard to overtighten it and cause the hammer to bind.  Of course it can’t tighten by itself so this problem would typically arise after a rebuild, but it it might also work initially but be so marginal that it may bind up after the nylon hammer swells due to moisture absorption.

[Scotchmo]

The first Acetal hammer that I made and used worked great but bound up (lost 200fps) when shooting on a 100+ degree day at an Oregon match. I now use a larger clearance on my plastic hammers.

I'm currently developing a 3D printed hammer for my Marauder. I have been using PLA (polylactic acid) for the prototypes. PLA is a modern (circa 2004) bioplastic, made from corn or sugar cane, and it is biodegradable. It is easy to 3D print, and has decent mechanical properties. However, it has a relatively high coefficient of friction, so is not that great for an airgun hammer. But it has performed OK in tests.

I just got in a roll of POM (polyoxymethylene) filament for my printer. It's an acetal resin like Delrin. I have not tried it yet. It is reported to have some difficulties when 3D printing. As soon as I come up with some good printer settings for POM, I plan on printing an Acetal hammer.

With dissimilar metal assemblies, thermal expansion properties need to be considered. With plastics, thermal expansion and hygroscopic properties need to be taken into account.

[WhatUPSbox?]

Moisture absorption and dry-out rates for thicker parts can be be quite long and it can be hard to know the condition of the material. Here are some curves for nylon 6 and 66. http://www.toray.jp/plastics/en/amilan/technical/tec_003.html

Looks like a .005 clearance, 1" part might be close if it started its life in Arizona and was in use somewhere wet.

Note the thin shell 1" tube OD will expand similarly to a 1" rod for the same moisture content (ignoring any press fit).   

[Scotchmo]

I did a little research on the subject. Included some of it in a new post:

https://www.gatewaytoairguns.org/GTA/index.php?topic=143168