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View Full Version : Bridgeport t-slots too snug



jmarkwolf
02-25-2013, 08:04 AM
I went to make my first chips on my new-to-me Bridgeport yesterday and found that my t-nuts don't fit the table slots the entire length of the table.

The t-nuts are from an inexpensive clamping kit I've used on my mill/drill for nearly 15 years.

I assumed that it was due to sloppy tolerance on the cheap import kit.

On further investigation I found that some of the t-nuts fit OK, but others didn't. The ones that don't fit tend to bind towards the center of the table, particularly on the center slot and slot closest to the column.

But then I found that my 0.625" guage pins don't fit the slots very well either. They'll tap fit at numerous places along the t-slot but only partially. They won't go to the bottom of the slot at any points along the slots. Don't know if this is typical.

The t-nuts that fit are 0.613" wide at the top, the t-nuts that don't fit are 0.616" wide.

Now I'm assuming the table slots have "mushroomed" from years of clamping forces.

Can anyone advise as to the nominal dimension of the typical t-nuts?

Should I consider "dressing" the table slot dimensions by taking a "skim" cut along the length of the slots, or modify the t-nut that don't fit?

Forrest Addy
02-25-2013, 08:25 AM
Chances are the bottom corners of the slot are deformed raising metal that interfers with passage of the T nuts. This is common in older machines and the cure is to dress the burrs with a flat file, Hold in the slot with the finger tips forcing metal removal at the burrs without more than incidental contact with the all-important slot faces.

Do not cut the T slots, If you do standard keys will not fit. You'll have to make new slot keys for every table gadget you get.

_Paul_
02-25-2013, 09:42 AM
Chances are the bottom corners of the slot are deformed raising metal that interfers with passage of the T nuts. This is common in older machines and the cure is to dress the burrs with a flat file, Hold in the slot with the finger tips forcing metal removal at the burrs without more than incidental contact with the all-important slot faces.

+1 on that

My old Taylor mill had a similar problem and on inspection it looked like some previous user had used hexagon bolts rather than Tee Nuts/Bolts which had damaged the underside of the slots I just filed them into the best shape I could.

Paul

polepenhollow
02-25-2013, 09:52 AM
I agree with filing the slots. That is common on older machines. My bridgeport was used with forged "T" bolts (not a T nut set) before I got it. The radiused heads on the bolts really messed up the T slots. An 8" medium file held in the slots and pressed pressed parallel to the existing table t slot wall. The slots would remain close to size and vertical, not radiused on the bottom and sloppy.

outlawspeeder
02-25-2013, 10:05 AM
+2 Just got a set of new "T" nuts three spots on the table would not allow them to pass. The "T" nut would rock on the bur. Clean the slot. Get the corners in the T slot if that doesn't get it grab a file, lay if flat and feel for the rocking. File, clean the file check, repeat until it slides.
PS The old set that came with it past these spots.

Jaakko Fagerlund
02-25-2013, 10:45 AM
Do not cut the T slots, If you do standard keys will not fit. You'll have to make new slot keys for every table gadget you get.
It is a nice feature when the T-slots are certain width and parallel to the X axis motion, lets you put key stock up in the slots to act as a back guide for a plate etc.

But, when I'm making fixtures that take their direction from the T-slots, I just put two round dowel pins in the bottom of the fixture. These dowel pins are 2 mm smaller in diameter than the T-slot widtht, so it is easy to install: Just drop it in, push it against the T-slot side and clamp it. Also makes it possible to use on other machines that have different T-slot widths :)

jmarkwolf
02-25-2013, 01:58 PM
Chances are the bottom corners of the slot are deformed raising metal that interfers with passage of the T nuts. This is common in older machines and the cure is to dress the burrs with a flat file, Hold in the slot with the finger tips forcing metal removal at the burrs without more than incidental contact with the all-important slot faces.

Do not cut the T slots, If you do standard keys will not fit. You'll have to make new slot keys for every table gadget you get.

Kinda confused.

If a standard 0.625" guage pin won't drop into the slot, then are "standard keys" likely to fit?

And why not take a skim cut with a side-cutting end mill rather than a hand file? Seems this would make for a precise fit.

Ron of Va
02-25-2013, 07:46 PM
Been there done that. Only one adjustment to the milling machine is necessary.

Take your best fitting T-nut and elevate in the mill vise to get to the BACK part of the T using a parallel.
Adjust your endmill to just barely kiss the side and bottom of the back of the good T-nut.

Now go through the rest of the T-nuts and skim off the back and bottom of each one. Flip it around after each pass and kiss the other side. When you are finished all of them will fit perfectly.

Do NOT try to do more than one at a time because they are not perfect and will not clamp the same, and one will definitely pop out.

If you have any binding after this milling operation, then lay them on their sides and do the up side. Flip and repeat.

Davo J
02-25-2013, 09:42 PM
And why not take a skim cut with a side-cutting end mill rather than a hand file? Seems this would make for a precise fit.

I agree with you, as long as your only skimming the bottom part of the T slot thats not true it would have to be more precise than a file. I would still use a file to knock off the sharp edge after milling it though. Just a light touch should do it.

Dave

Ohio Mike
02-25-2013, 10:48 PM
To me milling them out is like using a sledge hammer to swish ants. Filing is very appropriate here in my opinion. For all but the worst cases a light filing will correct the issue. There will likely be a couple of trouble spots that are much worse.

lakeside53
02-25-2013, 11:45 PM
Skim cutting also assume your gibs fit nicely all the way from end to end. In reality, they are probably are looser in the midddle than at the ends... Also, you'll have to do it in two passes and move the ram between, and tram each time.

