Bridgeport or Knee Mill, VMC, Surface Grinder Owners.

As it says in the "How to run a lathe book"
"many mechanics have a false sense of how tight is tight enough" or something like that
I think overtightening is common
Never buy a mill from a guy with arms bigger than your leg
 
Charles scozzari said:
Hi, is your table warped, and have you checked it ?. And yes, I have seen guys tighten clamps like they were gonna hold the crack of day light. This I would think is a thing related to Bridgeports and I bet nobody knew or cared what effect it had on the work in a production shop. I would love to know the condition of the table of a bench top mill. Charlie.
Click to expand...
I don't have a straight edge precise enough to check this. I have a Bridgeport clone (Ram) that has had very little use, and only five operators in it's life. I don't think it is warped. I have a Wells Index 847 that seems to be quite straight, despite apparently having been in a production shop long enough to wear out the center part of the x-axis screw. My third mill is a Bridgeport Series 1 that had been in a Job Shop, rode hard and put away wet. I did get the motor running and all moving parts operating properly. The right end of the X-axis screw is broken off and missing, so I'll have to fix that. The ways are quite worn, so table warpage is the least of my worries for this machine. Since it was free, I'm not about to complain.
 
One of the machinists at my first job (early 1970's) made good money betting that he could snap the fixed jaw off a 12" Craftsman adjustable. He was a very strong and sturdy guy, and he would do this by fitting the wrench on a 15/16" hex fastener and jerking on the wrench by throwing his body into the direction he was pulling. It popped the jaw off the wrench every time!

I suspected, but never proved, that he cheated by adjusting the wrench a little loose, so that the flats of the bolt acted like a ramp or wedge, multiplying the force trying to expand the jaws.
 
I think optimum clamping force could be calculated with some uncertainty. Off the top of my head, Maximum cutting force from spindle hp, RPM and cutter diameter. Assuming a vise with 2 hold down bolts, worst case will probably be with the cutter nearly above one vise bolt. Cutting force and the friction between CI table and vise gives the required clamping force. Clamping force (induced bolt tension) from bolt size and torque.

For practical experiment, how about clamping down the vise with a torque wrench, then take a heavier than normal cut. Put use a dial indicator on the vise to look for a small amount of movement. Repeat with less bolt torque until you see a small movement near the bolt. You would probably want to use more torque in practice but not 10x more.
 
At one time, we worked in an automotive machine shop, and one of our jobs was straightening crank and cam shafts.

We used a chisel and ball pien hammer, the chisel was placed at the low spot and tapped.

This relieved stresses, and the shaft would come TOWARDS the chisel.

I can see this here, the nut is pulled against the bottom of the T slot, and neither is the perfect fit, so there could be some point force that could cause some stress change that could cause that area of metal to grow resulting in the table changing shape.

Goes back to how much torque causes this?

Richard indicates he will have a table to work with.

I have no doubt of his statements as we used a related process to do something else.

The question is, at what point does it matter?

If 40 ft pounds is enough to secure the work, and 90 ft pounds is where something can be observed, then 60 ft pounds should he a safe place.


Sent from my SM-G781V using Tapatalk
 
wow, I'm new and learned something already. thank you
Is this something that should get fix if you buy a used mill ?
or just an FYI ?
 
I have been using 6061 -T6 aluminum for my 5" bridgeport vise on a clausing mill. I have indexing blocks, and the aluminum seems to lock into the tslot pretty good, so I don't need to gronk on it. I usually need to play with the tnut to get it to release. So I think the hard cast iron and softer aluminum lock it in pretty good. I wonder if that would also negate the situation. Can't tell as who knows how many owners and what type of work this mill did long ago.
 
People overtightening fasteners drives me nuts (pun intended). The point made well by others is that threaded fasteners hold - they really don't need to be refe'd down hard. I have made T-bolts for most of my vices - then it is easy to relieve the center portion of the head near the bolt shaft. I have also made T-nuts for each of my machines that: A. fit nicely, which distributes the load better (the standard T-nuts are very loosey goosey, every table is a bit different) and B. I make the web portion as tall as possible so there is some decent thread depth (when using standard bolts to secure that special component it is handy to have several nuts of different sizes and as long a female thread as possible so you don't have to cut the bolt to just the right length). Of course, the nut is not threaded quite through, so as to prevent "jacking" the table slots and breaking out a section. The taller web and better fitting nut/bolt will mitigate some of the coning effect that Richard is talking about. The nuts I have been making are about 50% longer than the standard nuts in a clamp kit. I can agree that 3" long would be even better, but even 50% longer is still a significant reduction in point loading.

Another of my huge pet peaves is when someone just grinds down a hex head bolt, or a carriage bolt and claims that is fine for a T-slot :mad:.
 
tq60 said:
At one time, we worked in an automotive machine shop, and one of our jobs was straightening crank and cam shafts.

We used a chisel and ball pien hammer, the chisel was placed at the low spot and tapped.

This relieved stresses, and the shaft would come TOWARDS the chisel.
Click to expand...
I have heard about this technique and tried it a little without much success, I guess like anything there is a learning curve. My question is if you had a long gradually bent shaft would you work on multiple points to get it straight or concentrate on one area? The reason I ask is I wonder if moving the vise to multiple spots on the table and overtightening at each spot might actually make things worse than leaving it at the same spot.

Provincial said:
There are ways to spread out this force. More fasteners, toe clamps, blocking to buttress the work (or vise) against the force applied, and other solutions exist. The point being, don't just accept that the two points built into the vise design are the only way to secure it against cutting forces.
Click to expand...
This is clearly key, each bolt will having certain amount of frictional clamping force, more bolts = more total clamping. The force from each bolt needs to be spread out over enough area to not damage the table but this will not have a significant effect on frictional clamping force (area is not in the basic equation for friction). More contact area by adding relief at the center should reduce damage to the surface but there is only so much that can be done given space constraints.
 
woodchucker said:
I have been using 6061 -T6 aluminum for my 5" bridgeport vise on a clausing mill. I have indexing blocks, and the aluminum seems to lock into the tslot pretty good, so I don't need to gronk on it. I usually need to play with the tnut to get it to release. So I think the hard cast iron and softer aluminum lock it in pretty good. I wonder if that would also negate the situation. Can't tell as who knows how many owners and what type of work this mill did long ago.
Click to expand...
After all my B.S. about there being no solution for tightening a vise is the reason for mill table warping I see now what I feel is a very possible cure that was present all the time. As woodchuck stated it's the keyed slot under the vise that IS the solution. Now I understand ( I think / hope ) what duty it plays in mounting a vise. Placing the vise on the table and setting the keys into the table slot keep the vise from rotating, and makes the tightening of the bolts down to I would think just over snug, say 50/60 foot pounds depending on the job and mentioned before in a reply, may relieve the stress on the table and may just be the cause of the warp. On my Bridgeport vise the key slot is on both the swivel base and the vise. Thanks, Charlie.
 
You must log in or register to reply here.
Back
Top