Anyone have success machining* NBR Rubber slabs into simple/clean shapes? 3

[WKTaylor]

I have a dilemma. Anyone... ?

Our turbine engine has fairlead blocks [for soft-capturing/holding tubing and wiring], made from NBR Rubber Shore A70+/-5 hardness.

On the Fairlead drawing, the preferred fab method is by rubber-molding: however it is optional to make [machine] them from thick rubber sheet... which seems like a valid option... except...

I have directed machining for most metals, a few ceramics, various plastics [various nylons, PTFE, etc] and various Fiber Reinforce Plastics [FRP]. However, I can't find anyone or any corporate experience or written guidance for machining slab rubber, such as: cutter geometry [teeth, angles, twist-angles, blade cut geometry/sharpness, etc], cutter-speeds, feed-rates, lubricant-flow, cooling, etc.

The only ideas that I could add to 'maybe' simplify the machining of a slab of rubber is to:

A. Chill the rubber block the to sub zero [but not below the brittle point] so it machines more like hard plastic. NOT sure of this possibility... just a SWAG.

B. Machine(a) ~80-to-90% thru on one-side; or (b) ~40-to-45%thru from both sides [mirror images].... then knife-cut-thru the thin remaining thick web to separate-out the intended part.

This logistics problem... no spares for almost a year and none 'due-in' in the foreseeable future... has become a [my] 'engineering problem'. Machining from readily available rubber slab material appears to the '$1M wraskley wrabbit' answer... if feasible.

Please advise!... quickly.


Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[3DDave]

Water jet can do pretty well on some rubber parts. At least on 1/8th inch gasket it goes through like a razor blade.

Over in the rubber section abrasive machining has been suggested. I presume with plenty of water to cool and remove fragments.

http://www.eng-tips.com/threadminder.cfm?pid=335

 

[MikeHalloran]

Got a steady hand?

https://www.amazon.com/SOUTHWEST-GROOVING-OFF-ROAD-MOTORCYCLES-MODIFIED/dp/B00M08TI7Q

Mike Halloran
Pembroke Pines, FL, USA

 

[berkshire]

I used to work for a DECAS certified company in Atlanta Ga that sadly, is no longer in business, that company used to do a lot of rubber machining. The preferred method was to cut parts oversized with a narrow knife blade on a bandsaw , then get to final size by grinding using lots of soluble oil coolant, nothing was ever cut dry, some round parts were chucked up in a lathe , then machined to size with a tailstock grinder, other parts were done in a fixture on a surface grinder, the idea was that the grinding introduced less distortion into the part, final passes would be made several times with no advance on the feed.
B.E.

You are judged not by what you know, but by what you can do.

 

[ornerynorsk]

Are you allowed an alternate material, such as urethane, which can be shop-cast more readily than nitrile? Machining anything elastomeric is a bit of a devil, and the waterjet and grinding are probably the 2 best methods for thick section, with knife cutting the best for thin section.



It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[WKTaylor]

3D, Burk... My head is hurting...

I see that water jet or abrasive grinding or a knife-blade saw or perhaps even a 'fine-wire saw'... with water-soluble oil lubricating coolant might work for most 2D Fairlead parts. Thanks! that helps for making a high percentage of the simple rectangular Fairleads we need when logistics fails to provide spares!

However the Rubber Fairlead I have to have made is a 'grommet-shaped' part which is illustrated in the attachment along with a 'simple rectangular Fairlead block'.

http://files.engineering.com/getfil...4ee24c&file=Rubber_Fairleads~Engine_Instl.pdf

Two Fairleads and U-Channels or Two Band-Clamps form the on-engine installs, as implied. I'll omit detailed info as-to WHY these Fairleads were made this way... suffice-it-to-say that these were designed for a prototype 'engine-upgrade Mod' that was started in in 1977 and was never revised thru production and now post-production.

Any specific thoughts/comments RE machining the '1/2-grommet-shaped Fairlead'. Should we try making the part as-shown [1/2]... or Make both halves as a single grommet... then split/slice it in half when machining is completed? Etc...


Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[btrueblood]

Mike,

How do you keep the rubber from undergoing reversion (i.e. becoming sticky/tacky along the heat-cut edge)? Maybe it doesn't matter for tires, when you want sticky traction anyway? Or maybe the risk of reversion is lower for SBR/nitrile or other tread rubbers, or with sulfur cured rubber? I know that peroxide cured and sulfur cured epdm will revert if it gets too hot. I've only had luck with abrasive grinding at making reasonably precise shapes, though I have seen some young guys at rubber molding shops carve rubber blocks with a sharp knife (and yes, I always speak very politely to them!

Wil,

That looks to be pretty standard grommet geometry - can you not find anything suitable by searching McMaster Carr etc.? Surprised you are getting no-quotes for molding, are you not willing to pay for tooling? How were the original parts tooled? I could point you to a couple of companies that might do the work.

 

[WKTaylor]

Btrueblood...

Good info to keep in-mind... and Yes I have been looking for a standard grommet: problem is that it has to be aerospace grade rubber and quality... and readily available from a vendor.

Closest that I've come for THIS install is MS35489-114X [split]... which would require a new saddle clamp design. Problem: this apparently simple design change begets a host of design changes that costs a LOT of $$s and time from at least 6 other engineers... not to mention USAF Tech data changes and T&E engine runs. Uhhhhhhggggg.

The original design intent appeared to establish a standard mounting install [metal brackets and saddle-clamp] common to a wide range of tubing diameters ... with only grommet-style Fairlead changes.

