Fastening and Attaching

On Tuesday (2/2/2016), we visited three different stations to learn different ways of fastening and attaching Delrin parts together. The different stations we visitied were:

Station A: Drill press, arbor press, piano wire fastening
Station B: Thermal press
Station C: Calipers, bushing measurements and tolerances, peg and slot measurements and tolerances

Station A:
My partners from the bottle opener project, Amy and Marissa, and I visited Station A first.

Drill Press and piano wire: Drill presses allow you to make holes of a certain size to fit (loose or tight depending on your preference) a piano wire in your delrin. For example, say you wanted to connect three pieces of delrin, and want one to be fixed while the other two rotate together. You can drill the holes such that the two rotating ones fit tightly around the piano wire and the fixed one fits loosely to allow for the rotation of the others.

Using the drill press, we learned a few safety tips like:

• keep your hair tied at all times (such that it doesn't pass your chins, in Larry's own words)
• Fix the Derlin piece aligned to the drill. Make sure it is tightly clamped to the stand so that it doesn't rotate with the drill. 
• And keep your fingers away from the drill until it has stopped rotating completely.

Arbor Press:
We use an arbor press to hold the piano wire aligned to the hole we made with the drill press and apply force on it for it to fit inside and connect our pieces.

Benefits and draw backs:
It is easy to accidentally drill holes that don't align or melt the Derlin from the heat of the drill, but if used with care and precision, this method works well for both tight and loose fit attachments that may or may not allow motion and can be taken apart if needed.


Station B: 
Next we visited the thermal press station where we melted two parts of Derlin together to attach them permanently. If the connection is meant to last forever, this is the best method out of the three to use. However, if one wants to rearrange the attachment later, the thermal press may not be the best option.

Safety tips around the thermal press machine:
You keep your fingers away from the tip of the heater while and after it is operating until you cool it down for 30 seconds by turning on the air supply.

The figure above shows the thermal press with its warning and the venting system with a pedal to step on (the fun part!).



Station C:
Here we used calipers to measure the dimensions of bushings, pegs and slots. These also allow tight (but not permanent) or loose attachments for pieces of Delrin. Due to uncertainties in measurement and the slight imprecision in the laser cutter, it is good to try small parts of Delrin and see how they fit before producing the life-size model. The laser cutter burns away parts and may burn slightly more than the intended. Also its intensity decreases with depth. So it cuts more of the top layer than the bottom (at an angle). Depending on which side you measure from, the width may be different.

We measured the diameter of a Delrin rod and the inside diameters of several bushings (loose, snug and tight fits).

Here I was measuring the diameter of the Delrin rod in millimeters. 

 Table 1: Measured diameters of bushings and Delrin rod

Item measured	Diameter (mm)
	Sample 1	Sample two
Tight fit bushings	6.35	6.33
Snug fit bushings	6.46	6.49
Loose fit bushings	6.65	6.64
Delrin rod	6.33

 Notice that the measurements within each type of fit are different. This could be because how hard I pressed the caliper isn't perfectly controlled or simply because the cuts aren't perfect fits. Trying parts to see if they fit is safer than fully relying on measurements.

We then measured the slots already labeled with dimensions to compare intended sizes to the final measurements.

Table 2: Labeled size vs measured size of slots produced by a laser cutter

Size written (in)	Samples	Size  measured (in)
0.135	Sample 1	0.1445
	Sample 2	0.1435
	Sample 3	0.1425
0.125	Sample 1	0.1345
	Sample 2	0.1345
	Sample 3	0.1335
0.115	Sample 1	0.1250
	Sample 2	0.1175
	Sample 3	0.1175

Notice that there are a range of values measured for slots labeled as the same size. But what all the measured sizes have in common is that they are all larger than the written values. This is because the laser cutter burns material slightly thicker than the line we make for it to follow. So we need to account for this when making slots for any type of fit.

We measured the pegs to fit in these slots and found:
Table 3: Pegs dimensions

Dimension (in)	Peg #1	Peg #2
Length	0.4930	0.4845
Width	0.1330	0.1394
Depth	0.1710

We then measured unlabeled blocks for the depth and width of slots and pegs and found:
Table 4: Slots and pegs (not previously labeled)

Dimension (mm)	Slots		Pegs
	# 1	#2	# 1	#2
Length	9.94	5.13	9.82	5.07
Width	5.13	4.76	5.18	4.76
Depth	5.02	5.07	4.95	5.10

 Note: The dimensions the thickness of the Delrin was equal to the width of the pegs, which I labeled as 3/16 in = 4.76 mm.

The connection was a snug fit.

From this station, I have learned that measurements are largely subject to uncertainties and that one needs to account for discrepancies and try the connections before producing entire models. 

Thank you for your time.

Best,

Meba