Made and used my first "flexture"

[FliesLikeABrick]

My dad has been hand-making some parts for customizing certain pieces of outdoor equipment. His historical process is just using a hacksaw and files to cut off and shape pieces of round aluminum stock.

As he has refined his design, he has also wanted more consistency batch-to-batch, and during a visit this week we were talking about how we can make some batches up in the shop here.

The part involves 3 main machining steps:
- Turning the diameters required (1/2" diameter round aluminum stock, turned down to .401"diameter except for a .125-long" shoulder left at the full 1/2" diameter at one end)
- Chamfering and Parting off
- Cutting a slot in one end

However, there is a problem with the order of operations. The overall length of the part is only 1.25" - too short to hold in a collet block with the shoulder at one end, which would have been the easiest way to hold it at the mill.

Holding the round stock in the mill (before cut to length) requires moving between the mill and lathe more often than is convenient for trying to batch 5 to 15 of these in a short run.

The answer we came up with was a jig that could hold a small quantity in the mill for slotting.

My dad was thinking a jig where the top is totally removed and clamps the parts, however I preferred a "flexture" where the jig could stay clamped and trammed in the mill.

The first step was determining the overall length and placement/spacing of the features. The length was dictated by what piece of aluminum bar I had in the scrap bin. The other features were:
- Set the centerline of the parts a known distance from the top for easier alignment of the slitting saw. We went with 0.5" from the top of the fixture
- 5 clamping screws (1/4-20 cap screws found in assorted inventory), biased towards the slotted side/front of the fixture
- 4 holding positions drilled with a Y drill (.404)
- Slot cut to ~ 2/3 the thickness of the fixture

The overall fixture length was ~ 5.25", leading to a pretty inflexible layout of the features.

The process started with approximate squaring of a piece of scrap from the bin, in the horizontal bandsaw. One side was flattened on the mill to provide a reference surface for the bottom of the fixture.

Holes were drilled from the top down for the 1/4-20 tapped holes; and with a Y drill from the front. The latter holes came out closer to .402 or .403 instead of .404, which gave perfect .001-002 clearance for holding the parts. If the hole had been larger, the required specs allowed for the turned diameter to be increased slightly (to .402 or .403 as-needed for proper clamping in the jig).

The holes were chamfered, and at this point ready for slotting. This is where I started taking pictures









Now tapping the 5x 1/4-20 holes from the top. Based on the length of the threads, I decided to install the tapping head in the drill press for this. The 1/4-20 tap I grabbed was a gun tap, which worked well for power tapping these through-holes.




After some deburring and drilling the 1/4" clearance hole in the top portion of the clamping screw locations, the fixture was ready for use. Here it is with a finished part for demonstration purposes:



And the next day in the mill with the first batch of parts:




This worked a treat. It is made of aluminum, so of course we need to be careful not to force any parts in and create burrs/galling. And if this was going to see more than occasional home shop use, it would be steel instead of aluminum to avoid wear.

The tolerances on the resulting part are very flexible, so this solution should work well when we occasionally want to make more. I am extremely pleased with how well it worked and how positively the clamping worked out, given that the proportions of the fixture were totally made up on the spot based on intuition (or lack thereof).

We greased the clamping screws on the threads and under the heads to reduce wear and prevent galling - but this also may have helped increase the amount of clamping at reasonable torques. We did not need to go more than moderately-snug on the clamping screws to hold the workpieces more firmly than we could budge - this seemed perfect.

You bet that if I ever need to slot small parts for a project of my own -- I will do my best to design such a project for those parts to be .401" on some feature to allow reuse of this fixture!

Thanks for reading.

Tools used:
- Logan 200 lathe
- Start-Rite H175W horizontal bandsaw
- Norsemen drill index
- Chicago-Latrobe tap+drill set
- Rockwell 21-100 mill
- cheap import 123 blocks
- The usual cheap strap clamp set
- Procunier #3 tapping head
- Powermatic 1200 drill press

 

[brino]

Very interesting!

I am not sure how I would have approached this clamping fixture. However, I will now remember the success you've shown above.
It's a very clean, simple, low-cost solution.

Also, it was extremely well presented.

Thanks for sharing this!

Brian

 

[Winegrower]

Interesting approach certainly. In my life, the word “flexture” has referred to a flexible printed circuit, commonly used for signals between moving and stationary parts. Is this term common in machining? if so, what would be a good definition?

 

[FliesLikeABrick]

I would describe it as a fixture where the elastic properties of the fixture construction/material are used to a functional advantage. In this case, not having the top be completely removable allows the parts to be exchanged without loosening the strap clamps that hold it down, or otherwise fumbling with a detachable upper portion to the fixture.

The key word is elastic -- not wanting to deform the fixture so much that the aluminum (or other material) starts to work-harden and risk losing springiness or becoming brittle and breaking. As a result, I knew it was critical to have only .001 to .002 clearance between the fixture and the work

 

[mmcmdl]

However, there is a problem with the order of operations. The overall length of the part is only 1.25" - too short to hold in a collet block with the shoulder at one end, which would have been the easiest way to hold it at the mill.

The use of a split bushing would eliminate the need for a dedicated fixture for your part . Bore or ream a .401 hole hole thru a whatever sized OD stock , split the bushing , insert it into a collet block or indexer with a collet stop .


Nice work on the fixture for ganging up the parts though .

 

[ConValSam]

Looks fantastic!

Hopefully at some point you can reveal what the part modifies; you have peaked my curiosity.

 

[FliesLikeABrick]

Looks fantastic!

Hopefully at some point you can reveal what the part modifies; you have peaked my curiosity.

Thanks.
My dad does a lot of outdoor clothing and equipment repair for colleagues and others in the world of hiking, climbing, ice climbing, etc. Amongst this, he also offers a service where he adds speed knobs to ice screws (safety anchors), without drilling or modifying the screw itself so as to not add any safety risk. More-expensive ice screws come with these knobs built in, but he has a way to add the knobs to almost all styles of older or cheaper screws. This piece is one of the components of the knob mechanism he attaches.

 

[FliesLikeABrick]

Tonight I finally decided to sit down and spend some time in OnShape, after not using CAD in the last ~12-15 years and having an itch to pick a CAD package back up. For 3D CAD/modeling I had mainly used SolidWorks in the past, and OnShape was pretty natural once I jogged some memories.

None of my ongoing projects really benefit from having anything drawn (mostly repairs to various things, building a greenhouse, etc) - so I decided to draw the part for this. OnShape is really pretty great, tons of videos on YouTube whenever I do have a question to use a feature for the first time. After about 30 minutes (5 minutes creating the part and 25 minutes dabbling with changing some lengths to parameters for overall vs head length, and tweaking the orthographic projection):

 

[ChazzC]

We greased the clamping screws on the threads and under the heads to reduce wear and prevent galling - but this also may have helped increase the amount of clamping at reasonable torques.

Nice setup, and thanks for the photos & details.

To prevent galling and increase torque I’d suggest using anti-seize rather than grease and also adding washers under the SHCS’s to eliminate direct contact with the flexture.