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Counter-sink for better alignment?

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homebrewed

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#1
I'm in the beginning phase of making a tooling plate for my benchtop mill. I flattened the plate with a fly cutter (it was warped and rocked on the table) and am now making the mounting holes for the plate. They are counter bored to place the bolt heads below the top surface.

I'm planning on making a number of low-profile work holders similar to what Harold Hall describes on his web site, but a thought occurred to me. If I bolt the work holders down using more counter-bored holes there will be a lot of possible variation in the alignment of each holder -- so in addition to aligning the tooling plate when it's installed, I will likely need to align each of the holders -- at least, if I'm milling some critical dimension on the X and Y axis. If I'm just thinning something down then alignment is not an issue (but the first thing I want to do DOES require good alignment on X). Anyway, I was thinking that if instead of counterboring, what if I countersink the holes and use flat-head bolts. The cone recess and cone-shaped bottom side of the bolt would force alignment between them ... at least to some degree. I THINK it would be better than a counter-bore.

So have folks here used this approach for the purpose of improving alignment? And if so, how did it work out? Is the manufacturing tolerance of the bolts too wide to make this a useful approach?
 

benmychree

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#2
Just key your fixture plate to the table slots, as is done with vises and dividing heads, etc.
 

homebrewed

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#3
Just key your fixture plate to the table slots, as is done with vises and dividing heads, etc.
That will work for the plate and is worthy of consideration. I'm talking about the work holders that are bolted to the plate. I suppose I could machine grooves in the top of the plate and key off them as well.....but I haven't seen that featured in any of the fixture plate designs I've looked at. For the most part they are a mix of D'd&T'd holes and blank holes for alignment pins. Number and spacing depend on the ambition of the maker.

Another approach would be to use alignment pins, since I'm planning on drilling/reaming some holes for them as well.

On the other hand, maybe I'm just over-thinking this.
 

mikey

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#4
I'm planning on making a number of low-profile work holders similar to what Harold Hall describes on his web site ...
We might be able to give you more useful input if you posted a pic of the kind of holder you're referring to. Searching Harold's site and guessing which one you mean can be confusing.
 

homebrewed

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#5
I'm thinking of something like this:
Low-profile clamps.jpg

-but replacing the bolts with socket-head flat top screws.
 

JimDawson

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#6
With the setup above I'm not sure you would gain much alignment by using flat head screws, but the flathead screws would give you additional head clearance. But counterbored screw holes would accomplish the same thing.

I use flathead screws for alignment in machine assemblies where the parts have to fit kind of close. If I need true alignment then I use counterbored standard screws and use dowel pins for alignment.

What we did on the Haas is to make a semi-permanent tooling plate with drill bushing in it. That way when we put the pallet on, the dowel pins in the pallet drop into the drill bushings to align the pallet. This is good for about +/- 0.002 over the length of the pallet. If anything closer is required, it's time to get out the indicator.
 

Cobra

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#7
The countersunk screws will give you sort of alignment but it will not be fine tuneable. If it needs to be exact I would stick with the counter bored fasteners so that the final adjustment can be made.
 

homebrewed

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#8
Thanks guys! I think my question has been answered, which boils down to: don't paint yourself into a corner.....
 

P. Waller

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