Help with linear rod drill press like design/materials

Question

I am putting together a basic machine for “pad printing” extremely small and precise parts. The design is very much the same as most homemade drill presses and consists of linear rods, flanged bearings and compression springs, but in place of a drill bit is a 30mm conical silicone rubber pad/stamp. The object being stamped will be positioned underneath on a precision XYR stage. A basic hand lever will be used to press the the rails/block downwards, applying minimal pressure. Scale will be approximately 10-15” tall. The blocks will be made of aluminium at this point and will be mounted to a very large solid base. Screws will he used to fix each component together. My goal, once set up, is for the pad to land in the exact same spot every time and with absolutely no movement or variation. It needs to be very rigid and with no slop.

I would like to know what diameter linear rods I should use, and if it makes much difference. I was originally looking at 40mm but this seems large?

Should I use standard steel rods? I have also considered introducing a ball spline shaft to prevent rotation.

I am not sure what size or type screws would be best suited for holding this thing together.

Are there any other factors I am overlooking that will help out?

I have come up with a few different layouts and I may be overthinking it. Do any of them make a difference to the overall result and accuracy of the press? Is there any particular direction I should be taking?

Thank you

Answer 1

it may be necessary to construct more than one design (known as iteration) in order to refine your final objective. "Very rigid and with no slop" is a rather general description. "A tolerance of 0.10 mm" would be more precise.

My understanding of pad printing is that one requires the impression pad to be re-inked from a source located near the destination. This suggests that your design requires a rotating head, which is another location of "slop" in the construction.

If you do not require maximum efficiency, you could get away with having the re-inker in the same location as the parts to be inked, with suitable alignment guides to ensure that the precision is maintained.

If you require the re-inker to be offset, your drawings don't show a precise enough option.

Generally speaking, for the most precision, use rectangular/linear assemblies and avoid cylindrical mounts for anything moving. My mini-milling machine has a rectangular column, while my drill press has a cylindrical column, as the drill press does not offer the precision of the mill. Related to this build question, the milling machine does not rotate about the z-axis and would not be capable of re-inking in the offset manner.

One of your drawings shows a pair of vertical rods. This approximates a rectangular support structure, but allows for torsion of the rods in the rotation of the z-axis. The 40 mm diameter referenced is not going to be "too big" for this type of structure, based on the description of the precision required.

Unrelated to this build, have you considered photo-lithography to accomplish your objective?

There are many details omitted from your design description, but enough of a basic question that this post may give you assistance.