I stripped the T-nuts on my PM-1030V compound

[erikmannie]

I saw this coming because one of these bolts was almost entirely stripped last week; I have always had a habit of overtightening hardware. In machining school, they said "Tight is good. Too tight is f***ed".

Today I was facing some hard steel (ER70S-6, only a .005" DOC) and the freshly sharpened 3/8" HSS facing tool was pushed back by the workpiece. This in turn rotated the AXA toolpost. The toolpost was rigidly mounted to the compound so the entire compound assembly rotated on the cross slide.

There is a circular scuff mark on the (precision ground flat) top of the cross slide where the base of the compound assembly rotated. The compound was oriented as it is in the photos (i.e. perpindicular to the spindle) so the workpiece had maximum leverage in which to exert force on the compound assembly.

I took it all apart, hoping to bypass the compound and mount the toolpost directly to the cross slide. I did not see a way to do that; I would be interested in doing that sometimes because I rarely use the compound.

Anyway, I had already ordered a selection of appropriately sized T slot nuts which are on their way (photo below). My plan is to use Grade 12.9 (metric) bolts to use in place of the aforementioned stripped bolts. I will need to reduce the height of the bolt head so that it will fit under the compound upon rotation (one needs to use a cone wrench to tighten these).

Any thoughts about this? Maybe I should use a torque wrench the first time so that I can get a feel for the proper torque spec? I was giving it well over 60 ft. lbs. When I need a torque spec, I just Google the spec for that sized (and material of) hardware. The bolt will be an M8 or M10; I have to wait for my selection of T slot nuts to arrive in order to see if it is an M8 or M10.

 

[erikmannie]

Here is a photo of the T slot nuts I plan on using and a chart for maximum torque for coarse steel bolts:

 

[erikmannie]

The explanation of when the lathe went down is not to be confused with what caused the bolts to strip: they were stripped by myself when I overtightened them earlier in an effort to achieve maximum rigidity.

The point I was trying to make is that a compound assembly held down by stripped bolts is likely to rotate. I do lock the carriage when I face. The .005" DOC on hard steel is typical. I do about .010" on mild steel and .020" on brass.

 

[erikmannie]

Looking at the values on the torque spec chart, I am 100% positive that I abused the factory T bolts.

The local steel yard almost always has Grade 8 or Grade 12.9 hardware in the size that I need.

 

[davidpbest]

Make yourself a solid tool post like I did. Look here. I rarely have the compound on my lathe now. The improved rigidity will astound you - especially deep parting operations and improved surface finishes.

 

[darkzero]

Don't feel bad, I did the same on my previous lathe. Not really your fault IMO, the Chinese t-nuts and/or studs are soft. My lathe did not have t-slots like yours but instead of replacing the t-slot studs I converted it to use SHCSs instead of a stud with nuts. So much easier using a hex key instead of a wrench.

When I got my PM1236 one of the first things I did was convert it to use SHCSs instead of a stud with nuts. I quickly got tired of using a wrench cause I was so used to using a hex key. This reminds me, I need to make new t-nuts, keep forgetting.

BTW I think you are over thinking this a bit, no need for a torque wrench. Just get quality hardware & chances are you won't have a problem with it again. There's no need to crank down on those nuts hard. Making them super tight is not going to gain you any rigidity. A good cinch on them is all that's necessary.

If you want to increase rigidity consider making a solid plinth like some people do. It will make a big difference in rigidity over your style compound. Me personally with the stuff I do I didn't want a solid plinth. I use my compound a lot. Good thing about having t-slots on your cross slide is if you decide to make a solid plinth but any time you need to use the compound, you can mount it on the back side of your cross slide & still use it with the spindle in reverse. May be a bit awkward to reach though.

 

[mikey]

I agree with you that you probably over-torqued those nuts but was it necessary? If the compound is not seated well on the cross slide or if the parts of the compound are not machined for a good fit then things can move, even with a lot of torque on the nuts. If you had to crank down because things slipped in use then its time to evaluate fits because it should not take a lot of torque to lock down the compound. The wrench for mine is 3" long and mine has never moved, even with 0.100" depths of cut in steel.

I would third the plinth idea.

 

[erikmannie]

Make yourself a solid tool post like I did. Look here. I rarely have the compound on my lathe now. The improved rigidity will astound you - especially deep parting operations and improved surface finishes.

I looked at your Flickr page. That seems like a good project to have a thread on; what material is that made of? I’m surprised that people don’t offer these for sale as an aftermarket add on. DIY would be fun, though; I have a mill.

 

[ttabbal]

Well worth the effort. My plinth almost never comes off. Just if I want to cut a taper with the compound. PM1127.

 

[erikmannie]

Don't feel bad, I did the same on my previous lathe. Not really your fault IMO, the Chinese t-nuts and/or studs are soft. My lathe did not have t-slots like yours but instead of replacing the t-slot studs I converted it to use SHCSs instead of a stud with nuts. So much easier using a hex key instead of a wrench.

When I got my PM1236 one of the first things I did was convert it to use SHCSs instead of a stud with nuts. I quickly got tired of using a wrench cause I was so used to using a hex key. This reminds me, I need to make new t-nuts, keep forgetting.

BTW I think you are over thinking this a bit, no need for a torque wrench. Just get quality hardware & chances are you won't have a problem with it again. There's no need to crank down on those nuts hard. Making them super tight is not going to gain you any rigidity. A good cinch on them is all that's necessary.

If you want to increase rigidity consider making a solid plinth like some people do. It will make a big difference in rigidity over your style compound. Me personally with the stuff I do I didn't want a solid plinth. I use my compound a lot. Good thing about having t-slots on your cross slide is if you decide to make a solid plinth but any time you need to use the compound, you can mount it on the back side of your cross slide & still use it with the spindle in reverse. May be a bit awkward to reach though.

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I have never heard of an SHCS; I will Google it. It looks like what happened to me is a common occurrence. I don’t imagine that I am exceeding the capability of this small import lathe by cutting steel. I also plan to cut Grade 5 titanium, so we’ll see if this benchtop 10” can handle it.