[Gncc50]

I am working on a fixture for a tool designer. This fixture is to locate parts that are to be assembled in an automated process. He wants me to machine the fixture to a +.004 tolerance.

He is locating most of the details on the fixture with flat head allen screws (no dowels). One of the details is two inches square and eleven inches high. It is bolted from the bottom through a plate with four flat head screws. Two other details are on a sub plate that is supported by four separate pieces one inch square and fifteen inches high. These are also bolted top and bottom only using flat head screws.

I am trying to convince him to make the bases wider and to use socket head cap screws and dowels. Am I being overly cautious? Are flat head screws used for location? Please give me your opinions.

Trying to get a designer to change his design is a difficult task.

Thanks, Pete

[Geof]

.....Are flat head screws used for location? Please give me your opinions.

Trying to get a designer to change his design is a difficult task.

Thanks, Pete

Your last comment is an understatement.

Flat head screws, countersunk flat head? I think they locate better than regular socket head cap screws but expecting them to get within 0.004" is a bit optimistic.

Why not do exactly what the designer wants but make it clear in writing that you do not guarantee the precision of the fixture if it is taken apart and re-assembled. That way you get paid for this job then if they eventually realise dowels are good idea you get to make a second one.

[rfwyzard]

I go along with Geof. Just remember SHOP RULES.
RULE #1: The Boss is ALWAYS RIGHT.
RULE #2: If the Boss is wrong, see Rule #1.

[vlmarshall]

What tolerance is the designer calling for on the threaded holes? How about the fasteners themselves?

I think he's crazy not to use dowels to locate parts.

[sergizmo]

So, are all the flat heads going to be put on the optical comparator to verify that the heads are tapered at the correct angle all the way around? To get anywhere near .004" repeatability, they will have to be. Also, look in the Machinery's handbook at the tolerance/size range for the size of flathead you are going to use and compare that to the tolerances for dowels. Hardware is all over the place.

Screws are not meant for locating. The only exception are the ones with ground shoulders. This is basic stuff.

Why is this guy so dead set against dowels? Drill three reasonably precise holes, ream, press in three dowels. How hard is that?

[RICHARD ZASTROW]

Pete, You might consider suggesting Spirol dowel bushings.

These are tubular bushings that function as spring pin type dowels. They are tubular to allow the bolt to pass through and fasten to the mating part.

You will not have to add additional locations, just open the bolt hole diameter to accept the "Spirols". They come in different lengths so you need only enough length to establish location.

The assembly can then be taken apart and re-assembled in the same location.

I would think this could hold your tolerance.

Just a thought.

Dick Z

add note: www.spirol.com

[Gncc50]

Thanks for your replies.

I am trying to keep in mind rfwyzards' two main rules about the boss always being right and also ending up with something that is going to work.

Thank you for the link on the Spirol dowels. I haven't seen roll pins used like that before. Seems like that would be a great idea when space is limited.

I am debating if I should refer him to this forum, so he could see the opinions of others.

Thanks again, Pete

[Geof]

Put in Spirol dowels on some of the bolts and tell him they are bolt strengtheners.

[mactec54]

Hi Gncc50

I would do as he wants there are not many flat head screws so just put them in a collet
clean up the taper true to the thread & you will get it better than .004 by doing this more like .001 I do it all the time on a product that I make I use Torque Head type flat heads as well these are better to get in & out if you have to

[Geof]

Hi Gncc50

I would do as he wants there are not many flat head screws so just put them in a collet
clean up the taper true to the thread ....

Doing this means that if the flat head screws need replacing for any reason, socket stripping for instance, things are very likely going to be out of alignment.

[mactec54]

Hi Geof

That is correct but it won't be his problem as he has built it to spec it is very hard to strip out the head of a torque head screw a regular hex flat head yes but not a torque flat head

The part attached are screws straight from the box with no machining of the head & they hold the part at better than .002 in true position checked on CMM

[svenakela]

I'm from a totally different school. I would avoid screws for holding (and absolutely not for positioning) on a fixture for as long as possible. I would only use screws if the fixture is a short term tool.

[Gncc50]

I'm from a totally different school. I would avoid screws for holding (and absolutely not for positioning) on a fixture for as long as possible. I would only use screws if the fixture is a short term tool.

I agree to not using flat heads for location. Just curious, what type of fastener would you use to assemble a fixture?

[svenakela]

Quick locks for example, been running thousands of operations with clamps only. Threads will be worn out on a production tool after a while, even if the fixture is hardened. The only threads that are ok (if you ask me) on a production tool are replaceable screws.