Bridgeport or Knee Mill, VMC, Surface Grinder Owners.

[Provincial]

The short answer is yes. I did a test and got a repeatable result. The table did deform in the area near the tee nut when the vise was clamped down. However, it was very slight, only .0001" (1/10,000") and the deformation went away when the nut was loosened up again. The vise in relation to the spindle did not move in the Z axis at all. So in a broader picture, the error is negligible and doesn't appear to be permanent, at least in my case.
How I did the test:
My mill is a RB-1 (Aka, First), basically a Bridgeport hybrid in a bed mill configuration, instead of a knee, It was manufactured in 1987 by Long Chang Machinery, Taiwan, It is very well made machine with Meehanite castings and chromed ways. It is in excellent condition.

After removing the vise I stoned and cleaned the table, I checked the table for any existing warpage. I used a 24" Starrett straight edge, I know its not a "scraped-in camelback" but I checked both edges on my granite surface plate, it was very flat not rocking or pivoting at any point, I could not get a .0005" shim under it at all. I got the same result on the mill's table, no discernible humps or high spots, the shim could not get under it at any point except in the flaking divots. When I deliberately put the shim under the rule it would distinctly pivot on spot. I moved the rule a few inches at a time, end to end and across. So I can safely say the table is flat to less than a half a thou in any 24"
View attachment 441870View attachment 441879
Next, I checked the way travel by setting a tenths reading indicator on a magnetic base set on the bed and swept the movable length of the table, there was a slight rise at the far limits of travel about .0013" with the table moved all the way to the right and about .001" when moved all the way to the left. This is expected as the weight of the table hanging off one end causes the opposite end to rise slightly, this error only appears in the last few inches of travel. For most of the travel, the table showed no average change. Note, the leftwards travel is limited due to the X-axis power drive unit, that's why its error was slightly less.
View attachment 441887View attachment 441893View attachment 441872
Next I mounted the vice back on, the center of the table first stoning and cleaning the bottom and then tramming it in. Both a vertical and horizontal sweep of the fixed jaw showed almost no movement with a Starrett last word test indicator .0005" res. Then I loosened stud nuts and set the indicator on the top center of the find jaw and retightened the nuts nuts, no visible movement on the indicator. Then I thought the quill's DRO has a 1μ .00005 (5/100,000") resolution, So I made a probe out of an inverted end mill and a ballbearing set in the center recess, I chucked it in the spindle and rested it on the top of the vise inline with the bolts. I put a bungee cord on the quill lever to put constant pressure I tightened the bolts and I got about .0001" rise. However on closer inspection I realized the ball bearing had made a small dent in the top of the vise and when I tightened the vise there was a slight movement on the Y axis causing the ball to ride up on the side of the dent and give the false reading. I then stoned the top of the vise and placed a square of carbide for the ball to rest on. Subsequent tests could not show any movement in the Z axis.
View attachment 441874View attachment 441871View attachment 441880
Next I checked the table around the area of the vice hold down nuts, with the probe, I got a .00005" rise but the diameter of the quill prevented me from getting in close and was in the way of the wrench. So I switched to tenths reading indicators, placing one as close to the nut as possible and one a few inches away as a control. Tightening the nuts showed a .0001" rise on the close indicator and no change on the far indicator. I repeated the test several times with and with out the probe on the vice and the nuts on both sides of the vice. All results were consistent, a one ten-thousandth bulge/distortion in the table surface at the immediate area around the nut, but no change in the vice itself.
View attachment 441878View attachment 441882View attachment 441885View attachment 441888
Given these results, I did not proceed to make larger tee nuts to see if the they would "help" as the distortion was so insignificant, went away when the bolt pressure was relived and didn't affect the vise in any way. I don't think it's an issue for me. Of course these are my results, on one machine. IMHO The theory is still viable as a different, weaker table could possibly distort more and even retain some of that distortion. Anyone concerned should conduct their own test.

Good test!

The one thing missing is the torque applied to the hold-down hardware. I see you used what appears to be a double box end wrench. That is a way of limiting torque, as the length of the wrench limits the torque applied.

I suspect that longer wrenches are often used, with resulting higher torque applied.