I agree that molding is 'best' FAB method... but the supposed 'no-bids' make me suspicious that there is contracting 'dis-interest or neglect' at best; or at-worst slow-rolling or malfeasance.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[btrueblood]

but the supposed 'no-bids' make me suspicious that there is contracting 'dis-interest or neglect' at best; or at-worst slow-rolling or malfeasance.

Ugh is right. I have forgotten how large aero companies work (PTSD?) having left the biz some 20 years ago. If you want a good company to work with, Rainier Rubber in Tukwila, WA does a bangup job for us (a tiny little valve company) and apparently keeps the doors open by making all sorts of rubbers seals and such for some aero company called Boing or something like that (dunno, sounds like a startup?). But, I do recall now getting in trouble as a young, impatient aero development engineer for "pre-engineering" jobs with vendors that I knew could do the work and were good folks to deal with vs. the run it through the purchasing/contracting mill and hope you see something in a year or three method.

 

[WKTaylor]

Hmmm... Bt... RE Your last post...

When all-others fail, engineers don't have that luxury... time for me to make noise about spares. Time for another well-earned 'aw-shit' award.



Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[WKTaylor]

bt...

NO comments RE freezing the rubber to stiffen it for Machining???

-----------
BTW... as an interim-grabbing-at-straws-measure... These parts are usually a 100% time/condition/overhaul change item. Duhhhh.

Just released a cleaning, inspecting and sealant brush over-coating process for used rubber Fairlead-halves. Expecting about 80+% failure-to-pass-serviceability-inspection rate... but a few 'better ones' may separate out as serviceable and be reinstalled.

I will also see work with a machine shop that was willing to try 'hogging-out' this part from slab rubber using methods/techniques mentioned above.

NOTE. While 'exploring' [stumbling around] this subject, I discovered SAE AIR5367 Machining of Composite Materials, Components and Structures, which will be useful for 'other work'.

PS. I've kicked-around various SWAG schemes to adapt the standard MS Grommet to the existing large saddle clamps. No-Go, so-far.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[MikeHalloran]

Yeah, for a tire groove, sticky is not a bad thing.

Re the no-bids,

My friend George ran a hydraulic supply company, with a store near a large, uh, launch complex.
He got a purchase order for a few o-rings, put them in the mail....
and got them back, with a note explaining that they didn't have the right paperwork.
More round trips ensued for the same parts, with increasing costs to meet expanded requirements,
including multilayer envelopes guaranteeing long shelf life, costing four dollars per envelope,
one to ship _each_ o-ring.

After months of frustration, he sent them a no-bid with a note that went like this:
"My store is near your gate.
At that store, the o-rings you want are in stock, and sell for a nickel.
Come and get them if you like."

And thereafter, guys in black pants, white shirts and black ties would show up and buy a handful of o-rings for cash.



Mike Halloran
Pembroke Pines, FL, USA

 

[DAVIDSTECKER]

With all of the advanced adhesives out there is the a chance that you can water jet out a central core and two "Flat Washers" and "glue" them together?
Make the core the width of the clamps then apply the flat washers to the face of the core for max surface area.
It would aide assembly by using a rod for alignment then slice them in half.

 

[btrueblood]

Wil,

I have never had the cojones to put a bunch of dry ice, or fire extinguisher spray, or LN2, onto a running lathe/milling machine. Freezing rubber does make it stiff, and easier to machine, but depending on how big a part you have, it may not stay stiff enough long enough. I do know that we have purchased lip seals in odd sizes from a company (Eriks West?) that does just that, i.e. lathe turn seals, and have heard that they do cryo processing to do some of the work. I can confirm the name for you if need be, I'd have to bug a colleague at our now sister company.

If you can find slab rubber that meets your material/traceability requirements, I would second/third/whatever waterjet cutting the block. Glueing rubber has never been very successful for me.

Mike, I worked with another young engineer who replaced some bespoke silicone damping compound that was ridiculously hard to work with (very short shelf life, expensive, long lead time for procurement) with a piece of silicone rubber from McMaster Carr for a vibration test. Naturally, the cheap rubber gasket material was several times better at reducing peak stresses in the part undergoing testing. He was summarily punished by many different people.

 

[WKTaylor]

Guys... good info and inspiring Fud-4-Thot.

When I mentioned freezing the rubber, I meant: 'deep-freeze-it'; remove from freezer; mount/machine a-bit-off; return to deep freezer, repeat, until done.

I think water-jet, wire-saw and molding are also 'viable options'.

NOTE.

In the interim, I've had several brain-thunderstorms, 'best' of which are thus...

1. Machine a simple 2-D shape... all original dimensions... except OMIT the 2-small strap retaining/centering flanges [so no strap-RECESS!!]... then bond this shape onto the ID of the cheap-to-make CRES sheet-metal saddle clamp. Use MMM-A-101 adhesive [several qualified products] for high-strength bonding of the vulcanized synthetic rubber to the CRES clamp surface. A few small holes punched along the CL of the saddle clamp would hold adhesive, enhancing axial 'push-out' resistance.

Anyone have experience with bonding already-vulcanized rubber to CRES? The adhesive/method mentioned above [MMM-A-101] seems to work well with thin/flexible rubber to CRES... but I have no experience with the same process and thick/stiff rubber. I wish I could vulcanize the rubber fairlead to the clamp in a hot [mold] vulcanizing process.

2. I also kicked-around building-up layers of synthetic rubber-cork tape [AMS-T-6841] on the saddle clamp ID... then trimming the built-up layers, across to-match the clamp mounting flanges using a band-saw knife blade [etc].

Gottagobacktowork.



Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[btrueblood]

Any reason you couldn't roll the edge of the stainless band to partially "encapsulate" the rubber? Or fold some tabs down from the edge of the band, similar idea.

 

[WKTaylor]

btrueblood... intriguing concept... another possibility, thanks!

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]