Concentricity of gun drilling
Concentricity of gun drilling
(OP)
I am machining a 16" cannulated shaft (.75 OD). There are 2 IDs which are gundrilled separate from each side and meet in the middle. I am having issues with the holes lining up, concentrically, where they meet in the middle. I am allowed a .006" tolerance on concentricity. Our machines are capable of making good parts, but about half appear to be out of tolerance (visual check looking down cannulation, at the shoulder).
Is there a method to measure this concentricity? I know I can make gages, but I don't have alot of time.
Thanks in advance for any guidance you can offer :)
Is there a method to measure this concentricity? I know I can make gages, but I don't have alot of time.
Thanks in advance for any guidance you can offer :)





RE: Concentricity of gun drilling
I ask because the word concentric in regular conversation is a little different than the GD&T definition, in terms of measurement.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Concentricity of gun drilling
They do have a .006 run-out tolerance from the smaller ID to the larger ID (the larger ID is the datum referenced)
Thanks for asking for clarification; in a nut shell, I need the two ID's to line up with in .006" when they meet in the middle.
RE: Concentricity of gun drilling
Powerhound, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Concentricity of gun drilling
RE: Concentricity of gun drilling
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Concentricity of gun drilling
Now, if you just want to make sure that both holes on each end to match in the center with a difference of no more than .006, then maybe a straightness tolerance of .006 beyond the MMC and make both IDs at the smallest allowable size. Now you can make a checking fixture which should be straight pin with a size that is .006" smaller than the smallest allowable size. Push the checking pin in one end of the hole and, hopefully, through to the other end. If it goes, then the step is less than .006. If it doesn't go all the way, then the step is too large or the hole is not straight.
Dave D.
www.qmsi.ca
RE: Concentricity of gun drilling
Powerhound, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Concentricity of gun drilling
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Concentricity of gun drilling
My other comment would be perhaps a change in process. Depending on the difference in diameters, could you gun drill the small hole all the way through, turn the O.D. on centers to true up the outside, then drill/ream the larger O.D. hole with standard tooling. The larger I.D. tooling should follow the gun drilled hole and you'll avoid a lot of the mismatch.
RE: Concentricity of gun drilling
As I understand the question, there is nothing about 2 different size holes drilled from each end but the process had each hole (probably the same size) drilled at opposite ends. Apparently, there is a mismatch in the centre. That is how I read it anyway. I don't believe there is anything in the original statement relating to the 0.750 OD except to state that it is 16 inches. Concentricity and positional require datums and it seems that we don't have any. Badger2011, we need more clarification. There are a lot of knowledgeable people here who can help. Are you only looking at the mismatch on the ID? Are you interested in the relationship of the ID to the OD (concentricity, circular or total runout, positional)? Clarification please.
Dave D.
www.qmsi.ca
RE: Concentricity of gun drilling
^^^This is all true^^^
Clarification: it is a .006 total run-out, not .012. My picture was incorrect, my apologies.
The 2 sizes on the ID, which meet in the middle, are Ø.375 and Ø.438.
My machinist wants to order a longer gun drill to drill the Ø.375 hole 9" deep, then follow it with the larger Ø.438 gun drill. This will ensure the holes are concentric with one another, BUT there may be a mismatch when we enter the opposite side and gun drill back towards the stepped hole with the Ø.375 gun drill.
Also, I've machined a functional gauge to check this concentricity run-out of .006 so we can check the parts coming off the machine.
My remaining question for all you, and thank you again for you help already, is: Is there any machining techniques that might further help us make good parts? We are running these on swiss machines.
Thank you!
RE: Concentricity of gun drilling
RE: Concentricity of gun drilling
Powerhound, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Concentricity of gun drilling
Twist drills (the kind with two flutes) are available in common sizes in lengths up to 12". They are called 'aircraft drills'. They are not gundrills, and using them is not gundrilling, it's just deep drilling.
Gundrill bits have only one flute, so they don't have a point angle in the usual sense. They are guided by the diameter of the hole already drilled, which also means that the hole must be started with a twist drill before the gundrill is inserted.
So, we have to ask for clarification of the OP: Are you just drilling a deep hole with extra long twist drills, or are you actually using gundrills?
Mike Halloran
Pembroke Pines, FL, USA