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Holes in a bent plate.

Try to Tack weld a piece of metal across the gap outside of the ears. Then drill.

That way the part is a box, not a U, shouldn't move as much.

I also would punch a small hole, say .5, then plunge a 1" endmill, then plunge a 1.5" endmill, Then pre-size it for your bore, then bore.

Those plunging endmills ought to be HSS for cost and longevity purposes. Carbide isn't your friend here. you probably know that.

Edit: i think you said you are already welding something like I am suggesting. didn't see that before now.
 
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Might you take a long round stock. drill a true hole at on end and press in a center punch. put in one part sidee hole at location, push your long center punch threw with the part squared up to spec and pinch the other side..might be good to make a extention drill to drill the other side.. then it would be like a line bore..
Might make the first hole and the second hole with a smaller drill so spotting location to +-.005 could be easy.

Ops, a close hole so use a reamer to finish.
Odd note sometimes a positiverake reamer can be griund with the back taper going the other way, so to become a negitive rake.Yes you need some OD stock to do that.
Also a reamed can be spun at the nose to have a pilot.
Most anything you do to a remer, it is good to keep the the front center for sharpening or other altering.

Use a 1 5/8 shell reamer ground to 1.6213 with .0002 PI back taper...Use the the reamer ID to be the line bore driver so to match one hole to the other. You can sharpen back 1" and still make spec size
Line boring off a driver one can make a sleeve to go on the reaner shank for the last hole, CRS sleeve will last a coon's age.


Here is one for $50
 
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You might make a special roughter out of a 1 5/8 x 4" houigen . grind the OD down to 1.600 or so.

 
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If you've tried boring through with no luck, then clamping distortion must be issue. What else could it be?

Put a couple of DTI on there with magbases, zero, then clamp. You'll easily see distortion. You may need to jury rig some sort of indicator holder.

Modify your clamping until you get acceptable distortion.
 
If you've tried boring through with no luck, then clamping distortion must be issue. What else could it be?

Put a couple of DTI on there with magbases, zero, then clamp. You'll easily see distortion. You may need to jury rig some sort of indicator holder.

Modify your clamping until you get acceptable distortion.
This is why I said we needed more info. post 7 and 10, something isnt flying, and we are spinning are wheels.
 
A U shaped spacer between the ears and c clamp that thing parallel. How do the expect your holes to line up when the ears are likely going to be moving all over when they install their bolts and tighten it down?
 
A U shaped spacer between the ears and c clamp that thing parallel. How do the expect your holes to line up when the ears are likely going to be moving all over when they install their bolts and tighten it down?
OP Said
"We have a support welded between the flanges to keep any flex down. It’s got a lot of parts welded in the gap for the assembly so the part shouldn’t really flex."
 
OP Said
"We have a support welded between the flanges to keep any flex down. It’s got a lot of parts welded in the gap for the assembly so the part shouldn’t really flex."
I was thinking more he needs to pre flex the out of parallel tabs so that they are parallel.
 
Change over to allied “thin wall” insert, with the pilot point.
Bore/ream/drill a piece of 1 1/2 xs pipe that is about a 1/16 smaller than the channel flange gap. Drill first hole, align pin pipe between flanges and small tack it into position. Drill second hole- pipe will guide bit and keep flanges in unstressed position alignment.

That hole size smells suspiciously like 1 1/2 std pipe hinge called out by somebody who thinks pipe is round and has no seam.
 








 
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