Thread: Hole tolerance for 3jaw chuck backplate

I have made a back plate for mounting a 150mm 3 jaw chuck. It is a SanOu brand which has 4 8M threaded holes for mounting. I have done the shoulder to fix into the jaw's register. The next step is to drill 4 holes for mounting the chuck. I painted the holes on the jaw and pressed against the back plate. It left an impression on the back plate. I assumed that it should be pretty close.

I have made centre holes first and measured again the distance between 4 holes. However, I have done the centre hole in one go and then 8mm drill bit for holes. I have difficulties on getting the 8 mm drill bit on the centre of the centre hole. I have tried my best to centre the drill. When holes are done, I have no luck of matching them with the threaded holes on the chuck. It needs a lot of precision.

My question is:
1. Should I use 8.5mm drill bit to build in some tolerance.
2. Should I do the centre drill, followed by 8mm drill bit. In that case, centre drill and 8mm drill should be in line
3. What is the best practice of doing this. I am sure that it really requires accuracy and tolerance consideration.
4. What is my option now, perhaps re-drill?

Easiest way if the holes are only out by a little would be to drill them bigger, say 9 or 10 mm and put a washer under the bolt head.

Would of been good to of marked a PCD line on the face of the backing plate whilst it was in the lathe.

After removing use dividers to mark the cordial distance between the holes.

Centre punch your marks the spot or centre drill your holes, you can spot or centre drill all your holes first and then finish with clearance drill, if clearance drill is to big to be used in one go a pilot hole may be required, if the pilot is being used it is better to drill your clearance hole in the same set up, chamfer holes or remove burs.

All of the drilling should be completed with the back plate held securely

[QUOTE=pipeclay;1990159

Would of been good to of marked a PCD line on the face of the backing plate whilst it was in the lathe.

[/QUOTE]

What is a PCD line?

Since it is 4 threaded holes, I can assume they are 90 degree from the centre, right? Yeah, just measure them, they are 90 degree from centre.

Will it be more accurate, if now I mark these 4 holes based on above information. It is a matter to determine how far the centre of the hole from centre. How accurate do I need to get. Within 1mm when measuring the distance from centre? Still probably not enough?

This is one example and will explain what PCD is and how to calculate it.

https://www.westerntowing.co.uk/pcd-calculation/

Do a search for chordal distance calculator.

If you own a pair of dividers mark out your PCD by jamming a piece of wood into your backplate bore, cut up a drink can or similar light metal, place a piece about 25 mm square in the centre of the wood, hold it there by bending the 4 corners 90 degrees and knock it into the wood so it holds, use your dividers from the outside edge of your back plate and scribe 4 radial lines across the material attached to the wood, ( does not have to cross at the centre but cross on the material.

After the lines are marked scribe lines from the opposing corners, where they cross will be the centre of your back plate.Then scribe the PCD with the dividers, put a centre pop mark on the PCD, set your dividers to what you calculated your chordal distance to be and then mark the position of your 3 remaining holes, the 1st one will be your centre pop, to check you are correct with the dividers scribe 3 more lines going in the opposite direction to how you scribed them the first time, the scribe marks should all cross on your PCD line.

A pair of Jenny calipers would make it easier to find your plate centre.

Someone on here might offer to do it properly for you, if they knew where in Australia you are located.

What about doing it like this?

https://www.arceurotrade.co.uk/Catal...ransfer-Screws

I just realise that the most important criterion is the register of the chuck which governs the concentricity of the back plate and the chuck. The holes of the bolt will be nice if they are perfectly match up. Otherwise enlarging the holes just to allow the bolts to be fastened will be do for the purpose of mounting.

Is my understanding correct?

The first answer related to that, I didn't realise you are a complete novice, I thought there was some prior knowledge.

This write-up on the lathes.co.uk website might be of use to you - Fitting a lathe Chuck

You are correct in saying the spigot machined on the backplate to match the chuck register is the most critical feature - the mounting bolt holes do not have to be close fits as described in the linked article above.

I fitted a similar rear bolted 125mm 4-jaw chuck to to a backplate some time back and for the 7mm mounting screws on that I used 8mm holes in the backplate. For that installation I used homemade transfer punches that worked the same as those that SurfinNev linked to above.

Ray

Someone also said that the front and back of the plate should be parallel as much as possible. Again what is the tolerance. I have measured 4 spots on the plate the thickness has a 0.04mm difference around the circumference.

Hi Dave, Guys,

The easiest way to ensure that the chuck is true, is to fit the backplate on the lathe spindle and take a fine cut across it. If you have already cut the register then only cut up to the register.

A trick that the more experienced use is to cut the register 10 thou small and then when the chuck is fitted and the fastening screws are just nipped. Then use a piece of precision round bar in the chuck and a gauge on the bar then tap the chuck to get it as true as possible, before tightening the screws firmly.

I have mounted the back plate on the chuck. In order to tidy up the back plate, I clamped the chuck in 3 jaw chuck on the lathe. I have about 02mm run out on the clamped chuck. I have swiped the 3 jaw chuck on the lathe, the runout is only 0.01mm. When in turn clamping another chuck, the run out is multiplied by 10 times. Do I need to get in clamped on a 4 jaw in order to dial in for for achieving less run out?

Are you able to post a photo/s of what you are trying to do here holding one chuck/backplate in another?

There are many types of backplates so I think some clear pictures of what you are doing (and why) would help explain this.

Ray

Originally Posted by davekwok

I have mounted the back plate on the chuck. In order to tidy up the back plate, I clamped the chuck in 3 jaw chuck on the lathe. I have about 02mm run out on the clamped chuck. I have swiped the 3 jaw chuck on the lathe, the runout is only 0.01mm. When in turn clamping another chuck, the run out is multiplied by 10 times. Do I need to get in clamped on a 4 jaw in order to dial in for for achieving less run out?

Hi Dave,

Clamping one chuck in another is never going to give you any accuracy ! Once you remove and refit it things will change.

All 3 jaw chucks have a few thou of runout and that will be multiplied by the runout of the second chuck ! 4 Jaw chucks are good simply because you can adjust them for minimum runout.