R8 Collet Chuck or R8 Collet

I have a newly acquired SX2P mill with an R8 spindle. I have obtained an R8 collet chuck as I already have a Vertex M3 Collet chuck with a full set of ER32 collets, which seem to be of reasonable quality. The M3 collet chuck was used quite successfully on my Warco Mini Mill which is now in pieces as part of a on-going CNC conversion. Hence the purchase of the SX2P.

In addition to a few HSS endmills and slot drills I have a set of three indexible milling cutters in 10, 12 and 16mm diameters all with 12mm shanks, so far unused.

The question is would I be better off holding these directly in a 12mm R8 Collet rather than in the collet chuck?

The R8 collet will technically give you superior results since it holds the cutters more rigidly (and a couple of recent threads suggest more consistently accurately) but assuming your chuck and ER collets are goodish quality, you might struggle to notice a difference.

I dont have a collet chuck as yet just some R8 collets and I know some argue that the chucks take up space in the Z but I have found that there can be a down side to using just the collets. If the workpiece is not very high and is bolted straight to the table the tool is so low its difficult to see whats going on. Not so bad when you are using the vice or the work is taller but anything low level is partly hidden.

Definitely get the collet chuck. You can hold drills and taps as well as end mills up to the capacity of the collet. It will be a much shorter setup than the Jacobs chuck so you will be able to drill or tap work that would not fit under the Jacobs.

Indexables put them in the R8 collets direct and never look back, same with any cutter or tool.
Will be stupidly quicker to change, more rigid and induce less strain on head etc due the the horrible ER being extended (leverage.)

There are clearly differences in use, but I reckon CT is likely not far off the mark. Workpieces can (usually) be raised from the table by using parallels if one is visually challenged, while using direct collets, at times. As I see it, generally just needs thinking about before starting the job.

A lockable spindle is likely a help because whether removing direct collets or chucked collets, some effort is required each time the tool size changes. That where the old reliable Jacobs chuck comes into its own whenever only axial loads are concerned. Most larger cutters would be sourced to fit directly in the spindle?

I don’t have R8, but do have a Clarkson, ER and some discrete direct collets - all 2MT. I use each as I feel fits the task, but would not now complete a full set of direct collets. ER gets used most.

No question IMHO, go for R8 collets direct in the spindle. Easy to torque up, I just apply a spanner to the drawbar and hold the qull in my hand, good yank on the spanner, done. I have never had a milling tool shift in the collet. To release just loosen drawbar nut and give the top of it a gentle tap with the flat of the spanner head. Yes there can be a problem reaching thin material clamped direct to the table with a short or small cutter, in that rare case need to use a longer cutter. Great to have an extra 40mm or so extra daylight under the quill.

As NDIY says it is really horses for courses, I mostly use my ER 32 but do have a few common shank size MT3 collets but they rarely get use due to wanting more rigidity, I'm more likely to use them on a tall part where headroom is an issue though I do have a couple of 3/4" shank indexables that I use in a MT collet but they don't see a lot of use.

Even with the work raised up direct can obscure your view when using small cutters or you can run into hold downs and clamps, this is the SX2.7 spindle that is about 60mm dia with a FC-3 cutter in it, even if the work were in a vice you would have a job seeing what you were milling

As you have the collets already I woud get an ER holder and a couple of tool shank size R8 collets for when you want to take off a heavy cut where the shorter extension will be a help.

Concerning tightness.

I believe that according to Bridgeport, R8 recommended drawbar torque is around 25 - 30 ft lb as bing "normal tightening torque" for the drawbar thread. However translation of normal tightening torque into actual ft/lb does vary over the years and is complete pain to track down. Found it on the internet a couple of times and wrote it down but source has now disappeared. I think Tormach also suggest about 30 ft lb too but can't track that source down either.

I did just find a handy dandy tightening torques for fasteners table form Norbar, the toque wrench people, at **LINK** which says 26.5 ft/lb for ordinary P grade fasteners and 50.3 for better S grade ones in the 7/16 - 20 (UNF) R8 collet thread. Norbar say P grade fasteners have 35 tons/psi Ultimate Tensile Strength. Bridgeport drawbar suppliers usually claim them to be made of cold rolled steel which, for the more ordinary grades, tends to be quoted as having a UTS of 70,000 psi which is near enough the same. So having gone all round Robin Hoods Barn it looks like 25 - 30 ft lb is reasonable.

According to Techniks main data site **LINK** or summarised in a nice table here **LINK** ordinary ER-25 is around 77 ft-lb but the ball bearing mini-nut only needs 29 ft / lb. I imagine other breeds of ball bearing nut are similar so looks to be a no brainer to use a ball bearing nut on a lightweight machines. One tends to think that ball bearing might make the nut more likely to back off but clearly this isn't the case, at least not for professional gear.

With ER collets its important to remember that the holding part, full length on larger sizes but only part length on smaller ones, must be filled and the collet torqued up properly if specified run-out is to be achieved. The ER system uses the nut to assist alignment duties as well as providing the tightening torque so, theoretically, accuracy is compromised by the very design of the system. In practice everything is a compromise of some sort or other. The designer selects which ones he (or she) can live with in the application so that if used properly there are no significant real world issues.

Not sure that 77 fl/lb torque on my Bridgeport spindle to tighten the E25 set up doesn't count as a real world issue but I use the ER's about once every third blue moon after second muck spreading so no great worries. If they were my go to system I'd have bought a ball bearing nut years ago.

Clive.

Edited By Clive Foster on 14/12/2018 14:09:40