Since the most common hold-down hardware is 1/2-13 NC, I checked a Case construction equipment service manual chart for recommended torques with lightly oiled threads:

Grade 5: 80-96 ft.-lbs.
Grade 8: 110-132 ft.-lbs.

I think the Grade 5 figure is more appropriate, and probably good for clamping steel, rather than cast iron. Since the mill table receives repeated cycles of compression, it makes sense to lower the fastener torque. I'd suggest looking for torque specifications for 1/2" NC cylinder head bolts on gas engines.

 

[Charles scozzari]

The short answer is yes. I did a test and got a repeatable result. The table did deform in the area near the tee nut when the vise was clamped down. However, it was very slight, only .0001" (1/10,000") and the deformation went away when the nut was loosened up again. The vise in relation to the spindle did not move in the Z axis at all. So in a broader picture, the error is negligible and doesn't appear to be permanent, at least in my case.
How I did the test:
My mill is a RB-1 (Aka, First), basically a Bridgeport hybrid in a bed mill configuration, instead of a knee, It was manufactured in 1987 by Long Chang Machinery, Taiwan, It is very well made machine with Meehanite castings and chromed ways. It is in excellent condition.

After removing the vise I stoned and cleaned the table, I checked the table for any existing warpage. I used a 24" Starrett straight edge, I know its not a "scraped-in camelback" but I checked both edges on my granite surface plate, it was very flat not rocking or pivoting at any point, I could not get a .0005" shim under it at all. I got the same result on the mill's table, no discernible humps or high spots, the shim could not get under it at any point except in the flaking divots. When I deliberately put the shim under the rule it would distinctly pivot on spot. I moved the rule a few inches at a time, end to end and across. So I can safely say the table is flat to less than a half a thou in any 24"
View attachment 441870View attachment 441879
Next, I checked the way travel by setting a tenths reading indicator on a magnetic base set on the bed and swept the movable length of the table, there was a slight rise at the far limits of travel about .0013" with the table moved all the way to the right and about .001" when moved all the way to the left. This is expected as the weight of the table hanging off one end causes the opposite end to rise slightly, this error only appears in the last few inches of travel. For most of the travel, the table showed no average change. Note, the leftwards travel is limited due to the X-axis power drive unit, that's why its error was slightly less.
View attachment 441887View attachment 441893View attachment 441872
Next I mounted the vice back on, the center of the table first stoning and cleaning the bottom and then tramming it in. Both a vertical and horizontal sweep of the fixed jaw showed almost no movement with a Starrett last word test indicator .0005" res. Then I loosened stud nuts and set the indicator on the top center of the find jaw and retightened the nuts nuts, no visible movement on the indicator. Then I thought the quill's DRO has a 1μ .00005 (5/100,000") resolution, So I made a probe out of an inverted end mill and a ballbearing set in the center recess, I chucked it in the spindle and rested it on the top of the vise inline with the bolts. I put a bungee cord on the quill lever to put constant pressure I tightened the bolts and I got about .0001" rise. However on closer inspection I realized the ball bearing had made a small dent in the top of the vise and when I tightened the vise there was a slight movement on the Y axis causing the ball to ride up on the side of the dent and give the false reading. I then stoned the top of the vise and placed a square of carbide for the ball to rest on. Subsequent tests could not show any movement in the Z axis.
View attachment 441874View attachment 441871View attachment 441880
Next I checked the table around the area of the vice hold down nuts, with the probe, I got a .00005" rise but the diameter of the quill prevented me from getting in close and was in the way of the wrench. So I switched to tenths reading indicators, placing one as close to the nut as possible and one a few inches away as a control. Tightening the nuts showed a .0001" rise on the close indicator and no change on the far indicator. I repeated the test several times with and with out the probe on the vice and the nuts on both sides of the vice. All results were consistent, a one ten-thousandth bulge/distortion in the table surface at the immediate area around the nut, but no change in the vice itself.
View attachment 441878View attachment 441882View attachment 441885View attachment 441888
Given these results, I did not proceed to make larger tee nuts to see if the they would "help" as the distortion was so insignificant, went away when the bolt pressure was relived and didn't affect the vise in any way. I don't think it's an issue for me. Of course these are my results, on one machine. IMHO The theory is still viable as a different, weaker table could possibly distort more and even retain some of that distortion. Anyone concerned should conduct their own test.

