Delrin vrs Bronze bushings

[BoxGods]

I was wondering if anyone had an opinion on the reletive merits of using delrin or bronze for the bushing material on the slide blocks of my CNC router. Currently in my design I have 1" thick aluminum blocks that are 3.25" square and there is a 2.5" dia Delrin plug pressed into the center of the block. ACME 1/2-10 to be threaded through.

Would it be better to just press in say a 1" long pre threaded cylinder nut? Bellow is a picture that might make it a bit clearer.

[NC Cams]

You need to be more specific when you say "bronze". There are literally hundreds of bronze alloys to choose from.

There are lead based bronze and tin based bronze. Some make good bushings and some make great pipe fittings but the alloys are not interchangeable.

A bit more specificity please.

[BoxGods]

Sorry, I thought "bronze bushings" pretty much excluded pipe fittings.

Silicone Bronze or one of the alloys that is typically 30% oil content, typically used for a bearing surface.

Thanks =)

[NC Cams]

Porous bronze that is capable of retaining oil is actually a sintered bronze. The porosity is good for holding/retaining oil but the void can reduce the ultimate load carring capabilities of the material, especially under high heat or high load conditions - thus it can yield more readily.

If you're looking for good siezure resistance and good physical strength, you may want to consider a leaded bronze. The bronze (copper + zinc) provides the strength/yield resistance whereas the lead provides the "slipperyness" (surface action) that helps reduce friction and/or scuffing.

Since almost any cast billet bronze should have a higher yield strength than delrin, it would have much less of a tendency to yield or "creep" under load. The slipperyness of the bronze versus the delrin would depend, again, on which bronze alloy you choose.

Keep in mind that sintered bronzes may work exceptionally well in motor bushings because there is a high/higher speed potential - the speed combined with the lube film from the impregnated bushing and oil applied to same will result in the development of a hydrodynamic oil film which will prevent/resist wear and friction.

In a ball nut or screw application, howwever, you're going to rely more upon the natural siezure resisitance of the material. This is where a bronze with high strength and load carrying capability along with good scuff resistance will serve you better.

A billet aluminum-sllicon bronze material (as opposed to sintered material) such as UNS C64200 would be my first choice. This material works quite nicely as bronze guides for automotive engine valves among other uses. It needs minimal lube and provides high wear resistance, very good creep resistance and high scuff resistance under conditions of marginal lubrication like that encountered in valve guides.

It is also pretty slippery when properly finished (reamed) and even lightly lubed with oil and it has very good wear properties.

[BoxGods]

Thank you very much. Bronze it is =)

[Zumba]

Keep in mind that you may WANT the compliance that Delrin gives you. If lubricity were equal between the two materials, there would still be less friction with the Delrin unless the alignment of all components were perfect. Misalignment + bronze = binding. Lots of it.

Companies like Legacy Woodworking (makers of the hand operated "4th axis" rotary carving gizmo) use Delrin nuts not only because they're cheaper, but it makes their machine work despite sloppy assembly (by the user since the machine is shipped as a box full of parts).

Besides the friction issue, there is also the accuracy issue. Milling machines and lathes use bronze nuts because they're heavy duty. While most good machines have split nuts, where backlash can be reduced, there's a sweet spot between "too much friction" and "too much backlash". Unfortunately, that sweet spot is always a compromise. You'll never get 0 backlash without too much friction. With manual metalworking machines, backlash is rarely in issue. Cuts are almost always unidirectional or point to point, and the DRO saves the day. Not so with CNCs.

Delrin allows you to take backlash down to zero thanks to its compliance, without creating a ridiculous amount of friction. However, that compliance will bite you in the ass if the Delrin is put under heavy load.

So there really is no correct answer without seeing your machine in person and testing it with both materials. If you want the best of both worlds, you gotta use ballscrews. That's why they exist.

[BoxGods]

You make an interesting point, especially in my case. A friend of mine is building this machine also so that we can just email gcode back and forth. He is in another state. Let's just say he is a "close enough" kind of guy =)

Throughout the design process I have worried wether or not he was up to assembling his machine, and your mention of the Legacy Woodworking machine example might give him a little bit of needed wiggle room.

Thanks