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HP 7475A Plotter: Roland Knife Adapter

Knockoff Roland drag knife blades and holders being cheap and readily available on eBay, it didn’t take long to figure out that they’re not drop-in replacements for HP pens:

HP 7475A - Roland knife holder vs HP pen

HP 7475A – Roland knife holder vs HP pen

The Roland Cutter Knowledge PDF shows that the blade must protrude just slightly beyond the holder shell, letting the flat end stabilize the media and regulate the cut depth, but some experimentation was in order just to get the mechanics worked out.

The central brass blade holder looks like it should fit neatly inside the pen body outline:

HP 7475A - Roland knife holder - internal

HP 7475A – Roland knife holder – internal

A small O-ring normally fits in the thread gap to provide some friction between the two metal parts, with the knurled nut locking them together at the desired setting.

The blade rides on a smooth bearing pushed upward against a stop by a spring exerting 220-400 g on that rounded shaft. I think a real vinyl cutter would have a spring-loaded pin pushing downward on that shaft to provide vertical compliance at the blade tip, but I’ve never seen such a thing in real life.

That suggests half a pound of downward cutter force that the HP pen holder definitely can’t provide; the spec is 19±10 g.

Applying a digital caliper to the blade holder produced the usual measurement array:

//-- Drag knife holder

ExpRK = 0.30;						// expand critical sections (by radius)
AdjLen = 2.0;						// allowance for adjustment travel

KnifeOutline = [
	[0,0],							//  0 blade point (actually 0.25 mm offset)
	[1.0/2,0.0],					//  1  ... blunt end
	[1.0/2,4.0],					//  2  ... cylinder
	[2.0/2,4.0],					//  3 shank
	[2.0/2,5.9],					//  4  .. at bearing
	[6.0/2,5.9],					//  5 holder - shell
	[7.3/2 + ExpRK,8.3],			//  6 holder - taper to body
	[7.3/2 + ExpRK,21.0 - AdjLen],	//  7 holder body
	[8.8/2 + ExpRK,22.0 - AdjLen],	//  8 holder - threads bottom
	[8.8/2 + ExpRK,25.0],[9.0/2 + ExpRK,26.0],		//  9 clear threads to reduce friction
	[9.0/2 + ExpRK,32.0],[8.8/2 + ExpRK,33.0],		// 11  ... end clearance
	[8.8/2 + ExpRK,42.5 - AdjLen],	// 13 holder - threads top = locknut bottom
	[12.5/2,42.5 - AdjLen],			// 14 knurled locknut - adjustment travel
	[12.5/2,45.8],					// 15 knurled locknut - top
	[11.0/2,45.8],					// 16 holder - adjusting knurl
	[11.0/2,52.0],					// 17 holder - top surface
	[3.0/2,52.0],[3.0/2,57.2],		// 18 spring post
	[0.0,57.2]						// 19 end of post
	];

ThreadLength = KnifeOutline[13][HEIGHT] - KnifeOutline[8][HEIGHT];

Which spins up into a solid model of the brass part:

HP7475A - Roland knife holder - solid model

HP7475A – Roland knife holder – solid model

The large ring is slightly larger than the actual knurled nut, to ensure it cuts off the top of the HP pen body.

The raised section in the middle of the threads provides a little relief, as screwing the holder into a sufficiently snug plastic sleeve turned out to require more effort than seemed reasonable. I don’t have a tap for what might be a loose 9×0.75 mm fine-pitch thread (the actual OD is 8.75), so it’s gotta form its own path.

Subtracting the holder from the HP pen body produced an adapter much like the Sakura pen adapters:

HP7475A - Roland knife adapter - solid model

HP7475A – Roland knife adapter – solid model

Split across the flange for building:

HP7475A - Roland knife adapter - build layout

HP7475A – Roland knife adapter – build layout

Running the plotter in Etch A Sketch mode, that little blade actually cut a sheet of paper:

HP 7475A - Roland knife adapter - first cut

HP 7475A – Roland knife adapter – first cut

However, it didn’t cut very well at all, mostly because the pen holder doesn’t grip the adapter tightly enough to resist the lateral forces required to drive the blade through the paper, nor does it provide enough downward force to maintain the cut; I cheated by pressing on the holder to encourage the blade to keep on cutting.

By design, the plotter pen lift / drop mechanism doesn’t (and really can’t) apply enough downward force. A sliding bar across the entire width of the plotter raises the holder through a mechanical tab and lowers the holder by releasing the tab. A small spring then provides all the downward force, overcoming a dashpot that slows the pen drop to prevent crushing the nib against the paper.

Just for fun, though, I figured I should see what happens with the blade firmly anchored in the pen holder…

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