Jlel12
Plastic
- Joined
- Dec 28, 2009
- Location
- Boston, MA, USA
Hi all,
My office has run into issues with bores on parts coming in under/oversized, and have decided to increase our in-house inspection capacity. I've been tasked with sourcing the equipment, and I'm currently trying to parse through the different options available to us - I was hoping to get some feedback and suggestions from the experts.
Here's our situation:
- We're a small engineering firm, and all of our parts are made by outside shops. We have some people with varying degrees of machining/metrology experience on-staff, but no dedicated machinists/QC folks, so we can't necessarily trust that the use of the tool we get will have the accumulated experience and "feel" of a professional machinist.
- Bores in our parts fall in one of three ranges: small (30-60 mm), medium (80-150 mm) and huge (around 250 mm - only a few parts this size, and all within a fairly close range). The aspect ratio of these bores is always less than 1, and is typically very small (shallow bearing recesses, for example). Finest tolerances that we call out are +0/-0.02 mm (basically +0/-.0007"). Walls around bores are rarely thinner than 2.5 - 3 mm.
- The purpose of these tools is a) to have a leg to stand on when we argue with our machinists that they've goofed ("I checked this +0/-.02 bore with my dial calipers" really doesn't hold water), and b) be able to determine, with reasonable certainty, whether bores are comparable across lots of parts. We don't want - and don't need - to spend a gazillion dollars to get a complete set of equipment.
The tools I'm looking at include (images in titles, for clarity):
- Telescoping spring gages: I personally have always been nervous about using telescoping gages (although that may be because I goofed while using them my freshman year of college, and blew a 3" tap). However, I've only had crappy gauges historically, and the surface finish on most of our parts is quite good (we call out 1.6 microinches on most parts). We have OD mikes that are good enough to measure these, and can get length standards.
- Internal calipers: I'm concerned that these will suffer the same issues as measuring with regular calipers - compliant arms, heat sensitive, prone to cosine error (I think it's a cosine error, at least - if the measurement isnt taken perpendicular to the bore axis). Not a fan, but can be convinced.
- Rod-type & tubular inside micrometers: Better than the calipers, but I'm still concerned about them not being perpendicular to the hole axis.
- Bore gage set: I've seen some of these with supposed "self-centering" wheels on one side, but I'm a little skeptical. The price point is attractive, though.
- Three-point bore micrometers (120-120-120): These seem like the best option, especially since they claim to be self-centering and self-aligning. Conveniently, they're also the most expensive.
Options beyond these are going to be excessive (i.e. Etalon 135-135-90 bore mikes, differential pressure gages, etc.). Regarding what sorts of errors we see/are likely to see on our bores (ovaline vs. 3-lobed) - I unfortunately don't have a good sense. It's about a 50-50 split between turned and milled parts (which I assume are interpolated and then finish-bored), for what that's worth. Let me know if there's any additional information I can provide to help clarify my situation.
Finally, apologies if I've goofed on terminology. I've read through a whole host of threads on this subject, and all the different names are starting to run together at this point...
My office has run into issues with bores on parts coming in under/oversized, and have decided to increase our in-house inspection capacity. I've been tasked with sourcing the equipment, and I'm currently trying to parse through the different options available to us - I was hoping to get some feedback and suggestions from the experts.
Here's our situation:
- We're a small engineering firm, and all of our parts are made by outside shops. We have some people with varying degrees of machining/metrology experience on-staff, but no dedicated machinists/QC folks, so we can't necessarily trust that the use of the tool we get will have the accumulated experience and "feel" of a professional machinist.
- Bores in our parts fall in one of three ranges: small (30-60 mm), medium (80-150 mm) and huge (around 250 mm - only a few parts this size, and all within a fairly close range). The aspect ratio of these bores is always less than 1, and is typically very small (shallow bearing recesses, for example). Finest tolerances that we call out are +0/-0.02 mm (basically +0/-.0007"). Walls around bores are rarely thinner than 2.5 - 3 mm.
- The purpose of these tools is a) to have a leg to stand on when we argue with our machinists that they've goofed ("I checked this +0/-.02 bore with my dial calipers" really doesn't hold water), and b) be able to determine, with reasonable certainty, whether bores are comparable across lots of parts. We don't want - and don't need - to spend a gazillion dollars to get a complete set of equipment.
The tools I'm looking at include (images in titles, for clarity):
- Telescoping spring gages: I personally have always been nervous about using telescoping gages (although that may be because I goofed while using them my freshman year of college, and blew a 3" tap). However, I've only had crappy gauges historically, and the surface finish on most of our parts is quite good (we call out 1.6 microinches on most parts). We have OD mikes that are good enough to measure these, and can get length standards.
- Internal calipers: I'm concerned that these will suffer the same issues as measuring with regular calipers - compliant arms, heat sensitive, prone to cosine error (I think it's a cosine error, at least - if the measurement isnt taken perpendicular to the bore axis). Not a fan, but can be convinced.
- Rod-type & tubular inside micrometers: Better than the calipers, but I'm still concerned about them not being perpendicular to the hole axis.
- Bore gage set: I've seen some of these with supposed "self-centering" wheels on one side, but I'm a little skeptical. The price point is attractive, though.
- Three-point bore micrometers (120-120-120): These seem like the best option, especially since they claim to be self-centering and self-aligning. Conveniently, they're also the most expensive.
Options beyond these are going to be excessive (i.e. Etalon 135-135-90 bore mikes, differential pressure gages, etc.). Regarding what sorts of errors we see/are likely to see on our bores (ovaline vs. 3-lobed) - I unfortunately don't have a good sense. It's about a 50-50 split between turned and milled parts (which I assume are interpolated and then finish-bored), for what that's worth. Let me know if there's any additional information I can provide to help clarify my situation.
Finally, apologies if I've goofed on terminology. I've read through a whole host of threads on this subject, and all the different names are starting to run together at this point...