I'd just fix the tiny problem areas.... with a file.

oldtiffie
02-25-2013, 11:46 PM
Some people use the t-slots as gauges to insert the keys from a rotary table or a vise to align them with the axix of the table.

That may or may not be the case to a new or little worn machine but not necessarily the case for tee-slots that have been "mushroomed" and have only had the burrs at the bottom of the sides of the "over-hang" removed with a file or what-ever.

I took all the alignment keys off my new vises and rotary tables and ditched (aka "binned") them long ago and have never missed them. FVices, rotary table and jobs are individually aligned as part of the job set-up.

Some use the sides of the tee-slot in conjunction with a tightening plate and screw as the sided of the tee-slots are assumed to be vertical and true - that may not be the case if the tee-slots have been deformed.

If the undersides of the tee-slot over-hangs have been distorted then the top faces of the tee-nut side projections may also be deformed and will have been compressed and only bearing on the inner edges of the tee-slot over-hang.

It is worthwhile once in a while to "true-up" the bearing faces of tee-nuts.

Other than using a woodruff key cutter or similar there is not much you can do about the distorted undersides of the slot over-hang.

Here are some pics of my badly machined tilting tables:

http://i200.photobucket.com/albums/aa294/oldtiffie/Tilt_table/Tilt_table7.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Tilt_table/Tilt_table8.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Tilt_table/Tilt_table6.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Tilt_table/Tilt_table6.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Tilt_table/Tilt_table4.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Tilt_table/Tilt_table9.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Tilt_table/Tilt_table1.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Tilt_table/Tilt_table11.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Tilt_table/Tilt_table12.jpg

There was an error of aboput 1.0mm (~0.040"_ between top and bottom and corner to corner (and some others too) when I started but all faces and edges are with 0.01mm (~ 0.0004" - "4 tenths") when I'd finished.

I am not at all sure that there is a reasonably "tight" spec for the sides of the table as regards being parallel to the "C" table axis dove-tails either.

There is too much "presumed" (which is bad enough) but "assumed" (which is worse) here without checking first if you are going to rely on it as a reference.

Same goes for rear jaw verticality and flatness and paralellsm of the top face (where jobs and parallel strips use as references) of a vice.

Same applies to shapers and surface/T&C grinders.

I can't compell anyone else to carry out these and similar checks periodically - and I am not going to try - but I check and where necessary retcify any significat errors in my machines, vices, magnetic chucks etc. or whaere I can't or haven't got the time to do it, I compensate for it.

I am only concerned about the machines etc. being able to keep to what ever tolerance/s I am using. "Finer" errors don't really matter.

Hence why amongst other things I work to as wide ("rough"??) a tolerance as I can so as the job works as intended.

There is a lot less worry and concern here then.

wbleeker
02-26-2013, 12:55 AM
I am not sure that the Bridgeport will even have enough X travel to mill the slots out in one setting, clean them up with a file as Forrest said, and if your t nuts still don't fit mill them to size. You don't show a location, some of these machines were made under licence by Adcock and Shipley and the one I had the T Slots were about .025" narrow.
Will

oldtiffie
02-26-2013, 01:34 AM
+1

Table travel (and tee slot length) are less the the length of the table.

Perhaps the best way is to mill the slots on a larger/longer mill or on a planer.

I'd leave it alone and "work around" what ever few real problems might arise with the tee-slots.

jmarkwolf
02-26-2013, 07:25 AM
Update:

Turns out most of the problem was "hardened caked grime" on the side walls of the T-slot. Under good light I was able to see it.

Filing it, understandably had been largely ineffective, so I took after it with a paint scraper, then scotchbrite. My T-nuts now slide the entire length of the slots.

I can still feel the t-nuts "try" to bind a little in some places. I think there is still some residual "hard cake". Next, I think I'll try little wire wheels on my Dremel, otherwise it will probably accumulate again, or swell the next time it gets wet with cutting fluid.

Thanks for all the responses guys, and yes, skim cuts would have been the sledge hammer approach to what turned out to be a "hygiene problem"!

Joe Rogers
02-26-2013, 09:08 AM
Don't they make t slot scrapers? One came with my clamping set bought cheaply from Enco.
Joe

jmarkwolf
02-26-2013, 11:06 AM
Yes, I tried a t-slot scraper first.

I find that they work for clearing chips out of the slots, not for cleaning cryptonite welded to the sides of the slots! :p

Mcgyver
02-26-2013, 11:12 AM
I find that they work for clearing chips out of the slots, not for cleaning cryptonite welded to the sides of the slots! :p

must have been some cryptonite, not even a file would touch it! This is a pretty funny thread - glad you didn't mill anything....and you're a good man for coming 'clean' on the real problem :D

outlawspeeder
02-26-2013, 11:15 AM
what turned out to be a "hygiene problem"!

What have you been doing to that mill. Brings a new meaning to tool pron. hahaaha

Glad you took an onther look.

jmarkwolf
02-26-2013, 12:32 PM
What have you been doing to that mill. Brings a new meaning to tool pron. hahaaha

Glad you took an onther look.

Haven't been doing anything to it yet, except clean it!

All I've done so far is buy it! ;)