Thursday, February 4, 2016

Sherline: Zebras This Time, Not Horses

The Y-axis stopped working again and it wasn't the parallel port connector.  The motor shaft connects to a coupler, which is held to the lead screw by a left hand Allen head screw 6-32.  I cannot get it tight enough with the Allen wrench I have, at least enough to do its job of turning the lead screw. Tomorrow: I will buy some LocTite to put in the screw to lead screw interface; the smallest lock washer I can find to hold the 6-32 screw in the coupler; and I will try to find the following: if I could use this very powerful power screwdriver I bought to torque down the screw that goes into the lead screw, I think it would be enough.  But I have Allen head bits too short for the deep dark hole in question.  My hope is for an extension,1/4" male for the power screwdriver chuck and 1/4" female to put the standard length bits into.  Or maybe some very long Allen head bits.  Perhaps the power nut driver for 1/4" nuts would, with some pressure, hold the 1/4" Allen head bits in it.  And some very long Allen head wrenches with longer arms to increase the force I can exert on this hex head nut.  I am tempted at the moment to get the Sherline working again, and look at something more serious.  Two days of cutting acetal should not requires four hours of maintenance.  I have asked Sherline's support guy to recommend a more maintenance free CNC mill.

I have it back together and it worked briefly.  Thinking of selling it for parts and just giving up on this whole CNC mill idea.  Maybe Sherline just doesn't have it together, but I am fast losing hope that CNC mills are actually a net gain.

Sherline thinks the thuid, thud thud noise is exceeding the range of Y and told me what to change in the LinuxCNC ini files.  Try that tomorrow.


Unknown said...

I might be barking up the wrong tree, but I'm thinking:
- unless the power screwdriver is an impact unit, it won't tighten things any more than hand power would -- after all a hand will be supporting the screwdriver. It took 6 weeks for a sprained finger to heal after I learned that my drill offers more torque than my fingers can handle.
- I've had really good success slipping a bar with a hole in the end, or a length of solid tubing, or a 6-way screwdriver with the bit removed, over the "L" end of an allen wrench to get more torque at a right-angle to the axis of twist. I've broken a few allen keys before breaking the screw loose, but they are cheap to purchase.

Will said...

You may not know that the 6-32 screw is the weakest fastener in regular use. They break long before you think something that size should. 6-40 is stronger, and used on firearms, but not used much in industry, so you pay a premium to specify it.

My default replacement for the 6-32 screw was to go up to the 8-32 in size, if there was sufficient material to work with. Unfortunately, designers seldom turned out to have much practical experience, which explains why they tend to perpetuate the same mistakes over and over.

The only engineers I've worked with that understood the realities of mechanical design had a background in hands-on wrench spinning. One had a Porsche 911 with a small block Chevy, and a sailboat. Another had raced sailboats and motorcycles. A third one raced motorcycles and a split-window Vette. One built kit aircraft. Hmm, now that I think about it, most of the best ones I encountered had motorcycles of some sort in their background.

I use a deepwall 1/4"x 1/4drive socket for that sort of problem. You can get a socket adaptor to run 1/4" sockets with drills. Most any hardware store should carry them. Horror Fright does. You can even get them with a wobble head. If the socket is too large to fit into the hole, you can turn it down on a lathe, to some extent. 12 point sockets tend to be thinner walled to start with.

Will said...

Just a reminder, it's not the CNC causing problems, so much as the unit is too small and lightweight for your needs. The Sherline is pretty much a toy. A commercial one the size of it would cost near the same as a full size machine. And would still have limitations. I don't think I've ever seen a Sherline in any industrial machine auction, or in a shop.