Fixture threads

[Houndogforever]

I'm making a set of fixtures for an aluminum part. I'm using a uniforce clamp to hold the material in place.
The uniforce clamp is forcing against two parts which in turn push against a bolted on steel stop block which is the location.


On the threaded holes that are for the OK vise and Uniforce clamps, I intend to use E-Z Lok inserts so that the threads actively being used will have steel rather than the aluminum fixture plate.

So my question. On the holes that are simply for the stop blocks, do I need to helicoil those threads? The block will be bolted in place and firmly set into a pocket and dowel pinned in place.
I would think for just one tightening to mount the block I can just use a form tap and then torque down the hold down bolts. 3/8-16 and 1/4-20.

Will that pull or wiggle loose with time? Would a helicoil or EZ-lok be better?

What do you say guys?

 

[Mtndew]

do I need to helicoil those threads?

If it's 6061, I would helicoil them. If it's 7075 aluminum then you're good for quite a while.
But if you're just tightening them down once and done, then no helicoil is needed. unless there is force or stress on that thread.

 

[GiroDyno]

If it's a permanent/dedicated fixture what about match drilling and adding a couple of roll pins to each bar.

 

[Houndogforever]

This is the first opp for the blank roughing and putting on a dovetail. The holes are the 3/8-16 that are holding the steel locator bars in place. The bars are snug fit into a pocket for location and support.


This is the second opp. The dark red block with two cap screw holes is the location block on this side. The only pressure will be the sideways pressure from the uniforce clamp and the stop block will be snug pressed into a 1/4" deep pocket and drilled for dowel pins after assembly.

 

[beege]

EZ lok's for screws you are always using, but I wouldn't use STI's in holes you're only using once. Consider using Torx cap screws instead of the supplied hex.

 

[Houndogforever]

EZ lok's for screws you are always using, but I wouldn't use STI's in holes you're only using once. Consider using Torx cap screws instead of the supplied hex.

What does the Torx gain me?

 

[AARONT]

I wouldn't bother with the inserts in the holes for stop blocks. Only the holes that will be repeatedly threading hardware in and out of.

 

[DavidScott]

Don't bother inserting the threads you don't cycle, my fixtures usually are retired before the threads wear out on my 8-32 holes for Uniforce type clamps at up to 5,000 cycles, and I find the torx screws in the fixed stops keep me from unscrewing them when I try, all those screws can get confusing when swapping parts.

 

[beege]

I find that the Torx screws are easier to "find" with a power torque screwdriver

 

[Orange Vise]

I wouldn't bother using threaded inserts for the stop block holes. Permanent bolted connections don't need them, even in aluminum. I would, however, tap the threads to at least 2xD.

Are the stop blocks in a machined slot / against a shoulder? That'll prevent shear forces from acting on the screws.

 

[Houndogforever]

Yes, the block is about 1-1/8 tall but only stands .85 above the part bottom. So .275 deep into a pocket. I also put a 3/8 bolt closest to the contact point to make sure it doesn't lift. I also elongated the block so it is 1" wide but 1-1/4 long so it makes it even less likely to move or lift. The blocks are made from 4140 at HRc 35-40.

 

[Houndogforever]

These are the stop blocks. Looking at the bottom. They will be flipped over, faced off and counterbored.

 

[DavidScott]

Your clamps aren't taller than your stop blocks are they?

 

[Houndogforever]

Looking at that picture, yeah, it shows ONE location with the proper height and the other 3 are all low.
Yes, the uniforce clamp top will be even with the top of the stop block, which is about 0.075 below feature level.

On the second opp fixture, there won't be any heavy milling. Just a light end cut removing the dovetail remnants. Then drilling and tapping a hole thru the 1.03 alum part.

 

[Houndogforever]

This will be the rotary fixture to go with this project. The lower fixture will prep the block and dovetail, then all 4 move to tombstone, then all 4 move to second opp and tap the sides.
I've been making this part since 1981 when I made them on a Jet bench top mill in my basement.
Once it is all running it should spit out 4 complete parts in about 20 minutes. Change over maybe 5 minutes. So 8 parts per hour I would say.

 

[D Nelson]

Use something besides uniforce they are too weak. They don’t last use pit bulls or something else. Of course that’s just my opinion I quit using them
Don

 

[DavidScott]

Use something besides uniforce they are too weak. They don’t last use pit bulls or something else. Of course that’s just my opinion I quit using them
Don

The basic design is good, just some of the details needed improving. I do love driving one screw to clamp 2 parts, with a pneumatic screwdriver.

 

[Houndogforever]

I've used the uniforce before and it holds fine. I only use them on light machining things like milling off the dovetail or drilling and tapping.

It is a 1" uniforce clamp that says it gives 2500lbs of force which should be plenty.

On the first opp where I am roughing all the material away I am using OK-vises with knurled face so I'm confident those will hold.