Thanks for taking the time to conduct such a compelling test. To me, it shows that common sense prevails.

 

[Charles scozzari]

First, understand this is not intended at/to ANYONE. Some times a problem is just overthought and balloons out of control. I try to remember to use the KISS solution, ( Keep It Simple Stupid. ) Thanks, Charlie.

 

[Eddyde]

Good test!

The one thing missing is the torque applied to the hold-down hardware. I see you used what appears to be a double box end wrench. That is a way of limiting torque, as the length of the wrench limits the torque applied.

I suspect that longer wrenches are often used, with resulting higher torque applied.

Since the most common hold-down hardware is 1/2-13 NC, I checked a Case construction equipment service manual chart for recommended torques with lightly oiled threads:

Grade 5: 80-96 ft.-lbs.
Grade 8: 110-132 ft.-lbs.

I think the Grade 5 figure is more appropriate, and probably good for clamping steel, rather than cast iron. Since the mill table receives repeated cycles of compression, it makes sense to lower the fastener torque. I'd suggest looking for torque specifications for 1/2" NC cylinder head bolts on gas engines.

Yes, I should have mentioned, I couldn't find my torque wrench, it may have been loaned out some time ago... I did put an even pull/push on the box-end wrench, even going a bit beyond what I would normally give those nuts, I also tried a breaker bar with a socket but I felt I could have easily striped or snapped the nut or stud. I've learned not to gorilla threaded fasteners...

 

[Richard King 2]

You coming to be a quest in the April Class? it's a weekend class. You can come down, no charge remember...

 

[ddickey]

Rich,.I'm in an outage right now until the first part if May. Working 60 hour weeks.
It would hard but I'll see if I can get down there fir a couple days. For sure later in the summer though.

 

[Charles scozzari]

I can't help myself. I know I said I was out, but I would like your opinion on this. My 1966 42" Bridgeport mill had a original Bridgeport table power feed. It hung there for the better part of 35 years before I purchased the mill and changed to the new style. That particular power feed was heavy enough ( I believe somewhere around 50 lb's ) to almost wipe the right side knee scraping marks clean off, meaning almost none of the scraping is visible while the left side is in very good shape with much of the original scraping marks still there. Before this conversation I attributed that to the weight of the power feed hanging from the right side of the table. To me, that would also be responsible for some degree of table bow. What are your thoughts on that.? I would think the tables you are going to use to demonstrate your cause are not in good shape to begin with.

 

[Provincial]

Here is an off-the-wall thought:

If tables are bowed low on the ends, and the center section has more wear and is higher, does the top of the table stay the same distance from the cutter as the table moves on the X-axis?

 

[Richard King 2]

I can't help myself. I know I said I was out, but I would like your opinion on this. My 1966 42" Bridgeport mill had a original Bridgeport table power feed. It hung there for the better part of 35 years before I purchased the mill and changed to the new style. That particular power feed was heavy enough ( I believe somewhere around 50 lb's ) to almost wipe the right side knee scraping marks clean off, meaning almost none of the scraping is visible while the left side is in very good shape with much of the original scraping marks still there. Before this conversation I attributed that to the weight of the power feed hanging from the right side of the table. To me, that would also be responsible for some degree of table bow. What are your thoughts on that.? I would think the tables you are going to use to demonstrate your cause are not in good shape to begin with.

That is a contributing factor. I agree with you but the T-slots are a factor too. (up early, can't sleep in hotel rooms worth a hoot/)

 

[Richard King 2]

Here is an off-the-wall thought:

If tables are bowed low on the ends, and the center section has more wear and is higher, does the top of the table stay the same distance from the cutter as the table moves on the X-axis?

It depends - Think about how a cylindrical grinder works. The part your grinding is round and as it turns it hits one spot on grinding wheel and it grinds round. A mill is a single point cutter and if you table is bowed .008" in 42" and you have a short part in a 8" vise, you will never have an issue. It you clamp a long part on the table it will bend - Here is a You Tube show of a guy planning a Bridgeport table. Listen to the cutter . It is only hitting in the center. That table is bowed up in the middle.