Solve This Mystery ?

GK1918

GK1918

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I'll try to make it short. The job; customer has a 1949 Ford F7 or F8. He already put a V8 diesel in it.
Now for disk brakes. He bought late fr & rear axles from a late GMC. This front GMC is too wide.
Found to be the F8 & GMC are both Rockwells. So the plan is putting GMC knuckles on the Ford (for
the original axle width. The GMC bushing bore is say 1.224 and the Ford bore is 1.359. The job was set
up and I chose my smallest Criterion boring head. I bored one bushing side that came out to 1.3595 which
is target for a press fit. The second side?? I am taking .005's and .010's. I got up to 1.349 now I got ta
sneak up to 1.359. So I'm cutting back to .002 cuts. I stick my snap gauge in and measure that, now I get
1.410????????? Where did .60 or so go. No way in hell will my little boring head chop .060????
Every cut was checked twice and written down, & mic was locked after measuring. Thats a day for not!
Any way I ran down & got a piece of CI water pipe & turned it to the 1.410, pressed it in the bore with
locktite, now to rebore that back to 1.359 and press the bushing to that. And yes my table is locked and
the DROs still on 0.0000. This is my first big time %(R$ up. So whats possessed? my mic (Starrett) -
snap gauges (Starrett) , even with old eyes, that lost .060 I can see- as the head is whizzing around.
No one in this shop can figure this one out. Also the mic & snap gauge was checked with a standard.
sam
A pic of the diesel we installed and then I now own the big 337 Flathead I love Flatheads
good luck

Diesel.jpg 337 CID.jpg
 
When using snap gauges, I always take 3 measurements just to be sure. I had a similar snafu in my younger days and now I always triple check.
 
I have to agree with Mac here, triple check. I've done the same thing.
 
I did this making the center hole in a cast iron face plate for the lathe.
I decided my problem was trying to sneak up on the final measurement.
I did some research then on machining cast iron, and took two basics away from it: use a zero rake tool, and never cut smaller than the radius of the tool.
It seems that cast iron can get really fussy when trying to take light cuts that are smaller than the nose radius, especially if your tool has a rake.

Since then, I just bore undersize, run a spring pass, measure three times, dial in final dimension and take it one shot. I try to hit between 0.004" and 0.007" DoC for the final pass. You paid big bucks for that Criterion head, why treat it like a cheap Chinese tool - it is meant to be used and trusted.
I am very far from being an expert on machining cast iron, but I have had no more issues hitting dimension since I started doing the above.
I am sure there are other considerations I am totally ignorant of, but I got this info from reading over modeling forums where folks are routinely having to bore very precise holes in cast iron on machines that are not super rigid.
 
A snap gage is really for someone in production or for a random inspection out in the field or something. And you cannot “feel” the measurement being made. Sorry, but it sounds like the gremlins stopped by because of it. I have a few snap gages but have never used them (I hoard tools, another topic). I use a standard micrometer or grab my dial calipers…Good Luck, Dave.
 
Using snap gages is not a problem. Could be just missing a turn of the mic? I have a kid here at the shop almost swear at me for telling him his measure was off .025. Finally stuck some calipers on it. He still thought I was off! Easy to do some times. We look at the thimble numbers and forget about the scale. DAMHIKT ;0
 
GK
lots of folks have experienced this many times.:black eye: And yes it is frustrating so don't beat your self up to bad.
By chance did you use the rocking method on the snap gauge?
The lock doesn't always lock firmly on snap gauges and as you adjust your micrometer "aka calibrated c clamp" it will change your measurement.

I use snap gauges often, but rely on inside micrometer for self assurance. Seeing that you are getting into making $$$ I am sure that you have looked into getting a bore gauge set or a hole micrometers. They will pay for them self time and again.
 
Everyone's on the on the right track, its just a machining power play I guess. Yes I snap it
several times & mic it several times. We do have a expensive bore gage but I do not want
to move the table even so we have dros & dials (that I learned 50yrs ago) right or wrong
that, stuck with me -the high school teacher saying do not touch or remove anything
till the job is done. Anyway yesterday I bored the insert (my same method) and came
on target. Also I checked my pockets swept the floor an still can't find where I lost
.060 ?? and my cat didn't do it cause he's not allowed until all chips are vac. so he don't
step on em LOL oh again forgot to snap a pic..
sam
forgot to say that I hog most of it with .006 quill feed, ending with several .002 spring
passes.
 
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