[samstu]

Hi. I'm new to forum and recently purchased a used Tormach 1100 series 3 with a fixture plate installed. As part of my cleanup, I pulled fixture plate and found at least 20 round gouges in the bed of the mill. These all lined up with the holes of the surface plate.

I'm sure these were caused by the cupped end of the usual chinese strap clamp / work holding kits that we've all seen and probably used. In normal circumstances, a stud contacting or damaging the floor of a t-slot is not a big deal, but with a surface plate (at least the one on my Tormach) not so. The cupped studs, when tightened (perhaps incorrectly) mill a nice circle until the nut on the other end turns. It's even worse if the upper nut runs out of threads before strap is tight, as the turning of the stud against the table will jack the fixture plate off the table at best and at worse damage both.

My thoughts on correcting this issue are:

1. Grind all the studs in the clamping set flat so that sharp cutter shape of cup is reduced or removed.

2. Don't run studs all the way home, leave about 1 turn above table to allow some safe twist.

3. Machine flats into studs so they can be held while upper nut tighted.

4. Design fixture plate with threaded holes only over t slot, or with steel inserts on table side to take the wear. Maybe less flexible but much more idiot proof.

I will attempt to post some pics.

Maybe this has been well covered, but in my searches I didn't find anything about it. It's also a bit disappointing as these surface plates are marketed as a way to protect a mill. Any other thoughts on avoiding this issue, please let me know.

Sam

[Jim Dawson]

Sounds like user error to me. With some care the studs should not be allowed to screw down against the table.

I think any or all of your ideas above would be useful.

[kstrauss]

I believe that the usual preventative approach is to not tap the holes to full depth and/or damage the bottoms of the tapped holes so that the studs cannot be fully screwed in. For example, T-nut threads are often nicked on the bottom with a chisel. As Jim mentions, all of your suggested ideas should help prevent the problem in the future.

[jwatte]

Sorry to hear that!

I don't believe in 1. Pressure is pressure, and will deform a surface.

2 and 3 seem like the normal approaches. Generally, the bottom of the threads will be deformed to wedge studs that go too far down.

4 seems like the idiot-proof way :-)

[mountaindew]

I believe that the usual preventative approach is to not tap the holes to full depth and/or damage the bottoms of the tapped holes so that the studs cannot be fully screwed in. For example, T-nut threads are often nicked on the bottom with a chisel. As Jim mentions, all of your suggested ideas should help prevent the problem in the future.

I was going to say! Those big surface plates should have short threads not thru. I remember years ago someone on Youtube was complaining about tool quality and the t nuts for a 4th axis surface plate required tapping thru. I watched saying to myself there is a reason for this and be careful you can damage the 4th axis plate or other fixtures. I see the need for tapped thru holes in fixture plates, t-nuts , straps nuts , whatever, but the user needs to be careful and aware you can damage things with them and mark them if possible.

An example:
I make my own custom t-nuts and fixtures for work holding. These are small ones for the type of t-track sold these days. They were designed to be wide and spread out the clamp force on the thin t-track profile. The studs bottom out on threaded holes "not thru" it would be easy to damage track if they went thru. But I did included a hole for a location Pin and a thru hole for a small position locking screw. These t nuts can be locked into position on t track and fixture plates can be located on them and clamped over and over without changing or losing offsets. The studs and pins can also be removed and the nut left locked in position below work surface. Then days or weeks later I can replace pin, stud and fixture plate and i'm back setup for that project. Easy and very handy with the hundreds of work offsets that can be used and stored in PP "with descriptions"

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