HS: Rough End Mill marks / finishes causing frame cracking by stress risers? - RimfireCentral.com Forums

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Old 07-22-2019, 04:09 PM
Gus1957

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HS: Rough End Mill marks / finishes causing frame cracking by stress risers?



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Has anyone seen info on the relation to poor finishes caused by end mills tearing the steel in slide release pocket versus where the cracks initiate? Has anyone polished out endmill marks to improve finish to prevent cracking?
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  #2  
Old 07-22-2019, 07:02 PM
SGVictor
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Quote:
Originally Posted by Gus1957 View Post
Has anyone seen info on the relation to poor finishes caused by end mills tearing the steel in slide release pocket versus where the cracks initiate? Has anyone polished out endmill marks to improve finish to prevent cracking?
Generally speaking the end mill marks are considered surface finish and a factor
is applied as a correction to the basic steel endurance limit. It is the change in section at the back of the magazine well and the pocket for the slide lock that are considered stress risers which also affect the endurance limit. I suspect the stress risers have more influence than the surface finish in this case. Just remember that anything you do now about the surface finish will likely result in a reduction of thickness where you can least afford it.
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Old 07-22-2019, 07:56 PM
Alan Aronstein

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RMS and Stress ???

I don't beleive that the RMS of the surface has any bearing on the stress conditions in the frame. I assume that the grooves are into the correct thickness and, they do not reduce the overall thicknesses that are specified. The pictures are clearly not 8 or even 16 RMS. Looks bad but the effect is questionable !!! - Alan Aronstein
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Old 07-22-2019, 11:49 PM
SGVictor
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Originally Posted by Alan Aronstein View Post
I don't beleive that the RMS of the surface has any bearing on the stress conditions in the frame. I assume that the grooves are into the correct thickness and, they do not reduce the overall thicknesses that are specified. The pictures are clearly not 8 or even 16 RMS. Looks bad but the effect is questionable !!! - Alan Aronstein
Alan,

In the textbook by Shigley that I used at Purdue in the late 1960's surface finish is indeed a factor.

The standard endurance-limit modifying factors were:

ka Surface factor
kb Size factor
kc Reliability factor
kd Temperature factor
ke Modifying factor for stress concentration
kf Miscellaneous-effects factor

For example at 30 Rockwell C the strength is approximately 140 kpsi and the Ka factor for:
a polished finish is 1.0
a ground finish is .89
a machined or cold drawn finish is .68
a hot rolled finish is .46
an as forged finish is .32

As the hardness goes up these factors would increase except for the polished and the ground finishes.

While there is an effect from the surface finish, the stress concentration is much greater in this case because there are multiple changes in section at this particular location in the frame.
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Old 07-23-2019, 12:37 AM
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hamden
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My coach and mentor told me to file the edges smoothe at the rear corner of the magwell. and at the bottom of the slide stop area. Sort of like a drilled crack stop. I say again that Coach worked with Joe Benner and was OIC if Army teams.

Another thing I do is to lightly stone the roughness under the trigger bar. Not to remove any machine marks. But to smoothe the bumps into slick bearing surfaces. The marks hold lube nicely.
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Last edited by hamden; 07-23-2019 at 12:45 AM.
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Old 07-23-2019, 07:04 AM
LDBennett
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It's my opinion that what matters is keeping the stress input into the crack prone area of the frame to a minimum. The nature of fatigue failures as we see in the frames is that the higher the stress the greater the likelihood of a fatigue failure over time. The curve describing the stress level inputs verses the number of inputs to metal failures is not linear. So for maximum life of the steel, keep the stress inputs to a minimum.

In a semi-auto gun if you choose the ammo right and the recoil spring right, the slide can be made to NOT pound on the frame slide stop abutment. Ideally you want the slide to travel fully and gently kiss the frame stop abutment. What stress level is there in the frame if the gun is made to only gently kiss the frame slide stop abutment? Some gun designs use buffers to get closer to this but unfortunately there seems to be no room inside the Hi Std frame for them.

So far in decades of Hi Std use it has been found that the original 5.5 pound spring and Std Vel ammo (like CCI SV) can minimize fatigue damage to the steel in the frame. But regular replacement of the recoil spring is required. Some say a 10% reduction in the power of the spring is recoil spring change time. Others count rounds and say 5 to 10 thousand rounds is when the recoil spring needs changeout.

Stress riser or not, reduced stress can help to extend the life of these Hi Std frames.

LDBennett
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Old 07-23-2019, 09:20 AM
Gus1957

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Thank you all for the input. I've only had High Standards since last fall and am learning a lot both good and bad.
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Old 07-25-2019, 08:27 PM
octalman
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Gus,

I have owned several pistols purchased used. The worst was much rougher than your pictures appear. No problems. However, the roughness can be indicative of a dull tool or poor operator. No way to know if there was localized heat effect that could introduce stress. Although, not likely
the roughness should cause concern. Machining that left a sharp corner is a problem waiting to happen. Shoot and enjoy.
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Old 07-26-2019, 06:36 AM
Gus1957

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I'm wondering how the Connecticut plants ran their manufacturing facilities. These type of "rough" finishes remind me of rushing production - like piece work by machine tool operators, not fully qualified machinists. I went thru a Machinist Apprenticeship back in the 70's and we taught that the finish on the part was part of your personal trade mark. Of course there is always the balance of tolerance & surface finish. Never hurt to make the part look good, but we got paid by the hour.
Then in the 80's worked at the Watervliet Arsenal as a production machinist on the 105mm breech block line by old manual machine tools with no CNC at that time. Finish requirements were always met and we did need to make a quota each shift.
Manufacturing quality certainly has improved with CNC & Finishing tools.

I did get burned on a Victor that had cracks on both sides of frame with previous weld repairs & ejector brazed in place, but still shoots. For $250 it still has the value in balance of parts if frame totally fails.

I learned my lesson not to buy a HS without removing barrel & slide to view frame even a gun shows. If they don't allow it - just pass.
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Old 07-26-2019, 11:10 AM
SGVictor
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The original two pistols in this thread represent a spread in time of about 4 years and one is by High Standard Manufacturing Corporation Inc. in Hamden and the other by High Standard Inc. in East Hartford.

The earlier pistol was probably machined by hand by an operator that was paid on a percentage based on a piecework quota. The shop routing for the part defined the speeds and feeds and when an operator changed these to benefit his production quota, he was given a suspension. Obviously speeds and feeds are factors in the resulting finish as are a number of other factors.

At some point, the did introduce CNC milling for the frames but I do not have an exact date. I suspect is was not until after the move to East Hartford. I do have a picture of a frame on a CNC mill in the East Hartford plant.

Today we might consider their machining methods crude like drill jigs and gang drills but I worked in aerospace all my working life and in the mid 1960's we were still using that method on a lot of aerospace parts.

The surface finish is a consideration and I gave those factory earlier but it is the geometry f the part that has factors that are much greater. These stress concentration factors are of much greater magnitude than the finish factor. That and where the frame hardness actually is with respect to the specified hardness range and where the actual frame dimensions fall with in the tolerance band are also factors.
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Old 07-26-2019, 12:02 PM
Gus1957

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Thank you for the follow up. I would like to see a history book on High Standards that included the machining processes and gaging / overall quality control to go along with the models/years made.
High Standards certainly have earned a place in the shooting field.
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