Dowel pins

  • Thread starter Charlie Griswold
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Charlie Griswold

Recently my company is having a machine built that uses Electric Cylinders that have no dowel pins on the base or carriage plates. Turns out most Electric Cylinder companies I have researched don't even offer dowel pin holes as an option. What is that? If you are buying a device for accuracy and repeatability of +/- .003" are you going to mount the things together on a cnn machine and lock tight them together?

How much cost and forethought would it be to include dowel pin holes and does any body know a manufacturer of such a device?
My company manufactures equipment that uses dowel pins as you describe. In some assemblies, we've had problems getting parts to align and
function properly when dowel pin holes are pre-drilled. The problem is the sum of tolerances on hole diameters and hole location of the two mating parts. If these assembly tolerances will affect your +/-.003 accuracy and repeatiblity, then I think you may pay dearly for super-precision dowel pin holes. In these critical applications, we drill holes and press pins after assembly.

David L.

Charlie Griswold

Match drilling, as you suggested, is the option of last resort. It only prevents parts from shifting once aligned. It "holds" the machine accurate and repeatable, but you have to get it that way by aligning mating parts on a mill.

Say you need to replace a part. Separating match pinned parts is nasty work. Plus, you will need to send the whole assembly to a machine shop to
re-align the mating parts and drill and pin again. What customer with a down machine would want to wait for that?

I agree dowel pin holes must be held to tighter tolerances than conventional fastener holes when you drill them in pre-assembly, but that is why they are there. You hold the accuracy between both pin holes and you make sure the pin holes are parallel (or perpendicular) to the actuator's travel. That way you know the mating part will be both perendicular and parallel when you put the dowel pins in, provided the mating surfaces are also machined that way. It requires a little more control and quality assurance on the
designer/machinist's part, but it automatically aligns the parts without using a mill at assembly, and holds accuracy without having to match drill.

That is one reason why you pay more for an actuator being +/- .003 accurate/repeatable verses +/-.010 accurate/repeatable. Accurate dowel pin holes cost a little more time and money to put in, but they save you having to align the mating parts on a mill; You can now replace parts out in the field without having to match drill (direct from factory); and your machine can not fall out of alignment over time unless the parts/actuators actually get bent.

Share this information with your marketing department. I would pay more for an actuator with dowel pins over one without and would make it a prime criteria in selecting a brand of actuator.