Lower Frame Assembly

Well, it's about time I posted again.  I'm on vacation this week, so it ain't like I don't have the time...

Last Friday, I got my lower frame assembly done.  I gotta say, it went together much like the MTW Build Guide said it would, but it used a total of 40 T-Nuts instead of 36 per the guide's lower frame materials needed section.  Any difficulties I encountered were due to some failure to read carefully on my part, but the pictures in the build guide kept me straight (for the most part).

One aspect of the build is still bugging me, though.  I have a slight twist in the assembly which I have managed to minimize but not quite eliminate.  I'm letting it sit for a few days and will tackle it again when I feel like it, maybe carving a bit on the plastic parts to see if I can bring this frame into a true and square relationship with itself.

I'm building on the kitchen table, the top of which is 3/8" glass plate and reasonably flat.  This gets covered with an old white bed sheet to enable me to see well enough when I drop small parts, etc., and to serve as a good lay-down area for parts in progress.

All I can figure is I've either got something in a slight bind, or have a defect somewhere I haven't yet seen, hence the time away.  When I come at it again, my eyes will be "fresh", and I might spot what was right in front of me the whole time.

I am getting ahead of myself, though, as I should probably mention that I used a straight fluted high-speed steel tap for all the 5mm x 0.8mm threads tapped into the extrusions.  I didn't bother with a tapping guide of any sort, just picked up a set of tapping wrenches at Harbor Freight, some WD-40, and an old shodo brush I trimmed down to use for cleaning the flutes between tappings, and went for it.

Now, the usual procedure for tapping in aluminum calls for a much more expensive cutting fluid like Tap Magic for Aluminum, but I found by working slowly and patiently, I was able to get straight starts, and achieve very clean looking cuts with the straight tap and WD-40 with minimal effort.  As I may have mentioned before, the center section of Misumi square extrusions has a pre-extruded hole that is perfect for 5mm X 0.8 threads.

NOTE:  ALWAYS wear eye protection when working with taps!  There is the potential for eye injury during tapping operations.

The biggie is to get the tap started squarely.  Once the first three or four threads are cut, check for squareness, and correct as needed.  A tap that is not square will cause binding and inaccuracies in your build.  Correct as needed NOW if the tap is not running straight.

Oh, and always use a tapping wrench, not a Crescent wrench, pliers or other jack-leg tool - the tap won't run straight, and bad things will happen.  The set I linked above is under $8 at Harbor Freight, and it's money well-spent!

Generally, run the tap in a half-turn or a bit more, then back out about a quarter-turn.  Go more once a few threads have been cut, but don't force it.  Better to go slower than faster.

 

If the tap starts to drag a little, more oil down the bore will help, but it doesn't take much, so don't waste it.  It may also help to back-turn and then go forward by a lesser amount than before a couple of times, then resume.  Patience is a real virtue here.

[NOTE:  NEVER FORCE A BOUND (STUCK) TAP, as taps are made of very hard, brittle metal and are subject to break off in the bore, leaving you an ugly mess to get out of.  The best way here is to go slowly and carefully and avoid getting a tap stuck in the first place.  There are special tools available to extract broken taps, but it can get really dicey getting one out.]

Once the tap is ran in a sufficient distance to support the fastener to be used, back it out, and clean the tap and the extrusion over the trash bin (canned air helps here) to get ready for the next tapping operation.

Now would be a good time to check-fit the tapped end with the correct fastener by running it in until it bottoms out.  If it does not run up all the way to the top of its threads, prep the tap and cut a few more threads until it does.

I did all my tapping and parts prep in about an hour and a half, with frequent short breaks in between to clean as I went so I didn't make a big nasty mess in the kitchen.

It took me another two hours or so to assemble the frame.  It's worth noting here that I used button head cap screws instead of plain old socket heads for a more finished look, and that I used an Allen bit set and screwdriver type handle that made assembly go very easily and kept me from over-tightening anything.

That brings me to my "oopsie-for-the-day".  It's embarassing, really.  After all my careful planning, I neglected to order any 5mm flat washers!  A quick trip around the corner to a nearby Fastenal store corrected that problem.  Under five bucks later, two bags of 100 each (one bag for me and one for Tony) were in my hot little hands, and I was on my way.  The guy (J.C.) at Fastenal was really quick finding what I needed, and got me processed in short order.  It's good to know they're now carrying metric stuff, and they're right around the corner (literally) from where I live, so I can hit them up if I forget to buy other stuff like this.

I'm not sure which direction I'll be going next time, whether I will get to hang out with Tony some more and give him a hand with his lower frame, or if we'll be talking about linear motion components next.  Only time will tell.

I've also got some ideas about small modifications to the design to make it easier to square up... but more research is really needed before I'm there just yet.

The only way to find out will be to stay tuned, folks...