Wednesday, 3 July 2019

Bearing retainer plate - CAM and machining

What is it?

This bracket retains the ball bearing at the bottom of the ballscrew. Modelled in a fetching chestnut brown here:

CAM for the bearing retainer bracket:

Found a bit of mystery metal that's about the correct size. Looks like cold rolled steel, about 1/4" thick. Should work nicely.

The CAM seems simple enough. Face off, drill and chamfer the fixings, machine the cavity, then contour the external profile, leaving support tabs which I will remove later. Finally, chamfer the hole and circumference.

This is what it should end up looking like:

Machining the bearing retainer:
The new Korloy APMT1604 PC5300 inserts arrived from China this week, c/o Aliexpress. Yes, they are a massive improvement on the Mitsubishi inserts. It illustrates clearly that there is a critical difference in the geometry, although it's not blindingly obvious at first glance when you compare them side by side. 

Yet again I've caught myself out by being too aggressive with stepdown and feedrate when facing off strip / thin bar. It managed to make a half decent job but squealed like a stuck pig and left a "suboptimal" surface finish. It's not rocket science but seems to be taking me a while to learn this simple lesson. It'll be OK for now.

The other issue which isn't obvious here is that the 8mm cutter slipped when machining the central pocket. The reason for that is fairly simple - the cutter doesn't have the Weldon flat on its shank, yet I insisted on using it in a side holder with grub screw. I've got away with it previously but here I was doing some fast plunges which caused the slippage. It required me to remeasure the tool length offset and repeat the operation (twice!). This is the final re-run, which at last gave the required result for the contour operation. I then had to repeat the operation for the central bore, which you can see didn't quite end up as a through hole.

The other issue highlighted here was that I had a rather aggressive plunge feedrate. And a rather aggressive speed and feed programmed for the cutter. Like 5000rpm, 800mm/min (feed) and 500mm/min (plunge). It was the plunge wot did it. When you add tabs to a 2D contour, the cutter plunges back in after passing the tab. Lucky I didn't bugger the cutter and / or work. And lucky it was a centre cutting end mill.

Here it is, ready for cutting the support tabs. Looks OK - apart from the chatter marks on the surface.

Seems to be as intended.

And yes, the pulley boss clears the central bore.

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