Denford Starmill – Spindle material and finish

[RainbowsParticipant @rainbows]

I’ve come into possession of a Denford Starmill with a R8 nose spindle, bit unfortunate as the 30 taper spindle would be much handier for tool changes. But the nice people at Denford, with no machines or spares to sell and thus no monetary stake left, have released all the manuals, assemblies and in this case even blueprints for their early mills.

 

Looking at the spindle I feel its a bit of a bugger to turn. I can steal the use of a nice big lathe and mill but no DROs. Turning the bearing seats and taper in one set up will give inherently closeish accuracy. The internal features are going to have to be a touch off on the way in and feel around blind with the dials.

However the original calls for some hardening and grinding. A cylindrical grinder I do not have. I can get the bearing finish with an external lap but warping from heat treat could still knock it out of line. The internal taper I haven’t worked out yet.

 

Could anyone recommend a good steel for this? I’m hoping someone out of nowhere shows me a steel that machines beautifully, has minimal internal stresses and somehow comes pre hardened at the same time. I’m vaguely aware some can have modern surface processes to harden them without any warping, but then minimum order quantities become a thing which I imagine may already be a problem getting anything fancier than mild steel.

[JasonBModerator @jasonb]

If it is just being done for tool changing have you thought of the Tormach TTS or copies? uses the R8 and smaller tool holders.

[David George 1Participant @davidgeorge1]

I would use EN24T It is pre hardened but machinable. I have recently made a spindle for my M yford M Type lathe from this and had no problems since. I used a fixed centre to hold the spindle whilst finnishing the od and bore etc.

[John HinkleyParticipant @johnhinkley26699]

Have you access to the drawings and dimensions for an R8 spindle?  If so, would it be possible to have the R8 taper location taken out to INT30 dimensions, subbing it out to an engineering firm, if necessary?

John

 

[JasonBModerator @jasonb]

Seem to be a few running the TTS on the denford forum

[FulmenParticipant @fulmen]

EN24T (34crnimo6) would be my first choice as well.

[JasonB]

On JasonB Said:

If it is just being done for tool changing have you thought of the Tormach TTS or copies? uses the R8 and smaller tool holders.

I was about to bite on TTS tooling until I saw a youtube series by CasualHaze. He’s installed a electro-hydraulic power draw bar on his slightly bigger more modern Denford Triac. Where 30 taper tooling needs something like 500kg drawbar tension he was last I saw hitting 1500kg. More searching had some people (on probably larger machines) recommending 2500kg. (Though that the Denford Forum guy has it on a manual lever subverts that, interesting)

On threaded manual drawbars not much of an issue, and Tormach has a triple stack air cylinder for their factory fitted machines. But having seen footage of hydraulic injection injuries I didn’t feel like copying CasualHaze, and hopefully the cost and faff of a new spindle will be approximate to that of buying TTS tooling and the high power pneumatics. 30 taper tooling seems to quite readily roll around ebay and autojumbles.

While the R8 spindle is the same outline so enough meat for conversion it’s missing the internals for the grabber and beville stack. It saves material cost, some hogging and getting the bearings to size but still quite a job. I have it in my head I can sell the old spindle to recoup some costs (possibly unreasonably optimistic) or keep it as a spare in case the new spindle catastrophically breaks (possibly unreasonably pessimistic).

EN24T seems to be the material consensus. Black EN24 of the required size comes in at £40 which seems fairly reasonable. We will see if I burn through £40 of carbide inserts trying to turn it while I’m at it :p

Hopefully this will later convert to a spindle build thread, assuming I get around to it before an angry mob burns the forum down

 

[JasonBModerator @jasonb]

I’ve seen the TTS with Belleville washers

[Rainbows]

On Rainbows Said:

We will see if I burn through £40 of carbide inserts trying to turn it while I’m at it

I found that EN24T turns beautifully, even on a [cough] mini lathe. (I used the CCGT “aluminium” inserts on it.)

I used it when I made the shaft for my model gas turbine, and (more relevant) an ER11 spindle for my little CNC.

 

Sequence of operations was a bit of a head-scratcher, and it was much more straightforward than your design.

I cut the taper before finishing the bearing journals and turned a matching male taper at the same settings to use as a live centre to mount the shaft between centres to finish the bearing journals.

I ended up with less than 4µm runout on the tool shank in the end, most of which came when mounting the bearings.

Link if interested:

Making a Stiffer ER11 Spindle

[ChrisLHParticipant @chrislh]

You don’t say what your intended use is but if it’s not heavy you could consider making your new spindle from En1A for easy machining !

All steels have the same Youngs modulus so stiffness would not suffer. It would, of course, not be as strong as the original which was made from good stuff but you would be hard put to it to stress such a dumpy part enough to bend it. It would be vulnerable to accidental dings though.

[FulmenParticipant @fulmen]

EN24T machines quite nice considering the strength. You will need a bit of patience and preferably some flood cooling, but you shouldn’t have any problems. You can machine it with HSS so if you start breaking inserts it’s your fault.

[EmgeeParticipant @emgee]

The Emco F1 cnc milling machine uses a manual toolchange with pressure from a Bellville washer stack, toolholder form is SKF30 but BT30 wth a separate M12 45deg draw stud fitted is a replacement.

You could continue to use the R8 with a pull stud fitted and mods to the spindle housing to accommodate the retaining mechanism.

Emgee

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