# Bolt strength for mounting caster/leveler to cast iron drill mill base??



## accokeek (Dec 28, 2014)

Greetings

I'm a relatively new member and am in the process of acquiring a PM932M-PDF drill/mill, I have what I guess is a classic noob question.  I am planning on using Foot Master GDR-80S leveling casters mounted to the cast iron base of the unit.  The manual (Thanks, Matt!!) shows the base with a stem mount and the specs for the caster shows an m12x1.75 bolt/threaded stud.  So my question is what level of bolt should I be using Grade 5 (8.8M) or Grade 8 (10.9) to mount these leveler/caster unit.  The PM-932M-PDF has an installed weight of about 1,050 lbs, which is why I'm planning to go with the GDR-80 versus the GDR-60 units, since conceivably there could be 200-300 lbs max load on the table at some point.  Guidance/experience/thoughts would be appreciated

Thanks


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## dracozny (Dec 28, 2014)

I typically would use a grade 8 for anything with any serious load, although I have used cheaper bolts. for a Caster I say go with peace of mind and stick with the Grade 8. 
http://tinelok.com/grade-5-vs-grade-8-fasteners/


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## Terrywerm (Dec 28, 2014)

Your description indicates that the casters are stem mounted, with the stem oriented in a vertical position, up through the mounting foot of the mill/drill stand. The weight of the machine sits directly on the caster swivel bearing or frame and the mounting stem is used only to keep the caster in place on the bottom of the stand. Additionally, a few hundred pounds per caster is next to nothing in the grand scheme of things, and you should be able to get by just fine with Grade 5 bolts or stems.

If the stems are already on the casters, I doubt that you will have any choice as to the grade of stem or bolt. To be honest, I've never seen a light or medium duty caster (up to 1000 lbs or so) that had a grade 8 stem.


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## awander (Dec 28, 2014)

The Foot Master casters have a tapped and threaded hole for the "stem", which in this case is an M12 bolt.

Because the caster wheel is offset from the axis of the bolt, the force is not simply straight down, but will tend to rock the caster and bend the bolt.

I would say go with the strongest you can find, the 10.9 fasteners. What is it going to cost, a dollar or so extra for the 4 bolts you need?


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## Tony Wells (Dec 28, 2014)

IMO, there is little to be gained by exceeding the tensile strength of the material you are threading into.


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## Terrywerm (Dec 28, 2014)

If the casters are bolted up tight to the bottom of the stand as they are meant to be, there is no chance of the bolt bending. At most the mounting pad of the caster would act as a fulcrum, and try to strip the threads out of the caster or off of the bolt. That simply is not going to happen with the machine that we are talking about here. It's just not heavy enough to do that.


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## accokeek (Dec 28, 2014)

terrywerm said:


> If the casters are bolted up tight to the bottom of the stand as they are meant to be, there is no chance of the bolt bending. At most the mounting pad of the caster would act as a fulcrum, and try to strip the threads out of the caster or off of the bolt. That simply is not going to happen with the machine that we are talking about here. It's just not heavy enough to do that.




Thanks to all for your quick responses, as awander pointed out the cost for higher strength bolts is at most a dollar or two, so since the 10.9 bolts are readily available maybe I'll go with them.  I really appreciate everyone's responses, it can be difficult to bootstrap into a new endeavor


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## extropic (Dec 28, 2014)

An 8.8 12mm (1.75 pitch) bolt has an ultimate tensile strength of 15,155 lbs. (67,412 newtons). You could *HANG* your mill and (I expect) your car from one bolt and have a very comfortable safety margin. 8.8 bolts for your casters will be trouble free under any rational circumstances.


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